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Abundant rodingite was found within serpentinized peridotite and spatially close to gabbro and diabase dykes in the Xigaze ophiolite of the Yarlung Zangbo suture zone. Two types of rodingite are identified based on field- and textural relationships as well as on mineral- and bulk-rock compositions. Type I rodingite with porphyroblastic texture occurs along extensional fractures within massive serpentinized peridotite and contains two subtypes representing the early stage of rodingitization. Type II rodingite is associated with a foliated serpentinized host rock and contains three subtypes, characterized by prehnite-absent mineral assemblages with fine-grained porphyritic to aphanitic texture. All rodingite II subtypes experienced a higher degree of rodingitization than Type I. Type I rodingite was formed by metasomatism of CaO-rich fluids from serpentinized peridotite during ocean floor metamorphism at ~120–130 Ma. Thermodynamic modeling shows that Type I rodingite originated at low P-T conditions of

Continental deep subduction after the closure of large oceanic basins is commonly ascribed to the gravitational pull of the subducting oceanic slab. However, it is not clear how continental lithosphere adjacent to small oceanic basins was subducted to mantle depths. The Sesia Zone in the Western Alps provides an excellent target for exploration of subduction dynamics in such a tectonic setting. Here we report the first finding of coesite in a jadeite-bearing orthogneiss from the Sesia Zone, providing the first evidence for deep subduction of the continental crust to mantle depths for ultrahigh-pressure (UHP) metamorphism in this zone. Three coesite inclusions were identified by laser Raman spectroscopy in two garnet grains. Based on zircon U-Pb dating and trace element analysis, the UHP metamorphic age was constrained to be 76.0 ± 1.0 Ma. The phase equilibrium modeling yields peak metamorphic pressures of 2.8–3.3 GPa, demonstrating the continental deep subduction to mantle depths of >80 km. The subducted continental crust was a rifted hyperextended continental margin, which was converted to the passive continental margin during seafloor spreading and then deeply subducted during the oblique convergence between the Adria microplate and Eurasian plate in the Late Cretaceous. Because the slab pull could only play a limited role in closing small oceanic basins for continental collision, the distal push of either continental breakup or seafloor spreading is suggested as the major driving force for the deep subduction of continental crust in the Western Alps. Therefore, deep subduction of the continental crust bordering small oceanic basins would have been induced by the far-field stress of compression, whereas that bordering large oceanic basins was spontaneous due to the oceanic slab pull. This provides a new insight into the geodynamic mechanism of continental deep subduction.

The behavior of Ti-Nb-Ta is crucial to reveal the genesis of island arc magmatism. However, mobilization and fractionation of Ti-Nb-Ta in subduction zone settings remain poorly understood. The discovery of felsic veins rich in coarse-grained rutile within retrograde eclogite of the North Qaidam UHP metamorphic belt provides a unique and novel opportunity to study age variation during rutile formation and alteration, as well as Ti-Nb-Ta mobility and fractionation during fluid/melt-rock interaction. Rutile high-resolution elemental mapping, and U-Pb bulk grain (ID-TIMS), and in-situ U-Pb geochronology have been utilized to focus on the properties of rutile in both, felsic vein and retrograde eclogite host to gain insight into possible similarities and differences. Three groups of rutile were distinguished according to its host rock, trace elements signature, and genetical connection to ilmenite: eclogite-hosted rutile (Rt-1), felsic vein-hosted rutile not associated with ilmenite (Rt-2a), and associated with ilmenite (Rt-2b). Field evidence and rutile trace elements characteristics document the source of vein-hosted rutile to be mainly derived from the eclogite during fluid/melt-rock interaction. Principal Component Analysis reveals that Nb, Ta, Sn, and W are more enriched in Rt-2a compared to Rt-1; Rt-2b has higher Nb, U, and Hf than Rt-2a. High-resolution mapping across large rutile grains shows the enrichment of high field strength elements (HFSEs) in rutile near to ilmenite, which indicates a HFSEs back diffusion from the rutile–ilmenite boundary during the replacement of rutile by ilmenite. The Nb/Ta ratios of Rt-2a are lower than those of Rt-1, which result from different partition coefficients of Nb and Ta during fluid/melt-rock interaction. The diffusion-influenced rutile exhibits suprachondritic Nb/Ta ratios and demonstrates that diffusion of Nb in rutile is higher than that of Ta under identical P-T conditions. Rutiles Rt-1 and Rt-2a yield consistent 206Pb/238U ages of 426-423 Ma, which is similar to the 433 ± 3 Ma determined by ID-TIMS results of bulk rutile grains. This indicates that Ti-Nb-Ta must have been mobilized during the exhumation of deeply subducted continental crust. However, the diffusion-influenced rutile shows a large variation of ages compared to the rutile not associated with ilmenite, demonstrating that the back-diffusion may affect the U-Pb system in rutile. Therefore, when rutile is partially altered into ilmenite or titanite, its dating should be used with caution. Thus, this study demonstrates volume diffusion is a very important geological process to result in extreme HFSEs fractionation and age variation of rutile on the mineral scale. The rutile aggerates that occur in the felsic veins in 3-5m distance to the adjacent retrograde eclogite suggest that Ti-Nb-Ta-rich melts/fluids were transported over a distance of at least several meters and that rutile does not represent a residual phase of the Na-Si-Al-, F- and CH4-bearing fluid/melt environment that formed during anatexis of the subducted continental crust. The formation of rutile-rich aggregates during the generation, transport, and crystallization of subducted continental crust-derived melts/fluids in the deep roots of orogenic belts may be a critical trigger for the depletion of HFSEs in arc magmatic rocks during the formation of the continental crust.

Fluid is a key agent for the mass transfer between the subducting slab and the mantle wedge, which greatly affects the evolution of the crust-mantle system at convergent plate boundaries. A geochemical study was carried out for coesite-bearing jadeite quartzite and its country rock granitic gneiss from the Dora Maira Massif in the Western Alps. The results provide new insights into the composition of metamorphic fluids and fluid-rock interaction processes in the continental subduction zone. Coesite inclusions are found for the first time in metamorphic zircons of the granitic gneiss. The ultrahigh-pressure (UHP) metamorphism was dated to occur at 34.7 ± 0.3 Ma, confirming that the country rock experienced the UHP metamorphism synchronously with the whiteschist and jadeite quartzite. The jadeite quartzite occurs as layers and boudins within the coesite-bearing whiteschist, and shows similar whole-rock REE distribution patterns to both granitic gneiss and whiteschist. In addition, relict domains of magmatic zircon in the three types of UHP metamorphic rocks exhibit similar U-Pb ages and δ18O values, indicating that they have the same protolith of granites. However, metamorphic zircons in the jadeite quartzite show significantly lower δ18O values of 6.1–7.3‰ than the relict magmatic domains of 9.4–10.6‰. Furthermore, the jadeite quartzite has whole-rock δ18O values of 7.9–8.7‰, significantly lower than those of 9.5–10.9‰ for the granitic gneiss but higher than 6.5–7.9‰ for the whiteschist. These differences suggest that the jadeite quartzite was probably formed through metasomatism of metagranite by external fluids with relatively low δ18O values. The jadeite quartzite shows high δ56Fe values of 0.69–0.87‰, considerably higher than those of 0.18–0.38‰ for the granitic gneiss, but falling within the range of 0.32–1.03‰ for the whiteschist from the same outcrop. The Fe isotope modeling results suggest that the metasomatic fluids responsible for the formation of jadeite quartzite were possibly derived from the host whiteschist under UHP metamorphic conditions. Such fluids are enriched in Si, Al and Mg, and have high δ56Fe but low δ18O values. Therefore, the jadeite quartzite records the action of metamorphic supercritical fluids at the subarc depth. If the metasomatized rocks in the continental subduction zone would dehydrate to generate fluids with variable geochemical compositions due to multistage fluid-rock interactions, the fluids could further infiltrate the surrounding rock and possibly the mantle wedge to result in further geochemical transfer from the subducting slab to the mantle wedge.

