Compositional Isopleths Research Articles

P-T path from garnet zoning in pelitic schist from NE Sardinia, Italy: Further constraints on the metamorphic and tectonic evolution of the north Sardinia Variscan belt

Mylonitic micaschists in the south-eastern sector of the Posada-Asinara Shear Zone in the Axial Zone of the Sardinia Variscan chain were investigated for the reconstruction of their metamorphic evolution and P-T history. Micaschists underwent polyphase ductile deformation consisting of an old D1 deformation (~345–340 Ma) associated to shearing and folding and to a penetrative S1 axial plane foliation. The S1 foliation is progressively transposed by the D2 phase, which is associated with upright up to NE verging folds and dextral shear zones. Micaschists are characterized by abundant centimetric garnet crystals with strong compositional zoning. The garnet porphyroblasts (~15 vol%) are associated with plagioclase, quartz, biotite, staurolite, white mica, chloritoid and retrograde chlorite. Garnet presents an internal foliation identified by the isoorientation of quartz inclusions sometimes arranged into a sigmoidal pattern suggesting rotation of the garnet during growth, discordant respect to the external S2 foliation. The S2 foliation is identified by the preferred orientation of micas and chlorite and by the alternance between quartzo-feldspathic and micaceous layers. The garnet core contains several small inclusions of quartz, rutile, apatite, and minor monazite and zircon. Additional inclusions, observed in the garnet domain around the core are ilmenite, chloritoid, staurolite and white mica. EMPA analyses reveal an even more complex chemical zoning consisting of garnet core, garnet mantle, and rim. The compositional isopleths that match the composition of the garnet core intersect with isopleths for the highest Si contents in K-white mica at about T 430–490 °C, P 1.3–1.4 GPa. Garnet rim isopleths and isopleths for the lowest Si contents in K-white mica indicate P-T conditions close to 560–630 °C/0.6–1.1 GPa. The resulting P-T path is clockwise, subdivided into two separate stages. The first stage is a prograde segment suggested by the garnet core to mantle compositional variation whereas the second reflects garnet rim growth during exhumation. • Garnet crystals up to 1 cm are characterized by core, mantle, rim, and outer rim • Clockwise P-T path shows a prograde segment up to pressure peak (1.7GPa) • High – pressure conditions pre-date Barrovian metamorphism in Sardinia

Metamorphic patterns and zircon U–Pb dating of the Xilingol complex in Inner Mongolia, China: Implications for rifting metamorphism and tectonic evolution in eastern Central Asian Orogenic Belt

The Xilingol complex is one of the maximal outcropped metamorphic terranes in the eastern segment of the Central Asian Orogenic Belt. Detailed petrological and geochronological study of the Xilingol complex will provide valuable insights into the orogenic process and closure of the Paleo-Asian Ocean. We present a comprehensive study of petrology, pseudosection modelling, and zircon U-Pb geochronology of representative rocks collected in Daqing Pasture, including two-mica quartz schist, epidote amphibolite, biotite plagioclase gneiss and quartzite. The Xilingol complex in Daqing Pasture can be preliminarily divided into the north and south segments based on field and petrological observations. The north segment comprises two-mica quartz schist, biotite quartz schist, amphibolite and minor epidote amphibolite, whereas the south segment consists mainly of biotite gneiss, amphibolite and quartzite, with varying degrees of migmatization developed. Combined pseudosection modelling and composition isopleths of garnet and biotite constrain a peak P–T condition of ~660 °C, ~5.5 kbar for the garnet-bearing two-mica quartz schist in the north segment, corresponding to low amphibolite facies metamorphism. The garnet-bearing biotite plagioclase gneisses in the south segment are interpreted to experience high amphibolite facies metamorphism at a peak condition of ~750–790 °C, ~5.5–8.0 kbar, followed by nearly isobaric cooling. The variations of peak P–T estimates within Daqing Pasture, coupled with the spatial heterogeneities of lithological associations between different outcropped terranes, reveal an overall northward decreasing metamorphic grade for the Xilingol complex. Metamorphic zircons from three biotite plagioclase gneisses in the south segment yield apparent 206 Pb/ 238 U age spreads of 397–348 Ma, 363–294 Ma and 341–301 Ma, respectively. These partially overlapping zircon ages are an indication of discrete metamorphic events involving localized thermal metamorphism at depth in the Middle Devonian and long-lasting Buchan-type metamorphism in the Carboniferous, with intermediate data probably related to a mixture of multiple growth domains. Five metamorphic zircons with low Th/U ratios from the epidote amphibolite in the north segment yield a concordia age of 284.8 ± 1.9 Ma, reflecting the diachronism of the Late Paleozoic Buchan-type metamorphism. Summarizing all U-Pb data of inherited magmatic zircons and metamorphic zircons, a distinct U-Pb age continuum is retrieved to elucidate a successive process from sedimentary diagenesis of forearc sedimentary formations to deep burial in the Early Devonian, and then to subsequent multi-stage metamorphic overprinting from the Middle Devonian to the Permian. The isograd patterns, P–T–t paths (clockwise paths and long-term duration) and spatial distribution of metamorphic terranes of the Xilingol complex are more compatible with an active continent rift setting rather than paired metamorphic belts in the arc-basin system, revealing post-collisional extension of the Early-Middle Paleozoic orogenic lithosphere of eastern CAOB in the Late Paleozoic. • The Xilingol complex underwent long-lasting Buchan-type metamorphism in the Carboniferous to Permian. • Metamorphic patterns of the Xilingol complex are compatible with continent rift setting. • Zircon U-Pb age continuum reveals a successive process from diagenesis to deep burial to multiple metamorphic episodes.

