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A new activity model for biotite is formulated in the system K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O2 (KFMASHTO), which extends that for the KFMASH system by introducing a titanium-biotite and a ferric-biotite end-member (tbio: K(TiMg2)[(O)2(AlSi3)O10] and fbio: K(Fe3+Mg2)[(OH)2(Al2Si2)O10]), as well as a pyrophyllite end-member (pyp: Al2[(OH)2Si4O10]) that accounts for the presence of octahedral excess-Al in natural biotites. Phonon calculations applying density functional theory (DFT) using the software Castep yielded the standard entropies of tbio and fbio as Sotbio = 328.06 J/(mol·K) and Sofbio = 301.69 J/(mol·K), and their heat capacity functions. From experimental phase-equilibrium data, the enthalpy of formation value of tbio was constrained as Δ H f , t b i o o = −6124.68 ± 3.33 kJ/mol. Natural data were used to derive Δ H f , f b i o o = −5935.3 ± 6.6 kJ/mol. The single-defect DFT method was applied to parameterize important macroscopic mixing properties (macro-W's) involving tbio and pyp end-members in the model (fbio was treated ideal). Castep-derived microscopic interaction energies (micro-w's) are presented herein for KFMASH-biotite. The octahedral same-site (M1) Mg–Al mixing micro-w (wMgAl(M1)), the same-site tetrahedral Si-Al mixing parameter (wSiAl(T1)) and the related cross-site term are: wMgAl(M1) = 82.5 kJ/mol, wSiAl(T1) = 95.6 kJ/mol (two T1-sites) and w M g A l A l S i (M 1 T 1) = 175.1 kJ/mol. The linear combination of these micro-w's gives a macroscopic Wphleas = 18.8 kJ/mol, that is not transferable to other mineral groups. Micro w's for Mg-Fe mixing in biotite (wMgFe(M1), wMgFe(M2), w M g M g F e F e (M 1 M 2) ), are all close to ideality. The biotite activity model of this study is thus a first example of next-generation activity models that use DFT- and thus physically based micro-w's and reassembled macro-W's for petrological calculations. Test calculations on 5 samples from low- to high-grade metamorphic environments covering metapelite to greywacke bulk-compositions using Perple_X suite of programs illustrate the performance of the new biotite activity model. Computed mineral-chemistries are in all cases in better agreement with measured compositions than resulting from published activity models of biotite. [ABSTRACT FROM AUTHOR]

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Integrated petrological, geochemical, isotopic and thermobarometric study of metasedimentary rocks from the amphibolite‐ to granulite‐facies transition zone of the Eastern Dharwar Craton (EDC), South India, has provided new insight into the evolution of the lower continental crust in this region. Phase equilibrium modelling of metapelites and metagreywackes suggests that they reached peak metamorphism at ~800–850°C and 6–7 kbar (corresponding to a paleodepth of ~20 km), with minor retrograde change occurring at ~700°C and 3–5 kbar during exhumation. U–Pb ages of conventionally separated zircon from metapelite samples range from 2.5 to 3.4 Ga, whereas garnet‐hosted zircons yield younger ages of 2.5–2.7 Ga. Zircon Th/U ratios and Hf isotopes reveal several significant pulses of zircon growth at 3.0, 2.95 and 2.7 Ga. Hf isotope data suggest the evolution of juvenile magma at around 3.2 Ga, while Hf model ages show that the crust building process also involved the recycling of pre‐existing Mesoarchean crust. Our study confirms the presence of a Paleoarchean component in the EDC lower crust, as well as older metamorphic events in the terrain and the gradational distribution of the metamorphic rocks. [ABSTRACT FROM AUTHOR]

U–Pb zircon and monazite geochronology are considered to be among the most efficient and reliable methods for constraining the timing of high‐temperature (HT) metamorphic events. However, the reliability of these chronometers is coupled to their ability to participate in reactions. A case study examining the responsiveness of zircon and monazite has been conducted using granulite facies metapelitic and metamafic lithologies in the Warumpi Province, central Australia. In some instances, metapelitic granulites from this locality are polymetamorphic, with an early M1 assemblage containing orthopyroxene, cordierite, biotite, quartz, ilmenite and magnetite, and an M2 assemblage represented by garnet, sillimanite, orthopyroxene, cordierite, biotite, sapphirine, ilmenite and magnetite. M2 metamorphism is linked to HT peak conditions of 8–10 kbar and 850–915°C. Detrital and metamorphic zircon and monazite from these rocks dominantly record U–Pb dates of 1670–1610 Ma and have trace element compositions suggesting they grew prior to peak M2 garnet in the rock. Lu–Hf geochronology from M2 garnet gives ages of c. 1150 Ma. Zircon and monazite are therefore suggested to have remained largely inert during HT metamorphism. We attribute the relatively minor response of zircon and monazite during high‐temperature Mesoproterozoic metamorphism to the localized development of refractory bulk compositions at c. 1630 Ma during M1 metamorphism. This created refractory Mg–Al‐rich bulk compositions that were unable to undergo significant partial melting, despite experiencing subsequent temperatures of ~900°C at c. 1150 Ma. In contrast, metapelitic and metamafic rocks in the area that did not develop refractory bulk compositions during M1 metamorphism were able to partially melt and record c. 1150 Ma accessory mineral U–Pb ages. These results contribute to a small, but growing number of case studies investigating the systematics of the U–Pb system in zircon and monazite in polymetamorphic HT terranes and their apparent resistance to isotopic resetting. Where disequilibrium is apparent, garnet Lu–Hf geochronology can form an important tool to interrogate the significance of accessory U–Pb ages. In the Warumpi Province in central Australia, c. 1640 Ma zircon U–Pb ages had previously been interpreted to reflect the formation of HT garnet‐bearing granulites during a collisional event. Instead, the garnet‐bearing assemblages formed at c. 1150 Ma during the Mesoproterozoic, calling into question the existence of a late Palaeoproterozoic collisional system in central Australia. [ABSTRACT FROM AUTHOR]

Hercynite-rich spinel is a typical mineral of both quartz-bearing and quartz-free metapelitic rocks that form under low-pressure high-temperature conditions in the granulite facies (Sengupta et al., 1991) or upper-amphibolite facies of regional and contact metamorphism (Pattison and Tracy, 1991). Temperature-pressure estimation of metamorphic terrains is necessary to understand how large-scale crust evolution has occurred. High-temperature phase equilibrium cannot be properly evaluated solely based on normal cationic couple geothermobarometers, as ferromagnesian cationic exchange undergoes processes related to retrogression during cooling after peak metamorphism. Instead, key mineral parageneses are preserved and standard petrogenetic grids study based on reaction textures and mineral chemistry can be very helpful in better estimating temperature pressure.Regional GeologyDorbeh's contact aureole is formed in West Azerbaijan province, south of Urmia, north of Oshnavieh, and the Sanandaj-Sirjan zone around Dorbeh diorite. The intrusion is one of the components of the Urmia Plutonic Complex (UPC) which is spread along with granites and alkali syenites/granites at the northwest end of the Sanandaj-Sirjan zone in the south of Urmia. This diorite intruded sedimentary units of shale and carbonate belonging to Permian to Triassic-Jurassic including the Doroud and Ruteh Formations and have metamorphosed them in contact aureole, causing hornfels and calc-silicate to sometimes scarn deposits in the area (Modjarrad and Mohamed, 2015). This intrusion is covered by Miocene units (Ghalamghash, 2009). The observed parageneses in Dorbeh aureole including Cld + Chl, Ctd + And, Alm + Cld, And+Alm, Hc+Alm, Chl + Hc + And + Sil, Alm + And + Sil, Alm + And + Sil + Hc. The peak metamorphism assemblage is Alm + And + Sil + Hc. The reactions responsible for the parageneses productions were introduced in the previous studies (Modjarrad and Mohamed, 2015).Materials and Methods 50 microscopic thin sections were prepared and petrographically examined. A reagent sample with the highest number of parageneses and experienced peak metamorphism was analyzed by JEOL JXA-8200 microprobes electron device for garnet, chlorite, chloritoid, spinel, iron oxide, aluminosilicate, and apatite at the University of Potsdam, Germany. For whole rock chemistry, several metapelite samples were analyzed by XRF at the ALS-Chemex Laboratory of Canada.Discussion and ConclusionsThe main purpose of the present paper is to investigate the parageneses, the mineral chemistry, and the temperature-pressure estimation in the metamorphic contact aureole around Dorbeh intrusive. This intrusion consists of amphibole, plagioclase, and clinopyroxene, is a part of the Urmia Intrusive Complex (UPC), and with dioritic composition was intruded the Upper Cretaceous into the Paleozoic pelitic and limestone sedimentary rocks and metamorphosed them. The metapelite section of the aureole has exhibited interesting parageneses due to the exceptional iron-rich composition of the protolith. The hornfels part of the Dorbeh aureole is dominated by chlorite + chloritoid + garnet + aluminosilicates + spinel + opaque minerals and less quartz in the matrix. Mineral chemistry studies showed that Fe-chlorite between Repidolite to Bronswigite, Fe-chloritoid, Almandine garnet, Hercynitic spinel, and ilmenite are the minerals composition which is in agreement with the Fe-rich content of the parent rock. There was no specific zoning pattern in the metapelites garnet grains. It was also found that there is a reverse relationship between Si content and iron, magnesium, titanium, and chromium in chlorite and aluminosilicate structures. The metamorphic conditions for Dorbeh aureole by the multi-equilibria method by THERMOCALC software have been estimated at 570±7°C, at a pressure of 2.8±0.2 kbar.Detailed investigation of phase relationships of metamorphic rocks can be considered a desirable method along with quantitative data-based methods to evaluate the occurrence of progressive metamorphism. This method is based on direct observation of reaction textures and is more objective in estimating metamorphic conditions by the method of the intersection of equilibrium reactions. Most of the time, secondary cationic exchanges penetrate from the mineral boundary to a considerable stratum and question the accuracy of the temperature and pressure calculated on the cationic couple's geothermometers to a large extent. The contact aureole with higher than normal temperatures specific to the margins of dry mafic intrusives is sometimes associated with the formation of special parageneses such as Grt+Sil+And+Hc. This paragenesis has also been reported from high-temperature (granolithic) regional metamorphic terrains, but its production conditions are more limited in contact aureoles and require a high molar fraction of iron/aluminum or mass dryness and/or aureole. Therefore, the composition of all ferromagnesian minerals is very close to the final iron end member and no specific temperature fluctuation in the aureole is understood from the garnet zoning pattern. Although staurolite has not been seen in the studied sections, the evidence suggests that hercynite in the area was caused by staurolite breakdown. With precision in the composition of analyzed chlorites, it was found that there is a direct relationship between Si and alkaline content in chlorite structure, but titano-ferromagnesian content is in contrast with Si content. An almost similar relationship was observed in the aluminosilicate crystals. The inverse relationship between silica and Fe, Ti, and Cr oxides prevails.