—This study presents an overview of the systematic petrography, mineralogy, and geochemistry of jadeitite and jadeite-rich rocks found as blocks in the serpentinite mélanges of the Rio San Juan Complex (RSJC) of the northern Dominican Republic. The RSJC is one of the remnants of the subduction/accretionary complex of the Great Caribbean Arc that once spanned the gap between North and South America, moved relatively eastward to its present position as the Lesser Antilles island arc, and left collisional fragments along the two continental margins. Our systematic collection of heterogeneous samples ranges from jadeitite s.str. (sensu stricto) with ≥90 vol.% jadeite to quartz-rich rocks with jadeite and lawsonite. Two suites of rock types can be recognized. In the matrix-quartz-free rock suite, albite is the principal vein-filling or interstitial phase. Quartz is present only as inclusions in the cores of some jadeite crystals. In the matrix-quartz-bearing rock suite, quartz is abundant and albite is relatively rare. The first-order question concerning jadeite-rich rocks is whether jadeite precipitated from a high-pressure aqueous fluid (“vein precipitation” or “P-type”) or whether the jadeite-rich rock formed through comprehensive metasomatic replacement of an igneous protolith (“R-type”). Some examples occur as discordant veins and are clearly P-type. For most, however, classification has been equivocal. The systematic data on the petrography and whole-rock chemistry of jadeite rocks from the RSJC presented in this paper leads to significant clarification. A major argument against R-type genesis is that the metasomatic mass transfer required to produce jadeitite and jadeite-rich rocks from any normal igneous protolith is prohibitively complex. Using whole-rock, major-element compositions, we show that many members of the matrix-quartz-bearing rock suite from the RSJC can be derived by isochemical HP/LT metamorphism of normal oceanic plagiogranites subducted together with oceanic crust. Isocon analysis shows, furthermore, that more jadeite-rich rock types and also members of the matrix-quartz-free suite can be derived from such plagiogranites primarily by straightforward desilication, a realistic scenario in a serpentine-rich environment. The quartz inclusions found in jadeite crystals of the matrix-quartz-free suite corroborate a genetic path in which the plagioclase in a plagiogranite protolith reacts to jadeite + quartz. Later desilication and the formation of albite in the Si-undersaturated rock matrix leave tell-tale quartz inclusions as relics in jadeite crystals.

Massive, boudinaged, rose-coloured layers up to one metre in thickness composed of 60–80 vol% spessartine-rich garnet and 15–40 vol% quartz occur in high-pressure, garnet-mica schist on Margarita I...

High pressure (HP) and ultrahigh pressure (UHP) oceanic rocks remain very scarce along the Indus-Yarlung Tsangpo suture zone in the Tibetan Plateau. Such rock types, however, could provide new and important insight into the understanding of the India-Asia collision tectonics and regional metamorphic processes. This contribution focuses on newly discovered HP garnet amphibolites that occur in the North Ayilari range along the Indus suture zone in the Western Himalaya. Petrographic and mineralogical studies as well as pressure–temperature (P–T) calculations show that the garnet amphibolites underwent at least three metamorphic stages. A prograde amphibolite-facies stage at 0.9–1.2 GPa and 605–680°C (M1) was followed by HP granulite-facies metamorphism at 680–750°C and 1.1–1.5 GPa (M2) and cooling to amphibolite-facies metamorphism at 628–650°C, 0.6–0.8 GPa (M3). Zircon and apatite U–Pb dating shows that the garnet amphibolites experienced granulite-facies conditions at 33–32 Ma and a cooling age of 12.5 ± 2.6 Ma at temperatures of 375–550 °C. The garnet amphibolites have light rare earth element (LREE)-depleted distribution patterns, low (87Sr/86Sr)i = 0.70727–0.70833, but high eNd(t) (+8.9 – +9.8) in whole-rock, and high eHf(t) (+13.9 – +16.0) values of zircon, which are similar to those of normal mid-ocean ridge basalt. This confirms that the protolith represents an oceanic slice that dominantly sourced from depleted mantle material. Geochemical, geochronological and geothermobarometrical data demonstrate that the garnet amphibolites represented a part of the Neo-Tethyan ocean, that experienced a profound amphibolite facies metamorphism before reaching HP granulite-facies conditions at ca. 32 Ma as a consequence of crustal thickening during the India–Asian collision. Thus, the studied HP oceanic-type slice represents an important puzzle piece that, together with other continental portions that experienced different P-T conditions such as HP and UHP eclogite and different metapelites and quartzo-feldspathic gneisses, helps to decipher the tectonic evolution of the Western Himalaya orogen, from oceanic subduction to continental collision and subsequent exhumation.

Fluids play a crucial role in element mobility and mass transfer at the slab-mantle interface in subduction zones. However, tracing the source and chemical composition of subduction zone fluids still remains challenging. High-pressure (HP) metamorphic leucophyllites, mainly composed of quartz, muscovite/phengite and Mg-chlorite, occur in several localities in the Eastern Alps and experienced significant Mg-metasomatism at forearc depths during the Eoalpine orogeny. They thus provide a good opportunity to explore the origin of Mg-rich fluids in the continental subduction zone. The leucophyllites are rich in Mg and poor in Fe, Na, and Ca and occur as lenses and layers in the country rocks of metagranites. Both leucophyllites and metagranites show similar whole-rock REE distribution patterns, and magmatic zircons from them exhibit similar U-Pb ages of ∼271 Ma, suggesting that both types of rock have the same protolith of early Permian granite. One metagranite that preserves magmatic composition shows a low δ26Mg value of −0.64‰, suggesting a contribution of carbonate-rich sediment when granitic melts were produced. A profile of samples from metagranite through transitional gneiss to leucophyllite in Hungary shows two trends in δ26Mg values. In the Vashegy sections, δ26Mg values firstly decrease from −0.64‰ to −1.30‰ during gneiss formation, and then increase to 0.29‰ during leucophyllite formation. In a second locality, the TV tower section, the δ26Mg values firstly decrease from −0.29‰ to −0.89‰ during gneiss formation, and then remain constant during leucophyllite formation. Two leucophyllites in Austria exhibit higher δ26Mg values of 0.05 to 0.09‰ compared to their country rocks of about −0.20‰. Based on the Mg isotope systematics and their relationships to whole-rock geochemical compositions, we propose two types of fluid in the subduction zone: (1) a low δ26Mg ( 0.3‰) fluid derived from dehydration of talc-rich serpentinite. This is for the first time to find subduction zone fluids with distinct δ26Mg values at forearc depths. Both fluids possibly originated from dehydration of the sediment-serpentinite melange at the slab-mantle interface. The fluids were heterogeneous in δ26Mg values and they metasomatized slices of continental crust along shear zones. The presence of fluids with δ26Mg values as low as −1.3‰ suggests that the dissolution of Mg-rich carbonates can be significant at forearc depths. Our results provide an excellent example of tracing different sources of subduction zone fluids by coupling Mg isotopes with other petrological and geochemical variables. They may also have great implications for the generation of magmas with variable Mg isotope compositions at convergent plate boundaries.