Clues from Garnet-Spinel-Bearing Peridotite and Pyroxenite Xenoliths about the Formation of the Subcratonic Lithospheric Mantle—a Case Study from the Orapa Kimberlite Cluster

Abstract A series of 18 ultramafic mantle xenoliths from the Orapa and Letlhakane mines containing both garnet and spinel were investigated petrographically and by means of classical and multi-equilibrium geothermobarometry in order to understand their P–T evolution and infer tectonic processes consistent with that evolution, assuming that samples from the shallow subcontinental lithospheric mantle (SCLM), may have remained cool enough to preserve memory of its formation. Samples broadly classify into peridotites, mainly harzburgites, and pyroxenites, mainly (olivine-) websterites with less than 15 vol% olivine. The websterites all show thin garnet coronas around xenomorphic spinel clusters and often coarse garnet and very fine spinel exsolving from pyroxenes. Classical geothermobarometry results vary consistently between the 40 mW/m2 geotherm at high pressures and the 50 mW/m2 geotherm at lower pressures but partly fail the reliability test indicating some degree of disequilibrium, particularly, as may be expected, toward lower P and T. Eleven samples were analyzed in detail for mineral modes, and pseudosections were calculated for the four compositionally most extreme samples. All four samples show systematic divergence of the mode and composition isopleths indicating overall chemical disequilibrium, but partial isopleth convergence in two distinct P–T regions that were interpreted as the main equilibration stage and a stage of late ‘freeze-in’ of a few composition isopleths at lower temperature and higher pressure. Specifically, the P–T changes between stage 1 and stage 2 were from 900°C and 13 kbar to 750°C and 23 kbar for a websterite and from 850°C and 24 kbar to 730°C and 33 kbar for a harzburgite. For an olivine-orthopyroxenite and -clinopyroxenite the change was from 910°C, 17 kbar and 770°C, 17 kbar for stage 1, respectively, to less well-constrained conditions for stage 2 of 650°C, 36 kbar and 630°C, 20 kbar, respectively. This interpretation of a protracted or stepwise freeze-in along a P–T path holds even after considering thermodynamic, analytical and, as far as possible, geological uncertainties. The result is consistent with a ‘subcretion’ or ‘buoyant subduction’ model for the formation of the SCLM, specifically with the down-drag of an already subcreted lithospheric platelet by a subsequently arriving platelet. In the framework of the crust–mantle evolution in the Orapa region this most likely occurred in the Neoarchean, between 2.8 and 2.5 Ga.

Construction of P–T–t paths for eclogite in the Tongbai orogen by combining phase equilibria modelling with zircon inclusion composition

AbstractThe interpretation of metamorphic ages for zircon domains in eclogite remains challenging because of the uncertainties in interpreting their rare earth element (REE) patterns and mineral inclusions. This has seriously hindered the construction of robust pressure (P)–temperature (T)–time (t) paths for eclogites from collisional orogens. In this study, a composite approach is used to obtain a reliable P–T path for eclogite from the Tongbai orogen and quantitatively attribute zircon U–Pb ages to specific P–T intervals. An integrated study using isochemical phase diagram section (pseudosection) modelling, multi‐equilibrium thermobarometry, and the Zr‐in‐rutile thermometry with petrographic observations indicates that the target eclogite experienced multistage metamorphic evolution. The evolution is characterized by a clockwise P–T path with compressional heating for peak eclogite facies metamorphism at 588 ± 20°C and 26.7 ± 2.8 kbar and two stages of isothermal decompression from an early stage of amphibole eclogite facies metamorphism at 15–10 kbar to a late stage of amphibolite facies metamorphism at pressures of <10 kbar. Metamorphic zircon domains in eclogite and quartz vein show relatively flat patterns of heavy REE without notable Eu anomalies but different cathodoluminescence responses and middle REE (MREE) contents. This indicates their growth in different stages of eclogite facies metamorphism, which are categorized into two groups. The first group is composed of bright rims or homogenous grains that are characterized by low MREE contents, suggesting the existence of abundant amphibole during the metamorphism. The compositional isopleths of amphibole inclusions indicate that this group of zircon domains would form by hydration during crustal exhumation to amphibole eclogite facies at 240 ± 2 Ma. The second group consists of dark sector mantles and shows high MREE contents, indicating the prograde to peak metamorphism with significant decomposition of amphibole or lawsonite during crustal subduction to eclogite facies at 245 ± 2 Ma. Therefore, the combination of the isopleth thermobarometry of mineral inclusions with the Zr‐in‐rutile thermometry and the zircon REE partitioning is an effective means to link the metamorphic P–T conditions to the ages of metamorphic zircons in the different stages of collisional orogeny.