Understanding the interplay between bulk composition and metamorphic grade underpins our interpretations of metamorphism in orogenic belts. The focus of this study is the regional garnet–staurolite–kyanite–sillimanite metamorphic sequence of the Whetstone Lake area, southeastern Ontario. In the kyanite and lower sillimanite zones of this area, there is exceptional diversity in metapelitic mineral assemblages that cannot be accounted for by differences in metamorphic grade. We present a data set of petrographic observations, phase proportions, whole-rock geochemical compositions, and mineral compositions, from thirty-two samples that encapsulate the range of assemblages found in these zones. Differences in bulk composition are the primary control on mineral assemblage development. Whole-rock XMg = molar MgO/(MgO + FeO) and |${\textrm{X}}_{\textrm{Fe}^{3+}}=\kern0.5em \textrm{molar}\ 2\times{\textrm{Fe}}_2{\textrm{O}}_3/\left(2\times{\textrm{Fe}}_2{\textrm{O}}_3+\textrm{FeO}\right)$| exert the greatest control on the observed mineral assemblages, whilst variation in MnO, K2O, and Al2O3 have a secondary influence. We use a set of quality factors (Duesterhoeft & Lanari, 2020) to test the ability of thermodynamic models to reproduce the observed mineral assemblages, modal abundances, and mineral compositions in the diverse bulk compositions at Whetstone Lake. Eight samples were selected for phase equilibrium modelling, for which two bulk compositions were calculated for each sample: (1) a whole-rock bulk composition based on an X-ray fluorescence analysis and (2) a carefully considered local bulk composition based on combining mineral proportions with representative mineral compositions, as obtained from a single thin section. Our modelling uses thermodynamic data set 6.2 (Holland & Powell, 2011) and the solution models of White et al. (2014a , 2014b) that incorporate several Fe3+ end members needed to model the natural data. Modelling in both types of bulk composition broadly predicted mineral assemblages that match those observed. In addition, predicted mineral assemblage fields overlap within uncertainty between 620°C and 675°C and between 6.5 and 7.5 kbar, consistent with the limited range of grade represented by the natural rocks. Predicted modal abundances better match those observed when phase diagrams are constructed using local bulk compositions compared to whole-rock bulk compositions. Despite the acceptable agreement between predicted and observed mineral assemblages, consistent discrepancies are found between predicted and observed mineral compositions. These include overestimation of |${\textrm{X}}_{\textrm{Mg}}^{\ast }$| in garnet, staurolite, and cordierite, overestimation of Ti in staurolite and biotite, underestimation of Si in biotite, and overestimation of Al and underestimation of Fe3+, Fe2+, and Mg in muscovite. The Whetstone Lake suite of this study will be useful to test the predictive capability of future thermodynamic models. [ABSTRACT FROM AUTHOR]

According to the core data from two deep holes, the following mineral assemblages were identified in metapelitic rocks of the Uni Suite in the central part of the Vyatka Belt, sandwiched between Archean blocks in the northeastern part of the Volgo–Uralian Segment of the East European Craton: (1) Pl + Ms + Bt + Qz + Kfs + And ± Chl and (2) Pl + Bt + Qz + Kfs + Grt + Sil ± Ms ± Chl. A set of methods including classical (Grt–Bt, GASP, Ti-in-Bt, and Ms–Bt) and multi-equilibrium thermobarometry (winTWQ 2.34), as well as the isopleth intersection method on pseudosection diagrams (GeoPS 3.2.2.128), was applied to estimate the P–T conditions for these assemblages. The calculated P–T parameters of metamorphism are 520–650°C (or up to 690°C, according to the isopleth method) and 2–5.4 kbar. Metamorphism of paragneiss containing assemblage (2) was accompanied by anatexis in the water-saturated system. Metamorphism of rocks of the Uni Suite belongs to the relatively shallow type of the andalusite–sillimanite facies series and amphibolite facies and relates to deformations during orogenes. [ABSTRACT FROM AUTHOR]

The conservation of archaeological heritage such as open‐air petroglyphs is a major challenge due to the vulnerability of surfaces exposed to local environmental conditions. A precise knowledge of their nature therefore constitutes the cornerstone of their preservation. The surfaces of the valleys of the Mount Bego are characterized by a chromatic contrast between the thin red layer covering the outcrops and the underlying greenish substrate, revealed by thousands of prehistoric petroglyphs. To establish the mineralogical nature of the red layer, a spectroscopic, crystallographic, and petrographic study was carried out. Collectively, the results of this study evidence that, unlike rock coatings, this layer formed through the mobilization of structural iron, in the form of Fe (hydr)oxides, from the clay minerals that constitute the rocks. We thus discuss how the succession of events along the geological history of the site has created the environmental conditions for its formation. This study eventually provides useful data on the actual state of the petroglyphs toward their preservation. In a more global context, the conclusions of this study bring insights into the physicochemical mechanism leading to the reddening of recently exposed glacial valleys. [ABSTRACT FROM AUTHOR]

Scapolite occurrences are widely observed in the metasedimentary rocks exposed around the Khetri Copper Belt and adjoining Nim ka Thana copper mineralized area in western India. Amoeboidal to well-developed and rounded/elliptical-shaped marialitic scapolite (Na-rich end-member) rich zones with variable Cl contents ranging from 1.0 wt % to 2.9 wt % have been identified in proximity to the ore-bearing hydrothermal fluid activity zones. Although scapolite is formed as a product of regional metamorphism in many places, in this study, we propose a strong possibility that scapolite was formed by hydrothermal ore-bearing fluid interaction with metasediments. The evidence of hydrothermal activity and Cl sourcing is attributed to (i) the absence of evaporite beds in the area and no Na-rich plagioclase as inclusions within the scapolite suggesting the formation of marialitic scapolite from sodic plagioclase in the metasediments with the interacting hydrothermal fluid; (ii) an epithermal to mesothermal hydrothermal fluid with moderate salinity responsible for the Cu mineralization that is ascribed to be the source of Cl for the formation of marialitic scapolite; (iii) diffusion of SO2 in the scapolite in close association with the sulfide mineral phase (chalcopyrite) supporting the involvement of ore-bearing fluid in the development of scapolite; (iv) the absence of zoned scapolite, the spatial distribution of scapolite in a particular lithology, the occasional incorporation of sulfur into marialitic scapolite and the texture/geometry in the scapolite suggesting a broad hydrothermal linkage instead of a pure metamorphic origin. [ABSTRACT FROM AUTHOR]

The origin of coeval magmatic and metamorphic rock associations is of great significance in tectonic interpretations. In this study, spatially associated cordierite granite (S-type), metapelite and diorite from the Qinghai Nanshan (NW China) area were dated to be coeval at ∼247–244 Ma. The cordierite granite and metapelite have almost uniform peak pressure-temperature (P – T) conditions of c. 3.8–5.0 kbar / c. 740–790 °C, indicating a geothermal gradient of higher than ∼40 °C / km. The zircon crystallization temperature of the diorite pluton is estimated to be c. 760 °C by Ti-in-zircon thermometer, putting a lower limit of temperature for dioritic magma. Both cotectic (phenocrysts) and restitic cordierite crystals were identified in the cordierite granite. Similar whole rock compositions of the coarse-grained cordierite granite and the metapelite in the Qinghai Nanshan area as well as the average / median pelite worldwide, imply formation of the granite was from almost complete melting of autochthonous metapelite, followed by in-situ recrystallization with negligible / without melt extraction. These data indicate that the cordierite granite and metapelite are both products of the contact aureole surrounding the diorite pluton. This study presents an example for better understanding the transition from high-grade metapelite to S-type granite. • Associated cordierite granite, metapelite and diorite record similar P-T-t conditions. • P-T conditions indicate a geothermal gradient that higher than ∼40 °C/km. • Cordierite granite and metapelite are products of contact metamorphism. [ABSTRACT FROM AUTHOR]

Graphite is essential to characterize biogenic (organic) and abiogenic (inorganic) sources of metamorphic carbon. This study includes a detailed petrological, Raman spectroscopic, and stable carbon isotopic analysis of graphite from the high-pressure (HP) and ultrahigh-pressure (UHP) metapelites in the southwestern Tianshan orogenic belt, China. Graphite occurs as foliation-parallel bands, a feature described for the first time in Tianshan, and has relatively high δ13C values (δ13C graphite of −14.8 to −12.5‰), indicating an abiogenic precursor. The structural characterization of the graphite morphologies using backscattered electron images and Raman spectroscopy reveals very high crystallinity. Thermodynamic modeling and zirconium in rutile thermometry constrained the P–T conditions of graphite formation during eclogite-facies peak metamorphism at approximately 27 kbar and 530 °C. The banded occurrence of graphite, the CH 4 -rich fluid inclusions associated with graphite, and thermodynamic modeling (by GFluid) suggest that graphite precipitated oxidatively from a CH 4 -rich fluid. This CH 4 -rich fluid may stem from the (U)HP carbonate-bearing eclogites that have been reported in previous studies. The oxidative precipitation of the studied graphite from CH 4 -rich fluid suggests that the ambient redox state is one of the key factors that control the fate of COH fluids in the deep carbon cycle. • UHP metapelites from the southwestern Tianshan (China) contain fluid-deposited graphite. • Oxidation of abiotic CH 4 caused graphite precipitation. • Ambient redox state is one of the key factors that control the fate of COH fluids. [ABSTRACT FROM AUTHOR]