Fluids released by subducting slabs are important to the geochemical evolution of Earth’s crust-mantle system. However, it is still not well constrained for the mobility of redox sensitive elements (like Fe, C or S) in subduction zone fluids, and its effect on the redox property of the overlying mantle wedge and its derived arc magmas. Iron isotope fractionation is sensitive to the redox state and speciation of Fe in fluids, which can potentially provide clues on the oxygen fugacity (fO2) of slab-derived fluids. Whiteschists at the Dora-Maira Massif in the Western Alps have experienced ultrahigh-pressure metamorphism at subarc depths. They are rich in both SiO2 (mostly >65 wt.%) and MgO (4–10 wt.%), and mainly composed of pyrope, quartz/coesite, talc, phengite and kyanite. They were demonstrated to have a granite protolith, but metasomatized by serpentinite-derived Mg-rich fluids at the slab-mantle interface in a subduction channel. Thus these rocks provide an excellent target to explore the fluid-rock interaction by Fe isotopes. The whiteschists show extremely high δ56Fe values of +0.32 to +1.22‰, being much higher than those of +0.10 to +0.38‰ for the surrounding metagranites. The Fe isotope composition of whiteschists significantly deviates from the igneous differentiation trend defined by rocks with basaltic to rhyolitic compositions. The heavy Fe isotope compositions must be produced by a metasomatic process. The much lower Fe2O3 and FeOt contents and Fe3+/∑Fe ratios but higher δ56Fe values for the whiteschists relative to their protolith suggest the reduction of Fe3+ to Fe2+ and the loss of isotopically light Fe, probably in the form of Fe(II)-Cl and/or Fe(II)-(HS) complexes, through saline fluids and/or HS- bearing fluids. The reduction of Fe3+ to Fe2+ and the possible presence of HS- suggests the occurrence of relatively reduced fluids. Such reduced fluids were probably derived from serpentinite dehydration at the slab-mantle interface in the subduction channel. Therefore, the Fe isotope results demonstrate that the fO2 at the slab-mantle interface can be locally highly heterogeneous, leading to various oxidation states in the mantle wedge and arc magmas.

The geological toolbox for the analysis of orogenic processes has seen substantial additions over the recent decades. Major advances have been made in the ability to simulate geological conditions and processes by computer modeling, we have a much improved knowledge of geochemical processes (trace elements and isotopes in particular), and, last but not least, the range and versatility of micro-analytical methods and instrumentation available to us has been expanded dramatically. The latter aspect has had a particular impact on metamorphic petrology as the conventional tools to determine pressure-temperature-time paths via rock and mineral analysis combined with isotope geochronology have generally gained in precision and accuracy through a much improved resolution. Geochronology has moved from whole-grain analysis to spot analyses on zoned single grains while analyzing trace elements and isotopes at the same time. It is therefore possible to recognize complex multi-stage orogenic events in single samples or single grains. Diffusion profiles in minerals provide us with time-scales of metamorphic and magmatic processes. Major and trace elements as well as isotope profiles of mineral grains give us insight into changing ambient conditions during mineral growth. This special issue focuses on pressure-temperature conditions and geochronology of metamorphic rocks, isotope studies of magmatic rocks, partial melting and metasomatic processes, all with a view at tectonic implications and orogeny. Much of the evidence has been gained from small-scale observation and analysis. Diligent field work and detailed microscopic examination remain indispensable, though, as demonstrated clearly by the “expansion” of the P-T space as applicable to crustal metamorphic processes towards extreme pressures and temperatures. As a consequence, much interest has thus focused on UHP and UHT rocks and the orogenic processes that generate them. There is petrological evidence for subduction to, and subsequent exhumation from, depths in excess of 250 km. While this impacts substantially on the large-scale geodynamic models of orogenic processes, the evidence is gathered primarily from small-scale petrological observation. The same applies in principle to UHT granulites. To generate temperatures in the 900 to 1 100 °C range within the sillimanite stability field in orogenic heat budget models remains a serious challenge, however. The large overlap of solid-state metamorphism, partial anatexis and magmatic crystallization in P-T space suggests close interrelationships, and the role and contribution of the diverse magmatic activities in forming orogenic belts, from the onset of subduction to late-orogenic delamination or slab-break-off is crucial for our understanding of geodynamics, with focus on geochemistry and geochronology. Aspects of metasomatism in the framework of orogeny are given some special consideration in this issue. The topics include the formation of sagvandite, pyrope quartzite (or whiteschist), jadeitite and rodingite. One of the key questions addressed is the source and the nature of the fluids that triggered the different types of metasomatic events. Finally, it is shown how small-scale methods contribute to a better understanding of the formation of deep-seated mantle xenoliths.

The Beloretsk Metamorphic Complex in the SW Urals formed at a convergent eastern margin of Baltica during the Neoproterozoic-Early Cambrian Timanide orogeny. It comprises three major units with lenses of facies-critical metabasites within metasedimentary rocks: A lowermost eclogite unit, an intermediate garnet amphibolite unit and an upper amphibolite-greenschist unit. Pressure (P)-temperature (T)-paths of four rocks from the two lowermost units were determined mainly by PT pseudosection techniques showing similar clockwise loops at different peak metamorphic, water-saturated conditions: A phengite-bearing eclogite shows peak PT conditions of 16.5–18.5 kbar/525–550 °C (stage I) followed by stage II at 11.5–13.0 kbar/585–615 °C. A garnet amphibolite from the intermediate unit yields lower peak conditions of 11.7–14.5 kbar/480–510 °C (stage I) followed by stage II at 9.5–11.0 kbar/535–560 °C. However, a granite gneiss in the eclogite unit shows similar maximum pressures as the eclogite, but higher temperatures at 15.6–16.2 kbar/660–675 °C, whereas a garnet micaschist contains comparable high pressure relicts, but underwent an advanced midcrustal reequilibration at 7.5–9.0 kbar/555–610 °C. We dated the eclogite by a 7-point Rb/Sr mineral isochron (phengite, omphacite, apatite) at 532.2±9.1 Ma interpreted as age of crystallisation of the eclogitic peak PT assemblage. This age is the youngest compared to the known Timanide metamorphic and magmatic ages.

In this study, we present new zircon U-Pb and Hf isotope data and geochemical analyses for Jurassic igneous rocks from the southern Zhangguangcai Range, NE China. In the context of our new U/Pb dat...

Blueschists are sporadically exposed as lenses within the Lancangjiang metamorphic complex, and represent unique components of the Paleo-Tethys. In this paper, we present geochemical and geochronological results of blueschists to decipher their origin and tectonic significance. The whole-rock geochemical analyses revealed strong similarities with ocean island basalt (OIB), and further discrimination diagrams confirm an affinity to a within-plate setting. Combined studies on blueschists using cathodoluminescence (CL) imaging, SHRIMP U-Pb dating of zircon domains and 40Ar/39Ar dating of phengite and glaucophane provide evidence of their magmatic origin and metamorphic evolution. Slightly oscillatory zoned or compositionally homogeneous zircon grains/domains, as well as structureless zircon rims, yield ages from 231.6 ± 3.7 to 225.3 ± 4.8 Ma, recording the blueschist facies metamorphic event. In contrast, the captured zircon grains and cores with a major age peak at ~241 Ma as well as several minor older age peaks indicate the multiple provenance of the zircons. 40Ar/39Ar step heating analyses on single grains of phengite and glaucophane separated from blueschists yield plateau ages ranging from 242.5 ± 1.4 to 228.7 ± 1.5 Ma which are interpreted to reflect high-pressure metamorphism. This study provides geochemical and geochronological constraints on the tectonic evolution of the Paleo-Tethyan ocean, which was closed and subsequently subducted as a result of the collision of the Simao and Baoshan Blocks. During subduction in Trassic (243 to 225 Ma), the protoliths of blueschists underwent blueschist facies conditions.

The Xigaze ophiolite (Tibet), which occurs in the central segment of the Yarlung Zangbo Suture Zone, exposes a complete portion of a mantle sequence that consists essentially of fresh as well as serpentinized peridotites. We studied a sequence beneath the crustal section that exposes fresh, Cpx-bearing harzburgites and dunites that are underlain by serpentinized Cpx-bearing harzburgites and dunites. The rocks at the bottom are crosscut by dykes that have undergone different degrees of rodingitization. The modal compositions of peridotite from both fresh and serpentinized sections plot in abyssal upper mantle fields, with clinopyroxene modes less than 5 vol. %. Although harzburgites and dunites indicate that melt has been lost relative to primitive mantle compositions, the trace element patterns carry signatures of enrichment in incompatible elements, such as (i) “bowl-shaped” patterns of trace elements in silicate-Earth normalized spider diagrams, (ii) positive anomalies in highly incompatible trace elements such as Rb, Th, U, Ta, and (iii) enrichment of LREE in the clinopyroxenes from dunites and harzburgites. These features are indicative of complex melt transfer processes and cannot be produced by simple melt extraction. Petrographic studies reveal that harzburgite and dunite contain interstitial polyphase aggregates of olivine + Cpx + spinel + Opx and olivine + Cpx + Spinel, respectively. Experimental studies (e.g. Morgan and Liang, 2003) suggest that these aggregates represent frozen melt-rich components, indicating that fertile melt was percolating through the depleted harzburgite – dunite matrix. Presence of such “melt pods” would explain the trace element enrichment patterns of the bulk rock, as well as features such as reverse zoning (core: Cr, Fe2+ rich, rim: Al, Mg rich) of spinels in polyphase aggregates in fresh dunites. These results show that melt extraction from the mantle is not a single stage process, and that evidence of multiple melt pulses that propagated through a rock are preserved in the petrographic features as well as in the form of chemical signatures that indicate refertilization of initially depleted rocks.