Kyanite-garnet granulite from the Andrelândia nappe system, Brasília belt, registers two late Neoproterozoic metamorphic cycles

The southern Brasília belt (SBB) evolved during the Neoproterozoic amalgamation of Western Gondwana. The Andrelândia nappe system (ANS) is part of the SBB and a key area to understand the tectono-metamorphic evolution of the SBB due to the ongoing controversy concerning whether peak metamorphism occurred at >650 Ma, >618 Ma or 605 Ma in the upper nappe of the ANS. A kyanite-garnet-bearing felsic granulite from the Três Pontas-Varginha nappe (TPVN) in the ANS was investigated to constrain the pressure-temperature (P-T)-time evolution. For this purpose, we applied thermodynamic modelling with P-T pseudosections contoured by garnet compositional isopleths and monazite in-situ dating by LA-ICP-MS. Early garnet (Gt I) formed in the P-T range of 11-20 kbar and 495-595 °C based on the garnet core composition. This growth was followed by resorption at retrograde conditions. Mineralogical changes during a second metamorphic cycle are discernible by new growth of garnet (Gt II) with a significantly different composition compared to Gt I. The P-T climax reached 13 ± 1.5 kbar and 800 ± 50 °C, accompanied by partial melting. The two metamorphic cycles are assigned to two successive burial-exhumation events that ended with nappe stacking and formation of the TPVN. Monazite occurring in garnet (Gt II) and in the matrix contains Y concentrations of 0.1-3.5 wt.% independent of the textural position, but positively correlated with HREE contents, supporting the formation of all monazite during corrosion of Gt I and early growth of Gt II. Dating of monazite yielded a concordia age of 617.1 ± 3.8 (2σ) Ma. We assign this age to the end of the first P-T loop and beginning of the second loop. Literature data suggest two ages, close to 650 Ma and 605 Ma, for the P-T climax in the TPVN. The age difference of ca. 45 Ma fits a multiple metamorphic evolution similar to Phanerozoic orogens, characterized by a difference in time between two metamorphic peaks in the range of tens of million years (15-60 Ma) within a single orogenic cycle. This points to an orogenic evolution in the late Neoproterozoic (Brasiliano/Pan-African orogenesis) that had the character of modern mountain-building processes.

Partial melting due to breakdown of phengite and amphibole in retrogressed eclogite of deep Precambrian crust: An example from the Algonquin terrane, western Grenville Province, Canada

Characterizing partial melting of eclogite in Precambrian orogens is critical to understanding the petrological processes that operate in the deep orogenic crust. Retrogressed eclogite from the Algonquin terrane in the western Grenville Province, Canada, was taken as an example to study such processes. The studied samples contained the assemblage of garnet, omphacite, amphibole, phengite, biotite, and rutile at peak pressure. Various exhumation- and melt-related textures formed show evidence for a complex post-peak pressure history. Rutile is only found in the core of garnet, whereas ilmenite-bearing assemblages predominate in the garnet rim and the matrix. Symplectite of diopside + plagioclase + amphibole replaced former omphacite. Symplectite of amphibole and plagioclase formed as a corona around garnet. Polymineralic inclusions of plagioclase + K-feldspar ± ilmenite ± biotite ± amphibole ± apatite ± sulfides in garnet crystallized from melt, which was captured during garnet growth and reacted with the host. Cusps of plagioclase into the boundaries of other phases (e.g. garnet and clinopyroxene) mimic the original melt-solid interface. Neoblasts of garnet with euhedral crystal faces were formed as a peritectic phase during eclogite melting. Leucocratic veins without external connection, present in the sampled mafic pod at outcrop scale, are interpreted as crystallized pockets of melt which derived internally. Pseudosection modeling of the studied samples along with compositional isopleths of chemically zoned garnet yielded an exhumation path from 1.7 ± 0.1 GPa, 680 ± 50 °C to 1.3 ± 0.1 GPa, 920 ± 50 °C. Along this path, the modal content of melt increased in response to the breakdown of phengite and amphibole along with involvement of omphacite and rutile. The melt crystallized during cooling and equilibrated at 0.6–0.8 GPa, 710–750 °C, estimated using empirical thermobarometry. This study shows that metabasite from Proterozoic collisional orogens can be partially melted on the exhumation and heating path through phengite- and amphibole-dehydration melting.