In this study, data from garnet‐kyanite metapelites in ultrahigh‐pressure (UHP) domains of the Western Gneiss Region (WGR), Norway, are presented. U–Pb geochronology and trace element compositions in zircon, monazite, apatite, rutile and garnet were acquired, and pressure–temperature (P–T) conditions were calculated using mineral equilibria forward modelling and Zr‐in‐rutile thermometry. Garnet‐kyanite gneiss from Ulsteinvik record a prograde evolution passing through ~690–710°C and ~9–11 kbar. Zircon and rutile age and thermometry data indicate these prograde conditions significantly pre‐date Silurian UHP subduction in the WGR and are interpreted to record early Caledonian subduction of continental‐derived allochthons. Minimum peak conditions in the Ulsteinvik metapelite occur at ~28 kbar, constrained by an inferred garnet+kyanite+omphacite+muscovite+rutile+coesite+H2O assemblage. The retrograde evolution passed through ~740°C and ~7 kbar, first recorded by the destruction of omphacite and followed by the partial replacement of kyanite and garnet by cordierite and spinel. Garnet‐kyanite metapelite from the diamond‐bearing Fjørtoft outcrop documents a polymetamorphic history, with garnet forming during the late Mesoproterozoic and limited preservation of high‐pressure Caledonian assemblages. Similar to the Ulsteinvik metapelite, zircon and rutile age data from the Fjørtoft metapelite also record pre‐Scandian Caledonian ages. Two potential tectonic scenarios are possible: (1) The samples were exhumed at different times during pre‐Scandian subduction of the Blåhø nappe, or (2) the samples do not share a history in the same nappe complex, instead the Ulsteinvik metapelite is a constituent of the Seve‐Blåhø Nappe, whilst the Fjørtoft metapelite shares its history within a separate nappe complex. [ABSTRACT FROM AUTHOR]

[Display omitted] • Three generations of garnets are identified in Kharta HP pelitic granulite. • Four-stages metamorphic evolution is revealed for the HP pelitic granulite. • Metamorphic Zircon coexisted with Grt-I rim/Grt-II in the granulite facies (M3) at 16–13 Ma. • The HP pelitic granulites were exhumed to middle-lower crustal prior to 14–13 Ma. • The exhumation of the HP pelitic granulites were due to the activity of STDS. HP pelitic granulites associated with eclogites from the Kharta Gneiss are found in the Thongmön area, central Himalaya, which provides significant constraints for the tectonic evolution of the Himalayan orogeny. Three generations of garnets are identified in pelitic granulites (DR15P15). The first generation of garnet is core sections of euhedral to subhedral porphyroblasts (Grt-I core) followed by growth of Grt-I mantle during the heating stage. The third generation of garnet is represented by thin rims of Grt-I and small grains (Grt-II) in the matrix, which likely nucleated and grew in response to peritectic reaction during exhumation. Petrographic evidence and P–T pseudosection calculation reveal a four-stages metamorphic evolution: 1) a prograde metamorphic stage (M1) represented by Grt-I core and its mineral inclusions rutile, ilmenite and quartz with metamorphic conditions of 740–760 °C and 13–16 kbar; 2) a near isobaric heating stage with slightly decompression to HP granulite-facies (M2, 820–840 °C, 13–14 kbar) indicated by the composition of Grt-I mantle and the increase of Zr concentrations of rutile in it; 3) a heating decompression stage (M3) supported by a decrease in grossular content in the Grt-I rim and higher X An in plagioclase, yielding T > 860 °C and P = 10–11 kbar in the sillimanite stability field; 4) a final decompression stage followed by cooling (M4, T ~ 800 °C and P = 6–8 kbar) indicated by biotite replacing sillimanite and lower Ti content in biotite in the matrix. Based on the zircon trace element geochemistry, zircon inclusions, and the REE partitioning between garnet and zircon, the metamorphic zircon rims with middle Miocene age (16–13 Ma) probably grew during Grt-I rim/Grt-II formation at granulite facies stage (M3). The P–T history of the pelitic granulites of the Kharta Gneiss indicates that they were subducted beneath the Asian plate and exhumed to middle-lower crustal depths prior to 14–13 Ma due to the activity of STDS. [ABSTRACT FROM AUTHOR]

Boron geochemistry can track fluid–rock interaction during metamorphic evolution and provides important insights into mass transfer processes in subduction zones. This study presents boron concentration and isotopic data for white mica (phengite and paragonite) and tourmaline in an ultra-high pressure (UHP) metapelitic schist from the western Tianshan (Xinjiang Province, China). The pelitic schist experienced dehydration during heating related to the onset of exhumation, which is recorded by phengite and tourmaline formed during this stage. Boron isotope ratios in phengite decreased from – 8.5 to – 16.0 ‰ (relative to NIST SRM 951) with increasing temperature from 525 to 575 °C. This is recorded in a correlated decrease of 11B/10B ratios, B content and Si content of phengite. Thus, the B isotopes of released fluids during decompression–heating evolved from 0 to – 6.9 ‰, consistent with a preferential loss of isotopically heavy B during dehydration. The formation of BSE-dark zones in tourmaline with relatively light δ11B values (– 9 to – 6‰) and high Mg# (0.65–0.68) could be related with fluids released during this stage. In a second stage, paragonite formed in a rehydration process during advanced exhumation. During interaction with external fluids, boron concentrations and isotopic values in paragonite increased: B concentration range from 72 to 232 μg/g, and δ11B increased from – 15.6 to – 2.5 ‰. Fluid-fractionation modeling demonstrates that the external fluid [(B) = 340 ± 20 μg/g; δ11B = + 8 ± 2 ‰] may have been derived from high-δ11B serpentinites that occur in the study area (δ11B between – 1 and + 8 ‰). In response to hydration, tourmaline likely developed BSE-light zones with heavier δ11B values (− 4 to − 2‰) and lower Mg# (0.62–0.64). Boron geochemistry of white micas and tourmaline improve our understanding of mass transfer during metamorphic processes in subduction zones; it allows us to identify the influence of both closed-system recrystallization events and the effect and likely source of externally derived fluids. [ABSTRACT FROM AUTHOR]

The cratonic basement in the Kabul district is dominated by gneiss, migmatite, schist, and amphibolite and is characterized by extensive outcrops. Orthopyroxene and garnet are the index minerals of some of the gneisse, hence, they are considered as charnockite. Biotite, quartz and feldspar are the rock-forming minerals of the migmatites and their modal abundances vary between melanosome and leucosome. The predominant minerals of the schistic samples are garnet, kyanite, quartz, feldspar, muscovite and biotite. Amphibole (pargasite), garnet and plagioclase are the essential minerals of amphibolitic samples. Based on thermocalc software, petrogenetic grade and conventional thermobarometers the calculated average pressure and temperature for charnockitic, schistic and garnet-amphibolitic samples are 7.03 kbar and 590 ℃, 9.94 kbar and 518 ℃ and 9.24 kbar, 664 ℃ respectively. The mineralogical paragenesis and the geothermal gradient resulted from thermobarometry calculations of metamorphic basement of the Kabul block is in accordance with metamorphic gradient of collisional orogenic belts. The cratonic basement of the Kabul block is possibly representing metamorphic rocks of the deep level of Proterozoic collisional orogenic belts during the formation of Columbia and Rodinia supercontinents.

We investigated an experimentally sheared (γ = 15, γ˙ = 3 × 10−4 s−1, 300 MPa, 750°C) quartz‐muscovite aggregate to understand the deformation of parent and new crystals in partially molten rocks. The scanning electron microscope and electron backscatter diffraction analyses along the longitudinal axial section of the cylindrical sample suggest that quartz and muscovite melted partially and later produced K‐feldspar, ilmenite, biotite, mullite, and cordierite. Quartz grains became finer, and muscovite was almost entirely consumed in the process. With increasing γ, melt and crystal fractions decreased and increased, respectively. Among the new minerals, K‐feldspar grains (highest area fraction and coarsest) nucleated first, whereas cordierite and mullite grains, finest and least in number, respectively, nucleated last. Fine grain size, weak crystallographic preferred orientations, low intragranular deformation, and equant shapes suggest both initial and new minerals deformed dominantly by melt‐assisted grain boundary sliding, which is further substantiated by higher misorientations between adjacent grains of quartz, K‐feldspar, and ilmenite. Plain Language Summary: The processes governing the deformation of minerals in partially molten rocks are poorly understood as we generally only see the end product. To focus light on this, we sheared quartz and muscovite aggregate to a large shear strain at high pressure and temperature, where these two minerals underwent partial melting and produced new minerals. Electron backscatter diffraction based microstructural investigations of an experimentally sheared partial melt reveal that even at elevated pressure and temperatures, and significant magnitude of deformation, the presence of melt, together with strain partitioning and low intergranular stress transfer, inhibited intragranular plastic deformation in the remaining starting materials and the newly grown crystals. Key Points: In an HPT torsion experiment (γ = 15), quartz‐muscovite melted partially and produced K‐feldspar, ilmenite, biotite, mullite, and cordieriteQuartz grain size reduced, muscovite was consumed entirely, K‐feldspar grains nucleated first while mullite/cordierite nucleated lastMelt‐assisted grain boundary sliding was the dominant deformation mechanism for the reactants and "in‐situ" melt‐crystallized phases [ABSTRACT FROM AUTHOR]

The tectono‐metamorphic evolution of the European Alps is still contentious. The Monte Rosa tectonic unit is a prominent nappe in the Central European Alps and estimates of its peak Alpine pressure (P) and temperature (T) conditions are essential for reconstructing its tectono‐metamorphic evolution. However, the reported peak Alpine pressure and temperature estimates vary considerably between 1.2 and 2.7 GPa and 490 and 640°C for a variety of lithologies. Here, we show petrology and pseudosection modelling of metapelitic assemblages from the western portions of the Monte Rosa nappe (upper Ayas valley, Italy). We present newly discovered staurolite–chloritoid‐bearing metapelitic assemblages. These assemblages exhibit an Alpine high‐P metamorphic overprint of a former contact‐metamorphic mineral assemblage generated by post‐Variscan granitic intrusions. Staurolite contains major amounts of Zn (up to 1.0 atoms per formula units), which is currently, in contrast to Fe‐ and Mg‐staurolite end‐members, not considered in any thermodynamic database. We employ two end‐member mixing models for Zn in staurolite, site mixing, and molecular mixing. Both models enlarge the pressure and temperature stability range for the observed assemblage, where site mixing has the largest influence of ±0.2 GPa and ±20°C. Our results for three metapelite assemblages, with and without staurolite, indicate peak Alpine pressure of 1.6 ± 0.2 GPa and peak temperature of 585 ± 20°C. These peak pressure estimates agree with previously published estimates for metagranites in the nappe, and are in stark contrast with peak pressure obtained from talc‐, chloritoid‐, phengite‐, and quartz‐bearing lithologies termed 'whiteschists' (>2.2 GPa). Our results confirm a variation of peak Alpine pressure of 0.6 ± 0.2 GPa between metagranite/metapelite lithologies and a nearby whiteschist lens (>2.2 GPa) within the metagranite. Field observations indicate that the studied region is structurally coherent and that the whiteschist is not a tectonic slice formed by tectonic mélange. We suggest that the consistent peak pressure for metapelite and metagranite assemblages represents the regional peak pressure and that the higher pressure recorded in the whiteschist lens is likely due to dynamic pressure, possibly resulting from tectonic and/or reaction‐induced stresses. If the calculated pressure of 1.6 ± 0.2 GPa represents regional peak Alpine conditions, then the Monte Rosa nappe was exhumed from a significantly shallower depth than previously assumed, based on peak pressure estimates > 2.2 GPa for whiteschist lithologies. [ABSTRACT FROM AUTHOR]