The supplementary data of Cyclicity of multistage anatexis of deeply subducted continental crust during the North Qaidam orogeny: Tracing the source, timescale, and evolution of pulsed fluids/melts

Four Cenozoic, rhönite-bearing alkali-olivine basalt samples from the Changle area (Shandong Province, China) show an intracontinental character and were generated in an extensional setting. Petrographic studies document different generations of rhönite. In three samples, rhönite occurs either as a reaction product surrounding coarse-grained corundum, spinel and phlogopite or along cleavage planes in phlogopite. In one sample rhönite forms disseminated crystals in a mantle xenolith, possibly formed by a reaction of coarse-grained orthopyroxene or spinel with a melt. Rhönite exhibits a wide range of compositions: 22.9 wt %–33.0 wt % SiO2, 13.3 wt %–19.0 wt % Al2O3, 9.4 wt %–19.9 wt % MgO and 10.210.2 wt %–24.5 wt % FeO. The derived primary substitutions include (1) SiIV + NaVII = (Al, Fe3+)IV + CaVII, (2) MgVI = (Fe2+, Mn2+)VI and (3) TiVI + (Mg + Fe2+ + Mn2+)VI = 2Fe3+VI. Rare-earth-element (REE) patterns of euhedral rhönite crystals from the mantle xenolith (sample SS17) and those surrounding spinel (sample CL04) have a concave-upward shape for the heavy rare-earth elements (HREEs) and are slightly enriched in the light rare-earth elements (LREEs). These patterns resemble those of kaersutitic amphibole and kaersutite reported from basanite, olivine nephelinite, transitional alkali-olivine basalt and hawaiite. In contrast, REE patterns of the other two samples containing fine-grained, anhedral and acicular rhönite crystals (samples CL01 and EGS03) are relatively steep, with lower HREE and higher LREE abundances, similar to those of ocean island basalts (OIBs). All types of Changle rhönite show positive Nb, Ti and V anomalies in spidergrams normalized to primitive mantle. Mineral assemblages of the studied samples indicate that rhönite crystallized at different stages within a temperature range from about 950 to 1180 ∘C and at pressures below 0.5 kbar, with fO2 below the NNO buffer. The chemical composition of Changle rhönite is interpreted to depend on the composition of the initial silicate melt, the redox conditions during crystallization and the composition of the minerals involved in reactions to form rhönite. Similar to metasomatic mantle amphibole, the compositions of Changle rhönites cover the I-Amph (I-amphibole) and S-Amph (S-amphibole) fields, indicating that they may have formed due to an intraplate metasomatic event, overprinting an older metasomatic subduction episode.

This study presents data of in-situ mineral major and trace element chemistry, whole-rock major and trace element and Sr–Nd isotope geochemistry, as well as zircon U–Pb dating and Lu–Hf isotope studies of andesite dykes from the Sulu orogenic belt of eastern China to evaluate their petrogenesis and thus to provide insights into crust–mantle interactions in a tectonic terrane that underwent continental subduction and was then overprinted by oceanic subduction. The andesites mainly comprise plagioclase, hornblende, clinopyroxene and magnetite as phenocrysts that are embedded in a groundmass of fine-grained quartz and K-feldspar, with minor amounts of biotite, magnetite, titanite, apatite and zircon. They possess intermediate concentrations of SiO2 (54.97–62.24 wt.%), Na2O + K2O (6.35–7.24 wt.%) and MgO (3.37–7.12 wt.%) with high Mg# values [Mg# = 100 × Mg/(Mg + Fe2+) molar] of 54–64, and are characterized by arc-like trace element patterns that are enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE) but depleted in high field strength elements (HFSE). The hornblende and clinopyroxene phenocrysts exhibit enrichment in Th and U but an obvious depletion of HFSE. The andesites have high initial 87Sr/86Sr ratios of 0.7073–0.7086 and negative ɛNd(t) values of −15.7 to −14.4 for whole-rock, and newly crystallized magmatic zircons show negative ɛHf(t) values of −27.0 to −17.6. U–Pb dating on syn-magmatic zircons yields consistent ages of 124 ± 2 to 116 ± 1 Ma, indicating eruption of the andesitic lavas during the Early Cretaceous. Inherited zircons are present and yield Neoproterozoic, Paleoproterozoic and Archean U–Pb ages. Taking into account all these geochemical and geochronological data, and the compositional features of contemporaneously formed mafic–andesitic igneous rocks from the Sulu belt and the adjacent continental crust of the North China Craton, we propose that the andesites crystallized from primary andesitic magmas derived from partial melting of enriched metasomatites in the subcontinental lithospheric mantle. The enriched metasomatites are inferred to be formed by a two-step process: firstly by crust–mantle interactions during the Triassic collisional orogeny and secondly by a subsequent modification by fluids/melts that mainly derive from seafloor sediments during subduction of the paleo-Pacific plate in eastern Asia. Slab rollback of the subducted paleo-Pacific slab and concomitant asthenosphere upwelling are an appropriate geodynamic mechanism for the generation of extensive arc-like magmatism during the Early Cretaceous in the Sulu belt.

The Precambrian basement of the Jiaobei terrane consists of Archean metavolcanic and metasedimentary rocks, tonalitic – trondjhemitic – granodioritic gneisses (TTGs), Paleoproterozoic metasedimentary sequences, as well as lenses of metamorphic mafic–ultramafic rocks. Petrological, geochemical and in situ zircon U-Pb studies were conducted on meta-calcsilicates, which occur as irregular blocks within TTG gneisses. Samples were collected at Nanshankou, Jiaobei terrane of Shandong Peninsula at the north-eastern part of the North China Craton (NCC).Two groups of meta-calcsilicate rocks were distinguished, which originate from a metavolcanic-metasedimentary series. The respective bulk rock composition is a result of different proportions of source rock material involved. One group which is higher in CaO and lower in Na2O demonstrates a higher contribution material from a sedimentary source, whereas volcanoclastic to pyroclastic precursor dominates in the second group. The meta-calcsilicates are generally composed of Ca-bearing silicates with a peak preassure assemblage of Grt-Cpx-Zo-Am-Qz-Ttn; they experienced epidote amphibolite facies metamorphism constrained by P-T conditions of 580°C / 1.08–1.13 GPa. Late prehnite-, albite-, calcite- and actinolite-bearing veins refer to the subgreenschist conditions at the late exhumation stage.In situ zircon U-Pb dating of 173 detrital zircons from the meta-calcsilicate rocks yield Archean ages up to 3.04 Ga; a further predominant age population clusters at ca. 2.68 Ga with subordinate clusters at ca. 2.87, 2.78, which are interpreted to represent magmatic ages. A subordinate cluster at 2.51 Ga associated with zircon rim domains reflects a metamorphic event. Further frequently textureless rims of zircon of an age range between ca. 2.5 and –2.1 Ga indicate subsequent metamorphic events. The deposition of the meta-calcsilicate precursors occurred around of shortly after 2.1 Ga on an Archaean basement.