Paleoproterozoic oceanic subduction in the North China Craton: Insights from the metamorphic P–T–t paths of the Chicheng Mélange in the Hongqiyingzi Complex

How and when modern-style plate tectonics started in the Earth’s history is one of the most important scientific issues. The Chicheng mélange in the north margin of North China Craton (NCC) presents a new insight into the Paleoproterozoic cold oceanic subduction and the beginning of modern-style plate tectonic regime. The mélange comprises blocks of retrograde eclogites, amphibolites and peridotites in a matrix of metasediments. The retrograde eclogites mainly contain garnet, amphibole and symplectites consisting of fine-grained plagioclase, clinopyroxene and amphibole. Three metamorphic stages were derived, involving the prograde, peak, and post-peak decompression stages. The P–T conditions for the prograde and peak stages range from ~17 kbar/660 °C to >22/~750 °C kbar based on mineral composition isopleths in P–T pseudosections, involving core–to–mantle increasing pyrope and decreasing grossular in garnet and Ti content in amphibole. The post-peak decompression is proposed to be isothermal under melt (fluid)-absent conditions, recorded a pressure of 8–11 kbar on the basis of symplectite assemblages. For the amphibolites pre-peak heating, peak temperature, and post-peak cooling metamorphic stages were derived. They experienced anataxis during this overprinting metamorphism when heated to above the solidus. The peak temperature conditions are constrained to be 5–7 kbar/780–830 °C on the basis of the contours of the maximum Ti in amphibole and anorthite of plagioclase in P–T pseudosections. Zircon U–Pb dating for the selected samples and a summary of published age data reveal that the eclogite facies metamorphism may have occurred at 1.85–1.80 Ga, and the eclogitic rocks exhumed to a lower crust level during or after subduction to be overprinted by multi-phase thermal events at ~1.66 Ga and 460–420 Ma with undefined conditions, and at 360–270 Ma under amphibolite facies. The eclogite facies metamorphism from the Chicheng mélange shows a geothermal gradient of ~9 °C/km, pointing to a cold oceanic subduction at late Paleoproterozoic that occurred along the northern margin of the NCC. The overprinting metamorphism with anataxis recognized from the amphibolites represents a low-pressure type with a geothermal gradient of ~35 °C/km, which was interpreted to occur at 360–270 Ma governed by extension.

Retrieval of P–T–t–d paths in a syn-metamorphic shear zone: Implications for P-T calculations and the Permian–Triassic orogeny on the Korean Peninsula

Crustal-scale shear zones developed on the boundaries of the Precambrian Gyeonggi Massif (GM) can play key role on understanding collisional orogeny during the Late Paleozoic to Early Mesozoic in the Korean Peninsula. Garnet schist formed on the boundary between the GM and Okcheon Metamorphic Belt (OMB) experienced contractional (Dn) and extensional (Dn+1) shearing combined with a clockwise pressure–temperature–deformation (P–T–d) path. Two distinct mineral assemblages were found in the schists, (1) garnet (pre- and/or syn-Dn) + staurolite (syn-Dn and/or inter Dn and Dn+1, and syn-Dn+1) + andalusite (syn-Dn+1)-bearing and (2) garnet (pre- and/or syn-Dn) + epidote-bearing assemblages, depending on bulk-rock compositions. Various compositions (An10–40 in the former assemblage and An45–70 in the latter one) and texture of plagioclase resulted from bulk-rock compositions, heterogeneous strain and dissolution and precipitation during the multiple shearing. The P–T conditions estimated for the early contractional shearing (top-to-the-north reverse shear) are 560 °C–600 °C and 5.5–7.0 kbar based on the intersections between compositional isopleths for pre- and/or syn-Dn garnet and plagioclase (An30–40 & An45–55) calculated in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O and MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 systems. Subsequent extensional shearing (top-to-the-south normal shear) was characterized by recrystallization of fine-grained syn-Dn+1 staurolite with growth of syn-Dn+1 andalusite porphyroblasts during cooling and decompression to 560 °C and 3.5 kbar. The contractional and extensional shearing probably occurred at the Permian and the Late Triassic to Early Jurassic, respectively, based on the previous geochronological data. Consequently, inferred P–T–t–d paths on the GM–OMB boundary probably suggest not only crustal thickening followed by thinning, but also multiple amalgamations of several terranes.