Understanding equilibrium relationships between minerals is a fundamental goal of metamorphic petrology, especially for the quantification of metamorphic conditions recorded by crustal rocks. Information on pressure (P) and temperature (T) can be derived from mineral compositions based on the a priori assumption that frozen‐in assemblages reflect chemical equilibrium at the time of formation. While the stable coexistence of minerals is commonly assessed from a combination of microstructural criteria and interpretation of phase diagrams, there is no robust modelling strategy available so far for the unambiguous recognition of minerals that grew at different conditions. This study uses iterative thermodynamic models and the computer program Bingo‐Antidote to propose a general strategy for assessing the stable coexistence of minerals and therefore for tracing multiple partial equilibrium relationships preserved in metapelites to reconstruct their P–T evolutions. First the principles of this approach are elaborated in a theoretical example. Application to a metapelite from the Western Central Alps reveals a detailed, clockwise P–T path with at least four different equilibration stages including a poorly defined peak pressure at 1–1.1 GPa, ~560°C, followed by peak temperatures of ~620°C at 0.9 GPa and a late amphibolite stage at ~580°C, 0.65 GPa. The investigation of the natural metapelite via iterative thermodynamic models (ITM) allows the observed mineral formation sequence to be combined with a higher accuracy with variable mineral compositions and results in a detailed P–T path. The metastable perseverance of one or more mineral phases can be statistically inferred via ITM. In the investigated metapelite, only ~50 vol.% of the rock volume equilibrated at peak temperature conditions of 620°C. In addition, ITM enables a more detailed investigation of partial re‐equilibration by diffusion during subsequent metamorphic stages, showing that prograde garnet growth could have been modified during subsequent 10 Ma of amphibolite facies metamorphism in the Central Alps. [ABSTRACT FROM AUTHOR]

Variations in the pressures and temperatures of metamorphism reflect the complexity of collisional orogenic systems. In the north‐west Sanandaj–Sirjan Zone of western Iran, migmatites in the Boroujerd area record earlier dynamothermal metamorphism followed by the development of a contact metamorphic aureole induced by the Jurassic Boroujerd Batholith. Integrated micro‐textural observations, mineral compositions, and the results of phase equilibrium modelling indicate that peak metamorphic temperatures reached 640–660°C in andalusite schists and ~700°C in cordierite–sillimanite migmatites. The pressure does not exceed 3.5 kbar, implying a maximum depth of ~13 km. A hairpin clockwise P–T path in the Boroujerd region was inferred from mineral relations (e.g., replacement of andalusite1 by sillimanite in the migmatites in the heating stage, which was followed by a retrograde episode with growth of andalusite2) and P–T pseudosections. Metamorphism of the Boroujerd area was coeval with voluminous intermediate–mafic magmatism, which provided the heat necessary for partial melting. This study demonstrates that metamorphic sequences of the north‐west Sanandaj–Sirjan Zone developed under similar metamorphic conditions in the Lower to Middle Jurassic and record an extensional back‐arc setting related to the subduction of Neo‐Tethys. [ABSTRACT FROM AUTHOR]

• Metapelites of the Fenzishan Group were formed in a back-arc basin of an active continental margin. • Three age groups of detrital zircon: ∼ 3019–2816 Ma, 2761–2610 Ma and ∼ 2261–2128 Ma. • The minimum depositional age of metapelite precursor is ∼ 2261 – 2128 Ma. • Apatite U-Pb age ∼ 1923 – 1893 Ma reflects to the post peak collisional metamorphism. • Metapelites record the UHT metamorphism, showing an anticlockwise P-T path. Metapelites from the Fenzishan Group of the Zhengjiapo banded iron formation (BIF) iron mine studied have undergone high-pressure and ultrahigh-temperature (UHT) metamorphism. The rocks occur in the Changyi metallogenic belt that belongs to the Jiaobei terrane, Jiao-Liao-Ji Belt, North China Craton. P-T estimates using pseudosection modeling, mineral compositio isopleths and two-feldspar geothermometry of observed mesoperthite and antiperthite intergrowth textures of Bt-Grt-Mag-Amp gneisses, three different metamorphic stages were obtained: A prograde stage M1 at ∼ 600–610 °C/5.5–6.1 kbar, a peak metamorphic stage M2 at ∼ 899–1062 °C/8.6–9.15 kbar, an early retrograde stage M2́at 780–792 °C/8.6–9.15 kbar and a retrograde decompression stage M3 at 466–493 °C/4.5–5.8 kbar. Thus an anticlockwise trajectory can be defined indicating that granulite facies UHT-metamorphic peak conditions have been reached, followed by fast cooling during rapid exhumation. Geochemical characteristics of the metapelites indicate that the Fenzishan Group of the Zhengjiapo BIF iron mine was formed in a back-arc basin system of an active continental margin in a convergent/collisional geological setting.. Magmatic U-Pb ages (LA-ICP-MS) of detrital zircon from metapelites are ∼ 3096 – 2816 Ma and 2761 – 2610 Ma, indicating a source of the Archean basement which delivered clastic material for pelitic protolith. A zircon age of ∼ 2261 – 2128 Ma constrains the minimum depositional age of the metapelitic precursor, while an apatite U-Pb age of ∼ 1923 – 1893 Ma dating demonstrates a metamorphic age of ∼ 1923 – 1893 Ma, is indicated to represent a late Paleoproterozoic post peak metamorphic event that corresponds to the collision of the Jiao-Liao-Ji orogenic belt. [ABSTRACT FROM AUTHOR]

[Display omitted] • Framboidal pyrite contains measurable amounts of trace elements (As, Au, Ag, etc.) • Cooked carbonaceous matter is rich in Fe and S and depleted in metallic elements. • Pyrite and carbonaceous matter textures evolve with increasing metamorphic grade. • Trace element removal during transformation of framboidal pyrite to euhedral pyrite. • Removal of trace elements except Se from pyrite by pressure solution. The Cretaceous Luk Ulo Complex (LUC) in Central Java, Indonesia, hosts orogenic gold-type ores with S derived from metapelite. The host rock consists of accreted metapelite, metamorphosed under temperature up to 510 ± 30 °C. We investigated the textural and chemical evolution of pyrite and carbonaceous matter (CM), and analyzed the hosts phases for trace elements (TEs) such as As, Pb, Cu, Sb, Te, Ag, Zn, Bi, Se, Hg, and Au, with the aim of documenting the behavior of these elements during prograde metamorphism and evaluate the implications on gold mineralization. With increasing metamorphic grade, the texture and chemical composition of pyrite evolved through different stages: 1) framboidal pyrite and pyrite nodules in the sedimentary–diagenetic facies, 2) anhedral–subhedral pyrite in sub-greenschist facies (SGSF), 3) euhedral pyrite in lower greenschist facies (LGSF), 4) elongated pyrite in middle greenschist facies (MGSF), and 5) replacement of pyrite by pyrrhotite in upper greenschist facies (UGSF). The framboidal pyrite and pyrite nodules contain measurable TEs. Recrystallization of such pyrite into euhedral pyrite in LGSF resulted in significant depletion of TEs, except for Se, by pressure-solution. With increasing metamorphic grade, the recrystallization of organic matter to "cooked" CM in LGSF was accompanied by significant depletion in most TEs, with only Fe and S being retained. The transformation of cooked CM into graphite in MGSF, and of elongated pyrite into pyrrhotite in UGSF resulted in further removal of remaining TEs. These observations and the absence of para -amphibolite in the LUC suggest that remobilization of TEs and S from sedimentary pyrite and CM might have occurred in the LGSF, much earlier than previously proposed (UGSF-amphibolite transition). [ABSTRACT FROM AUTHOR]

It is known that different metamorphic rocks may have different abilities in recording metamorphic processes. Here, we report microscopically interlayered garnet amphibolite and metapelite layers in a rock from the Danba dome in eastern Tibetan Plateau, SW China. Both rock types exhibit similar garnet zoning patterns, biotite and plagioclase compositions, as well as peak P-T conditions (7.3–8.0 kbar / 700–725 °C for metapelite and 7.1–8.4 kbar / 690–710 °C for garnet amphibolite, respectively), indicating a global thermodynamic equilibrium state and the same metamorphic record of the two distinct layers. However, medium-sized garnet exhibits homogeneous core compositions and biotite preserves lower Ti and Fe# [= Fe2+ / (Fe2+ + Mg)] at the lithological interface, possibly indicating more intense chemical homogenization at the metamorphic peak and retrograde modification at the interface caused by the possible presence of vast fluid. Sensitive high-resolution ion microprobe (SHRIMP) U Pb dating of metamorphic zircon show a wide dispersion of ages (c. 200–155 Ma), overlapping previous age data reported for this region. These results indicate that differences in petrochemical compositions may not lead to significant distortion in the metamorphic records, including garnet zoning patterns and peak P-T conditions. This further suggests that metamorphic rocks originating from diverse protoliths, which exhibit varying peak P-T conditions within the same locality, could either be a result of the tectonic juxtaposition of distinct tectonic slices or the influence of varying fluid conditions. • Intercalated amphibolite and metapelite layers in one rock record identical P-T conditions • The lithological interface indicates faster peak homogenization and retrograde modification • Different peak P-T conditions in a same outcrop/locality may attribute to other reasons [ABSTRACT FROM AUTHOR]

