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Calcium (Ca) isotopic composition of the mantle has been previously estimated as δ44/40Ca. However, δ44/40Ca of the mantle records both its stable isotopic composition and excess/deficit in radiogenic 40Ca that can be produced by the decay of 40K. Here, we present the high-precision Ca isotopic compositions of 16 Archean komatiites from five representative localities (Barberton, Munro, Abitibi, Yilgarn, and Taishan). Due to the high degrees of partial melting that led to their formation, these komatiites are used here to simultaneously estimate ε40/44Ca, δ44/40Ca, and δ44/42Ca of the bulk silicate Earth. The komatiites exhibit a resolvable negative ε40/44Ca relative to SRM915a, with an average of −0.6 ± 0.1 (2SE, 95 c.i.% if not specified). Given the lack of significant differences among the samples and the non-zero value of ε40/44Ca, we suggest using δ44/42Ca to report the stable Ca isotopic composition in this study. Excluding one sample (SD6/400), which may have been affected by alteration, the remaining komatiites display relatively homogeneous δ44/42Ca, ranging from 0.39 ± 0.02‰ (2SE, N = 16) to 0.49 ± 0.02‰ (2SE, N = 10), defining a mean of 0.44 ± 0.02‰ (2SE, N = 15). The absence of correlations between δ44/42Ca and LOI, Sr/Nd, (Dy/Yb) N , and (La/Sm) N among the komatiites indicates that post-eruption alteration, the influence of residual garnet, as well as prior melt extraction from the mantle source have had a negligible influence on their stable Ca isotopic compositions. Based on available experimental petrological data and isotope fractionation factors, it is demonstrated that komatiites were insignificantly fractionated from their mantle sources (i.e., <0.02% in δ44/42Ca). Their δ44/42Ca represents the ambient mantle at the time of segregation. Accordingly, we suggest that the average δ44/42Ca and ε40/44Ca of the komatiites measured here are representative of the bulk silicate Earth. Our new estimate can serve as a reference point for Ca isotopes as a two-dimensional tracer for sources and geological processes. • Komatiites exhibit homogeneous radiogenic and stable Ca isotopic compositions. • A radiogenic 40Ca deficit in the mantle relative to SRM915a has been confirmed. • The bulk silicate Earth, on average, has a δ44/42Ca of 0.44 ± 0.02‰ (2SE) and an ε40/44Ca of −0.6 ± 0.1 (2SE). [ABSTRACT FROM AUTHOR]

With the most prominent topography on Earth, the Tibetan Plateau has profound influences on the hydrologic cycle and climate dynamics in Asia. However, the regional uplift history of the Tibetan Plateau remains highly debated. Here, we use europium anomalies and LREE/HREE ratio in detrital zircon, combined with whole-rock La/Yb N ratio, to constrain the crustal thickness and topographic evolution of the Gangdese mountains in southern Tibet. Europium anomaly, controlled by pressure-sensitive minerals plagioclase and garnet, has been shown to correlate with crustal thickness. Our results reveal contrasting crustal thickening histories of the eastern (east of ∼88°E) and western (west of ∼88°E) parts of the Gangdese. Prior to the India-Eurasia collision (∼60–55 Ma), the crust of the eastern Gangdese thickened continuously from ∼40 km to nearly 60 km, while the western Gangdese maintained a mildly thickened crust of ∼50 km. Both parts underwent substantial thinning (to 40–45 km) immediately before the India-Eurasia collision, but the eastern Gangdese re-thickened rapidly afterwards. In the western Gangdese, post-collisional thickening was delayed until ∼40–20 Ma, which resulted in a mild elevation of ∼2000 m for most areas of southern Tibet until the late Oligocene. This diachronous uplift of the Gangdese after collision may have resulted from more mantle-derived magma additions in the eastern Gangdese before collision, which could rheologically weaken its lithosphere. We suggest that the diachronous uplift of the Gangdese after the India-Eurasia collision may have dominated the moisture transfer pattern across southern Tibet through the late Eocene and intensified the South Asian monsoon through the Oligocene-Miocene boundary (∼25–20 Ma). • Eastern and western Gangdese show different crustal thickening histories. • Uplift of western Gangdese was delayed by at least 20 Myr after collision. • Diachronous uplift of Gangdese might impact the regional and South Asian climate. [ABSTRACT FROM AUTHOR]

Late Ediacaran to Cambrian metagranitoids from the northern Veporic unit of the Western Carpathians show imprints of three metamorphic events, which can be assigned to the Cenerian, Variscan, and Alpine orogenies based on electron microprobe dating of monazite. Metamorphic monazites in the metagranitoids are mostly of the Lower to Middle Ordovician age (480–460 Ma). The Ordovician monazite formed in equilibrium with the metamorphic assemblage garnet, biotite, kyanite, ilmenite and quartz at P-T conditions of 6–7 kbar and 550–570 °C, thus providing clear evidence for Cenerian metamorphism in the Western Carpathians. Variscan metamorphism caused minor monazite growth/recrystallization at 364 ± 13 Ma and produced a new mineral assemblage garnet, kyanite, rutile and phengite at P-T conditions of 20–22 kbar and 670–690 °C. The low-grade Alpine metamorphism is recorded by monazite of Cretaceous age (96 ± 23 Ma), but only in orthogneiss of extremely low-Ca composition. The Veporic metagranitoids show incomplete transformation from magmatic stage, still preserving remnants of plagioclase, K-feldspar and high-Ti biotite. This indicates that the metagranitoids remained relatively dry and failed to complete metamorphic reactions due to kinetic factors at fluid-deficient conditions. Nevertheless, the metagranitoids likely underwent significant geochemical changes during their metamorphic evolution including a severe loss of CaO and Na 2 O. • First recognition of Ordovician metamorphism as the result of Cenerian orogeny in the Western Carpathians • Ordovician, Carboniferous and Cretaceous orogenic events revealed by monazite dating, mineral assemblages, P-T conditions • Polymetamorphosed metagranites with preserved magmatic and multiple metamorphic stages due to fluid-deficient conditions [ABSTRACT FROM AUTHOR]

Based on petrological association, cathodoluminescence (CL), trace element signatures and orientation relationships, two generations of kyanite are distinguished in a high temperature, high pressure garnet-biotite-aluminosilicate bearing migmatite of South Harris, NW Scotland. The migmatite shows a garnet and biotite rich domain (Grt-Bt domain) which is cut at a low angle by a dominantly coarse-grained plagioclase-quartz leucosome. In addition, a fine-grained plagioclase-quartz-kyanite domain (Plag-Qtz-Ky domain) is present intercalated with the Grt-Bt domain and subparallel to the plagioclase-quartz domain. Type 1 kyanite is coarse grained and associated with Bt clusters and garnet within the Grt-Bt domain. It grew relatively early, syn - to post-garnet growth, in a suprasolidus environment resulting in crystallographically determined oscillatory CL and trace element zoning. Grains record evidence of progressive deformation in the crystal plastic regime, where deformation is accommodated by dislocation glide, climb and deformation twinning. The dominant activated slip system is (100)<001> with minor component of <100>, while deformation twins show a ∼ 180° rotation around ∼<001> axis and a twin plane near (001). Grains appear to be impervious to deformation induced diffusion with all zoning remaining sharp despite crystal plastic deformation. Grains in direct contact with the Plag-Qtz-Ky domain show late modification of the CL and trace element signature suggesting melt-mediated interface-coupled dissolution-precipitation reaction. This modification resulted in crosscutting lobate high trace element regions and irregular rims with low Cr and V content. These rims show similar CL and trace element characteristics as Type 2 kyanite which are exclusively seen within the Plag-Qtz-Ky domain suggesting that Type 2 grains are cogenetic with Type 1 rims. Type 2 grains are finer grained than Type 1 grains and show near uniform CL and trace element distributions with rare oscillatory zoned and relatively higher Cr & V bright cores. Type 2 show either no or very localized internal deformation features. However, they exhibit a clear shape preferred orientation which coincides with a crystallographic preferred orientation where the longest shape axis is parallel to <001>. We propose that Type 2 kyanite grains underwent melt-present deformation by rigid body rotation in an externally derived melt with different trace element chemistry than the host rock. This melt thus interacted chemically by melt-mediated interface-coupled dissolution-precipitation reactions with the surrounding rocks forming the Type 1 rims. Our study shows detailed analysis of kyanite is an important tool for giving constraints on the deformation, P–T and melting history of high-grade metamorphic rocks and migmatites. [Display omitted] • Two kyanite types defined by microstructure, paragenesis, EBSD, CL & trace elements. • T1 is from local partial melting, T2 is related to externally derived partial melt. • External melt modifies CL signal and trace elements in T1 kyanite. • Kyanite is resistant to crystal plastic deformation induced trace element diffusion. • Detailed investigation of kyanite yield independent process & evolution constraints. [ABSTRACT FROM AUTHOR]

The Vitim volcanic field, comprising Cenozoic basaltic lava flows associated with the Baikal Rift volcanism, contains abundant mantle xenoliths that offer insights into the subcontinental lithospheric mantle (SCLM) beneath the region. This study investigates the mineralogical and geochemical compositions of these mantle xenoliths, entrained in Miocene and Pleistocene basalts. The xenoliths, predominantly garnet-, spinel-, and garnet-spinel-bearing lherzolites, exhibit high modal content of clinopyroxene and low fosterite content of olivine, suggesting a relatively fertile SCLM. Distinct variations in CaO and Al 2 O 3 observed in spongy rims and cores of clinopyroxenes likely result from decompression-induced partial melting. Trace element patterns of clinopyroxene reveal three types: light rare-earth elements (LREE) depleted, LREE enriched, and transitional LREE types. Whole-rock trace element compositions of the four Vitim lherzolites showing flat to LREE and large ion lithophile elements enrichments are entirely consistent with those of their clinopyroxenes. The LREE depleted type indicates that these lherzolites were residual mantle after melt extraction, whereas the LREE enriched type suggests that they were metasomatized after residual mantle formed by partial melting. The transitional LREE type showing in-between features among the above two end-members could represent that those lherzolite underwent less or incomplete metasomatism, thus clinopyroxene cores still retain primary depleted LREE type feature of residual mantle. Most Vitim lherzolites were affected by cryptic metasomatism with less stealth metasomatism, whereas only those with secondary amphibole and apatite could be influenced by modal metasomatism. The lithospheric mantle beath the Vitim volcanic field representing by these lherzolites was metasomatized predominantly by hydrous fluids with minor silicate melts. Using the clinopyroxene melting model, it was found that most Vitim lherzolites have experienced <10% partial melting. Geothermal gradients estimated from mineral geothermobarometry indicate that lherzolites in the Pleistocene basalts equilibrated at shallower depths with higher temperatures compared to those in the Miocene basalts with deeper depths and lower temperatures. Sr Nd isotopic ratios, combined with previous results (Ionov et al., 2005), demonstrate that the Miocene lherzolites have a broader range from depleted to enriched components, whereas lherzolites in the Pleistocene basalts show lesser enrichment. It is proposed that the Pleistocene basalts captured shallower SCLM lherzolites experienced less degrees of melt metasomatism than those deeper SCLM lherzolites hosted by Miocene basalts. Considering the deeper SCLM is more vulnerable to metasomatism by ascending melts, such temporal and geochemical variation further emphasizes progressive asthenosphere upwelling beneath the Vitim region from the Miocene to Pleistocene time. • Vitim lherzolites show relatively fertile lithospheric mantle features. • They were mainly affected by cryptic, with minor stealth and modal metasomatism. • Vitim residual lithospheric mantle experienced <10% partial melting. • Shallower, less metasomatized lherzolites captured in Pleistocene than in Miocene. • The temporal, chemical variations reflect progressive upwelling of asthenosphere. [ABSTRACT FROM AUTHOR]

Garnet–orthopyroxene granulites from the Rauer Islands (East Antarctica) provide a spectacular example to investigate the late fluid evolution, metamorphic duration, and behavior of monazite and zircon in response to metamorphic reactions and fluid–rock interaction. Here, we characterize the secondary fluid inclusions in peritectic garnet and orthopyroxene, which occur as multiphase inclusions along micro–fractures. Inclusions are composed of siderite, pyrophyllite, calcite, quartz and residual CO 2 , representing stepdaughter phases resulting from the interactions between C–O–H fluid and its hosts at variable temperatures during retrogression. Zircon grains show clear core–rim structure, which yield 206Pb/238U ages of 540–507 Ma and 527–490 Ma, respectively. Index inclusions and internal structures suggest that the cores document the timing of peak and post–peak decompression while the growth of rims corresponds to melt crystallization during the final cooling. The U Pb systems in zircons are considered to have not been obviously affected by fluid or melt–mediated modification. The unusual formation of monazites in garnet–orthopyroxene granulites may be linked with the elevated phosphorus budget as a result of apatite dissolution during the prograde melting of the rocks. Detailed investigations suggest that the crystallization of monazites occurred both at peak and post–decompression stages, whose isotopic systematics has been completely reset due to melt–mediated dissolution–precipitation. The spurious dates for monazites (522–495 Ma) are highly coinstantaneous with the dating results for zircon rims, further supporting this view. Therefore, we conclude that the late carbonic fluid influx cannot result in marked U(Th)–Pb resetting in zircon and monazite. Instead, anatectic melt may have played an important role in the disturbance of isotopic systematics in monazites, especially for long–lived high–grade metamorphic terranes. Combined with previously published data, we propose that the Pan–African metamorphic event in the Rauer Islands may have reached the peak at around ∼540 Ma, followed by a protracted post–peak evolution that lasted for at least ∼50 Myr. This study highlights the importance of an integrated investigation of fluid and index mineral inclusions, as well as the chemical signatures of zircon and monazite, to interpret chronological data correctly. • Secondary fluid inclusions are reported from the garnet-orthopyroxene granulites. • Peak metamorphism occurred at ∼540 Ma, and the UHT metamorphic event lasted for >50 Myr. • Monazite dates of 522–495 Ma may be spurious due to melt-mediated Pb-loss. [ABSTRACT FROM AUTHOR]