Decompressional equilibration of the Midsund granulite from Otrøy, Western Gneiss Region, Norway

Abstract The Western Gneiss Region (WGR) of the Scandinavian Caledonides is an archetypal terrain for high-pressure (HP) and ultrahigh-pressure (UHP) metamorphism. However, the vast majority of lithologies occurring there bear no, or only limited, evidence for HP or UHP metamorphism. The studied Midsund HP granulite occurs on the island of Otrøy, a locality known for the occurrence of the UHP eclogites and mantle-derived, garnet-bearing ultramafics. The Midsund granulite consists of plagioclase, garnet, clinopyroxene, relict phengitic mica, biotite, rutile, quartz, amphibole, ilmenite and titanite, among the most prominent phases. Applied thermodynamic modelling in the NCKFMMnASHT system resulted in a pressure–temperature (P–T) pseudosection that provides an intersection of compositional isopleths of XMg (Mg/Mg+Fe) in garnet, albite in plagioclase and XNa (Na/Na+Ca) in clinopyroxene in the stability field of melt + plagioclase + garnet + clinopyroxene + amphibole + ilmenite. The obtained thermodynamic model yields P–T conditions of 1.32–1.45 GPa and 875–970 °C. The relatively high P–T recorded by the Midsund granulite may be explained as an effect of equilibration due to exhumation from HP (presumably UHP) conditions followed by a period of stagnation under HT at lower-to-medium crustal level. The latter seems to be a more widespread phenomenon in the WGR than previously thought and may well explain commonly calculated pressure contrasts between neighboring lithologies in the WGR and other HP–UHP terranes worldwide.

Testing the fidelity of thermometers at ultrahigh temperatures

AbstractA highly residual granulite facies rock (sample RG07‐21) from Lunnyj Island in the Rauer Group, East Antarctica, presents an opportunity to compare different approaches to constraining peak temperature in high‐grade metamorphic rocks. Sample RG07‐21 is a coarse‐grained pelitic migmatite composed of abundant garnet and orthopyroxene along with quartz, biotite, cordierite, and plagioclase with accessory rutile, ilmenite, zircon, and monazite. The inferred sequence of mineral growth is consistent with a clockwise pressure–temperature (P–T) evolution when compared with a forward model (P–T pseudosection) for the whole‐rock chemical composition. Peak metamorphic conditions are estimated at 9 ± 0.5 kbar and 910 ± 50°C based on conventional Al‐in‐orthopyroxene thermobarometry, Zr‐in‐rutile thermometry, and calculated compositional isopleths. U–Pb ages from zircon rims and neocrystallized monazite grains yield ages of c. 514 Ma, suggesting that crystallization of both minerals occurred towards the end of the youngest pervasive metamorphic episode in the region known as the Prydz Tectonic Event. The rare earth element compositions of zircon and garnet are consistent with equilibrium growth of these minerals in the presence of melt. When comparing the thermometry methods used in this study, it is apparent that the Al‐in‐orthopyroxene thermobarometer provides the most reliable estimate of peak conditions. There is a strong textural correlation between the temperatures obtained using the Zr‐in‐rutile thermometer––maximum temperatures are recorded by a single rutile grain included within orthopyroxene, whereas other grains included in garnet, orthopyroxene, quartz, and biotite yield a range of temperatures down to 820°C. Ti‐in‐zircon thermometry returns significantly lower temperature estimates of 678–841°C. Estimates at the upper end of this range are consistent with growth of zircon from crystallizing melt at temperatures close to the elevated (H2O undersaturated) solidus. Those estimates, significantly lower than the calculated temperature of this residual solidus, may reflect isolation of rutile from the effective equilibration volume leading to an activity of TiO2 that is lower than the assumed value of unity.

Compositional zoning in garnet and its modification by diffusion during pressure and temperature changes in metamorphic rocks; an approach and software

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

Ca-rich garnets and associated symplectites in mafic peraluminous granulites from the Gföhl Nappe System, Austria

Abstract. Mafic peraluminous granulites associated with the mantle-derived peridotites of the Dunkelsteiner Wald provide evidence of the tectono-metamorphic evolution of the Gföhl Nappe System, Austria. They contain the primary assemblage garnet + Al-rich clinopyroxene + kyanite. Large Ca- and Mg-rich garnets are embedded in a granoblastic matrix of Al-rich clinopyroxene, Ca-rich plagioclase and minor hornblende. They were partially replaced by different generations of symplectites: (a) corundum + sapphirine + spinel + plagioclase formed around kyanite inclusions, (b) orthopyroxene + spinel + plagioclase ± hornblende formed at their rims and (c) clinopyroxene + orthopyroxene + spinel + plagioclase ± hornblende formed within cracks. Large garnets show complex compositional structure comprising several repeatedly occurring garnet types, which are characterized by specific compositions. The areal extent and the cross-cutting relations observed in element distribution maps allowed for the derivation of the relative timing of the formation of the different garnet types. The compositional features of the garnets indicate post-formational modification by intra-crystalline diffusion and metasomatic agents. The garnet composition isopleths in equilibrium assemblage diagrams are in line with compositions modification as indicated by the element distribution maps. They confirm the deviation of composition from equilibrium for all garnet types. Furthermore, at least the youngest garnet types show evidence of metasomatic (Fe + Mg) loss affecting their Ca content. Pressure–temperature (P–T) estimates are based on equilibrium assemblage diagrams that reproduce satisfactorily the observed mineral assemblages and measured mineral compositions. Criteria for checking the existence of preserved equilibrium compositions are suggested. The results call into question the invariability of the assumption that the Ca content and/or zoning in garnet preserves primary P–T information from garnet growth in every case. Recrystallization and compositional readjustment of the reactive garnet volume during symplectite formation led to the development of pronounced, secondary diffusion-induced zoning profiles overprinting the different garnet types and post-dating the complex garnet compositional structure. The primary assemblage is stable between 760 and 880 °C and pressures > 11 kbar. The bulk composition of the crack symplectites is almost isochemical to the oldest, broken-down garnet type. These symplectites were formed above 730 °C and pressures between 5 and 7.5 kbar. The rocks studied underwent more or less isothermal decompression from pressures above 11 to ∼ 6 kbar at temperatures of about 800 °C. Crack and rim symplectites were formed after decompression during the early stage of approximately isobaric cooling under conditions of low differential stress. Due to limited availability of fluids promoting symplectite formation, the timescale of symplectite formation calculated from secondary diffusion profiles associated with crack symplectites is shown to be geologically very short (< 0.5 ka).