Metamorphic rocks associated with ophiolitic rocks occur on the eroded surface of a NW–SE-trending anticline in the Allahyarlu area, NW Iran, between the Caucasus and Zagros orogenic belts. Metapelitic rocks consist mainly of quartz, muscovite chlorite, altered biotite and garnet. S1 is the pervasive schistosity, wrapping garnet, which is folded by the second schistosity (S2). The amphibolite records only one phase of deformation as the main lineation. The rocks experienced metamorphism up to the amphibolite facies, then overprinted by greenschist facies condition. Thermobarometry indicates an average pressure of c. 5 kbar and an average temperature of c. 600 °C for the amphibolite facies metamorphism, corresponding to a ∼33 °C km−1 geothermal gradient in response to a thick magmatic arc setting. Greenschist facies metamorphism shows re-equilibration of the rocks during exhumation. Amphibolites whole rock geochemistry shows trace elements patterns similar to both island arc and back-arc basin basalts, suggesting that the protolith-forming magma of the amphibolites was enriched at shallow to medium depth of a subduction system. Negative Nb anomaly and slight enrichment in light rare earth elements (LREE) and large-ion lithophile elements (LILE) of the amphibolites indicate arc-related magmatism for their protolith and a back-arc sialic setting for their formation. 40Ar–39Ar dating on muscovite separated from two gneiss samples, and hornblende separated from three amphibolite samples, documents a Variscan (326–334 Ma) age. The magmatic and metamorphic rock association of the Allahyarlu area suggests the existence of an active continental margin arc during the Variscan orogeny, without clear evidence for a continental collision. [ABSTRACT FROM AUTHOR]

Garnet is one of the most robust and ubiquitous minerals that record element zoning during crustal metamorphism. In addition to major elements, zoning in trace elements can provide a wealth of information to document the changing conditions of garnet growth and modification. However, mapping trace elements at low concentrations, over large areas and with high resolution has remained a major challenge. We present a comprehensive investigation of the TE distribution in garnet from three Alpine samples that underwent a complex evolution at different metamorphic conditions. The TE distribution in garnet grains is mapped in 2D in thin section with a novel approach using laser ablation inductively coupled plasma time of flight mass spectrometry (LA-ICP-TOFMS) to achieve a lateral resolution of 5 µm and limits of detection for the heavy rare earth elements (REE) down to 0.2 µg/g. Comparison with major element zoning measured by electron probe microanalysis and trace elements measured by conventional LA-ICPMS spot analysis testifies to the accuracy of the measurements. Garnet in an amphibolite-facies metapelite from Campolungo, Central Alps, that recorded metamorphism to 600 °C preserves Y + REE trace element zoning that closely matches that of Ca. In this sample, there is no notable diffusive modification for trace elements. Y + REE zoning is dominated by Rayleigh fractionation in the core and by the sporadic breakdown of accessory phases producing annuli in the rim of the garnet. A granulite-facies garnet from Malenco, Eastern Central Alps, formed during subsolidus heating, followed by peritectic melting reactions up to temperatures of 800–850 °C. Major and trace element zoning are decoupled indicating diffusional resetting of major elements, whereas trace elements still largely document the growth history. Enrichment of trace elements in the garnet mantle may be related to the consumption of biotite (V, Cr) and the dissolution of zircon (Zr) and monazite (Y + REE) in the melt. Diffusion of Y + HREE at the core–mantle boundary occurred over a length scale of ~ 200 µm. Garnet in an eclogite from the Sesia Zone, Western Alps (P ~ 2 GPa, T ~ 600 °C), displays pronounced fluid-related veinlets, visible in FeO, MgO and MnO, which cross-cut the primary growth zoning. Surprisingly, complex Y + REE and Cr zoning is not affected by the veinlets, indicating that they did not form by a crack-seal mechanism but are rather related to a selective replacement process. The trace element maps provide a detailed insight into the growth and modification of garnet and thus allow assessment of equilibrium versus disequilibrium processes, and assist in determination of P–T conditions, garnet dating, diffusion modelling as well as documenting fluid-induced modifications. [ABSTRACT FROM AUTHOR]

The cratonic basement in the Kabul district is dominated by gneiss, migmatite, schist, and amphibolite and is characterized by extensive outcrops. Orthopyroxene and garnet are the index minerals of some of the gneisse, hence, they are considered as charnockite. Biotite, quartz and feldspar are the rock-forming minerals of the migmatites and their modal abundances vary between melanosome and leucosome. The predominant minerals of the schistic samples are garnet, kyanite, quartz, feldspar, muscovite and biotite. Amphibole (pargasite), garnet and plagioclase are the essential minerals of amphibolitic samples. Based on thermocalc software, petrogenetic grade and conventional thermobarometers the calculated average pressure and temperature for charnockitic, schistic and garnetamphibolitic samples are 7.03 kbar and 590 ℃, 9.94 kbar and 518 ℃ and 9.24 kbar, 664 ℃ respectively. The mineralogical paragenesis and the geothermal gradient resulted from thermobarometry calculations of metamorphic basement of the Kabul block is in accordance with metamorphic gradient of collisional orogenic belts. The cratonic basement of the Kabul block is possibly representing metamorphic rocks of the deep level of Proterozoic collisional orogenic belts during the formation of Columbia and Rodinia supercontinents. [ABSTRACT FROM AUTHOR]

The anatectic mechanism is the most fundamental criterion in determining the petrogenetic relationship between migmatite, granulite and granite at convergent plate margins. Although two main anatectic mechanisms, namely water-absent (dehydration) and water-fluxed (hydration) melting, have been identified in many collisional orogens, their spatiotemporal relationship and genetic connection remain obscure. To address this issue, we conduct an integrated study of forward phase equilibrium and trace element modelling for partial melting of metapelite in the Himalayan orogen. The results are used to decipher changes of phase relation and melt composition in response to changes of temperature, pressure and water content. Pressure and water content mainly affect the solidus and evolving residual mineral assemblages, whereas temperature controls the anatectic reaction. Hydration melting at lower pressure and temperature would produce anatectic melts with relatively high SiO 2 , FeO T + MgO, CaO, K 2 O, and Ba contents and SiO 2 /Al 2 O 3 ratios, but low Al 2 O 3 and Na 2 O contents and Na 2 O/K 2 O and Rb/Sr ratios. In contrast, dehydration melting at higher pressure and temperature would generate anatectic melts with relatively high Al 2 O 3 and Na 2 O contents and Na 2 O/K 2 O and Rb/Sr ratios, but low SiO 2 , FeO T + MgO, CaO, K 2 O, and Ba contents and SiO 2 /Al 2 O 3 ratios. A comparison of geochemical compositions between the modelled melts and the two groups of well-characterized leucogranites in the Himalayan orogen indicates that the coeval dehydration and hydration melting would likely occur at the deeper and shallower crustal levels, respectively. In combination with the secular evolution of metamorphic facies series in the Himalayan orogen, we propose a spatiotemporally coupled dehydration-hydration melting mechanism. The early metamorphism at lower geothermal gradients would result in metamorphic dehydration in the lower crust and hydration in the upper crust during the syn -collisional period in the Early Cenozoic. As a result, the later metamorphism at higher geothermal gradients would lead to dehydration melting in the lower crust and hydration melting in the upper crust during the post-collisional period in the Late Cenozoic. This study highlights the applicability of phase equilibrium and trace element modelling to reveal the geochemical variations of anatectic melts in response to changes of P–T –H 2 O conditions. This provides new insights into the relationship between dehydration and hydration melting in collisional orogens. [ABSTRACT FROM AUTHOR]

Low-grade metamorphic rocks are known for their low mechanical strength and high weatherability. In the Quadrilátero Ferrífero, an important mineral province located in southeastern Brazil, this set of features results in frequent issues with mining and road slope instability, mainly associated with phyllites. Despite the significant iron ore production that occurs in this province, little is known about the geotechnical properties of the materials constituting the weathering front. Here, we report work aimed at investigating the changes in the basic properties of phyllites at four weathering grades. The methodology used included the field classification and mapping of four weathering grades in order to assess their spatial distribution on a road slope, as well as sampling for geological characterization by petrography and X-ray diffraction (XRD). In addition, the authors performed a physical characterization of all weathering grades, including index properties, mercury porosimetry, and strength data obtained with a Schmidt hammer both in the field and in the laboratory as well as using the point load test (PLT). Results showed the relevance of structural features such as discontinuities and metamorphic foliation observed in the weathering morphology in situ, with different weathering intensities occurring along the slope. Changes in the porosity and pore size distribution in the weathering front were nonlinear, as were resistance variations. The difficulty involved in performing the macroscopic identification of weathering grades due to the very fine texture of the rock as well as microstructural heterogeneities are likely causes of the observed variability in phyllite properties. This fundamental knowledge may aid the prediction of short-term and long-term scenarios for slope stability based on rock weatherability. [ABSTRACT FROM AUTHOR]

• Both zircon and monazite record Neoproterozoic and Permian ages in the Miyi area. • Synchronous magmatism supports a magmatic-thermal overprint scenario. • Neoproterozoic Kang-Dian orogen underwent differential Phanerozoic overprinting. Metapelite within a ∼ 2.5 km-long transect from the southern Neoproterozoic Kang-Dian Orogenic Belt (SW China) was investigated. Metamorphic zircon and monazite of a plagioclase absent sample yielded Permian ages of 268 ± 4.5 Ma and 265.4 ± 1.5 Ma, respectively. Metamorphic zircon from the other three samples yielded Neoproterozoic ages of ca. 874–778 Ma but no Permian age, while monazite from one of them yielded both Neoproterozoic (811 ± 8 Ma) and Permian (259.3 ± 1.7 Ma) ages. The garnet growth / breakdown and plagioclase consumption / K-feldspar growth might account for the different Y and heavy rare earth element (HREE) content and negative Eu anomaly trend in monazite over time. The garnet was interpreted to be formed in the Neoproterozoic, while the preserved mineral assemblages represent the reworked products during Permian overprint. The synchronous magmatic activities might account for the Permian ages of monazite and zircon and the resorbed and diffused garnet zoning. These data indicate the Permian magmatic-thermal pulse affected the preexisted Neoproterozoic metamorphic rocks differentially. Such phenomenon is important and should be carefully considered in orogenic research. [ABSTRACT FROM AUTHOR]