In the North Qaidam ultrahigh-pressure (UHP) metamorphic belt, impure marble and interbedded eclogite represent a particular sedimentary provenance and tectonic setting, which have important implications for a controversial problem - the dynamic evolution of early Paleozoic subduction-collision complexes. In this contribution, detailed field work, mineral chemistry, and whole-rock geochemistry are presented for impure marble to provide the first direct evidence for the recycling of carbonate sediments under ultrahigh-pressures during subduction and collision in the Yuka terrane, in the North Qaidam UHP metamorphic belt. According to conventional geothermobarometry, pre-peak subduction to 0.8–1.3 GPa/485–569 °C was followed by peak UHP metamorphism at 2.5–3.3 GPa/567–754 °C and cooling to amphibolite facies conditions at 0.6–0.7 GPa/571–589 °C. U-Pb dating of zircons from impure marble reveals a large group with ages ranging from 441 to 458 Ma (peak at ~450 Ma), a smaller group ranging from 770 to 1000 Ma (peak at 780 Ma), and minor >1.8 Ga zircon aged ca. 430 Ma UHP metamorphism. The youngest detrital zircons suggest a maximum depositional age of ca. 442 Ma and a burial rate of ca. 1.0–1.1 cm/yr when combined with P–T conditions and UHP metamorphic age. The REE and trace element patterns of impure marble with positive Sr and U anomalies, negative high field strength elements (Nb, Ta, Zr, Hf, and Ti), and Ce anomalies imply that the marble had a marine limestone precursor. Impure marble intercalated with micaschist and eclogite was similar to limestone and siltstone protoliths deposited in continental fore-arc or arc setting with basic volcanic activity. Therefore, the Yuka terrane most likely evolved in a continental island arc setting during the Paleozoic. These data suggest that metasediments were derived from a mixture of Proterozoic continental crust and juvenile early Paleozoic oceanic and/or island arc crust. In addition, their protoliths were likely deposited in a terrigenous-dominated forearc marine basin rather than an intracontinental basin environment, further evidence that some continental arc volcanic rock may have been the source of eclogite in the North Qaidam. These sediments, formed in a forearc basin close to the Qaidam Block to the north, were transported in the subduction zone to 100–110 km depth with UHP metamorphism prior to exhumation. Meanwhile, the new results suggest that subduction erosion occurred along the active continental margin during the Qaidam Block with north-dipping subduction, indicating that the North Qaidam UHP metamorphic belt may have formed during continental-arc collision.

Garnet is one of the most significant minerals in metamorphic rocks, that provides key information on prograde, peak-metamorphic and retrograde parts of the pressure-temperature (PT) path. Such results require a detailed knowledge of its different growth domains. For iron-poor compositions, the cathodoluminescence (CL) microscopy is an important and often overlooked method and allows to identify the internal structures of all garnet grains in one thin section within only a few seconds. The advantage of the CL-microscope is to deliver low magnification images in true color, not only of garnet but also, for instance, of other rock forming silicates, carbonates, sulfates, etc., of metamorphic, but also of sedimentary and magmatic origin, using polished thin sections. Internal structures of grossular from Mexico and pyrope from the Italian Alps were characterized and visualized by CL-microscopy. The different growth domains were additionally studied using CL-spectra and electron microprobe (EMP) analysis. Grossular shows a patchy zonation in its core while in mantle and rim zones oscillatory zoning is observed. It contains zones of anomalous birefringence, zones of orange and bluish luminescence and zones lacking luminescence. Different but low amounts of the activator elements Mn2+ and Eu2+ are responsible for the orange and bluish luminescent domains. Pyrope is also characterized by oscillatory growth zones, shows a dull luminescent core with a change of crystal morphology during growth, and displays an increase of brightness from core towards rim—the outermost rim, however, is lacking luminescence. The different luminescent zones are characterized by different amounts of Dy3+, Tb3+, Sm3+ and Sm2+ as activator elements. Because of slow diffusion rates of activators such as the REEs Sm, Dy and Tb, it can be still possible to visualize possible prograde and/or peak pressure stage growth domains of garnet, even if later high temperature events may have homogenized the major element profiles. Such domains may help to identify respective assemblages of mineral inclusions, and hence these results can represent an integral part of a detailed PT path. Thus the CL-information can be used as an important pathfinder prior to supplementary investigations, as for instance EMP, ion probe, mineral or fluid inclusion studies.

Freezing-point depression was measured in aqueous fluid inclusions to determine salinities in six samples of jadeitite and jadeite-rich rock from the Jagua Clara serpentinite melange of the Rio San Juan Complex, Dominican Republic. The melange represents a fossil subduction-zone channel from a cold, mature subduction zone with a geothermal gradient of ~6 °C/km. One hundred and twenty-five determinations of salinity in primary inclusions hosted in jadeite, quartz, apatite and lawsonite range between extremes of 1.2 and 8.7, but yield a well-defined mean of 4.5 ± 1.1 wt% (±1 s.d.) NaCl equiv, slightly higher than mean seawater (3.5 wt%). In one sample, eight additional fluid inclusions in quartz aligned along grain boundaries yield slightly lower values of 2.7 ± 1.3 wt% NaCl equiv. Homogenization temperatures were also measured for 47 fluid inclusions in two samples, but primary entrapment densities are not preserved. It is significant that the suite includes two types of samples: those precipitated directly from an aqueous fluid as well as examples of metasomatic replacement of a pre-existing magmatic rock. Nevertheless, the results indicate identical salinity for both types and suggest a much stronger genetic link between the two types of jadeitite and jadeite-rich rock than has previously been assumed. Based on the results of conductivity measurements in modern subduction zones, we envision a pervasive fluid in the subduction channel that evolved from salinity levels lower than those in sea-water up to the measured values due to on-going but largely completed serpentinization in the subduction channel. The present data represent a reference marker for the subduction channel of the Rio San Juan intra-oceanic subduction zone at 30–50 km depth and after 50–60 Myr of operation.

The Xigaze ophiolite represents one of the most prominent occurrences of Neo-Tethyan ocean remnants in the central part of the Yarlung-Zangbo suture zone, southern Tibet, which marks the collision of the Indian and Eurasian plates during the Jurassic and Cretaceous. Abundant veins and bands of rodingite, which are spatially related to gabbro and diabase dykes and intermingled with serpentinized mantle peridotite, were found in the ophiolite sequence, extending from Jiding to Luqu, Bairang and Baigang. The gabbro as well as diabase formed in a back-arc basin that developed by subduction of the Tethys ocean in response to the convergence of India and Eurasia. Both rodingite and gabbro/diabase show the same geochronological and geochemical behavior, thus demonstrating the genetic relationship of these lithological types with oceanic lithosphere. Rodingitization can be seen as a Ca-involving metasomatic process that is genetically linked to serpentinization; the Xigaze rodingites typically show a gain in Ca and a loss of Na and K. The mineral assemblages of the rodingites are highly variable and can be divided into two groups. Group I rodingite, where prehnite is absent, consists of the subgroups clinozoisite rodingite, garnet–diopside rodingite and garnet–vesuvianite rodingite. The grossular-rich (hydro) garnet may contain up to 13.7 wt% TiO 2 , essentially as morimotoite component (23 mol%). In group II rodingite, prehnite is present and vesuvianite absent; subgroups are prehnite rodingite and prehnite–garnet rodingite. Group I rodingites mainly occur in deformed schistose serpentinites and have experienced a more intense rodingitization in a SiO 2 undersaturated environment than those of group II. Group II rodingites occur along extensional fractures in serpentinized peridotite. Their setting is SiO 2 saturated and reflects a later metasomatic fluid/rock interaction that occurred at shallow crustal levels. Group I rodingites presumably formed within the oceanic crust and were later involved in a subduction-zone environment at high pressures in the early stages of the geodynamic evolution of the Xigaze ophiolite, with a final late-stage imprint at shallow crustal levels. Group II rodingites are interpreted to have formed during late-stage ductile and brittle deformation events at shallow crustal levels, when the uplift process of the mantle section was nearly completed. Rodingites may thus represent an important chronological marker within ophiolites, which may help to evaluate the sequence of processes such as early magmatic intrusions, rodingitization, serpentinization, late basic intrusions, shallow crustal level alterations.