Challenges in constraining theP–Tconditions of mafic granulites: An example from the northern Trans‐North China Orogen

AbstractSome mafic granulites in the Sanggan area of the northern Trans‐North China Orogen (TNCO) have a relatively simple mineralogy with low energy grain shapes that are compatible with an assumption of equilibrium, but the rock‐forming minerals show variations in composition that create challenges for thermobarometry. The mafic granulites, which occur as apparently disrupted dyke‐like bodies in tonalite–trondhjemite–granodiorite gneisses, are divided into two types based on petrography and chemical composition. Type 1 mafic granulites are fine‐ to medium‐grained with an equilibrated texture and an assemblage of plagioclase+clinopyroxene+garnet+magnetite+ilmenite and sometimes minor hornblende±orthopyroxene. Type 2 mafic granulites are coarse‐grained and hornblende bearing with a peak assemblage of garnet+clinopyroxene+plagioclase+hornblende and variably developed coronae and symplectites of plagioclase+hornblende+orthopyroxene partially replacing porphyroblastic garnet±clinopyroxene.SIMSU–Pb dating of metamorphic zircon from two type 1 mafic granulites yields metamorphic ages ofc. 1.84 and 1.83 Ga, consistent with published ages of the type 2 mafic granulites. Based on phase equilibrium modelling, we use the common overlap ofP–Tfields defined by the mineral assemblage limits, and the mole proportion and composition isopleths of different minerals in each sample to quantify the metamorphic conditions. For type 1 granulites, overlap of the mineral proportion and composition fields for each of three samples yields similarP–Tconditions of 710–880°C at 0.57–0.79 GPa, 820–850°C at 0.59–0.63 GPa and 800–860°C at 0.59–0.68 GPa. For the type 2 granulites, overlaying the peak assemblage fields for three samples yields commonP–Tconditions of 870–890°C at 1.1–1.2 GPa. For the retrograde assemblage, overlap of the mineral proportion and composition fields for each sample yields similarP–Tconditions of 820–840°C at 0.85–0.88 GPa, 860–880°C at 0.83–0.86 GPa and 880–930°C at 0.89–0.95 GPa. TheP–Tconditions appear distinct between the two types of mafic granulite, with the mineralogically simple type 1 mafic granulites recording the lowest pressures. However, there are significant uncertainties associated with these results. For the granulites, there are uncertainties related to the determination of modes and composition of the equilibration volume, particularly estimation of O and H2O contents, and in the phase equilibrium modelling there are uncertainties that propagate through the calculation of mole proportions and mineral compositions. The compound uncertainties on pressure and temperature for high‐Tgranulites are large and the results of our study show that it may be unwise to rely onP–Tconditions determined from the simple intersection of calculated mineral composition isopleths alone. Since the samples in this study are from a limited area—a few hundred square metres—we infer that they record a singleP–Tpath involving both decompression and cooling. However, there is no evidence of the high‐Pgranulite facies event at 1.93–1.90 Ga that is recorded elsewhere in theTNCO, which suggests that the precursor basic dykes were emplaced late during the assembly of the North China Craton.