Abstract: Two phengite–garnet‐bearing metapelites from the Nevado‐Filábride Complex, Betic Cordillera, one from the Ragua unit (sample 23085) and the other (sample 23098) from the Calar Alto unit, were studied in detail to elucidate their metamorphic evolution. We calculated various P–T pseudosections for different O2 and H2O‐CO2 contents and Fe3+/Fe2+ ratios with PERPLE_X . On the basis of the compositions of the garnet core and the highest Si content in potassic white mica, different peak pressures of 12.7 ± 0.7 kbar at 525 ± 10°C (23085) and 17.4 ± 1.2 kbar at 526 ± 17°C (23098) resulted (errors refer to an estimated 1σ range). The suggested clockwise P–T loop was followed in both cases by another loop caused by a reheating process. On the basis of garnet rim compositions, peak temperatures of 596 ± 10°C at 10.7 ± 1.4 kbar (23085) and 629 ± 11°C at 8.5 ± 0.7 kbar (23098) resulted, supported by Zr‐in‐rutile thermometry (23085: 591 ± 11°C; 23098: 607 ± 10°C). In situ dating of monazite (23085) with the electron microprobe yielded ages between c. 48 and 14 Ma. On the basis of the monazite composition and histogram analysis, two age populations with mean ages of 40.2 ± 1.7 (1σ) and 24.1 ± 0.8 Ma could be defined. Low Y2O3 contents (<0.6 wt%) in monazite point to its formation after initial growth of garnet. The older generation was therefore assigned to the formation of the garnet core and the early P–T loop. The younger monazite population was related to the late P–T loop. A geodynamic scenario is hypothesized that assigns the Eocene metamorphism to subduction of a continental margin to depths of 50–65 km, followed by exhumation in an exhumation channel accompanied by mixing of slices of different rock types including mantle rocks. This process ended with stacking of such slices causing reheating of the studied rocks in the Early Miocene. The noted time gap of c. 15 Ma between subduction and nappe stacking is similar to the continent–continent collisional orogeny of the Himalaya. [ABSTRACT FROM AUTHOR]

We carried out combined Nd and Li isotopic studies on the Neoproterozoic biotite granites, late Mesozoic evolved two-mica/muscovite granites and associated the Shuangqiaoshan Group metapelites from the Jiangnan Orogen (South China), which, along with petrological and geochemical data, were used to constrain the lithium isotopic behavior during partial melting of metapelites, the origin of the REE tetrad effect in evolved granites, and implications for related rare-metal mineralization. The Neoproterozoic biotite granites show features typical of S-types (e.g., high A/CNK ratios), and have ε Nd  = −6.9 to −10 and δ 7 Li = −2.2–−4‰, comparable to the Shuangqiaoshan Group metapelites (with ε Nd  = −6 to −11 and δ 7 Li = −2–−3.7‰, respectively), suggesting that the biotite granites were derived from melting of the metapelites and the Li isotopic fractionation during the melting process appears negligible. The two late Mesozoic Shimensi and Zengjialong granites occur as small stocks, showing high silica, peraluminous, low CaO and Sr contents, variably enrichment in volatiles (F, B), and are accompanied by W-Cu and Sn mineralization. The Shimensi granites have ε Nd (t) values of −7.0–−8.6 and δ 7 Li = −2.7‰–−3.9‰, similar to the Shuangqiaoshan Group metapelites, whereas the Zengjialong granites have relatively high ε Nd (t) values (−5.1–−7.3) and heavy Li isotopic features (mostly δ 7 Li = −0.18‰–+1.53‰), and particularly, REE tetrad effect is seen in the latter. We argue that the late Mesozoic evolved granites formed as a result of re-melting of the granulitic residues left in the lower crust after extraction of the Neoproterozoic biotite granites from the metapelites. Such re-melting of the residues was triggered by underplating of basaltic magma, with more mantle input (and F, B as well) in the source of the Zengjialong granites. The elevated δ 7 Li values of the Zengjialong granites were probably caused by intense interaction between melt and external fluids during magma emplacement, which, in turn, resulted in the formation of the REE tetrad effect in the granites. Isotopic modeling suggests that fluid exsolution from evolved granites is minor, and is less likely to have been responsible for the extensive rare-metal mineralization as traditionally thought. [ABSTRACT FROM AUTHOR]

The Darreh Sary metapelitic rocks are located in the northeast of Zagros orogenic belt and Sanandaj-Sirjan structural zone. The lithological composition of these rocks includes slate, phyllite, muscovitebiotite schist, garnet schist, staurolite-garnet schist and staurolite schist. The shale is the protolith of these metamorphic rocks, which was originated from the continental island arc tectonic setting and has been subjected to processes of Zagros orogeny. The deformation mechanisms in these rocks include bulging recrystallization (BLG), subgrain rotation recrystallization (SGR) and grain boundary migration recrystallization (GBM), which are considered as the key to estimate the deformation temperature of the rocks. The estimated ranges of deformation temperature and depth in these rocks show the temperatures of 275-375, 375-500, and >500°C and the depths of 10 to 17 km. The observed structures in these rocks such as faults, fractures and folds, often with the NW-SE direction coordinate with the structural trends of Zagros orogenic belt structures. The S-C mylonite fabrics is observed in these rocks with other microstructures such as mica fish, σ fabric and garnet deformation indicate the dextral shear deformation movements of study area. Based on the obtained results of this research, the stages of tectonic evolution of Darreh Sary area were developed. [ABSTRACT FROM AUTHOR]

In many metapelitic assemblages, plagioclase is either CaO-deficient or even absent. In such cases, all the widely applied, well-calibrated plagioclase-related geobarometers lose their usage. Fortunately, it has been found that a net-transfer reaction including intracrystalline Fe2+−Ca2+ exchange in garnet is pressure-sensitive, therefore, a garnet geobarometer can be empirically calibrated under pressure−temperature (P−T) conditions of 430~895 °C and 1~15 kbar. The chemical composition range of the calibrant garnet is XCa = 0.02~0.29 and XFe = 0.42~0.91, and covers the majority of garnet in metapelite. The total error of this geobarometer was estimated to be within ±1.3 kbar. The application of this garnet geobarometer to metamorphic terranes certifies its applicability, and this geobarometer can play a unique role, especially when plagioclase is absent or CaO-deficient. Metamorphic P−T conditions can be simultaneously determined by the garnet−biotite pair through the application of the present garnet geobarometer in combination with a well-calibrated garnet-biotite geothermometer.

Two garnet-chloritoid bearing metapelites from the northern Malpica-Tuy unit (MTU) and a staurolite-andalusite bearing metapelite from the underlying parautochthon were studied to decipher their pressure–temperature (P–T) evolution, mainly based on chemically zoned minerals, such as phengite, using P–T, P–O 2 , and T–O 2 pseudosections. In addition, monazite ages were determined with the electron microprobe in the metapelite from the parautochthon. The chloritoid schists from the MTU experienced peak P–T conditions around 21 kbar at 570 °C and a subsequent isothermal exhumation. Thus, two high pressure rock types can be distinguished in the MTU: orthogneiss (peak pressures: 13–16 kbar) and eclogite + metapelite (peak pressures: 20–23 kbar). The latter rock types were exhumed in a subduction channel environment. The staurolite-andalusite bearing schist from the parautochthon experienced only peak pressures around 8 kbar at a temperature around 570 °C since garnet and potassic white-mica with Si contents above 3.2 per formula unit are lacking. This rock represents sediments that were originally at the surface of the downgoing continental plate and metamorphosed in early Variscan times (around 364 Ma) similar to the MTU rocks. Our mean monazite ages of 339 and 320 Ma might be related to backthrusting of the MTU rocks from deeper crustal levels onto the parautochthon. [ABSTRACT FROM AUTHOR]

The Palaeo-Mesoproterozoic metapelite granulites from northern Garo Hills, western Shillong-Meghalaya Gneissic Complex (SMGC), northeast India, consist of resorbed garnet, cordierite and K-feldspar porphyroblasts in a matrix comprising shape-preferred aggregates of biotite±sillimanite+quartz that define the penetrative gneissic fabric. An earlier assemblage including biotite and sillimanite occurs as inclusions within the garnet and cordierite porphyroblasts. Staurolite within cordierite in samples without matrix sillimanite is interpreted to have formed by a reaction between the sillimanite inclusion and the host cordierite during retrogression. Accessory monazite occurs as inclusions within garnet as well as in the matrix, whereas accessory xenotime occurs only in the matrix. The monazite inclusions in garnet contain higher Ca, and lower Y and Th/U than the matrix monazite outside resorbed garnet rims. On the other hand, matrix monazite away from garnet contains low Ca and Y, and shows very high Th/U ratios. The low Th/U ratios (<10) of the Y-poor garnet-hosted monazite indicate subsolidus formation during an early stage of prograde metamorphism. A calculated P–T pseudosection in the MnCKFMASH-PYCe system indicates that the garnet-hosted monazite formed at <3 kbar/600 °C (Stage A). These P–T estimates extend backward the previously inferred prograde P–T path from peak anatectic conditions of 7–8 kbar/850 °C based on major mineral equilibria. Furthermore, the calculated P–T pseudosections indicate that cordierite–staurolite equilibrated at ~5.5 kbar/630 °C during retrograde metamorphism. Thus, the P–T path was counterclockwise. The Y-rich matrix monazite outside garnet rims formed between ~3.2 kbar/650 °C and ~5 kbar/775 °C (Stage B) during prograde metamorphism. If the effect of bulk composition change due to open system behaviour during anatexis is considered, the P–T conditions may be lower for Stage A (<2 kbar/525 °C) and Stage B (~3 kbar/600 °C to ~3.5 kbar/660 °C). Prograde garnet growth occurred over the entire temperature range (550–850 °C), and Stage-B monazite was perhaps initially entrapped in garnet. During post-peak cooling, the Stage-B monazite grains were released in the matrix by garnet dissolution. Furthermore, new matrix monazite (low Y and very high Th/U ≤80, ~8 kbar/850–800 °C, Stage C), some monazite outside garnet rims (high Y and intermediate Th/U ≤30, ~8 kbar/800–785 °C, Stage D), and matrix xenotime (<785 °C) formed through post-peak crystallization of melt. Regardless of textural setting, all monazite populations show identical chemical ages (1630–1578 Ma, ±43 Ma). The lithological association (metapelite and mafic granulites), and metamorphic age and P–T path of the northern Garo Hills metapelites and those from the southern domain of the Central Indian Tectonic Zone (CITZ) are similar. The SMGC was initially aligned with the southern parts of CITZ and Chotanagpur Gneissic Complex of central/eastern India in an ENE direction, but was displaced ~350 km northward by sinistral movement along the north-trending Eastern Indian Tectonic Zone in Neoproterozoic. The southern CITZ metapelites supposedly originated in a back-arc associated with subducting oceanic lithosphere below the Southern Indian Block at c. 1.6 Ga during the initial stage of Indian shield assembly. It is inferred that the SMGC metapelites may also have originated contemporaneously with the southern CITZ metapelites in a similar back-arc setting. [ABSTRACT FROM AUTHOR]