Metarodingites are commonly garnet-bearing metasomatized mafic dykes within serpentinized mantle rocks. Garnet in metarodingites has the potential to preserve compositional and chronological information about the entire metamorphic-metasomatic evolution, from ocean floor formation to deep subduction. In this study, we investigate the chemical and chronological record of garnet, titanite, and zircon from metarodingites and garnet veins in chloritized blackwall, from the Zermatt-Saas zone in the Western European Alps. Garnet in the chlorite-rich metasomatic rind (blackwall) displays re-crystallization and new growth textures and therefore represents a second generation of garnet (Grt2), after the one in the metarodingite core (Grt1). Major and trace element mapping shows that Grt2 is richer in andradite and Ti-andradite components compared to the more grossular-rich Grt1 in the metarodingite. Moreover, we found that in both Grt1 and Grt2, Ti and U content are positively correlated, with U abundances up to 1.5 μg⋅g−1. Grt2 does not show rare earth element (REE) zonation, suggesting that REE transport was facilitaed by the presence of a fluid phase. We propose that the metasomatizing fluids from which Grt2 precipitated, were serpentinite-derived high-temperature brines capable of transporting Fe3+, Ti and REE. LA-ICPMS U–Pb dating of metarodingite garnet Grt1 samples yielded overlapping ages between 43.6 ± 0.9 and 44.1 ± 1.3 Ma, which are consistent with previous estimates of peak metamorphic conditions. Our data indicate that the metarodingite Grt1 grew within <2 Myr, suggesting a rapid burial rate of the unit or fast and pulsed garnet growth aided by the presence of fluids in the intergranular medium. Titanite from the blackwall sample yields an age of 45.0 ± 1.7 Ma and zircon rim an age of 48.0 ± 1.1 Ma, indicating that fluid release and chloritization occurred at peak conditions. U–Pb ages of garnet in the blackwall samples (Grt2) are significantly younger, yielding ages of 40.1 ± 0.9 Ma and 38.4 ± 0.8 Ma, respectively. The presence of multiple generations of metasomatic garnet in the blackwall suggests that periods of increased rock permeability occurred repeatedly, plausibily aided by deformation, as supported by the consistency of our multi-mineral geochronology. We show how garnet from metarodingites and blackwalls records the metamorphic-metasomatic history and can be used to obtain information on (de)hydration reactions and fluid flow during subduction. [Display omitted] • U–Pb dating of grossular-andradite garnet from eclogite facies rocks is a robust geochronometer. • During metamorphic hydration reaction and new garnet growth, REE distribution is transport limited. • High temperature brines transport Fe3+, Ti and REE. • Different ages of garnet veins indicate multiple events of fluid-rock interaction and pulsed fluid circulation. [ABSTRACT FROM AUTHOR]

P - T path of a contact aureole can provide important insights into the magma emplacement mechanism for nearly concentrically zoned granitic plutons. The Mesoproterozoic Xiamaling formation immediately adjacent to the southern Fangshan granitic Plutons in the Western Hills of Beijing, China, developed three contact metamorphic prograde staurolite zone, sillimanite zone and K-feldspar zone. These zones preserved corroded pink-andalusite wrapped by pale-andalusite, corroded garnet inclusion inside the K-feldspar porphyroblasts and staurolite, which add to well-established sequences of mineral growth. These sequences allow reconstruction of the complete P-T path for each prograde zone on the pseudosection diagram obtained by full system composition-based phase equilibrium modeling The resultant prograde P-T paths are composed of two segments, with the first one isobaric heating and the second distinct compression and heating. The staurolite zone shows the P-T path initiating from ∼2.0 kbar/350 °C to ∼1.8 kbar/510 °C and ending at ∼3.3 kbar/580 °C. The sillimanite zone path began from ∼2.0 kbar/350 °C to ∼1.5 kbar/500 °C then ended at ∼3.8 kbar/580 °C. The K-feldspar zone path started from ∼2.0 kbar/350 °C to ∼1.7 kbar/510 °C, then terminated at ∼3.5 kbar/670 °C, approaching the initial partial melting temperature condition, then documented a retrograde path cooling down to ∼630 °C. The two prograde P-T path segments of the three zones suggest a in situ heating followed by a ductile return flow from ∼7 km down to ∼11 km depth of the metapelites from the Xiamaling formation, responsable to a diapiric emplacement of the magmas for the zoned composite Fangshan Plutons. [Display omitted] • The zoned Fangshan Plutons developed contact aureole in the Xiamaling Formation. • Complete prograde P-T paths were reconstructed for three contact prograde zones. • The strong loading paths reveal wallrock deep downflow and magma diapir emplacement. [ABSTRACT FROM AUTHOR]

Here we investigate the evolution of a focused melt extraction horizon exposed in lower crustal metapelites of the Serre Massif in Calabria (southern Variscan belt, Italy) and consider the effect of melt drainage on the rheology of the orogenic lower crust. Our study shows that large (up to >10 cm long) ellipsoidal garnet aggregates associated with a small volume of leucosome in this migmatitic horizon represent clogged melt extraction channels. Using microstructural evidence, particularly electron backscatter diffraction (EBSD) data, we demonstrate that the garnet aggregates formed by coalescence of small peritectic crystals, carried along in anatectic melt migrating towards pull-apart structures, where the melt with entrained garnet pooled as it was progressively squeezed out. Phase equilibrium thermodynamic modelling of melt extraction events indicates that a minimum of 30 vol% of melt was drained out during nearly isobaric heating at average peak pressure (P) conditions of 0.97 GPa, from ∼700 °C to peak temperatures of 800–840 °C. We use numerical simulations of measured quartz crystallographic preferred orientations (CPOs) to show that melt extraction occurred under pure shear regime. Our results highlight how orogen rheology is conditioned along critical horizons where melting occurs and melt extraction is localized, leaving behind a hard residue. • Large ellipsoidal garnets form by coalescence of pooled single grains. • Coalescence of peritectic minerals occurs during melt extraction. • Melt wetted interfaces facilitates crystallographic misorientation reduction between adjacent crystals. • Lecosomes rich in large garnet represent clogged melt extraction channels. [ABSTRACT FROM AUTHOR]

Worldwide, Ultrahigh Pressure (UHP) oceanic units are rare and to date only three were recognized: Tianshan (China), and Lago di Cignana and Lago Superiore Unit (Western Alps, Italy). The UHP oceanic units represent the only geological object directly exhumed from mantle depth and they record fundamental information about processes occurring in the deepest portions of the subduction interface. In this work, we describe the occurrence of a new UHP oceanic unit within the Western Alps (mid Susa Valley, Internal Piedmont Zone). We here report the finding of a UHP index mineral, i.e., coesite and we provide a detailed study of garnet inclusions in metapelites and metabasites part of the meta-sedimentary cover of the meta-ophiolites cropping out in the mid Susa Valley. The samples were investigated via optical microscope, Raman spectroscopy, Elastic Geothermometry and classic thermometry (Zr-in-rutile) in order to constrain the P-T evolution of the area. The finding of a new UHP unit in the Internal Piedmont Zone, together with the already described Lago di Cignana and Lago Superiore units, points towards the possible existence of a UHP oceanic slice that reached a similar peak-P (i.e., the return point) along the same subduction gradient. This slice was then removed from the slab and dismembered during exhumation, and it is now exposed as coesite-bearing units juxtaposed with lower pressure eclogite-facies ophiolites. Moreover, the large occurrence of coesite along the entire Internal Piedmont Zone significantly increases the extension of the UHP oceanic units. Hence, the model of a localized non-lithostatic pressure is at the state of the art difficult to apply to the oceanic units of the Western Alps. • We report a new occurrence of coesite within the eclogitic meta-ophiolites (Internal Piedmont Zone) of the Western Alps. • Elastic Geobarometry and Zr-in-rutile thermometry results confirm the PT path proposed in literature with pseudosections. • We propose a unique UHP level inside oceanic slab by linking the three UHP oceanic units outcropping on the Western Alps. [ABSTRACT FROM AUTHOR]

Determining whether high-grade metamorphic eclogites and surrounding metasediments underwent a shared subduction-exhumation cycle or distinct metamorphic paths before their juxtaposition at shallow levels is crucial for deciphering metamorphic processes in orogenic belts. Constructing pressure-temperature-time (P–T- t) paths for both rocks is the most straightforward approach. However, current time constraints overwhelmingly rely on dating accessory mineral like zircon, rutile, and monazite, which may only record a portion of the entire metamorphic history. Here we present an integrated petrochronological analysis using garnet Lu–Hf and Sm–Nd, and zircon U–Pb on two eclogites and their surrounding micaschists from the Changning–Menglian orogenic belt in southeast Tibet. Pseudosection modelling and conventional thermobarometers suggest a clockwise P–T path for the eclogites, from ∼1.4 GPa/∼520 °C to ∼2.5 GPa/∼600 °C, followed by decompression to <∼0.7 GPa, and peak conditions of 2.0–2.2 GPa/570–620 °C for the micaschists. Lu–Hf dating for eclogites yields ages of ca. 238–241 Ma, indistinguishable from the Sm–Nd dates of ca. 239 Ma. However, the Lu–Hf ages for micaschists (ca. 249–251 Ma) are older than the corresponding Sm–Nd dates (ca. 240 Ma). The well-preserved growth zoning in garnet indicates a short growth period in eclogites and at least ∼10 Myr in micaschists, bracketing the timing of protracted the two distinct zircon growth phases at around 238 Ma and 250 Ma. The similarity in peak metamorphic conditions and the consistent high-pressure metamorphic ages suggest a coherent subduction and exhumation process for both eclogites and micaschists. The Lu–Hf ages of the micaschists constrain the subduction of the Lancang Group dates back to ca. 250 Ma. The narrow temporal gap between the initiation of subduction and the age of the protoliths implies a rapid shift from the formation of the protoliths to the subduction of oceanic slabs and the scraped-off sediments to depths exceeding 80 km during the Triassic—a hypothesis that warrants further exploration. • Zircon and garnet ages are obtained on eclogites and associated schists. • Similar peak conditions and ages indicate coherent subduction and exhumation. • Close protolith and metamorphic ages may reflect rapid tectonic shifts. [ABSTRACT FROM AUTHOR]

The tectonic-thermal mechanism of crustal melting provides vital information on continental differentiation and orogenic evolution. This study aims to understand the evolution and tectonic setting of newly discovered lower crustal leucosome-rich granulites of the Qilian Orogen, NW China. Tonalitic leucosome within garnet amphibolite occurs in net-like arrays or as stringers and has diffuse boundaries with the host, with enriched LILE, and high Sr and Sr/Y ratios, similar to that of adakite, formed by water-fluxed melting. However, granitic leucosome within garnet granitic gneiss shows sharp contacts with the host, and has low Sr, Ba, K and Pb but high Rb, Rb/Sr, with negative Eu anomalies, consistent with dehydration melting. Petrographic observations, geochemical data and pseudosection modelling suggest that garnet amphibolite experienced water-fluxed melting and garnet granitic gneiss underwent dehydration melting, respectively. LA-ICP-MS zircon and titanite dating indicate the anatectic event proceeded at ∼480–470 Ma. Phase equilibrium modelling suggests the garnet amphibolite and garnet granitic gneiss share a similar P-T path involving decompression heating from ∼12.1 kbar/711–746 °C to reach peak temperatures of 798–836 °C at 10.4 kbar, and subsequently decompression cooling to 4.6–7.6 kbar/600–672 °C. The calculated P-T conditions suggest Barrovian-type metamorphism with thermal gradients of 24.2 °C/km for the Central Qilian lower crust. We interpret a retro−/back-arc setting resulting from the southward subduction of the North Qilian oceanic lithosphere. [Display omitted] • Water-fluxed melting of hornblende and dehydration melting of biotite in the crust root. • An anatectic event proceeded at ca. 480–470 Ma in the Central Qilian belt. • A hot retro−/back-arc setting is proposed to provide enough heat for anatexis and metamorphism. [ABSTRACT FROM AUTHOR]

Garnet has been proposed to be an important fractionating phase during magmatic differentiation in thick volcanic arcs because garnet fractionation can reproduce the hallmarks of continental crust by driving residual magmas to higher silica, lower total iron, and higher iron oxidation states. Recently, Holycross and Cottrell (2023b) have measured the partition coefficients for Fe2+ and Fe3+ between garnet and melt, which allow for more precise modeling of the oxidation effect of garnet fractionation. Using their new experimental results combined with fractional crystallization modeling, Holycross and Cottrell (2023a, 2023b) concluded that garnet crystallization has little influence on arc magma oxidation. However, these conclusions are based on conceptual errors in modeling the effects of garnet. Firstly, they ignored garnet fractionation beyond 50% crystallization which would cause substantial Fe3+ enrichment. Limiting garnet fractionation to the initial 50% crystallization of arc basalts is inconsistent with existing experiments including their own experiments. Secondly, Fe2+ partitioning was indirectly and incorrectly parameterized using Fe Mg exchange coefficients, inconsistent with experimental partition coefficients. Application of these recent models leads to unrealistic Fe2+ partitioning behavior with progressive fractionation. When these models are corrected with proper mass balance and partitioning relationships, garnet as a driver of oxidation remains a viable and testable hypothesis. • Latest experiments allow parameterization of Fe partitioning in garnet. • Incorrect use of K D leads to erroneous model results. • Model using parameterized Fe partition coefficients shows strong oxidation. • Garnet-driven magma oxidation remains a viable hypothesis. [ABSTRACT FROM AUTHOR]