High‐Tand low‐Pmetamorphism in the Xilingol Complex of central Inner Mongolia, China: An indicator of extension in a previous orogeny

AbstractThe Xilingol Complex comprises biotite gneisses and amphibolite interlayers with extensive migmatization. Four representative samples were documented and found to record either two or three metamorphic stages. Phase modelling usingthermocalcsuggests that the observed assemblages represent the final stages that underwent cooling from temperature peaks, and are consistent with a fluid‐absent solidus inP–Tpseudosections. TheirP–Tconditions are further constrained to be 5–6 kbar/680–725°C and 4–5 kbar/650–680°C for two garnet‐bearing gneiss samples, 4–5 kbar/660–730°C for a cordierite‐bearing gneiss sample, and 4–5 kbar/680–710°C for an amphibolite sample based on mineral composition isopleths, involving measured Mg content in biotite, anorthite in plagioclase, grossular and pyrope in garnet and Ti content in amphibole. The peak temperature conditions recovered are 760–790°C or >760°C at 5–6 kbar based on the composition isopleths of plagioclase, biotite, garnet and especially the comparison of melt contents between the calculated and observed. A pre‐peak heating process with slight decompression can be suggested for some samples on the basis of the core–rim increase in the plagioclase anorthite, and the stability of ilmenite. Zircon U–Pb dating using theLA‐ICP‐MSmethod provides systemic constraints on the metamorphic ages of the Xilingol Complex to be 348–305 Ma, interpreted to represent the post‐peak cooling stages. Moreover, metagabbroic dykes that intruded into the Xilingol Complex yield 317 ± 3 Ma from magmatic zircon, and are considered to have played a significant role for heat advection triggering the high‐Tand low‐Pmetamorphism. Thus, the clockwiseP–Tpaths involving pre‐peak heating, peak and post‐peak cooling recovered for the Xilingol Complex are consistent with an extensional setting in the Carboniferous that developed on a previous orogen in response to addition of mantle‐derived materials probably together with upwelling of the asthenospheric mantle.

High‐Pressure Granulite Facies Overprinting During the Exhumation of Eclogites in the Bangong‐Nujiang Suture Zone, Central Tibet: Link to Flat‐Slab Subduction

AbstractThe geometric transformation of a descending plate, such as from steep to flat subduction in response to a change from normal to overthickened oceanic crust during subduction, is a common and important geological process at modern or fossil convergent margins. However, the links between this process and the metamorphic evolution of the exhumation of oceanic (ultra)high‐pressure eclogites are poorly understood. Here we report detailed petrological, mineralogical, phase equilibria, and secondary ion mass spectrometry zircon and rutile U‐Pb age data for the Dong Co eclogites at the western segment of the Bangong‐Nujiang suture zone, central Tibet. Our data reveal that the Dong Co eclogites experienced peak eclogite‐facies metamorphism (T = 610–630°C, P = 2.4–2.6 GPa) and underwent multiple stages of retrograde metamorphism. P‐T pseudosections and compositional isopleths of garnet define a complex clockwise P‐T‐t path (including two stages of decompression‐dominated P‐T path and one of isobaric heating), suggesting varying exhumation velocities. Combining previous studies with our new results, we suggest that the transformation from rapid to slow exhumation is dominated by the transition from steep to flat subduction. The flat‐slab segment, caused by subduction of buoyant oceanic plateau, led to an extremely slow exhumation and a strong overprinting of HP granulite facies at a depth of ~50 km at ~177 Ma. The slab rollback that followed in response to a substantial density increase of the eclogitized oceanic plateau resulted in another rapid exhumation process at ~168 Ma and triggered the formation of abundant near‐simultaneous or later magmatic rocks.

Pressure–temperature estimates of the blueschists from the Kopina Mt., northern Bohemian Massif, Poland – constraints on subduction of the Saxothuringian continental margin

The blueschist occurrence at the Kopina Mt. is situated at the eastern margin of the Karkonosze–Izera Massif in the West Sudetes of the northern Bohemian Massif. Metabasic rocks with continental crust affinity occur as lenses within the prevailing metasediments. The metabasites consist mainly of garnet, blue amphibole, epidote, chlorite-I, titanite, hematite and quartz forming the high-pressure assemblage. Synkinematic garnet exhibits prograde zoning, a feature allowing for assessment of the prograde course of the pressure–temperature ( P–T ) path. Phase equilibrium modelling has been used to estimate prograde and peak metamorphic conditions. Based on the P–T pseudosections, calculated in the system Na 2 O–CaO–K 2 O–FeO–MgO–MnO–Al 2 O 3 –SiO 2 –H 2 O–TiO 2 –O 2 (NCKFMMnASHTO), the garnet + glaucophane + epidote + chlorite + white mica(?) + Fe oxide + quartz ± titanite assemblage was formed between 12–15 kbar and 480–520 °C. These results are based on garnet and Na-amphibole compositional isopleths. Garnet shows a prograde zoning expressed by the rapid rim-ward decrease of spessartine, moderate increase of almandine and as light increase of other components. It is inferred here that white mica must have decomposed to secondary albite–chlorite-II–K-feldspar. The obtained results point to the formation of the Kopina blueschists along a low-temperature P–T gradient of 8–10 °C/km, typical of rocks from a subduction–exhumation channel. We postulate that the Kopina blueschists were formed in a subduction system developed along the eastern termination of the Saxothuringian Ocean and operating beneath the Tepla–Barrandian upper plate. When combined with previous data on continental crust affinity for the protolith, our results demonstrate a derivation of the Kopina blueschists from subducted part of the Saxothuringian margin.