Pelitic migmatites are texturally and mineralogically heterogeneous owing to variable proportions of light and dark coloured domains (leucosome and melanosome), the degree to which the two domains are segregated from one another and the variable composition of the leucosome (tonalite- trondhjemite to alkali feldspar granite). We use thermodynamic modelling to (1) provide insights into the variation in leucosome composition and solidification conditions for different melting and crystallization processes and (2) address the practical problem of how to sample heterogeneous migmatites for the purpose of constraining the pressure-temperature conditions of their formation. The latter is challenging because the appropriate bulk composition is affected by the above heterogeneity, as well as by the possibility of melt loss. Both dehydration melting and excess-water melting (for 2.0wt % excess water) are simulated for both equilibrium and fractional melting endmember processes. Three end-member processes are considered during the crystallization stage: (1) fractional crystallization of the melt; (2) equilibrium crystallization of just the melt in isolation from the solid phases; (3) crystallization during which melt and solids maintain chemical communication. Loss of melt during heating and cooling are also considered. Some of the key results of our simulations of pelitic migmatites are as follows. (1) Leucosome composition is primarily a reflection of the melt composition and to lesser degrees the crystallization process and the temperature of melt loss during cooling. Granite (sensu stricto) is the most common leucosome type, arising from many melting and crystallization processes, whereas tonalite or trondhjemite leucosome is generally indicative of excess-water melting. (2) Melts lost from partially molten regions, which have the potential to coalesce and form granites at shallower crustal levels, show less compositional variation than leucosomes in migmatites. Extracted melts are monzogranitic, or rarely granodioritic at low temperatures. (3) Comparing plagioclase compositions between leucosome and melanosome is potentially an effective means of distinguishing between crystallization processes, as well as the degree of retrograde melt-leucosome-melanosome chemical interaction. Fractional crystallization produces zoned plagioclase in the leucosome, isolated equilibrium crystallization produces plagioclase of different composition in the leucosome and melanosome, whereas chemical interaction causes the leucosome and melanosome plagioclase compositions to remain similar during crystallization. (4) The peak temperature of heterogeneous migmatites is most reliably constrained from an equilibrium phase assemblage diagram calculated using just the melanosome composition. Addition of leucosome to the melanosome composition can lead to peaktemperature estimates that differ from actual peak conditions by -25 to +50°C. In extreme cases, such as in rocks containing high proportions of leucosome to melanosome, or in which K-feldspar was absent as a solid phase at peak-temperature conditions, the assemblage developed in the rock at peak temperature may not appear in the phase diagram at all. (5) The solidification temperature (the temperature at which a partially molten rock changes to a completely solid migmatite, owing to either crystallization of the last melt or physical loss of the melt) can be accurately constrained if the melt and solids chemically communicated during cooling. If the melt crystallized in isolation from the melanosome, the solidification temperature is the temperature at which the last melt was physically lost or that of a water-saturated granitoid solidus (625-645°C). In the latter case, the solidification temperature may be much higher, but can only be estimated with a precision of tens to hundreds of degrees Celsius. [ABSTRACT FROM AUTHOR]

Turkey's Early Paleozoic tectonic evolution has received much attention in recent years. Early Paleozoic intrusive rocks are mainly distributed across Europe, Turkey (İstanbul-Zonguldak Zone, Strandja Zone, and Tauride Block), and Iran. However, the Early Paleozoic rocks have not yet been identified in north Turkey's Sakarya Zone, and thus the Early Paleozoic paleogeography of the Sakarya Zone remains enigmatic. Here, we provide new field observations, petrography, geochemistry, and geochronology of the western Sakarya Zone to identify the nature, origin, and tectonic setting of the Ordovician magmatic rocks cropping out in the Söğüt area (Bilecik, northwest Turkey). Our Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) U Pb analyses on zircons suggest that the Söğüt metagranites were emplaced at ∼484–461 Ma (except two zircons with ages of 490 and 494 Ma). There are abundant inherited zircon cores in the Söğüt metagranites, with ages of >500 Ma to ∼2.2 Ga. The inherited Cadomian cores give concordant ages of 588 and 572 Ma. The Söğüt metagranites are felsic and have high SiO 2 (72–76 wt%) and K 2 O (4.1–5.3 wt%) contents, with high K 2 O/Na 2 O ratios (1.2–1.9), and low Mg# (17–35) values. The samples are Ca-poor (CaO = 0.52–0.68 wt%) and show peraluminous [A/CNK = molar Al 2 O 3 /(K 2 O + CaO + Na 2 O) = 1.15–1.28] and high-K calc-alkaline geochemical characters, with high modal content of muscovite (∼10%), consistent with S-type compositions. The samples have low REE concentrations, with negative Eu anomalies. Ordovician zircon rims show εΗf (t) values of +3.6 to −35.4, whereas inherited zircon cores have εΗf (t) values of +33.9 to −26.1. Our data, in conjunction with previous studies, suggest that the Söğüt metagranites were generated by fluid-absent muscovite-dehydration melting of metasediments accumulated in a back-arc setting. This back-arc basin was characterized by a denuded continental crust with abundant pelite-dominated sediments and minor input of juvenile mafic melts. Our results, combined with previously published data and paleogeographic reconstructions, allow us to conclude that the Ordovician S-type granites were formed by progressive back-arc extension and crustal thinning prompted by the rollback of the southward subducting oceanic-lithosphere slab. This led to the opening of the Rheic Ocean on the northern side of Gondwana in response to the final closure and termination of the Tornquist Ocean as a branch of the Iapetus Ocean. We argue that the Ordovician S-type granites from the northern side of Gondwana could signal significant crustal reworking away from any clear continental collision zone. [Display omitted] • Ordovician S-type granites have been reported for the first time in Sakarya Zone. • The S-type granites were formed by the melting of a metapelitic source. • Our data confirm that the S-type granites originated in a subduction setting. • We propose that the crustal reworking may occur away from continental collision. [ABSTRACT FROM AUTHOR]

The migmatitic metapelites of the Lapland granulite belt (LGB) in the NE part of the Fennoscandian Shield represent an arc-related greywacke basin metamorphosed in the granulite facies. Detrital zircons from migmatitic metapelites are derived from 1.94 − 2.9 Ga old acid source rocks (U-Pb SIMS ages). The clustering of detrital zircon ages between 1.97 and 2.2 Ga is problematic, because abundant felsic crust of this age is absent in the shield. The metasediments are characterized by Sm-Nd model ages of ca. 2.3 Ga. A younger, 1905 − 1880 Ma population of homogeneous zircons was formed during regional metamorphism. The peak high-grade metamorphism took place at ~1900 Ma and the latest chronological record from subsequent decompression and cooling phase is from ca. 1870 Ma. The norite-enderbite series of the LGB represents arc-magmas, which were intruded into the metasediments at ~1920 − 1905 Ma ago according to zircon U-Pb ages and were probably an important heat source for metamorphism. Older, zoned zircon grains in a quartz norite vein, initial εNd values of 0 to +1 and the continuous spectrum of LILE enrichment in the enderbite-series probably reflect assimilation of metasediments by magmas. Monazite U-Pb ages of migmatitic metasediments in the range 1906 − 1910 ± 3 Ma overlap the late stage of enderbite intrusion and growth of early metamorphic zircons. Garnet-whole rock Sm-Nd ages from leucosomes in the range 1880 − 1886 ± 7 Ma are concurrent with the growth of the youngest metamorphic zircons and probably indicate the crystallization of leucosomes or the influence of a fluid liberated from them. Isotopic and petrologic data reveal that the evolution of Lapland Granulite belt from the erosion of source rocks to the generation of a sedimentary basin, its burial, metamorphism and exhumation took place within ca. 60 Ma.

The Palaeoproterozoic Lapland granulite belt was juxtaposed between Archaean and Proterozoic terrains in the NE part of the Fennoscandian Shield concurrently with the accretion of Svecofennian arc complexes at ~1.9 Ga. The belt consists mainly of aluminous migmatitic metagreywackes. Abundant noritic to enderbitic magmas were intruded concordantly into the metasediments and were probably an important heat source for metamorphism, which took place during the crystallization of the magmas. This is supported by structural and contact relations of metasediments and igneous rocks, and by the lack progressive metamorphic reaction textures in the igneous rock series. The peak of metamorphism took place above the dehydration melting temperature of the biotite-sillimanite-plagioclase-quartz assemblage at 750 − 850 °C and 5 − 8.5 kbar which lead to formation of a restitic palaeosome and peraluminous granitic melt in metapelites. Subsequently, the rocks were decompressed and cooled below the wet melting temperature of pelitic rocks (650 °C) under the stability field of andalusite coexisting with potassium feldspar (2 − 3 kbar). Cooling was accompanied by the crystallization of the neosomes, often carrying aluminium-rich phases. Postmetamorphic duplexing of the LGB is clearly seen in the distribution of calculated PT conditions.