We have explored the effect of temperature (T) and pressure (P) on Ni partitioning between garnet and olivine in well-equilibrated mantle xenoliths from on-craton, marginal-craton and off-craton settings and in high- P–T experiments at natural Ni abundance. Contrary to previous evaluations, the xenolith and experimental data indicate that the P effect is not negligible, consistent with the significant volume change of the garnet–olivine Mg-Ni exchange reaction. The recognition of a P effect satisfactorily resolves the discrepancies observed using previous calibrations of the Ni-in-Grt geothermometer and provides a solution to the long-standing controversy as to which Ni-in-Grt geothermometer is best applied to natural chromian pyrope compositions. A recalibrated, P -dependent Ni-in-garnet geothermometer reproduces the pyroxene T estimates for the xenoliths and the T conditions of the experiments with a standard error of estimate of 44 °C. The P dependence (ca. 40 °C/GPa) is comparable to that of the garnet–olivine Fe–Mg exchange geothermometer. A small tendency to overestimate at T < 900 °C relative to two-pyroxene thermometry is observed, which is unrelated to garnet compositional parameters. A set of simplified, geotherm-referenced calibrations permit traditional use of the Ni-in-garnet geothermometer as a single-mineral method, provided the local geotherm is known or can be estimated or inferred. • Ni partitioning between garnet and olivine is not independent of pressure. • Current versions of the Ni-in-Grt thermometer are unsatisfactory. • A P-dependent recalibration of the Ni-in-Grt thermometer is proposed. • Simplified P-independent formulations are proposed for reference geotherms. [ABSTRACT FROM AUTHOR]

Accurate dating of metallogenesis is important for understanding the origin of mineral deposits including skarn. This study presents LA-ICPMS U Pb dating and trace element analysis of garnets from three skarn deposits in the Tongling district, namely Xiaotongguanshan, Guanyinchong, and Baoshantao. All six skarn garnets give U Pb ages of 141–146 Ma, which are consistent with the zircon U Pb ages of the corresponding pluton. Besides, the garnet and zircon U Pb ages of Baoshantao skarn are consistent within error. In particular, the garnet U Pb age of the Xiaotongguanshan skarn suggests that it is the first stage of the Xiaotongguanshan magmatism that is directly linked with the formation of skarn. The correlation of garnet U concentrations with REE and/or andradite endmembers indicates that U entered garnet structure instead of being adsorbed by crystal surface during growth or captured in mineral or fluid inclusions. The U concentrations in Baoshantao skarn garnet correlate positively with LREE, which indicates that U is incorporated into garnet in the form of U4+ and occupies the X site following this equation: [U4+]VIII + 2[Al, Fe3+]IV - [Ca2+]VIII + 2[Si4+]IV. In the garnet of the Xiaotongguanshan skarn, U positively correlates with Ti, Zr, and Hf, which mainly occupy the octahedral Y site, indicating that U substitutes the Y site in the form of U6+. The substitution mechanism can be expressed as [U6+]VI + 3[Al, Fe3+]IV - [Al, Fe3+]VI + 3[Si4+]IV. The U contents in the garnet of the Guanyinchong skarn show good correlations with X2+ (REE and andradite) and Y3+ (Ti, Zr, and Hf), which indicates that both substitutions worked. The varied oxygen fugacity in different skarns might be responsible for different substitution mechanisms of U. • Similar garnet U Pb ages of 141–146 Ma for three different skarn deposits of Tongling. • The first stage of the Xiaotongguanshan magmatism is directly linked with the formation of skarn. • Garnets from endoskarns tend to have higher U concentrations than those from exoskarns. • Varied oxygen fugacity results in different substitution mechanisms of U. [ABSTRACT FROM AUTHOR]

Almandine-spessartine garnet from the Moldanubian Zone in the Bohemian Massif (Austria) records microstructural and compositional features of crystal growth imprinted during the transition from pegmatoid magmatic to subsolidus state. Directed garnet growth results in asymmetric compositional, morphological and microstructural zoning. In addition, the garnet core shows sector zoning reflected by the predominance of phosphate inclusions in {110} sectors, and rutile inclusions in {112} sectors of garnet. These sector-specific abundances of inclusions are also expressed in the electron probe microanalytical data of the non-equivalent garnet sectors. The contribution of the submicron and nano-inclusions of phosphates and rutile is of crucial importance for thermobarometric methods based on P, Na and Ti in garnet. The sector-specific abundance of inclusions is referred to the surface configuration of individual garnet facets. Contrastingly, the microstructural growth banding observed in the {112} garnet sectors supposedly results from compositional fluctuations of a boundary layer at the garnet-melt interface. These fluctuations control the details of the inclusion microstructures, which differ even in crystallographically equivalent sectors. • Microstructural and compositional zoning of pegmatoid garnet records directed growth. • Sector zoning with rutile prevailing in {112}, phosphates in {110} sectors of garnet • Formation of sector-zoning is referred to facet-specific surface configurations. • Compositional boundary layers at garnet-melt interface cause growth banding of garnet. • Inclusions contribute to Ti, P, Na data from electron beam microanalysis of garnet. [ABSTRACT FROM AUTHOR]

In hydrothermal tin (Sn) systems, it remains unclear whether cassiterite precipitates from reduced or oxidized fluids. To resolve this issue, the geochemistry of magmatic garnet and cassiterite separated from fractionated muscovite-garnet granite in the Paleocene Bawapin Sn W deposit was systematically investigated. CaO contents in Mn-rich garnet (spessartine) decrease from core to rim in single crystals and are negatively correlated with MnO/(MnO + FeO) ratios. These features suggest that garnet CaO content may be a good differentiation index for granitic magma evolution. Moreover, the Sn content in Mn-rich garnet increases with increasing Ca content and then decreases sharply at Ca contents of approximately 4300 ppm. Combined with evidence of Ta-rich magmatic cassiterite, the decreasing Sn content likely reflects the crystallization of magmatic cassiterite from the more evolved Sn-rich melts under oxidized conditions (ƒO 2 > ΔFMQ +1.5), in contrast to the reduced characteristics of the less-fractionated biotite monzogranite (ƒO 2 = ΔFMQ - 0.5). Late-stage oxidation might be attributable to fluid exsolution in the water-rich and Fe-poor granitic melts. This further indicates that hydrothermal cassiterite could precipitate in exsolved Sn4+-bearing fluids without Sn oxidation. This conclusion may provide a new perspective to our understanding of granite-related hydrothermal Sn systems worldwide. • The Paleocene muscovite-garnet granite has the characteristics of more evolved melt. • CaO content in Mn-rich garnet may be a good differentiation index of granitic magma evolution. • Sn variation in garnet and crystallization of magmatic cassiterite indicate late-stage oxidation in granitic melt. [ABSTRACT FROM AUTHOR]

Garnet with complex, discontinuous zoning is a common occurrence in metamorphic terrains, and the relationship between major and trace element zoning can provide insight into the metamorphic evolution of the host rock. Mylonitic micaschists along the Posada-Asinara Shear Zone in the Axial Zone of the Sardinia Variscan chain contain garnet porphyroblasts, enveloped by the S2 schistosity, with distinct core and rim domains. A large garnet porphyroblasts was investigated by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICPMS) mapping. The major element compositional variation follows a bell-shaped zoning, with Ca and Mn contents progressively decreasing, and Fe and Mg increasing, from the core to the outer rim. LA-ICPMS mapping revealed a thin and sharp annular enrichment zone in Y, Sc, Dy, Ho, Er, Tm at the mantle-rim boundary. The trace element (TE) compositional profiles show a central enrichment area for HREE (Tm, Yb, Lu). This enrichment decreases progressively, as a function of atomic number, for Er, Ho and Dy. Elements with even lower atomic number (Tb, Gd, Eu and Sm), are depleted in this central domain, but their content increases in broad shoulders towards the garnet rim. The position of these lateral shoulders migrates progressively rimwards with decreasing atomic number. The REE distribution, trend and behavior in the growth zones of the garnet is an example of TE control during a continuous growth ruled by diffusion-limited REE uptake. The Y + HREE annular enrichment zone, interpreted as resulting from a decrease in the garnet growth rate, reflects a short-lived episode in the garnet growth history. • The major and trace element compositional zoning in garnet follow different pattern. • REE distribution in growing garnet was controlled by diffusion-limited REE uptake. • Y + HREE annulus reflects a short-lived episode in the garnet growth history. [ABSTRACT FROM AUTHOR]

In subduction zones, the mantle wedge is the domain where liquid phases released by the oceanic subducting plates cause mantle hydration, metasomatism, and partial melting, producting the arc crust. However, direct petrologic information on mantle wedges overlying active subduction zones remains scarce. Here we constrain key aspects of the petrological evolution of the supra-subduction mantle underneath the active volcanic district of Southern Colombia (Mercaderes) by studying a unique suite of peridotite and pyroxenite xenoliths hosted by andesites and lamprophyres. These garnet-bearing peridotite, websterite, clino- and orthopyroxenite xenoliths equilibrated between 2.5 and 4 GPa and 1100–1200 °C. The studied rocks display considerable heterogeneity in terms of lithotypes, textures, mineral abundance and extent of metasomatic overprint. Most samples display fine-grained mylonitic textures overprinting coarse-grained peridotite and garnet pyroxenite precursors; the EBSD study of undeformed garnet websterite samples suggests crystallization from melts and/or melt-rock reaction processes. Modal enrichment in clinopyroxene as well as in the alkalis and LREE contents in some garnet peridotite implies interaction with alkaline basaltic melts, whereas enrichment in silica, Pb and Sr in the garnet-bearing orthopyroxenites records interaction with metasomatic, slab-derived liquids. Some garnet clinopyroxenite xenoliths display marked positive Sr anomalies and minor Eu anomalies, indicating derivation of these rocks from Mg-rich, plagioclase-bearing (and garnet-absent) low-pressure cumulates. Their equilibration in the garnet field implies that after crystallization these clinopyroxenites were transported to deeper levels in the mantle, as the result of foundering into the mantle of the dense arc roots, as already documented in Mercaderes and elsewhere in the Andes. In conclusion, the Mercaderes xenoliths probe a heterogeneous and chemically active domain of the Colombian mantle wedge, where deep sinking lower crustal rocks, melt metasomatism by distinct venues of chemically different melts, and deformation predate fast exhumation by volatile-rich melts. • Direct petrologic records of mantle wedge from presently active subduction zones are still few. • Garnet peridotite and pyroxenite xenoliths disclose the evolution of the mantle wedge underneath Southern Colombia. • The xenoliths recrystallized at 2.5-4 GPa and 1100-1200 °C and display heterogeneous rock types and texture. • Metasomatism led to enrichment in alkalis and LREE of garnet peridotite and in silica, Pb, Sr of garnet orthopyroxenite. • Positive Sr and Eu anomalies in garnet clinopyroxenites suggests foundering of low-pressure plagioclase-bearing cumulates. [ABSTRACT FROM AUTHOR]

Two metasedimentary migmatite samples from the same outcrop in the northern margin of the Oak Creek pluton in the Wet Mountains, Colorado, have distinct mineralogy, textures and major and trace element geochemistry. Sample W6, a garnet-biotite migmatite, has trace element characteristics consistent with a sedimentary protolith, this sample is also rich in silica and lacks sillimanite and cordierite. Sample W6A, a garnet-sillimanite-cordierite migmatite, has strong heavy rare earth element (HREE) enrichment, low silica, and contains abundant aluminum-rich minerals. Both rocks are relatively oxidized, consistent with sedimentary protoliths for both. Based on textural and geochemical evidence, we interpret the differences in mineralogy between the two samples to be the result of differing degrees of melt extraction and variations in protolith composition. Such differences were assessed using isocon analysis and calculated P-T phase diagrams. Melt retention in sample W6 resulted in the absence of sillimanite and cordierite, and back-reaction between minerals and melt in the peak assemblage. By contrast, melt extraction in W6A left garnet in the residuum, and imparted a strong heavy rare earth element enrichment on the sample, allowing preservation of the peak mineral assemblage. Consequently, W6A records peak conditions of 743 °C and ~7.6 kbar, whereas W6 yields lower P-T estimates that may represent the prograde and retrograde metamorphic paths. Our results indicate that melt extraction can be heterogeneous even at outcrop scale, and differing mineralogy may reflect variable proportions of melt loss and local variations in the composition of the protolith. Moreover, at least at small-scale, migmatization of iron-rich protoliths is a plausible mechanism for the production of ferroan, alkaline granitic magmas. • Petrological and geochemical differences attributed to variable protolith composition and differing degrees of melt loss. • Melt retention leads to back-reaction between the melt and the solid residuum. • Substantial melt loss enhances preservation of the peak assemblage. • Protolith composition and migmatization processes control melt geochemistry. [ABSTRACT FROM AUTHOR]