The Liaohe Group is an important Paleoproterozoic lithostratigraphic unit of the Jiao-Liao-Ji orogenic belt and traditionally subdivided into the North and South Liaohe Groups. However, their tectonic setting and evolution are still a matter of debate. The precise depositional and metamorphic data of the studied metasedimentary rocks from the South Liaohe Group provide important information on the controversial discussion of the tectonic setting and evolution. Based on cathodoluminescence (CL) imaging and connecting LA-ICP-MS U-Pb geochronology, zircon grains separated from metasedimentary rocks of different formations of the South Liaohe Group were subdivided into detrital and metamorphic zircons or zircon domains. Igneous zircon cores of detrital zircons of the Li’eryu, Gaojiayu, Dashiqiao and Gaixian Formations yield U-Pb age peaks at ∼2130 Ma, ∼2520 Ma, ∼2160 Ma/∼2500 Ma, and ∼2040 Ma, indicating that their sediments were transported from the Liaoji Granitoids, the Archean basement rocks of the adjacent Complex. In addition, deposition age of the sedimentary precursors was investigated which must have taken place at some time after ∼2.05 Ga, ∼2.07 Ga, ∼2.04 Ga and ∼1.92 Ga, respectively. These new geochronological data can be well explained by (1) a continent-arc collision model which suggests that the South Liaohe Group formed an intervening island arc and foreland basin sandwiched between the Longgang and Nangrim Complexes, and (2) a northward subduction of the Nangrim Complex source rocks of the South Liaohe Group; these source rocks were successively deposited along the margin of the two Complexes.

Comprehensive studies of depositional age and source material of the Liaohe Group are significant to the tectonic evolution of the Jiao-Liao-Ji Belt (JLJB). The Gaixian Formation is widely distributed and stratigraphically defined as the uppermost formation of the Liaohe Group. However, our new work reveals that not all meta-sedimentary rocks of the Liaohe Group deposited before the late Paleoproterozoic orogeny. Based on the geochronological results, these meta-sedimentary rocks can be subdivided into two categories. Detrital zircon from the category A meta-sedimentary rocks yield U-Pb age peaks at ca. 2.50 Ga, 2.18 Ga and 2.06 Ga. The youngest age peak of ca. 2.06 Ga, together with the metamorphic age of 1.95–1.84 Ga yielded by pervious studies, indicate their depositional ages of 2.06–1.95 Ga. Compared to the category A, detrital zircon from the category B yield similar 207Pb/206Pb age peaks at ca. 2.54 Ga, ca. 2.17 Ga and ca. 2.05 Ga, but importantly also at ca. 1.87 Ga which was never observed for the category A. The late Paleoproterozoic zircon (~1.87 Ga) include magmatic cores and weakly oscillatory zoned or unzoned rims. These rims yield two age peaks of ca. 1.88 Ga and ca. 1.80 Ga, which are interpreted as inherited partial melting ages. Therefore, the category B meta-sedimentary rocks deposited at some time after ca. 1.80 Ga. These new geochronological data reveal that the category B meta-sedimentary rocks are significantly different with those from the traditional Gaixian Formation. In combination with regional stratigraphy data, it is recommended that the category B meta-sedimentary rocks should be ascribed to the Changhai Group rather than the Liaohe Group.

Three half-meter-wide pelitic granulite interlayers are found within mafic, intermediate, and felsic granulites in Chicheng of the Trans-North China Orogen, they help to understand the assembly of the Eastern and Western blocks of the the North China Craton (NCC). Detailed petrological study shows that all have identical high-pressure mineral assemblages of garnet + kyanite + K-feldspar + plagioclase + quartz + rutile. Inclusions of mesoperthite in garnet indicate the former presence of ternary feldspar at peak conditions, which, in the matrix, have recrystallized. Kyanite is commonly surrounded by a narrow corona or chain of small garnets, enveloped by a zone of plagioclase and an outermost rim of potassium feldspar. Phase equilibria modeling using THERMOCALC yiels a peak metamorphic pressure of 1.6 GPa at 880 °C (M2a) and a peak temperature of 950 °C at 1.5 GPa (M2b). The ternary feldspar thermometer also indicates a peak temperature of 900–950 °C. The pelitic and mafic granulites exhibit the same peak metamorphic pressure, while the pelitic granulites show a 150 °C higher peak metamorphic temperature than the interlayered mafic granulites of 750–790 °C. Absence of sillimanite in the Chicheng pelitic granulite and orthopyroxene in the mafic granulite indicates a fast exhumation process of cooling, which is different from other high pressure rocks from the Trans-North China Orogen. Analysis of metamorphic monazite and zircon yields concordant SIMS U–Pb ages of ca. 1.91 Ga. This study suggests that the Chicheng HP granulite terrane was subjected to fast tectonic exhumation after the collision that occurred at ca. 1.91 Ga. The results provide a reliable metamorphic record for the final collisiion of the Paleoproterozoic Trans-North China Orogen in the NCC.

The Bushmanland Domain of the western Namaqua-Natal Metamorphic Province encompasses Paleoproterozoic to Mesoproterozoic intrusives as well as metamorphosed volcano-sedimentary supracrustal rocks. In this largest domain of the Namaqua Sector of the Namaqua-Natal Province, high-grade metamorphism prevails, with granulite-facies rocks occupying the central and southern parts. Previous studies of the granulites have revealed evidence for peak metamorphism reaching conditions that are at least transitional to ultrahigh temperatures (UHT). We present first evidence for coexisting sapphirine and quartz, and related peak conditions of 1000℃ – 1050℃ and 7 – 8 kbar. The contrast to previous P-T estimates and a clockwise P-T path possibly point to an older M1 metamorphic cycle that predates the well-established 1040 – 1020 Ma Namaquan (M2) cycle, characterised by lower-T and lower-P granulite-facies metamorphism. This M2 metamorphism shows a general anticlockwise P-T path as recorded in mafic rocks of the study area, with peak conditions of 770 – 830℃ and 4.9 – 6.4 kbar. Zircon and monazite geochronology yields two distinct age groups, with a Late Mesoproterozoic group confirming the ~ 1040 to 1020 Ma metamorphic age of the Namaquan orogeny related to M2, and a Paleoproterozoic group with a relatively narrow age range of ~1787 – 1773 Ma. This latter group encompasses the zircon cores and relates to magmatic crystallisation of felsic host rocks. The magmatic zircons had been through a stage of clastic transport before obtaining their metamorphic overgrowth within the Bushmanland Group supracrustal sequence. In the compiled zircon age spectrum for the Bushmanland Domain, this particular age group had not shown up prominently before. It points to a significant source area which is either not exposed, or could not be localized yet

Following the first petrological studies on UHT metamorphic pelitic granulite of the Tuguiwula area, Jining complex, North China Craton, considerable work has been done in the past decades that revealed an anti-clockwise P-T path, reaching a maximum pressure range at ca.10 kbar. In this study, we re-investigated pelitic granulite from this locality and determined four distinct metamorphic stages, Pmax (M2), Tmax (M3), cooling (M4), and decompression (M5), which document a clockwise P-T trajectory, providing evidence for an early high-pressure metamorphic scenario at pressures >11.8 kbar. The Pmax (M2) conditions were calculated using garnet domains with highest XCa (grt) isopleths, which contain inclusions of kyanite. The entire mineral assemblage in equilibrium is Grt + Ky + Qz + Rt + Kfs + liq ± Ilm, suggesting minimum pressures of 11.8 kbar. The Tmax metamorphism (M3) is characterized by the assemblage Grt + Qz + Pl + Ilm + Kfs + Sil + liq. Geothermobarometrical studies which include two-feldspar geothermometry and Zr-in-rutile thermometry yields a P-T range of 990 °C−995 °C and 9–10 kbar, indicating an ultra-high temperature (UHT) metamorphic overprint. The subsequent retrograde metamorphic stages M4 and M5 are characterized by the formation of Bt + Sil + Qz + symplectites and cordierite replacing garnet rims, suggesting P-T conditions of 840 °C−960 °C/8.5–9.5 kbar and 800 °C−850 °C/5.5–6.8 kbar, respectively. SHRIMP U-Pb age dating provided a weighted mean 207Pb/206Pb age of 1911.6 ± 4.5 Ma that represents the timing of the UHT metamorphic stage. The current study on pelitic granulite documents two novel results which have a significant impact on the geotectonic evolution: (1) a clockwise P-T path instead of an anticlockwise one was derived, and (2) P-T estimates are much higher than those reported from previous studies of both, pressure and temperature. The available P-T data of the pre-HP metamorphic stage together with the clockwise P-T trajectory, UHT-, cooling- and decompression-stages of the current study reflect a possible collision or subduction origin.