The garnet-biotite-muscovite-plagioclase (GBMP) barometer was empirically revised for P-T conditions of 1-14 kbar and 450-840 °C, using 263 metapelitic rock samples from all over the world. This barometer is based on activity models for garnet, biotite and plagioclase identical to those of the well-calibrated garnet-biotite thermometer and the garnet-aluminosilicate-plagioclase-quartz (GASP) barometer. The GBMP barometer is less temperature dependent than the GASP barometer and can be applied to either Al2SiO5-absent or Al2SiO5-bearing metapelites. The total error of the GBMP barometer is estimated to be about ±1.2 kbar on considering input temperature error and analytical errors of chemical compositions of the phases involved. The random error of the GBMP barometer is evenly distributed with respect to pressure, temperature and mineral composition. Simultaneous application of the GBMP barometer and the garnet-biotite thermometer identifies the correct stability field for Al2SiO5-bearing metapelites. Application of the GBMP barometer to metapelitic rocks within the same geological terranes or thermal contact aureoles yielded similar pressures within error. A spreadsheet for implementing the proposed GBMP geobarometer is supplied on the journal's website. [ABSTRACT FROM AUTHOR]

Garnet-bearing metapelites are cropped out in the Zanhuang metamorphic complex located in the central section of the N–S trending Trans-North China Orogen. Petrological studies indicate that these garnet-bearing metapelites preserve three distinct metamorphic assemblages (M 1 –M 3 ). The cores of garnet porphyroblasts and fine-grained inclusions of plagioclase + quartz ± biotite ± apatite ± magnetite ± rutile ± ilmenite define the prograde metamorphic stage (M 1 ). The assemblage of the peak metamorphic stage (M 2 ) consists of biotite + plagioclase + quartz ± kyanite ± hornblende ± magnetite ± ilmenite in equilibrium with the lowest- X Mn garnet rims. The peak metamorphism was followed by a near-isothermal decompressional stage (M 3 ) and the development of symplectitic coronas of biotite + plagioclase + quartz ± hornblende ± magnetite surrounding garnet porphyroblasts. Quantitative phase equilibria modeling and traditional thermobarometric estimations were applied to yield P – T conditions of 4.5–5.9 kbar and 551–596 °C for stage M 1 , 9.6–12.3 kbar and 770–830 °C for stage M 2 , and ∼4.7 kbar and ∼635 °C for stage M 3 . The combination of the mineral inclusions, mineral compositions, and metamorphic reactions in the Zanhuang metapelites defines a clockwise P – T path with nearly isothermal decompression, which is considered to have been related to the amalgamation of the Eastern and Western Blocks to form the North China Craton. Abundant U–Pb spot analyses of metamorphic zircons from the gneiss and amphibolite reveal two discrete, meaningful ages of metamorphism within the Zanhuang complex: the first age dates to ∼2507 Ma, and the second age dates to ∼1839 Ma. The first age group (∼2507 Ma) was most likely associated with a regional Neoarchean granulite-facies metamorphic event within the North China Craton, whereas the second age group (∼1839 Ma) is interpreted as the result of a Paleoproterozoic orogenic event that occurred in the Trans-North China Orogen (TNCO). [ABSTRACT FROM AUTHOR]

The Palaeoproterozoic Svecofennian bedrock of the Seinäjoki-Ilmajoki area, western Finland, is largely composed of porphyroblastic metapelites. In the area, the regional metamorphic grade increases towards the southwest. Over a distance of 15 km, andalusite mica schists gradually grade into migmatitic garnet cordierite-sillimanite mica gneisses with a facies-series of the andalusite-sillimanite type. Five regional metamorphic zones are present: andalusite, sillimanite-muscovite, sillimanite-K-feldspar, cordierite-K-feldspar and garnet-cordierite-K-feldspar. The primary layering (S0) of the mica schists is deformed by an isoclinal fold phase (F2), which is synchronous with the main metamorphic phase and the growth of micas. S1 is very weak and subject to interpretation. The S2 schistosity is deformed by intense late-metamorphic F3 and F3b folds, which have formed under slightly different metamorphic conditions: practically no metamorphic micas have grown parallel to axial planes while within F3b folds there are a few granitic veins parallel to these planes. The F3 and F3b folds probably belong to one phase. S2 dominates in the mica schists while S3 and S3b dominate in the mica gneisses. The metapelites are also deformed by younger minor fold phases (F4 and F5). A composite schistosity (S0±S1±S2±S3, or S3b) commonly occurs in the metapelites. The peak of regional metamorphism has been associated with the intrusion of 1.89-1.88 Ga old tonalite plutons. Geothermometric estimates for regional metamorphism are c. 730 °C at an assumed pressure of 5 kbar. Neosomes in the high-grade mica gneisses occur as patches rather than as elongated, narrow veins. Garnet coexists with cordierite, but the minerals are rarely in equilibrium. Muscovitization and the formation of retrogressive andalusite did not occur in the high-grade mica gneisses, but there is minor kyanite indicating that the crust probably underwent near-isobaric cooling. The area of highest-grade regional metamorphic conditions contains a post-kinematic mangeritic quartz monzonite (1.87 Ga), which has a narrow granulite-grade contact aureole. The chemical composition of biotites in the metapelites varies as a function of metamorphic zoning, but the chemical composition of metapelites remains mostly unchanged.

New activity-composition ( a- x) relations for minerals commonly occurring in metapelites are presented for use with the internally consistent thermodynamic dataset of Holland & Powell (, Journal of Metamorphic Geology, 29, 333-383). The a- x relations include a broader consideration of Fe2O3 in minerals, changes to the formalism of several phases and order-disorder in all ferromagnesian minerals where Fe-Mg mixing occurs on multiple sites. The a- x relations for chlorite, biotite, garnet, chloritoid, staurolite, cordierite, orthopyroxene, muscovite, paragonite and margarite have been substantially reparameterized using the approach outlined in the companion paper in this issue. For the first time, the entire set of a- x relations for the common ferromagnesian minerals in metapelitic rocks is parameterized simultaneously, with attention paid to ensuring that they can be used together to calculate phase diagrams of geologically appropriate topology. The a- x relations developed are for use in the Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O2 (NCKFMASHTO) system for both subsolidus and suprasolidus conditions. Petrogenetic grids in KFMASH and KFMASHTO are similar in topology to those produced with earlier end-member datasets and a- x relations, but with some notable differences. In particular, in subsolidus equilibria, the FeO/(FeO + MgO) of garnet is now greater than in coexisting staurolite, bringing a number of key staurolite-bearing equilibria into better agreement with inferences from field and petrographic observations. Furthermore, the addition of Fe3+ and Ti to a number of silicate phases allows more plausible equilibria to be calculated in relevant systems. Pseudosections calculated with the new a- x relations are also topologically similar to equivalent diagrams using earlier a- x relations, although with many low variance fields shifting in P- T space to somewhat lower pressure conditions. [ABSTRACT FROM AUTHOR]

High-grade terranes showing multiple episodes of leucogranite magmatism are ideal to assess the fertility of crustal protoliths. Two episodes of Paleoproterozoic leucogranite magmatism occur in the Mahalapye Complex high-grade terrane, at the northern edge of the Kaapvaal Craton. The older one (leucogranite 1) at ∼2.05 Ga represented by leucosomes, thin veins and dykes is related to the melting of ∼2.06 Ga biotite-bearing diorite gneiss. In comparison, the younger episode (leucogranite 2) at ∼2.03 Ga forms massive bodies. Leucogranite 1 is associated with the migmatitic diorite gneiss and crosscut by the ∼2.04 Ga granodiorite. Leucogranite 2 contains xenoliths of diorite gneiss and granodiorite, as well as inclusions of quartz-poor and quartz-rich metapelites (Bt + Crd + Pl + Kfs + Spl ± Sil ± Grt + Ilm + Qz). Phase equilibria modelling of a representative quartz-poor metapelite indicate that its partial melting occurred at ∼810 °C and ∼4.8 kbar in the presence of aqueous fluid. The peraluminous, ferroan and alkalic composition of the predicted melt is comparable to the chemistry of leucogranite 2. The larger volume of anatectic melt for the estimated P-T condition corresponds to the occurrence of leucogranite 2 as massive bodies. The lower temperature of in-situ anatexis (<750 °C) at a deeper level (∼7.3 kbar) and lack of additional fluid, likely did not generate significant melt fraction during the formation of leucogranite 1. The contrast in the fertility of the two protoliths with respect to production of leucogranite melts was also related to different setting of melting. Melting of diorite gneiss is related to the local influence of mafic dykes and sills, whereas melting of metapelite is related to regional collision. It follows that high pressures, water-deficient regimes and localized heat sources produce limited volumes of partial melt, whereas high temperatures, fluid-present regimes and a large-scale heat source produces higher volumes of melts. [Display omitted] • Two episodes of leucogranite (LG1, LG2) magmatism in the Mahalapye Complex. • Dehydration melting of diorite gneiss (∼730 °C/∼7.2–7.5 kbar) formed the less dominant LG1. • Fluid-present melting of metapelite (∼810 °C/∼4.8 kbar) formed the dominant LG2. • Phase equilibria modelling indicates large content of anatectic melt for LG2 than LG1. • Temperature, presence/absence of fluid, scale of heat source, affect fertility of protoliths. [ABSTRACT FROM AUTHOR]

Abstract: Pelitic and semipelitic anatectic granulites form one of the major lithological units in Kan Nack complex of the Kon Tum massif (in south-central Vietnam), which comprises HT metamorphic and magmatic rocks including granulites and charnockites is classically regarded as the older part of the Gondwana-derived Indosinia terrain. Metamorphic evolution study of pelitic granulite, the most abundant among granulites exposed in this massif, facilitates to understand that tectonic setting take place during the Indosinian time. The paragenetic assemblages, mineral chemistry, thermobarometry and P–T evolution path of pelitic–semipelitic granulites from Kon Tum massif has been studied in detail. Petrographic feature demonstrates that the pelitic granulite experienced prograde history, from pregranulitic conditions in the amphibolite facies up to the peak granulitic assemblages. Successive prograde reactions led to the temperature-climax giving rise to assemblages with cordierite-hercynite and cordierite-hercynite-K-feldspar. Then, as attested by the mineralogic association occurring in cordieritic coronas, these rocks have been affected by retrograde conditions coeval with a decrease of the pressure. Thermobarometic results show that the highest temperature obtained by ksp/pl thermometry is 850°C and the highest pressure obtained by GASP (Garnet Alumino-Silicate Plagioclase) is 7.8kbar. The obtained clockwise P–T evolution path involving heating decompression, then nearly isothermal decompression and nearly isobar cooling conditions shows that high temperature–low pressure metamorphism of the studied pelitic anatectic granulites of Kan Nack complex occurred possibly in extensional setting during the Indosinian orogeny of 260–240Ma in age. [Copyright &y& Elsevier]