The mechanism of magma generation during continental collision remains enigmatic. Appinite–granite suites commonly occur within collisional orogenic belts, providing an opportunity to monitor lithospheric evolution and the geodynamic processes involved. This paper presents geochronological, mineralogical, and geochemical data for the Eocene (~50 Ma) Pengcuolin appinite–granite suite, which is the first such suite to have been found in the southern Lhasa Terrane, southern Tibet. Two distinct populations of plagioclase are observed in the appinitic rocks: high-An plagioclases that crystallized as an early phase from H 2 O-saturated mafic magmas, and low-An plagioclases that formed as a late phase at pressures of ~3 kbar and ~5 kbar, as calculated from equilibrium thermobarometry using low-Al and high-Al amphiboles, respectively. The amphibole-rich appinitic rocks have low SiO 2 contents and moderate values of Mg#, and are enriched in large ion lithophile elements and depleted in high field strength elements. They have homogeneous initial 87Sr/86Sr ratios of 0.705174–0.705330, ε Nd (t) values of 0.85 to 1.47, and zircon ε Hf (t) values of 3.3 to 7.9. These features suggest the appinites were derived from the melting of mantle peridotites that had been permeated by slab-derived fluids. Whole-rock geochemical data, equilibrium melting calculations, and ABS5 modeling results indicate that the primary melts of the appinitic rocks were generated from garnet-stabilized mantle, and that the melts then ascended with crystallization taking place under H 2 O-saturated conditions and under variable temperatures and pressures. The associated granites have a high and narrow range of SiO 2 contents, and they exhibit a high-K calc-alkaline character, and low A/CNK ratios and Mg# values. These geochemical characteristics, plus their widespread distribution and depleted isotopic compositions (87Sr/86Sr i = ~0.705, ε Nd (t) = 1.12–1.59, and zircon ε Hf (t) = 6.9–9.2), are comparable to those of Cenozoic granitoids elsewhere in southern Tibet and consistent with the partial melting of juvenile mafic lower-crustal materials. This appinite–granite suite not only attests to a flux of slab fluids through the lithospheric mantle during breakoff of the Neo-Tethyan slab, it also sheds light on the vertical profile of magma evolution from the deep mantle to the shallow crust. The emplacement of the appinite–granite suite was facilitated by regional faults, but the rising hydrous basaltic magmas were often trapped by the thick overlying continental crust, and those underplating magmas provided the heat and water that caused the partial melting of the continental crust, finally leading to the widespread Cenozoic felsic (and rare mafic) magmatic rocks of southern Tibet. Unlabelled Image • Slab-fluid fluxing of lithospheric mantle occurred during Neo-Tethyan slab breakoff. • Melting P–T conditions established for magma generation after slab breakoff. • Underplating of hydrous basaltic magmas facilitated partial melting of overlying continental crust. [ABSTRACT FROM AUTHOR]

A large dataset of minor elements (Ni, Mn, Co, Zn, Ca, Ti, Al, Cr and Na) in olivines from orogenic peridotites is reported to investigate the application of these elements as sensitive tracers of mantle processes in subduction zones. Orogenic mantle peridotites can be distinguished from crustal peridotites by overall higher Ni and lower Co and Zn contents in olivine. In particular, the Ni/Co ratio of olivine can successfully discriminate between the two types of peridotites. When mantle peridotites underwent transfer from hot lithospheric mantle to cool subduction zones, olivine Ca, Al and Cr contents dramatically decreased, and Ca in olivine can be a useful geothermometer for orogenic peridotites. The Na contents of olivines from garnet-facies mantle peridotites (mostly >20 ppm) are typically higher than those from spinel-facies mantle peridotites (mostly <11 ppm), indicating that Na in olivine is a potential pressure indicator for orogenic peridotites. The high Ti contents of olivine may be caused by subducted crustal metasomatism, producing the wide occurrence of Ti-clinohumite and rutile in strongly metasomatized garnet lherzolites. Olivines from these high-Ti garnet lherzolites also have Mn contents deviating from the normal mantle olivine range, implying high sensitivity of Mn in olivine to crust–mantle interaction in subduction zones. Our data highlight the broad prospects for olivine geochemistry in the realm of subduction zone geodynamics. • A large dataset of olivine minor elements in orogenic peridotites is reported. • Olivine Ni/Co ratio can discriminate between mantle and crustal peridotites. • Ca in olivine is a useful geothermometer for orogenic peridotites. • Na in olivine is a potential pressure indicator for orogenic peridotites. • Olivine Ti and Mn are sensitive to mantle wedge metasomatism. [ABSTRACT FROM AUTHOR]

This study reports zirconium-in-rutile thermometry data from the Himalayan ultrahigh-pressure eclogites and their retrogressed counterparts. The Zr contents of three textural varieties of rutile i.e. matrix rutile, inclusion phase, and overgrown by ilmenite/titanite from coesite-bearing eclogites, slightly retrogressed eclogites, highly retrogressed eclogites, and amphibolites were analyzed for Zr contents using the electron probe micro-analyser. Majority of the rutile grains exhibited homogeneous chemical compositions in all the aforementioned textural varieties; however, some of grains were relatively heterogeneous. Average temperature values of 729 °C, 727 °C, and 706 °C were obtained for the matrix, inclusion, and overgrown rutiles from UHP eclogites and of 705 °C, 699 °C, and 707 °C for those from HP eclogites, respectively. Slightly retrogressed eclogites yielded similar average T values of 703 °C, 706 °C, and 706 °C, respectively, whereas highly retrogressed eclogites resulted in relatively low average temperatures, ca. 690 °C, 691 °C, and 679 °C, respectively. When compared with these eclogites, lower average T values of 672 °C, 630 °C, and 656 °C were obtained from garnetites, and of 665 °C, 658 °C, and 640 °C from garnet amphibolites, respectively. The relatively homogeneous Zr contents and higher temperature values from the matrix rutile in HP and UHP eclogites suggest peak or near-peak metamorphic crystallization, whereas grains with low Zr contents, yielding low temperature values, were likely to be affected by the late-stage retrogression. • Zirconium-in-rutile thermometry was applied to rutile in the Himalayan UHP eclogites and their retrogressed counterparts. • UHP eclogites preserved peak or near peak metamorphic temperatures that were partially reset in retrogressed rocks • The data are consistent with the conventional thermobarometry • The single mineral thermometry is robust for metamorphic rocks that have insufficient mineral assemblages [ABSTRACT FROM AUTHOR]

In this study, we utilize high-alumina pelitic schist samples from the Holenarsipur Greenstone Belt, a component of the ca. 3.30–3.15 Ga Sargur Supergroup in the Western Dharwar Craton, South India to establish a Mesoarchaean metamorphic history in the terrane. On the basis of integrated textural, mineral compositional, metamorphic reaction history and P-T pseudosection modelling studies, garnet-monazite trace element thermometry and coupled monazite electron probe micro-analyzer and sensitive high-resolution ion microprobe U-Pb dating results, we constrain for the first time a Mesoarchaean metamorphism along a hairpin clockwise metamorphic P-T path in the Dharwar Craton. The path reveals a two-stage prograde segment with an initial phase of heating accompanying burial (dP/dT gradient ~ 30 bar/°C) and a later phase of steep burial (∆P ~ 1.1 kbar, dP/dT gradient ~110 bar/°C), peak metamorphism at 7.5 kbar, 640 °C and a post-peak, steep decompression stage (P~6.3 kbar at 635 °C). Chemically zoned monazite grains shielded within garnet or occurring in low-strain matrix domains were formed in two stages during the Mesoarchaean metamorphism: (a) Monazite cores with relatively lower Y concentrations, higher Th/U ratios, a strong negative Eu anomaly; their REE partitioning relationships with the garnet inner rim reveal their crystallization was synchronous with the growth of garnet. (b) Monazite rims with higher Y and uraniferous compositions and lower Th/U ratios which, combined with Y and HREE enrichments in the outer rim of garnet, indicate their formation during the post-peak decompression stage associated with coupled decompositions of garnet and zircon. We correlate U-Pb monazite core and rim ages at 3144 ± 7 Ma and 3105 ± 10 Ma respectively with the timings of peak and retrograde metamorphism. Our findings, when evaluated with the available geological, isotopic, geochronological data of Meso- to Palaeoarchaean extant and extinct felsic crust as well as thermo-mechanical models, predict the ca.3.14–3.11 Ga metamorphism in the Holenarsipur Greenstone Belt as part of a widespread tectono-thermal event in the Western Dharwar Craton. The event marks late stages of sagduction tectonics in the craton as it evolves from a predominantly vertical to a horizontal shortening mode. • We have studied a Mesoarchaean metamorphism in the western Dharwar craton, S India. • High-alumina pelitic schists from the Holenarsipur Greenstone Belt record a hairpin CW P-T loop. • Peak metamorphism (T Max ~ 640 °C at ~7.5 kbar) is dated at ca. 3.14 Ga. • The Mesoarchaean metamorphism is part of a widespread tectonothermal event in the WDC. • The event marks a shift in tectonic style from initial sagduction to later horizontal plate motions. [ABSTRACT FROM AUTHOR]

Compositional zoning in garnet, mineral inclusions and the application of the Zr-in-rutile thermometry on rutile inclusions in garnet in combination with conventional geothermobarometry and thermodynamic modelling allows a reconstruction of the prograde pressure-temperature evolution in felsic and mafic high-pressure granulites from the Moldanubian Zone, Bohemian Massif, Lower Austria. Most garnets in these rocks show homogeneous core compositions with high grossular contents (~30 mol%), while their rim zones have a markedly reduced grossular content. Rutile inclusions in the grossular rich garnet cores have low Zr concentrations (400 to 1300 ppm) indicating a formation temperature of ~810–820 °C which implies that the garnet host grew at these temperature conditions as well. Based on numerous polycrystalline melt inclusions, high Ti-biotite relics and a generally high Ti concentration in garnet cores, the peritectic biotite breakdown reaction is considered to be responsible for a first garnet growth, now observed as high-grossular garnet cores. The corresponding pressure is estimated to be in the range of 1.6 to 2.5 GPa, based on experimentally determined biotite breakdown reactions, thermodynamic modelling and the occurrence of high-Ti biotite in garnet cores. Rutile inclusions in low-Ca garnet rims contain significantly higher Zr concentrations (1700 to 5800 ppm) resulting in ultrahigh temperatures of ~1030 °C. Similar temperature as well as corresponding pressure estimates of 1000 ± 50 °C and 1.60 ± 0.10 GPa were obtained by geothermobarometry and thermodynamic modelling using garnet rim and re-integrated ternary feldspar compositions. These high pressure and ultrahigh temperature conditions are well known from literature for these granulites. The proposed two-phase garnet growth is not only seen in different temperatures obtained from rutile inclusions in garnet core and rim areas, but also in discontinuous trace (Cr, Ga, P, Ti, V, Zr) and heavy rare earth element profiles across garnet porphyroblasts, implying a different reaction mechanism for garnet rim growth. This second phase of garnet growth must have occurred during near isobaric heating to the ultrahigh temperature peak, most likely even at slightly lower pressures compared to the garnet core growth. By applying a binary Fe-Mg diffusion model to strongly zoned garnet grains a maximum timescale of 5–6 million years was estimated for the exhumation and cooling process, assuming a linear cooling path from 1000 °C at 1.6 GPa to 760 °C at 0.8 GPa. This short-lived ultrahigh temperature event corresponds to cooling and exhumation rates of 40–50 °C Ma−1 and 5.3–6.6 mm y−1, respectively. • Rutile and Ti-biotite inclusions in garnet core define a PT event at 810–820 °C, 1.6–2.5 GPa • Rutile inclusions in garnet rims record a following UHT stage (~1030 °C) • Garnet major and trace element zoning display two-stage garnet growth • Crystallized melt droplets are present in garnet core and rim • Fe-Mg diffusion modelling on garnet profiles demonstrate rapid exhumation and cooling (~4.4–5.3 mm y−1 and ~40–50 °C My−1) [ABSTRACT FROM AUTHOR]

Pyroxenite is a relatively rare but important rock type of mafic-ultramafic series, and provides a useful tool to understand deep crustal and crust mantle processes. Here we present petrology, phase equilibrium modelling and P-T estimates, zircon U Pb geochronology and Hf isotopes on a recent find of garnet websterite from the Jambumalai hillock, Kolli-massif, southern India. The rock types in this location include metamorphosed orthopyroxenite, clinopyroxenite, garnet websterite and garnetite among which, garnet websterite is the most abundant variety. Petrological studies show that the peak metamorphic mineral assemblage of the garnet websterite is Grt + Cpx + Opx ± Ilm. The garnet is Fe rich (X M g = 0.39–0.41) whereas the clinopyroxene composition varies from augite to sub-augite (X M g = 0.72–0.79). The orthopyroxene shows a compositional range between ferrosilite and hypersthene (X M g = 0.61–64). Clinopyroxene in some of the garnet websterite samples exhibits exsolution microstructure with blebs of orthopyroxene. The orthopyroxene is Ca poor and Fe rich and displays the features of 'inverted pigeonite'. Phase-equilibria modelling in the NCFMASH system yielded a P-T estimate of c.22 Kbar and c.980 °C. Zircon U Pb analysis using SIMS yielded a weighted mean age of 2526 ± 38 Ma, which is regarded as the age of metamorphism. Zircon Lu Hf isotope data shows initial εHf values between +2 and −7.7, indicating a dominantly reworked crustal source for the protolith. Our results suggest high pressure and high temperature metamorphism at the base of a thickened Archean continental crust. • Granulite-grade garnet pyroxenite (c.980°C and c. 22kbar) reported for the first time from Kolli-massif, southern, India. • The SIMS U-Pb zircon metamorphic age of garnet pyroxenite is ca. 2526 Ma, is the oldest reported from the Koll-massif. • The crustal growth processes of the Kolli-massif include arc magmatism ca. 2550 Ma followed by metamorphism ca. 2526 Ma. • Negative ε Hf values (2 to -8) and model ages of ca. 3500 Ma of the zircons indicates reworking of a Mesoarchean crust. [ABSTRACT FROM AUTHOR]