The Mg-rich silicic crustal rocks characterized by high SiO2 and MgO contents are widespread in the Alps, providing rare targets to investigate the origin of Mg-rich fluids in the continental subduction zone. However, it is a prerequisite to elucidate the protolith nature of such Mg-rich rocks, including the possible alteration process experienced before the Alpine metamorphism. For this purpose, we carried out a detailed study of trace elements, O-Hf isotopes and U-Pb ages for zircons from the coesite-bearing whiteschist and its country rock metagranite at Dora-Maria in Western Alps. The whiteschist is characterized by much higher MgO contents than the adjacent metagranite. Relict magmatic zircons from the two types of rocks give essentially the same U-Pb ages of ~ 262 Ma, similar REE contents and patterns, the same range of eHf(t1) values from − 6.4 to 2.8 and δ18O values from mostly 9 to 11‰. These features provide compelling evidence that the whiteschist and metagranite share the same protolith, i.e., the Permian granite. All the above geochemical compositions are consistent with the argument that the high MgO content of the whiteschist was acquired from high Mg fluids from dehydration of the serpentinized peridotite at the slab-mantle interface in the continental subduction channel. The hypothesis of a pre-Alpine hydrothermal alteration of the granitic protolith to cause the MgO enrichment is not favorable. Zircon in the metagranite does not show any metamorphic overgrowth during the Alpine orogeny, in contrast to the extensive overgrowth of metamorphic rims at ~ 35 Ma in the whiteschist. The metamorphic rims show relatively low REE contents with variably flattened HREE patterns, different from the relict magmatic zircons that exhibit relative high REE contents with steep HREE patterns. This difference suggests that the breakdown of hydrous minerals plays a crucial role in dictating mineral reactions for zircon growth during the continental subduction-zone metamorphism. This interpretation is also consistent with the local occurrence of garnet coronae along the grain margins of biotite in the metagranite. Therefore, this study provides not only the constraints on the protolith nature and metasomatic process of Mg-rich whiteschist from the Western Alps, but also insights into the role of metamorphic fluids in controlling mineral reactions for the metamorphic growth of both zircon and garnet in the continental subduction zone.

Cathodoluminescence (CL) microscopy studies were performed to unravel internal structures and zonations of carbonates from 6 unaltered hypabyssal kimberlites from several Mesozoic - Paleozoic kimberlite pipes of the Siberian craton. Different generations of prograde and retrograde carbonate grains/domains were identified. Most investigated kimberlite samples are unusually fresh and contain abundant unaltered olivine. Only minor amounts of carbonates of a primary magmatic origin were found; most carbonates are of secondary origin formed during late metasomatic or hydrothermal events. A rare carbonatite xenolith discovered in the Malokuonapskaya kimberlite contains abundant apatite and monazite; an apatite inclusion was detected in one zircon grain. Some apatites contain oriented lamellae of monazite. Early studies of the carbonate minerals in this xenolith demonstrated a trace amount of Sr; all these characteristics are typical of carbonatite. We conclude that carbonate minerals play a significant role in the evolution of hypabyssal kimberlites of the Siberian craton. The CL-studies of these minerals provide significant insights into their growth history and help to differentiate different parageneses, as either primary or secondary. Carbonatite xenoliths are of vital significance for the discussion of a possible genetic interrelation of kimberlite and carbonatite melts in the mantle. The present results may serve as a starting point for future studies of these carbonate minerals, using electron-microprobe, micro-PIXE or other high-resolution techniques. Future in-situ oxygen and carbon isotope analyses, as well as inclusions of melt, fluid, and minerals in different carbonate domains, will add essential new data with which to further unravel the genesis of the kimberlite-carbonatite association.

Carbonaceous chondrites are affected to different degrees by thermal and aqueous metamorphism on their parent bodies. However, the degree of alteration has been categorized mainly by relative scales and achieving quantitative information about metamorphic temperature by conventional mineral thermometry is problematic for low petrologic types. We have developed a general approach to estimate the metamorphic peak temperature experienced by type 3 chondrites from diffusion zoning in minerals, and have applied this approach to olivine in type I and type II chondrules of CO3 chondrites. To obtain metamorphic temperatures from diffusion zoning, we have combined diffusion modeling with thermal modeling of the meteorite parent body. The integrated diffusion coefficient over time ( Γ ) was identified as a useful parameter to quantify the extent of chemical change by diffusion occurring in a mineral during a given thermal history. Knowing the temperature dependence of the diffusion coefficient, Γ values can be calculated for each thermal history and be compared to the Γ values obtained from diffusion modeling. For thermal histories realistic for the parent body, Γ depends primarily on the metamorphic peak temperature, so that Γ values determined from diffusion profiles in meteorite minerals can be directly related to the metamorphic peak temperature. This general approach is relatively insensitive to uncertainties in the input parameters for the thermal model. We found that chemical zoning in type I and type II chondrule olivine of the CO chondrites Kainsaz and Lance was largely influenced by solid state diffusion, which is evident from the observed correlation of zoning anisotropy with the crystallographic orientation. Chemical zoning in type II chondrule olivine is mainly igneous for CO chondrites of petrologic types up to at least 3.2 (Kainsaz) and was influenced only minor by diffusion during parent body metamorphism. Fe–Mg zoning in type II chondrule olivine and around sealed cracks in type I chondrule olivine yields similar Γ values, indicating a formation of both zoning features during a common thermal history on the parent body. In addition, Γ values for type II chondrule olivine correlate with metamorphic grade. The application of this approach on Fe–Mg zoning in type II chondrule olivine of CO3 chondrites yields estimates of maximum metamorphic peak temperatures ranging from 653 to 849 K for different petrologic subtypes. The Fe–Mg zoning of type I chondrule olivine is not consistent with the peak temperature estimates from type II chondrule olivine, suggesting an additional contribution of solar nebular processes to type I chondrule olivine zoning prior to accretion into the parent body.

High pressure (HP) and ultrahigh pressure (UHP) metamorphic rocks play a key role in understanding the tectonic evolution of orogenic belts. They have typically experienced complex changes during subduction and exhumation processes arising from recrystallization, deformation, fluid–rock interactions and even partial melting, and may therefore carry valuable records of evolving geodynamic systems in an orogenic belt. This special publication addresses the current work on HP–UHP metamorphism and its relation to the tectonic evolution of orogenic belts. This special publication contains fifteen papers covering the important orogenic belts of the Himalaya, Dabie–Sulu, Tian Shan, North Qaidam and others that have been grouped into three parts: (I) new developments in the determination of metamorphic pressure–temperature (PT) conditions and their timing, (II) overview papers of well-known HP–UHP metamorphic belts and (III) research papers for some newly discovered HP–UHP belts.