The eclogites of the North Muya complex (Eastern Siberia) are located within the Early Neoproterozoic metasedimentary and felsic rocks of the Baikal-Muya Fold Belt (BMFB). The eclogites show subduction-related affinity, with large-ion lithophile (LILE) and light rare-earth element (LREE) enrichment and high field-strength element (HFSE) depletion signatures, similar to the exposed plutonic and volcanic rocks of the Early Neoproterozoic (Early Baikalian) subduction setting in the BMFB. Coupled Nd (εNd(T) of +6 to −1.4) and Sr (87Sr/86Sr ratio of 0.705–0.708), along with key trace-element indicators, imply progressive crustal recycling (up to 5–10%) from the Early Precambrian continental rocks to a depleted mantle source or equivalent crustal contribution via intracrustal contamination. Mineral δ18O data (+3.9 − +11.5) indicate that the contaminant or recycled crustal substrate might be represented by rocks altered at both low and high-temperature, or result from variable fluid-rock interaction in the subduction channel. Pseudosection modelling of eclogites, coupled with zircon U Pb geochronology (~630 Ma) suggest that the Ediacarian high-pressure metamorphic event for different rocks shared a maximum depth corresponding to 2.5–2.7 GPa with variable temperature range (560–760 °C), reflecting their potential relation to distinct slices of the subducted crust. The estimated metamorphic conditions for both the burial and exhumation of rocks indicate a continental subduction setting, but with a relatively cold geotherm (~20–25 °C/kbar). These conditions resulted from the continental subduction of the Baikal-Muya composite structure beneath the relatively thin and immature overlying arc lithosphere of southern Siberia. Some carbonate-bearing eclogites and garnet-pyroxene rocks, metamorphosed under T below 700 °C and a minimum P up to 1.4 GPa, exhibit LREE-enriched patterns and low εNd(T) values of −7 to −16. These rocks have Paleoproterozoic to Archean model ages and may support the existence of a Paleoproterozoic or older lithosphere in the Baikal-Muya Fold Belt, but their subduction history and origin remain uncertain due to geochemical and isotopic signatures probably overprinted by carbonate metasomatism. Unlabelled Image • North Muya eclogites mostly have subduction signatures and chemistry of the Early Neoproterozoic arc rocks of the BMFB. • Elemental and Nd-Sr-O isotope evidence of crustal recycling/contamination and fluid-rock interaction in a subduction channel. • PTt evolution reached peak conditions of 560–760 °C and 2.5–2.7 GPa at 630 Ma in a continental subduction zone. [ABSTRACT FROM AUTHOR]

Experimental modeling of the interaction between garnets of mantle parageneses with H 2 O–CO 2 fluid was performed in order to determine the characteristic features and trends in garnet compositions due to metasomatic alterations, and to study the patterns of formation of carbon and carbon-bearing phases in these processes. Experiments were carried out in the lherzolitic garnet–CO 2 –H 2 O–C and eclogitic garnet–CO 2 –H 2 O–C systems at 6.3 GPa in the temperature range of 950–1550 °C using double graphite+platinum ampoule on a multi-anvil high-pressure apparatus of a split-sphere type (BARS). The interaction of mantle garnets with H 2 O–CO 2 fluid was found to cause garnet recrystallization and formation of new phases, namely magnesian carbonate, coesite, kyanite, corundum–eskolaite solid solution and silicate–carbonate melt, as well as metastable graphite (950–1550 °C) and diamond growth on seeds (1250–1550 °C). It was established that the garnet-fluid interaction was accompanied by the bivalent cations' redistribution between recrystallized garnet, carbonate and carbonate–silicate melt. In these processes calcium was preferentially partitioned in the melt, while magnesium was partitioned in garnet and carbonate. Recrystallization of garnets of lherzolitic and eclogitic parageneses under effect of H 2 O–CO 2 fluid resulted not only in compositional changes, but in the formation of carbonate, kyanite, coesite, graphite, corundum–eskolaite solid solution and carbonate-silicate melt inclusions in these garnets. The revealed patterns of variations in garnet compositions, as well as the typical inclusions in them, can be regarded as indicators of metasomatic alteration of garnets of mantle parageneses involving H 2 O–CO 2 fluid. The obtained results allow one to infer that H 2 O–CO 2 fluids play a crucial role in redistribution of divalent cations under the P-T parameters of the lithospheric mantle and control over the composition of carbonate–silicate melts coexisting with the eclogitic and lherzolitic assemblages. Another important result of this study is the experimental demonstration that graphite capsules on their own fail to ensure reliable hermeticity with respect to the fluid and melt phases in the high-pressure, high-temperature experiments. It was substantiated that the procedure involving a hermetically sealed platinum ampoule with inner graphite capsule prevents uncontrollable loss of matter in long-term petrological experiments and is optimal for modeling systems containing melts and fluids. • Experimental modeling of mantle garnets + H 2 O-CO 2 fluid interaction was performed. • Garnet-fluid interaction results in the formation of carbonate-silicate-oxide assemblage and Ca-rich carbonate-silicate melt. • Characteristic features of garnets metasomatized with H 2 O-CO 2 fluid are Ca/Mg zoning and inclusions of newly formed phases. • In garnet-H 2 O-CO 2 -C system graphite crystallizes at 6.3 GPa and 950–1550 °C. • Diamond growth on seeds in garnet-H 2 O-CO 2 -C system occurs at 1150–1550 °C (6.3 GPa). [ABSTRACT FROM AUTHOR]

In Tanzania, the deep lithospheric mantle (> 70 km depth) is characterized by significantly higher electrical conductivity within the cratonic root than in the Mozambique belt. Such contrasts are typically attributed to changes in volatiles and/or melt content, with changes in mineralogy deemed insufficient to impact conductivity. To test this assumption, electrical conductivity measurements were conducted at pressure-temperature conditions relevant to the Tanzanian lithosphere (1.5 and 3 GPa; from 400 to >1500 °C) on dunite (depleted) and pyroxenite (fertile) xenoliths from Engorora, Northern Tanzania. Once garnet becomes stable in the fertile mantle rock (> 60 km, 1.7 GPa), it nucleates at grain boundaries, forming the backbone of a conductive network. At 3 GPa, such garnet-rich networks increase conductivity by a factor of 100 regardless of temperature. Numerical models demonstrate that the observed low (< 10−2 S⋅m−1) and high (> 10−1 S⋅m−1) conductivity values can be explained by low and high degrees of garnet connectivity, respectively. Such high electrical conductivities in cratonic roots can be explained by the presence of connected garnet clusters or garnet pyroxenites, suggesting mantle fertilization. This new source for electromagnetic signal generation at appropriate pressures and temperatures must be factored in where interpreting magnetotelluric signals at relevant depths in the lithosphere. The deep root of the Tanzanian craton is characterized by high electrical conductivity. In this study, we reproduce this anomaly in the laboratory. We use powders of Tanzanian xenoliths, which are natural rocks similar to what is expected in the deep Tanzanian lithosphere. Two extremes of mantle composition are tested: dunite and pyroxenite, respectively typical of a depleted mantle (already produced melts) and fertile mantle (high chemical diversity). Experiments at high pressures and temperatures representative of the deep Tanzanian lithosphere reveal that the high electrical conductivity is generated by the establishment of a connected network of garnet pyroxenites. Minerals such as garnet and pyroxenes are known to conduct electricity much better than olivine – the major component of most mantle rocks – especially when they contain limited amounts of hydrogen. This "water" is incorporated in crystals and at their boundaries and promotes high-conductivity pathways. A significant increase in conductivity occurs once the pressure is high enough for garnet to become stable. These results suggest that the composition of the deep cratonic root in Tanzania is closer to a pyroxenite than a peridotite. This difference in composition compared to the average mantle is expected as an impact of the East African superplume. • Electrical conductivity measurements at high pressures and temperatures on dunite and pyroxenite xenoliths from Tanzania. • High electrical conductivity is associated with networks of garnet pyroxenites, with natural H contents limited < 0.1 vol%. • The high electrical conductivity in the root of the Tanzanian Craton would highlight current plume impingement. [ABSTRACT FROM AUTHOR]

A suite of lavas from northwest Mull, British Palaeocene Igneous Province, exhibit major and trace element characteristics that are best explained by very high pressure (>1.8 GPa) crystallization of an assemblage comprising aluminous clinopyroxene (Al-Cpx) plus garnet. The resulting series of consanguineous magmas are mildly Si-undersaturated. The trace element effects of this crystallizing assemblage are manifested in increasing light rare earth element (LREE) enrichment and heavy REE (HREE) depletion with decreasing whole-rock MgO, a predictable consequence of the bulk distribution coefficient (D) of this assemblage being > > 1 for HREE but <<1 for the LREE. Early crystallization of Al-Cpx and replacement of plagioclase feldspar by garnet in the crystallizing assemblage also results in increasing or near constant Al 2 O 3 abundances in cotectic compositions with MgO < ∼7 wt%. Garnet as opposed to plagioclase crystallization is also reflected in the incompatibility of Sr and compatibility of Y resulting in very high Sr/Y lavas (up to 100) with high Sr (∼1200 ppm) and low Y contents (∼12 ppm). Lavas also have high Zr/Y (up to 30). Major element constraints suggest the crystallizing assemblage comprised ∼40% garnet and 60% Al-Cpx. Magmas that fractionated at ∼1.8 GPa rose to the surface without interacting with the continental crust. Immediately underlying basalts and picrites show evidence of crustal contamination and assimilated fusible portions of the crust, therefore effectively lining the plumbing system and allowing later magmas to rise to the surface without crustal contamination. A local change in tectonic regime from extension to passive appears to be linked to a change from low pressure to very high-pressure crystallization of magmas. Whilst evidence for garnet fractionation in continental flood basalts is very rare, this paper provides a characterization of its geochemical consenquences. • New geochemical data has been collected for lavas from north Mull, Scotland. • REE patterns are explained by fractional crystallization of garnet+pyroxene. • This requires crystallization at base-lithospheric depth or ∼ 1.8 GPa. • These observations are consistent with laboratory melting experiments. • Pressure of crystallization is tectonically controlled. [ABSTRACT FROM AUTHOR]

In NE Asia, the early stage (T-J) subduction history of the Paleo-Pacific oceanic plate was largely reconstructed by studies on the Heilongjiang complex, a blueschist-containing N-S trending high-pressure metamorphic belt, which is suggested to represent the "suture zone" between the Jiamusi Block and the rest of the Central Asian Orogenic Belt. However, recently more accretionary complexes with similar geochronological data were reported on both sides of the "suture zone", making the tectonic affinities of the "suture zone" unclear. Here, we select the mylonitic meta-sedimentary rocks in Huanan, one of the potential accretionary complexes near the central Jiamusi Block to reveal its depositional and metamorphic history and discuss the tectonic implications. Our results show that these rocks were deposited in/after the Late Triassic (defined by the youngest detrital zircon group) and accepted clastics from both the Jiamusi and Songliao blocks. Before the Early Jurassic (c. 178 Ma by apatite U Pb dating results in this study), they experienced high-P/low-T metamorphism (510–520 °C and 10.5–11 kbar), and then were heated to about 610 °C at 8.5–9.2 kbar during exhumation. The meta-sedimentary rocks in Huanan are components of the early Mesozoic accretionary complex in NE China. However, their present location is c. 85 km to the east of the previous proposed "suture zone" from Luobei to Mudanjiang. Considering the spatially scattering distribution of other similar accretionary complexes, such as the Zhangguangcai complex (c. 40 km to the west of the "suture zone"), we suggest that these complexes dispersedly exposed throughout NE China were either nappes transported by post-exhumation thrusting, or remnants of archipelagic oceanic crusts in the NW Paleo-Pacific region during the early Mesozoic. • Garnet micaschist in Huanan experienced high-P metamorphism in subduction channel. • Huanan complex is a Mesozoic accretionary complex near the central Jiamusi Block. • Accretionary complexes were scattered due to thrusting or archipelago ocean closure. [ABSTRACT FROM AUTHOR]

The southern Qiangtang (SQT) in central Tibet is characterized by sporadic Late Cretaceous magmatic rocks whose petrogenesis and tectonic implications remain unclear. Here, new zircon U Pb ages and Hf isotope compositions, together with whole-rock element and isotope data have been collected for the Duoma volcanic rocks from the SQT, central Tibet. Zircon U Pb dating indicates that the Duoma volcanic rocks crystallized at ca. 98 Ma. The basalts of the Duoma volcanic rocks have low SiO 2 (51.49–53.36 wt%), high total alkalis (4.65–6.25 wt%), and display OIB-like isotopic signatures (87Sr/86Sr) i = 0.7051 to 0.7058, ε Nd (t) = +0.04 to +1.10), overall similar to intra-plate alkali basalts. Moreover, basalts exhibit slight depletion of high-field-strength elements (HFSEs; e.g., Nb, Ta, and Ti), but enrichment of light rare earth (La N /Yb N = 6.7–7.6) with no negative Eu anomalies (Eu/Eu* = 0.91–0.99). These geochemical features, together with their high La/Nb (1.56–1.70) and Zr/Hf (44.70–45.87) but low La/Ba (0.08–0.09) and Ca/Al (0.43–0.65) ratios, suggest that the Duoma basalts originated from partial melting of garnet clinopyroxenite that was metasomatized by depleted mantle-derived melts. The coexisting rhyolites have high SiO 2 (69.78–69.92 wt%) and exhibit significant enrichment in large-ion lithophile elements (LILEs) and depletion in HFSEs (Nb, Ta, and Ti). Furthermore, the Duoma rhyolites are characterized by the decoupled Nd (ε Nd (t) values ranging from −4.4 to −11.1) and Hf (ɛ Hf (t) values ranging from −0.7 to +2.7) compositions, that character indicates that they were probably derived from the partial melting of the lower continental crust. The compositional bimodality and heterogeneous source of the Late Cretaceous volcanism indicate that they were formed in an extension setting, presumably triggered by lithospheric delamination beneath the SQT. • The Duoma volcanic rocks have compositional bimodality and were erupted at ca. 98 Ma. • The basalts originated from partial melting of the garnet clinopyroxenite. • The rhyolites were derived from partial melting of the lower continental crust. • The Duoma bimodal volcanic rocks erupted in a post-collisional extensional setting. [ABSTRACT FROM AUTHOR]

In order to gain insights into continental arc magmatic processes, we have conducted a petrological and geochemical study of major and trace elements and Sr, Nd, and Pb isotopes of the Ampato-Sabancaya compound volcano, which belongs to the Andean Central Volcanic Zone (CVZ). Whole-rock compositions for Ampato and Sabancaya range from andesites to dacites (56.7–69.3 wt% SiO 2) and both belong to a medium- to high-K calk-alkaline magmatic series. Ampato-Sabancaya samples are characterized by high contents of large-ion lithophile elements (LILE; e.g. , K, Rb, Ba, Th), low concentrations of high field strength elements (HFSE; e.g. , Nb, Zr) and heavy rare earth elements (HREE; e.g. , Yb), with consequently high La/Yb and Sr/Y ratios. An increase in these ratios is usually interpreted as a result of magmatic differentiation in the presence of garnet in the deep crust. A detailed analysis reveals that the rocks of Ampato-Sabancaya display three different compositional groups. (1) The first, composed mainly of andesites (56.7–59.8 wt% SiO 2), corresponds to lavas from the early stage of the Ampato Basal edifice, as well as pyroclastic deposits from the Ampato Upper edifice. (2) The second group corresponds to andesitic and dacitic compositions (60.0–67.3 wt% SiO 2) from the Ampato Basal edifice (Moldepampa stage), the Ampato Upper edifice, and the Sabancaya edifice. (3) The third group corresponds to dacitic compositions (65.0–69.3 wt% SiO 2) associated with the Corinta Plinian fallout and pyroclastic flow deposits from the Ampato Upper edifice. This last group of dacites, erupted during the Ampato Upper edifice stage, have drastically different compositions from the other groups with Sr/Y (<27) and Sm/Yb (<4.7) ratios lower than other lavas and lacking evidence of amphibole and/or garnet fractionation during their genesis. As a whole, Sr, Nd, Pd isotopic ratios suggest that mantle-derived magmas are significantly affected by assimilation processes during their evolution, due to the thick (65–70 km) continental crust beneath the CVZ in southern Peru. In summary, the magmatic evolution of group 1 and 2 can be explained by a two-step model in which primitive magmas evolved in the deep crust in the so-called melting-assimilation-storage-homogenization (MASH)-type reservoirs by assimilation-fractional crystallization (AFC) processes involving garnet and/or amphibole. Then, amphibole-dominated upper crustal AFC processes and magma mixing are responsible for the geochemical diversity of the main ASCV trend. In contrats, the group 3 dacites followed an upper crustal AFC process (without amphibole) from a different primitive magma, which did not suffer the high pressure, garnet-dominated AFC processes. This evolution highlights the complexities associated to magma genesis and differentiation at continental arcs contructed on a thick crust. • Ampato-Sabancaya medium-high-K calc-alkaline suite range from andesites to dacites. • Trace elements and isotope ratios indicate crustal assimilation processes. • Geochemical modelling shows that AFC and magma mixing have controlled the evolution. • Magma evolved by lower and upper crustal differentiation. • The assimilated crust is the Arequipa Proterozoic basement. [ABSTRACT FROM AUTHOR]

Kyanite, an aluminous phase common in high- to medium-pressure metapelitic rocks, is critical in revealing complex tectonometamorphic histories of orogenic belts. So far it has not been documented in the well-established low-temperature – ultrahigh-pressure (LT-UHP) metamorphic belt of Chinese southwestern Tianshan. By detailed petrographic investigations, kyanite is recognized as minute inclusions or porphyroblasts in two different types of metasedimentary rocks. In omphacite-garnet phengite schists of the Habutengsu area, kyanite only forms needles associated with acicular phengite and lawsonite (pseudomorphed by paragonite and clinozoisite), and coesite (or its pseudomorph) in the extremely low-Ca core domain of well-zoned garnet. The acicular inclusion assemblage in garnet cores probably witnessed large overstepping of peak-stage reactions in metapelitic rocks in a cold subduction zone. The absence of matrix kyanite in this rock type may be ascribed to elevated Na and Ca activities during the growth of high-Ca garnet rim. This is supported by the common presence of sodic and calcic phases (e.g., paragonite and omphacite) in the matrix or the garnet rim domain, with absence in garnet core. In a suite of calcschists of the Muzhaerte area, kyanite occurs as a matrix and/or inclusion phase. In relatively fresh, less foliated chloritoid-ankerite calcschists, kyanite is a major matrix phase with coesite inclusions and locally surrounded by prophyroblasts or fine-grained aggregates of both paragonite and phengite. In altered chlorite-white mica calcschists, kyanite is rarely embedded in white mica aggregates but commonly enclosed in Fe-rich chloritoid porphyroblasts. The mantling of ankerite by acicular to prismatic Fe-rich chloritoid suggests locally increased Fe2+ activity and Al transport by fluids along grain boundaries. The common white mica corona around kyanite in the calcschists suggests elevated Na and/or K activities postdating kyanite formation. Outcrop-scale variations of kyanite textures and whole-rock alkali element contents support the infiltration of Na- and/or K-bearing fluids, probably a dominant mechanism responsible for the lack of kyanite in the Tianshan UHP metasedimentary rocks. P-T estimates suggest the kyanite corrosion took place at 15–21 kbar and 530–610 °C, corresponding to the early exhumation stage. This study suggests that kyanite may be a major peak phase of the Tianshan UHP metasedimentary rocks, and the complete understanding of its prograde evolution is a prerequisite to unravel the fate of subducted sediments and their chemical geodynamics in cold subduction zones. • Kyanite is revealed for the first time from the UHP belt of Chinese SW Tianshan. • Kyanite and the coesite inclusions suggests their UHP experience as a rigid pressure vessel. • The rarity of kyanite in Tianshan UHP belt is probably due to the infiltration of aqueous fluids. [ABSTRACT FROM AUTHOR]

Here, we present a case study of an apatite-calcite segregationary kimberlite from the Andriesfontein pipe (Kaapvaal craton) using EPMA and LA-ICP-MS data of olivine macrocrysts and groundmass phases to constrain magma evolution. The primary objectives are to first constrain magma evolution in the SCLM using olivine followed by late-stage evolution and the timing of fluid exsolution using groundmass phases and importantly, apatite. Olivine cores display a range in Mg# (87.2–93.6) with no distinguishable gap in compositions. Aluminium, however, clearly divides the cores into two populations: 1) high-Al cores (>100 ppm Al; >400 ppm Ca; Mg# 87.2–92.7; 52% of cores) interpreted to predominantly reflect the disaggregation of megacrysts, and 2) low-Al cores (<100 ppm Al; <200 ppm Ca; Mg# >92; 48% of cores) interpreted to reflect sampling of garnet peridotite at depths of 84.5–138 km estimated using the Al-in-olivine thermometer. Groundmass phlogopite and spinel are well-zoned recording evolution of the kimberlite melt with increasing oxidation state indicated by the presence of MUM spinel and tetraferriphlogopite rims. Perovskite trace elements concentrations are typical of kimberlite perovskite worldwide. Apatites are observed in late-stage segregations as both equant grains and radiating clusters of acicular laths, however, their compositions are identical (SrO ∼ 2.2 wt%, SiO 2 < 1 wt%, and low REE e.g., La 2 O 3 + Ce 2 O 3 - <0.01 wt%). The relatively high SrO and low SiO 2 suggests formation from a fluid phase rather than directly from the melt, consistent with the presence of apatite in segregations and predominantly acicular habit. Chondrite normalised REE profiles for apatite display sinusoidal patterns, which can be modelled by melt evolution through small degrees of predominantly perovskite fractionation. Ultimately, we present a five-stage model for melt evolution from melt-SCLM interactions to magma evolution through groundmass crystallisation and late-stage fluid exsolution, highlighting the complexity of kimberlite magma evolution from source to surface. • Petrology of an apatite-calcite segregtaionary kimberlite. • Olivine tracks melt-SCLM interaction and mantle sampling depth. • Apatite EPMA and LA-ICP-MS compositions track late stage fluid exsolution. • Melt evolution tracked from source to surface. [ABSTRACT FROM AUTHOR]

The Southernmost Brasília Orogen surrounds the southern margins of the São Francisco Craton. It exposes a structural window with a WNW-ESE direction formed by the Archean-Paleoproterozoic basement reworked by the Campo do Meio Shear Belt. Using metamorphic modeling, geothermobarometry, and U Pb dating of zircon and monazite, this study established the pressure-temperature-time (P-T -t) evolution of the Colônia metapelite, one of the main units of the Campo do Meio Shear Belt. This revealed a clockwise path with metamorphic peak conditions at 706 ± 16 °C and 9.7 ± 0.7 kbar on a staurolite-kyanite-muscovite-garnet-biotite schist, with cooling at ∼600 °C and ∼ 6 kbar. Forward metamorphic modeling based on the whole-rock composition of this sample supports partial melting in the staurolite stability field, facilitated by water influx. Water-fluxed anatexis promoted dissolution of detrital zircon grains. These processes, occurring during prograde metamorphism, are influenced by temperature, crystal size-distribution, bulk-rock composition, and the permanence of the rock at the thermal peak. Analysis of the zircon overgrowth zones, which were linked to peak metamorphic conditions, produced an age of 605 ± 7 Ma. The monazite age of 580 ± 5 Ma was associated with later syntectonic melt crystallization. The clockwise P-T -t path of the Colônia metapelite is similar to the evolution of Neoproterozoic units from the Southernmost Brasília Orogen. However, the degree of dissolution in detrital zircon grains restricts accurate evaluation of the possible stratigraphic correlations proposed in the literature. • Staurolite stability can exceed the solidus curve in the fluid-saturated melt. • Kyanite stabilization can occur after melting into low-Al metapelites. • Water-fluxed melting promotes dissolution-recrystallization in detrital zircon. • Metamorphic ages overlap the ages of likely source areas. • Sedimentary geochronological approach for melting-bearing rocks may be unfeasible. [ABSTRACT FROM AUTHOR]

The Kon Tum Massif in the eastern Indochina Block (ICB) contains extensive Ordo-Silurian and Permo-Triassic metamorphic rocks. These two episodes of metamorphism not only affected the Kon Tum Massif, but were also widely developed in the whole of Southeast and East Asia, including the South China Block (SCB). Comparisons of the metamorphic characteristics among these blocks can reveal the crustal evolution and their affinity in supercontinents. Two gneiss samples in the eastern part of the Kan Nack Complex of the Kon Tum Massif have similar peak metamorphic ages of 488–482 Ma and mineral assemblages of garnet + plagioclase + K-feldspar + biotite + sillimanite + ilmenite + quartz. P-T calculations and thermodynamic modelling constrain a pressure peak (M1) at 770–820 °C and ∼1.1–1.2 GPa, and a temperature peak (M2) at 790–850 °C and 0.85–0.98 GPa, suggesting a near-isothermal decompression. The post-peak M3 assemblage developed at 650–760 °C and 0.48–0.74 GPa. The clockwise P-T-t path with medium and variable geothermal gradients probably reflects an orogeny process from compression to extension, which was triggered by the closure of the Tam Ky-Phuoc Son Ocean in the northern flank of the Kon Tum Massif. The P-T-t path and tectonic setting of this period of metamorphism in the Kon Tum Massif is similar to that in the SCB, but occurred a little earlier than that in the SCB, suggesting that the ICB and the SCB were adjacent on the northern margin of East Gondwana and together experienced a diachronous early Paleozoic orogeny. A granulite sample from the western Kan Nack Complex contains garnet + orthopyroxene + plagioclase + quartz + ilmenite ± K-feldspar. Decomposition of garnet to plagioclase and orthopyroxene suggests a decompression process. Coupled with features of a gneiss sampled at the same spot, P-T calculations and thermodynamic modelling indicate a pressure peak of 885–960 °C and 0.83–0.95 GPa and a temperature peak at 890–970 °C and 0.61–0.73 GPa, corresponding to ultrahigh-temperature (UHT) metamorphism. LA-ICP-MS monazite U-Th-Pb dating constrains a peak metamorphic age of 254 Ma, and Sm-Nd mineral isochrons date the retrograde stage at 243–236 Ma. This UHT metamorphism with a clockwise P-T-t path probably resulted from the excess heat supplied by asthenospheric upwelling following the break-off of the subducting slab of the Paleo-Tethyan Ocean. The Permo-Triassic metamorphism and other geological records in the ICB are significantly different from those in the SCB, implying that the geological processes in the two blocks were controlled by different geodynamic regimes during Late Permian to Early Triassic. • Two periods of high-grade metamorphism are identified in the Kon Tum Massif, Vietnam. • Early Paleozoic metamorphism was controlled by the subduction-collision event. • Permo-Triassic metamorphism was induced by the break-off of the subducting slab. • Early event is similar to that in South China, implying the same tectonic regime. • Late event is distinct, suggesting different tectonic regime controlling them. [ABSTRACT FROM AUTHOR]