The age of peak-metamorphic conditions of the ultrahigh-pressure ( UHP ) metamorphic rock sequence constituting the Dora-Maira Massif is well established at 35 Ma. In order to understand the behaviour of excess argon during sequential pressure–temperature stages of exhumation from >3.5 to less than 0.5 GPa, distinct generations of chemically and petrographically well-characterized white mica were studied with the multigrain, stepwise-heating 40Ar/39Ar dating technique. Mica separates were obtained from boudins of coesite-bearing UHP pyrope quartzite and phengite schist enclosed within them, from retrograde assemblages evolving from the pyrope quartzite, and from biotite–phengite orthogneiss surrounding the boudins. Assuming the 35-Ma age as a benchmark, it is possible to reconstruct directly the quantity of excess argon during these various stages of the exhumation history. The amount of excess Ar roughly correlates with the Si content of phengite, which is an indicator for the pressure at which phengite incorporated excess Ar. All mica samples crystallized at pressures >1.0 GPa exhibit significant excess argon and aberrant ages significantly higher than 35 Ma. Between 1.0 and 0.5 GPa excess Ar diminishes and realistic ages are approached. Phengite (Si3.55) separates from two localities of fresh coesite-bearing pyrope quartzite equilibrated at ca . 3.8 GPa show similar U-shaped spectra with high apparent ages in the first or second degassing steps, dropping to a plateau-like value of 71.0±1.1 Ma in one case and yielding an apparently trustworthy plateau age of 103.4±1.0 Ma in the second. Retrograde phengite generations from pyrope quartzite crystallized between 3.2 and 1.5 GPa also display a U-shaped spectrum with a broad depression corresponding to a minimum age of 96.9±1.6 Ma. Retrograde phlogopite, representing part of the breakdown assemblage of pyrope and formed between 1.9 and 1.2 GPa, documents several sharp steps in age values from 548 Ma towards 850 Ma, whereas further degassing steps are characterized by a nearly linear increase of age values from 852 to 973 Ma. A plateau age of 56.3±0.9 Ma was calculated for phengite from a phengite-schist inclusion within pyrope quartzite. The spectra of phengite from the country-rock biotite–phengite gneiss surrounding the pyrope quartzite boudins are hump-shaped. The total degassing ages decrease with decreasing grain size (and decreasing metamorphic pressure from 1.6 to 0.3 GPa, as signalled by a concomitant decrease of Si per formula unit) from 43.25±0.60 Ma (1–2 mm) to relatively realistic values of 33.50±0.50 Ma (90–180 μm) and 31.55±0.50 Ma (

The Jiao-Liao-Ji Belt (JLJB) is one of the most important Paleoproterozoic orogenic belts, which is intervening between the Longgang and Nangrim-Liaonan blocks. The Liaohe Group is an important research subject for understanding the Paleoproterozoic tectonic setting and evolution of the JLJB. However, the relationship of depositional spatial and chronological distributions between the North and South Liaohe groups are still under debates. In this paper, we present geochronological and zircon Lu-Hf isotopic results of meta-sedimentary rocks from the Liaohe Group to decipher their provenance depositional ages and environment, as well as their tectonic significance on the JLJB. The detrital zircons from the corresponding formations, including the Li’eryu, Gaojiayu and Dashiqiao formations of the North and South Liaohe groups, show similarities in age patterns and Hf isotopic compositions. The detrital zircons from the Li’eryu Formation of the two groups display unimodal 207Pb/206Pb age peaks at ca. 2.2–2.1 Ga. On the other hand, detrital zircons from the Gaojiayu and Dashiqiao formations of the two groups, yield two dominating age peaks at ca. 2.2–2.1 and ~2.5 Ga. In summary, all detrital zircons with age peaks at ca. 2.2–2.1 Ga, have eHf(t) value of −6.99 to +9.41 and two-stage Hf depleted mantle ages (TDM2 model ages) of 3143 to 2137 Ma. The ~2.5 Ga detrital zircons however, have eHf(t) value and TDM2 model ages which widely range from −9.37 to +10.13 and from 349 to 2367 Ma, respectively. Geochronological results indicate that ca. 2.2–2.1 Ga zircons were derived from the Liaoji granitoids. The adjacent basement rocks of the Longgang and Liaonan-Nangrim blocks provide the major sources to detrital zircons that yield an age peak at ~2.5 Ga. Combined with previous studies, the new geochronological data suggest that the protoliths of the corresponding formations of the North and South Liaohe groups deposited their detritus progressively in a back-arc basin. The Liaoji granitoids are the main sources providing detritus for the Li’eryu Formation. Subsequently, clastic sediments transported from the Longgang and Liaonan blocks, as well as from the Liaoji granitoids, were successively deposited and provided sources to the Gaojiayu and Dashiqiao formations.

The studied granitoid gneisses from the Daqingshan complex in the Khondalite Belt of the North China Craton consist of tonalitic, trondhjemitic, and granodioritic (TTG) gneisses, as well as monzogranitic and granitic (MG) gneisses. The respective melts forming these granitoid gneisses are geochemically similar. The granitoid gneisses are generally enriched in large-ion lithophile elements (LILEs) such as Rb, Ba, Nd, La, Ce and depleted in high field strength elements (HFSEs) such as Nb, Ta, and Ti. Chondrite-normalized rare earth elements (REE) patterns indicate that both TTGs and MG gneisses illustrate various fractionation with (La/Yb)N ratios of 7.43–58.29 and 3.59–87.82, respectively. However, most TTG gneisses show positive Eu anomalies, and relatively negative Th and positive Sr anomalies, while MG gneisses have opposite characteristics. These geochemical characteristics suggest that TTG gneisses were derived from a magma source at higher pressures than that to have formed MG gneisses. Some TTG gneisses yield high Sr/Y (36–139) and (La/Yb)N ratios, indicating a subduction-related affinity. The whole series of granitoid gneisses from the Daqingshan complex belong to fractionated calc-alkaline I-type granitoids, suggesting their igneous protoliths akin to island arcs and continental magmatic arcs. LA-ICP-MS U–Pb dating of zircons from four representative granitoid samples indicates that tonalite and granodiorite exhibit weighted mean ages of 2416 ± 12 Ma and 2348 ± 10 Ma, respectively, whereas trondhjemite provides two groups of zircon ages yielding weighted mean ages of 2437 ± 15 Ma and 1876 ± 3 Ma, respectively. The studied granitic gneiss reveals an upper intercept of 1947 ± 45 Ma. Integration of cathodoluminescence images, Th/U ratio, and REE patterns of zircons indicates that all zircons of granitoid gneisses have a magmatic origin. Therefore, three major episodes of granitic magmatism at ca. 2.35–2.42, 1.95, and 1.88 Ga can be inferred. The ages of 1.95 and 1.88 Ga are coeval with widespread metamorphic events recorded by mafic and pelitic granulites in the Khondalite Belt. The Khondalite Belt could have undergone long-term arc-continent accretion along the southern margin of the Yinshan Block during early to late Paleoproterozoic time, and subsequent continental collision between the Yinshan and Ordos Blocks occurred at ca 1.95–1.88 Ga.

The rapid subduction and exhumation of high-pressure (HP) and ultrahigh-pressure (UHP) rocks exposed in collisional belts play a critical role in understanding the evolution of geodynamics and plate tectonics in subduction zone environments. Herein, ophiolitic fragments, eclogites, and amphibolites, from a subduction channel exposed in the Dulan terrane, North Qaidam orogen were investigated. Garnet compositional zoning from the studied eclogites records only a single cycle of subduction and exhumation. New uranium–lead (U–Pb) zircon ages of the current study combined with previous rutile U–Pb ages from eclogites indicate a rapid cycle of events, starting with the generation of the magmatic protolith at ca. 446 Ma, its subsequent subduction reaching a depth of ca. 100–110 km at ca. 435 Ma, and its exhumation into shallow portions of the crust which occurred at ca. 416 Ma. Thus a rapid 30 million years (Myr) protolith generation–subduction–exhumation cycle could be recorded. According to an ophiolitic rock association with island arc basalt (IAB)-like and mid-ocean ridge basalt (MORB)- to ocean island basalt (OIB)-like trace element compositions, the slab-derived components determined by Sr–Nd–Hf isotopes and the short time span of only about 11 Myr between photolith generation and UHP metamorphism in eclogites, the protolith of eclogite is best explained as a supra-subduction zone ophiolite. This finding likely records the closure of the South Qilian ocean and the initiation of a continent-arc collision at ca. 446 Ma. Furthermore, starting with subduction erosion during continent-arc collision in the early Palaeozoic, ophiolitic and arc-related materials become subducted in the North Qaidam. It was inferred that such a rapid succession of events from protolith formation to UHP overprint and final exhumation, could be explained by movements of rock material in a subduction channel. Thus, this study provides a new view in understanding the mechanics of different episodes of subduction systems in North Qaidam UHP metamorphic belt.