The use of equilibrium phase diagrams such as pseudosections has become widespread in recent years, but few studies tested the phase relations predicted by these models. This work aims at comparing predicted phase relations with mineral assemblages observed in experimental products and in natural samples, in this latter case using as reference metamorphic conditions pressures and temperatures determined for metapelites metamorphosed under the same conditions as the mafic rocks. Pseudosections were calculated with THERMOCALC for six mafic bulk-rock compositions, in the range 425–700 °C and 2–8 kbar. The resulting diagrams can be divided into two groups: one in which a classic sequence of mineral assemblages occur and another where quartz is unstable and diopside is stable throughout. The results compare well to the independent data in three of the six studied cases. The main inconsistencies found in the pseudosections are underestimation of the stability field of garnet and overestimation of the stability fields of diopside and hornblende, which is at odds not only with data specific to the rocks studied here, but with the literature on metamafic rocks in general. In the case of garnet, the most likely case of discrepancy is the absence of Mn in the activity-composition models available. On the other hand, the issue with diopside and hornblende is much harder to evaluate, as it may be related to any of the activity-composition models, or to relationships between two or more of them. Nonetheless, we can affirm that the overestimation of the diopside stability field is related to SiO 2 content, although there are other factors that influence it. • Pseudosections for some of the studied mafic compositions yield inconsistent results. • Main problems relate to stability of diopside, garnet and hornblende. • Diopside and hornblende overstability are linked to SiO 2 content. [ABSTRACT FROM AUTHOR]

Himalayan leucogranites provide some of the best examples in the world of highly felsic and strongly peraluminous granites, and they have been paid special attention with regard to investigating the evolution of continental crust. Here, we describe our mineralogical investigations of a late-Miocene leucogranite complex within the Ramba Dome of the Tethyan Himalaya Belt that have helped us decipher the petrogenesis of these high-silica peraluminous granites. The Ramba leucogranites range from two-mica granites (biotite + muscovite) to muscovite leucogranites (muscovite ± garnet ± tourmaline), with the two-mica granites constituting the main part of the pluton and the muscovite leucogranites commonly occurring as late-stage dikes that are located within the pluton or invaded the country rocks. Variations in the mineral assemblages of the two types of leucogranite are associated with systematic changes in bulk-rock geochemistry, and the distinctive mineralogical features demonstrate a genetic relationship between the two types of leucogranite that involved fractional crystallization, with the muscovite leucogranites being more evolved than the two-mica granites. Chemical data for the minerals in the two-mica granites record a long period of differentiation during which a normal granitic melt developed into a highly evolved magmatic system with high flux components (e.g., H 2 O, F, B). In contrast, the mineralogical data for the muscovite leucogranites record the crystallization of a very late residual melt, i.e., a volatile-rich, highly fluid melt. According to recent studies of differentiation processes in silicic magma reservoirs, we can interpret the Ramba leucogranites as having undergone a high degree of differentiation by in situ fractional crystallization in a crystal mush environment. The two-mica granites represent a congealed crystal mush that was composed of 'cumulate crystals' and a trapped interstitial liquid, and the muscovite leucogranites represent the almost pure liquid that was extracted from the crystal mush. Modeling using the trace elements Sr and Ba shows that the extraction probably occurred when the crystallinity of the mush was ~59–64%, at least for the most evolved muscovite leucogranite sample. The two-mica granite corresponds to a residual crystal mush that had a terminal porosity of ~20–26% filled with a trapped interstitial liquid. Collectively, fractional crystallization can account for the geological, mineralogical, petrological, and geochemical variations between the two types of leucogranite. This work highlights the important role of crystal fractionation in the generation of Himalayan leucogranites. • The genetic link between the two types of leucogranite is fractional crystallization. • The two-mica granites are less evolved and have a crystal mush composition. • The muscovite leucogranites are highly evolved derivatives from the crystal mush. • Fractionation process is important in the generation of Himalayan leucogranites. [ABSTRACT FROM AUTHOR]

Chemical disequilibrium of coexisting garnet and zircon in pelitic migmatites (Aoyama area, Ryoke belt, SW Japan) is shown by microtextural evidence and their heavy rare earth element (HREE) patterns. In zircon, two stages of metamorphic rim growth is observed under cathodoluminescence image, although their SHRIMP U Pb zircon ages are similar at ca. 92 Ma. Inner and outer rims of zircon tend to show steep HREE patterns irrespective of the U Pb age. The inner rims tend to give higher U content than the outer rims; some rim analyses give various Th/U ratios of 0.02–0.07 compared to the very low (<0.02) values seen in the rest of rim analyses. The higher-Th/U values are ascribed to the mixed analyses between thin prograde domains and thick retrograde overgrowths. Zircon grains with inclusions similar to previously-reported melt inclusions are further enclosed in garnet, supporting the growth of thin zircon domains coexisting with garnet during the prograde metamorphism. Garnet rims are commonly replaced by biotite-plagioclase intergrowths, indicating a back reaction with partial melts. Garnet exhibits decrease in HREE and Y concentrations towards the rim, pointing to its prograde growth. The garnet cores have prograde xenotime inclusions, show steep HREE patterns, and yield growth temperature of ~530–570 °C by a YAG-xenotime thermometer. On the other hand, the garnet rims have no xenotime inclusion and show flat HREE patterns. Rare garnet domains including sillimanite needles also show flat HREE patterns and low Y concentrations, which is interpreted as a product of dehydration melting consuming biotite and sillimanite at near-peak P - T conditions (~800 °C and ~0.5 GPa). One such garnet domain gives nearly-equilibrium REE distribution pattern when paired with the matrix zircon rims. Retrograde xenotime is present in the cracks in garnet and in the biotite-plagioclase intergrowths, suggesting that retrograde breakdown of garnet released HREE and Y to form it. Considering the availability of HREE and Zr and presence of melt inclusions in zircon rims, most part of the zircon rims with positive HREE patterns likely grew during the melt crystallization stage, meaning that the zircon rims and presently-preserved garnet domains did not grow in equilibrium. The above scenario was tested by the array plot analysis and it gave a result consistent with microtextural and traditional REE distribution constraints. Combination of microtextural and the array plot analyses may become a powerful tool to reliably correlate the zircon ages to the P-T evolution of the high-grade metamorphic rocks. • REE patterns of garnet and zircon from the high- T , low- P Ryoke metamorphic belt are determined. • HREE pattern of zircon rims are steep, whereas HREE pattern of garnet is steep in the cores and flat in the rims. • Zircon rims of prograde origin may be thin and retrograde crystallization from melt is more abundant. • Disequilibrium REE distribution between garnet and zircon rims is due to different growth timings of the two. • Combination of microtextural and array plot analyses is a powerful tool to reconstruct the metamorphic P-T-t evolution. [ABSTRACT FROM AUTHOR]

This contribution presents U-Pb geochronological data for Ca-Fe-Ti garnets from compositionally diverse alkaline and carbonatitic intrusive rocks ranging from Neoarchean to Permian-Triassic (Cinder Lake and Eden Lake in Manitoba, Canada; Belaya Zima and Odikhincha in Siberia, Russia; Afrikanda in the Kola Peninsula, Russia) obtained using isotope-dilution thermal-ionization mass spectrometry (ID-TIMS) and their trace-element compositions measured by laser-ablation inductively-coupled-plasma mass spectrometry (LA-ICPMS). The studied garnets yield U-Pb concordant or subconcordant ages obtained with a precision of <0.5% owing to their relatively high content of U and negligible common Pb. The new ID-TIMS data are in excellent agreement with the previously reported zircon, baddeleyite and perovskite ages. The results of the present work demonstrate that postmagmatic alteration does not disturb or reset the U-Pb isotopic budget of these minerals, and that garnets representing the andradite-schorlomite-morimotoite system can serve as a robust reference material for micro-analytical geochronological studies of a wide spectrum of igneous and contact-metasomatic rocks. The new LA-ICPMS data demonstrate that the abundances of rare-earth, high-field-strength and other trace elements in calcic garnets from alkaline and carbonatitic rocks vary by at least two orders of magnitude and can therefore be used as reliable magma-evolution tracers. The rare-earth budget of these minerals is best described in terms of the chondrite-normalized ratios (Sm/La) cn and (Sm/Yb) cn , which are sensitive to lanthanide fractionation, and Y/Ho, which is interpreted to respond to garnet re-equilibration with a fluid. • U-Pb ID-TIMS study of calcic garnets yields precise and reliable ages. • Postmagmatic alteration does not disturb the U-Pb isotopic budget. • Ca-Fe-Ti garnets can serve as a robust isotopic reference material. [ABSTRACT FROM AUTHOR]

Accessory phosphate minerals, such as monazite and xenotime, are commonly used as geochronometers to provide timing constraints on tectonometamorphic events recorded by the host rocks within which they occur. However, the formation and recrystallization of accessory minerals, and their interaction with major mineral phases, are still poorly understood. As a consequence, linking ages obtained from accessory minerals to the metamorphic pressure-temperature (P-T) paths obtained primarily from major mineral phases, such as garnet, remains challenging. While there have been studies that have advanced our understanding of the behaviour of various accessory minerals through thermodynamic modelling, limited examples are available to test their reliability in natural metamorphic rocks. This study incorporates phosphate minerals into one of the most commonly used thermodynamic data sets for phase equilibria modelling. This refined methodology is tested by modelling the detailed P-T-time (t) paths for rocks from two different regions of the Himalaya, one subsolidus and one that experienced suprasolidus conditions. The results obtained from our integrated models yield direct information on the behaviour of monazite and xenotime growth/breakdown along the calculated P-T paths. This allows us to tie different age populations obtained from the accessory minerals directly to the P-T paths derived from major mineral phases and facilitates a refined understanding of the P-T-t histories of those rock specimens. • Phosphate minerals are modelled with silicate minerals using thermodynamic modelling approach. • Results yield direct information on behavior of monazite and xenotime along calculated P-T path. • Robust tool to link the ages from those minerals to the P-T path to get a well constrained P-T-t paths. • Provides insight into the trace element partitioning and equilibrium between garnet and monazite. • This approach aids in better understanding of the metamorphic evolution of the studied rocks. [ABSTRACT FROM AUTHOR]

Abstract We present an approach and software for modelling the Compositional Zoning in Garnet and its Modification by diffusion (CZGM), which combines pseudosection modelling using Perple_X and multicomponent diffusion in garnet during metamorphism. CZGM uses the initial compositional profile, obtained by Perple_X with application of garnet fractionation, to generate a population of garnets of prescribed size-distribution and to apply multicomponent diffusion along a selected P-T path during prograde metamorphism and subsequent decompression and/or cooling. As an example, CZGM was applied on average pelitic rocks subjected to metamorphism at different temperatures (600 to 800 °C) and time duration of 1–10 Ma. CZGM provides an easy-to-handle tool for petrologists to examine hypothetical scenarios of garnet development along prograde and retrograde paths and to constrain the P-T path of the whole metamorphic event. By treatment of laboratory measured garnet-bearing metamorphic rocks, it allows to improve the accuracy of pressure and temperature conditions, usually estimated only from compositional isopleths of garnet. Graphical abstract Unlabelled Image Highlights • A software combining pseudosection modelling and diffusion in garnet is developed. • Predicting compositional zoning in garnets along arbitrary P-T paths. • Time duration of metamorphic process based on diffusion in garnet. [ABSTRACT FROM AUTHOR]

Abstract Widespread Cenozoic basalts, erupted at ca. 24–0.16 Ma in central Inner Mongolia, eastern China, are significant for understanding the potential links between deep subduction of oceanic slabs and the genesis of continental intra-plate magmatism. Here we examine the whole-rock geochemical and isotopic data, as well as olivine compositions, for Cenozoic alkali and tholeiitic basalts from the Abaga, Huitengliang, and Chifeng areas in central Inner Mongolia. The results suggest that fractional crystallization of olivine + clinopyroxene (in the alkali basalts) and olivine (in the tholeiites), played a key role in their magmatic evolution, without significant crustal contamination or post-magmatic alteration. Geochemical and petrogenetic analyses show that pyroxene and garnet were dominant phases in the residual mineral assemblage for both alkali basalts and tholeiites, implying a pyroxenite source for these basalts. We propose that a pyroxenite-dominated source may be the product of reaction between recycled oceanic slab-derived melts and surrounding mantle peridotites. The spatial and geochemical variations of the Cenozoic basalts in central Inner Mongolia, together with recent geophysical and geological observations in eastern China, imply that the origin and evolution of large-scale Cenozoic continental intra-plate magmatism can be related to subduction of the Paleo-Pacific and Paleo-Asian oceanic slabs. Incorporation of recycled oceanic materials from these two subducted slabs thus influenced the mantle source lithologies and deep-Earth geodynamic processes beneath eastern China. Graphical abstract Unlabelled Image Highlights • A pyroxenite source is inferred for the Abaga, Huitengliang and Chifeng basalts • Recycled altered oceanic crust contributed to the generation of the basalts • Geochemical variation relates to recycled Paleo-Asian and -Pacific oceanic slabs [ABSTRACT FROM AUTHOR]

Abstract The Zhujiafang ductile shear zone is located in the central Hengshan Complex, a key area for understanding the Paleoproterozoic tectonic evolution of the Trans-North China Orogen, North China Craton. Detailed metamorphic and geochronological analyses were carried out on metapelite and garnet amphibolite from the Zhujiafang ductile shear zone. The metapelite preserves two phases of mineral assemblages: early kyanite-rutile-bearing assemblage and late chlorite-staurolite-bearing assemblage in garnet–mica schist, and inclusion-type muscovite (high-Si) + kyanite assemblage and late sillimanite-bearing assemblage in sillimanite–mica gneiss. Garnet in the metapelite occasionally exhibits pronounced two-stage zoning characteristic of a diffusion core with irregular pyrope (X py) and grossular (X gr) contents and a growth rim with X py and X gr increasing outwards. The isopleths of the maximum X gr in garnet core and Si content in inclusion-type muscovite in the P–T pseudosections suggest that the early mineral assemblages underwent medium-high-pressure type metamorphism with pressures up to 12–14 kbar at 700–750 °C. The late assemblages and the growth zoning of garnet rim predict a late separated clockwise P–T path with peak conditions of 6.5 ± 0.2 kbar/620 ± 10 °C (medium-low-pressure type). The garnet amphibolite is mainly composed of garnet, hornblende, plagioclase, ilmenite and quartz, without overprinting of late mineral assemblages except for localized corona textures. Phase modeling suggests that the rock has experienced high-amphibolite facies metamorphism with peak conditions of 10.5 ± 0.8 kbar/770 ± 50 °C, which is broadly consistent with the early-phase metamorphism of metapelite. Zircon U Pb dating on metapelite yields two metamorphic age groups of 1.96–1.92 Ga and 1.87–1.86 Ga which are interpreted to represent the timing of the two separated phases of metamorphism. Two separated orogenic events may have occurred respectively at ~1.95 Ga and ~1.85 Ga in the Hengshan–Wutai area. The older phase of orogeny was resulted from continental collision and the younger phase of orogeny may be caused by within-plate deformation. The final exhumation of the high-grade rocks formed in the older (i.e. 1.95 Ga) orogeny should be related with the younger deformation/metamorphic event. Highlights • Zhujiafang supracrustal rocks underwent medium-high-pressure type metamorphism at 1.95 Ga. • Overprinting of a late medium-low-pressure type metamorphism in the Zhujiafang shear zone occurred at 1.85 Ga. • The final exhumation of the North Hengshan granulite terrane is related with the younger shear metamorphism. [ABSTRACT FROM AUTHOR]

Abstract Rare earth element (REE)-enriched scheelite is widely distributed in many ore deposits, and has been utilized in a broad range of geochronological and geochemical studies. In an effort to decipher skarn and quartz vein-hosted W mineralization process, scheelite from Triassic and Jurassic granites, skarn ores and quartz vein-hosted ores in the Xitian ore field (South China) were analyzed for major and minor element content and fluid inclusion microthermometry. All scheelite grains show homogeneous backscatter electron and cathodoluminescence response and uniform W and Ca distribution. The chondrite-normalized REE patterns of scheelite from Triassic granites to skarn ores show similar Pr–Sm-enrichments and negative Eu anomalies but different HREE contents. In contrast, the chondrite-normalized REE patterns of scheelite from Jurassic granites to quartz vein-hosted ores changes from flat with slight LREE enrichment, to concave with remarkable LREE depletion. The homogenization temperatures and salinities of fluid inclusions from the scheelite in Triassic granites and skarn ores are similar, both reduce significantly from the Jurassic granites to quartz vein-hosted ores. Combined with pressure, pH and mineral assemblage analysis, we propose that the REE signatures in the ore-forming fluids of skarn-type W deposits are mainly controlled by crystallization of REE-enriched skarn minerals (e.g., garnet, diopside, epidote). The similarity in LREE patterns and Y/Ho ratios between the Triassic granite and skarn ore scheelite indicates that the skarn ore-forming fluid was derived from the Triassic granite, while the distinct REE patterns and variable Y/Ho ratios in scheelite from Jurassic granites and quartz vein-hosted ores imply possible fluid-mixing for Jurassic W mineralization. In light of these results, REE-rich scheelite could serve to discriminate fluid source and document mineralization processes in a complex ore-forming system. Graphical abstract Unlabelled Image Highlights • Four types of scheelite with different REE patterns were identified in Xitian. • REE substitution mechanisms within the scheelite are different in granites and ores. • Scheelite REE signatures were inherited from the ore-forming fluid. • A highly evolved fluid is suggested for the Jurassic W mineralization. [ABSTRACT FROM AUTHOR]

Abstract In this study, garnet–sillimanite gneisses at Hongshaba in the eastern segment of the Khondalite Belt, North China Craton (NCC) are interpreted to have experienced ultrahigh-temperature (UHT) metamorphism (980–1040 °C) followed by post- T max cooling at pressures of 8–9 kbar to the solidus (810–830 °C), consistent with rare sapphirine-bearing assemblages in surrounding regions. This interpretation is mainly based on the combination of P – T fields and garnet X gr (=Ca/(Ca + Mg + Fe2+)) isopleths on the pseudosection of three garnet–sillimanite gneiss samples. Spinel tends to be enclosed in the outer margins of garnet, commonly closely associated with quartz. We interpret this to reflect the partial break down of garnet along the prograde path during heating with decompression followed by new garnet growth during cooling along an overall clockwise P – T evolution. Although Fe-rich UHT metapelites tend to contain neither diagnostic mineral assemblages nor orthopyroxene from which to extract T via Al-in-orthopyroxene thermometry, isopleths of Ca in garnet may aid in retrieving UHT conditions from these compositions. This is attributed to Ca diffusion in garnet being much slower than Fe and Mg diffusion, leading to little change in Ca contents during post- T max cooling. LA-ICP-MS U-Pb dating of metamorphic zircon in one garnet–sillimanite gneiss sample yields a mean 207Pb/206Pb age of ca. 1.91 Ga, which is interpreted to record the timing of cooling of the UHT rocks to the solidus. This UHT metamorphism is interpreted to have been generated by mantle-derived magma during a tectonic extension from ca. 1.95 to 1.92 Ga within a post-orogenic setting. Highlights • Metapelites at Hongshaba, North China Craton have experienced UHT metamorphism • Ca in garnet may aid in retrieving UHT conditions from Fe-rich metapelites • An overall clockwise P – T evolution is likely for Hongshaba UHT rocks • The timing of cooling of Hongshaba UHT rocks to solidus is ca. 1.91 Ga [ABSTRACT FROM AUTHOR]

Abstract Garnet–(olivine) websterite xenoliths from the lithospheric mantle of the central and northeastern parts of the Siberian Craton contain exsolution microstructures after Si- and Ti-rich precursor garnets. We petrographically, geochemically, and thermobarometrically investigated 13 such xenoliths from the Mir, Obnazhennaya, and Udachnaya kimberlite pipes. All samples contain garnet grains with needle- to lamellae-shaped precipitates (up to 3.0 vol%), including Ti-oxide and/or pyroxene. Orthopyroxene and clinopyroxene grains host oriented lamellae of complementary Ca-rich and Ca-poor pyroxene, respectively, in addition to lamellae of garnet and Ti- and/or Cr-oxides. The common exsolution lamellae assemblages in garnet and pyroxene imply that exsolution occurred during cooling from high-temperature precursors. Exsolution is unlikely to have resulted from variations in pressure, given experimental and thermodynamic constraints. Host mineral partitioning of transition metal and lanthanide elements with different diffusivities record temperatures that range between those of local geotherms and a dry pyroxenite solidus. Inferred magmatic minimum temperatures of 1500–1700 °C satisfy the physical conditions predicted from experimental studies of the solubility of excess Si and Ti in garnet. Granular inclusions of all major minerals within each other imply an overlapping crystallisation history. The reconstructed compositions of the websterite whole-rocks have high MgO contents (15.7–35.7 wt%). A plot of MgO/SiO 2 versus SiO 2 forms an array, apart from the compositions of natural websterites that formed by interaction of peridotite with basaltic or siliceous melts. The array overlaps the compositional range of komatiite flows from Commondale and Barberton, South Africa, including spinifex, massive, and cumulate subtypes of komatiites. Other major and minor element abundances and ratios of the Siberian websterite suite resemble those of South African Al-enriched komatiites and are distinct from melt–rock reaction websterites. Therefore, the mineral microstructures and geochemistry of the Siberian websterites are suggestive of the former presence of a thermal anomaly. We propose that mantle plume activity or a similar form of lower-mantle ascent played a major role in stabilising cratonic nuclei before amalgamation of the present-day Siberian Craton. Highlights • Exsolution microstructures in SCLM websterite record a mantle thermal anomaly. • Major elements of websterite resemble those of high-Mg melts. • Siberian Craton stabilisation involved whole mantle convection. [ABSTRACT FROM AUTHOR]

Abstract In the Migmatite Complex from NE Sardinia, a large lensoid body of coarse-grained, dark-green amphibolite with a schistose to weakly massive aspect crops out. Within this amphibolite centimetre-sized layers locally occur which contain millimetric porphyroblastic garnet. We investigated the amphibolite and the layers applying microstructural analyses and thermodynamic modelling in the NCKFMASH+Ti + Mn system in order to reconstruct the pressure-temperature (P-T) metamorphic evolution. The amphibolite underwent a burial path, recorded by the compositional zoning of garnet, that started at pressures of 0.8 GPa and showed only a slight increase in temperature leading to peak P-T conditions. The garnet rim records peak P-T conditions of 1.3–1.4 GPa at 690–740 °C. As the early exhumation of the amphibolites occurred already at lower temperatures than the burial, an anticlockwise P-T path results which is in contrast to the typical clockwise P-T paths reported for several high-pressure metamorphic rocks from NE Sardinia. We interpret the anti-clockwise path by the location of the studied rocks in the lowermost part of the upper plate and their burial to depths of around 45 km during the Variscan continental collision between Laurussia and Gondwana. This process could have affected some rock slices of the upper plate only owing to tectonic erosion by the downgoing plate. The subsequent uplift occurred in an exhumation channel where these slices were continuously cooled by the upper portion of the lower continental plate. Highlights • First occurrence of anticlockwise P-T path in Sardinia • Anti-clockwise path indicates location in the lowermost part of the upper plate. • Eclogites and amphibolites experienced similar decompression from amphibolite facies. [ABSTRACT FROM AUTHOR]

Abstract Garnet-bearing migmatites and associated leucogranites and leucosomes of the Trivandrum Block in the Kerala Khondalite Belt were formed through granulite facies dehydration melting of metasedimentary protoliths. Significant trace element depletions in Cs, Zr, Nb and the compatible transition metal elements V, Cr, Ni, Cu and Zn relative to average post-Archaean shales and model middle and upper crust, recorded in all samples, require their sedimentary protoliths to have been impure sandstones and greywackes, rather than shales. Leucogranites (70-75 wt%) and leucosomes (SiO 2 : 68-70 wt%), which are uniformly peraluminous and classed as S-type on the basis of their A/NK and ASI relations, form a compositional array that shows strong correlated variations between TiO 2 and Fe 2 O 3 , and TiO 2 or Fe 2 O 3 with Co and Y. These reflect coupled variations in modal garnet and ilmenite and require the presence of up to 15–20 wt% of entrained peritectic garnet in the higher Y and HREE leucosomes. The leucosomes have REE patterns with normalised La/Sm of 10, negative Eu anomalies (Eu* < 0.8) and flat HREE at 6–20 times chrondrite, whereas leucogranites range to much lower HREE (1–5 times chondrite) with higher La/Sm and large positive Eu anomalies (Eu* = 1.4–3.4). Despite broadly similar major element compositions that lie within the granite minimum melt field in terms of felsic components, leucogranite Zr contents are highly variable, ranging down to 4–20 ppm in the lowest HREE and high Eu* cases, resulting in Zr saturation temperatures (544–647 °C) that are lower than any feasible melt. These geochemical features, coupled with covariations between Nb,-Ta and Y Yb, collectively support petrological and field observations that the leucosomes are mixtures between former melt and entrained peritectic garnet and ilmenite. The leucogranites are the products of melt extraction and migration on metres to several metres lengthscales. Leucogranite Nb Y, Ta Yb, Eu* and Sr Ba relationships demonstrate that their chemical diversity is principally controlled by variation in peritectic phase assemblage, i.e. proportion of entrained garnet, ilmenite and K-feldspar, with near-situ feldspar-dominated fractional crystallisation and accumulation coupled with the loss of fractionated melt being secondary controls. Age-corrected heat productions of the best-segregated leucogranites that most closely approach melt compositions are 2.7 ± 0.8 μWm−3, somewhat greater than the weighted average, 1.7 ± 0.5 μWm−3, of preserved migmatite, in-situ leucosomes and leucogranites of all types. Correction of the 550 Ma heat production to account for an estimated 30% of migration and loss of melt from this middle crustal migmatite complex yields a syn-orogenic heat production of c. 2.0 ± 0.6 μWm−3 which, though elevated compared with model middle-crust, is unlikely to have been high enough to have facilitated the near-UHT to UHT metamorphism in the Trivandrum Block without the addition of heat external to the orogenic crust itself. Highlights • Geochemical variations in leucogranites are principally controlled by peritectic minerals assemblage entrainment. • •Cumulate leucogranites feature highly positive Eu* at low total HREE, and depletions in the HFSE, including Zr. • 550 Ma heat production in the Trivandrum Block was 2.0 ± 0.6 μWm−3, elevated but insufficient to cause UHT metamorphism. [ABSTRACT FROM AUTHOR]