High-pressure metamorphism in the Chinshuichi area, Yuli belt, eastern Taiwan

Tectonic blocks and slabs of mafic–ultramafic rocks are distributed discontinuously in the Yuli metamorphic belt of Taiwan. The blocks include rare omphacite metagabbros and garnet–epidote blueschists in the Wanjung and Juisui (Tamayen) areas, respectively. Such high-pressure (HP) mineral assemblages have been attributed to a mid-Miocene subduction event. However, the surrounding psammitic, pelitic and chloritic schists are the dominant greenschist-facies lithologies of the Yuli belt. In the Chinshuichi area, tectonic blocks are enclosed in garnet-bearing metapelites, suggesting elevated pressures. In this area, we recently discovered meta-plagiogranite containing the assemblage glaucophane+omphacite (XJd up to 0.39)+rutile+quartz, indicating P–T conditions near 13kbar/550°C. New equilibrium phase modeling of a garnet–paragonite mica schist and compositional isopleths for peak assemblage minerals garnet and phengite (Si=3.33–3.37pfu) indicate metamorphic conditions of 15.5–17kbar/530–550°C. These P–T estimates are higher than previously reported in the Yuli belt and suggest that both tectonic blocks and host metapelites underwent HP metamorphism. The juxtaposition of tectonic blocks and metapelites apparently occurred during the formation of a subduction–accretionary complex, followed by exhumation facilitated by a collisional event. These new findings imply that HP metamorphism was not limited to tectonic blocks, and instead played a significant role attending orogenesis in eastern Taiwan.

Multiple metamorphic episodes recorded in the Paleozoic Pyeongan Supergroup on the northeastern margin of the Yeongnam massif, South Korea: Implications for the Songrim (Indosinian) orogeny

Metasedimentary rocks of the Pyeongan Supergroup, which occur as roof pendants on the northeastern margin of the Yeongnam massif, South Korea, preserve a record of the impact of the Late Permian–Triassic Songrim (Indosinian) orogeny in the eastern Eurasia margin. Metamorphic results of this paper have revealed that the metasedimentary rocks were affected by three metamorphic events during the Middle Permian to Middle Mesozoic. Representative mineral assemblages from three metamorphic zones I, II and III are Cld+And±Ky±St, Grt+St+Ky+Bt±And, and And+Sill+Bt±Grt±St, respectively. Kyanite and sillimanite paramorphs after andalusite occur in zones I and III, respectively. Metamorphic P–T conditions and paths were determined from conventional geothermobarometers and compositional isopleths of chloritoid, staurolite, and garnet on P–T pseudosections in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (MnNCKFMASH) and KFMASH–T (TiO2)–O (Fe2O3) systems. In the study area, the Pyeongan Supergroup experienced medium pressure regional metamorphism (M1 and M2) belonging to greenschist to amphibolite facies followed by thermal metamorphism (M3). The M1 metamorphism occurred at 450–500°C and 2–4kbar and produced the kyanite paramorph after andalusite. The M2 metamorphism occurred at 520–600°C and 4–6kbar, and was associated with the growth of garnet, staurolite, and kyanite. The later thermal metamorphism occurred at ca. 580°C and ca. 4kbar, and was caused by intrusion of Jurassic granitoids. The medium pressure regional metamorphism (M1 and M2) have occurred around 270Ma during the Songrim orogeny based on the SHRIMP U–Pb zircon ages.

Petrology and zircon U–Pb geochronology of metagabbro from the Highland Complex, Sri Lanka: Implications for the correlation of Gondwana suture zones

The basement in the Sri Lankan fragment of Gondwana is composed of upper amphibolite- to granulite-facies rocks that underwent latest Neoproterozoic – Cambrian high-grade metamorphism. A recent tectonic model suggests that the Highland Complex in the central part of the island corresponds to a suture zone formed by the closure of a Neoproterozoic ocean between the Wanni and Vijayan Complexes. In order to test the model and further unravel the complex tectonic evolution of this region, we present new petrological, pressure–temperature, and zircon U–Pb geochronological data on garnet-bearing metagabbros from the Highland Complex. The metagabbros, which occur as blocks or lenses within metasediments of the Highland Complex, contain granulite-facies mineral assemblages of garnet+clinopyroxene+orthopyroxene+ilmenite+hornblende+plagioclase+quartz (sample HC7) and garnet+plagioclase+clinopyroxene+orthopyroxene+pargasite+quartz+rutile+ilmenite (sample HC8). Application of phase equilibria modeling in the system NCFMASHTO indicates peak P–T conditions of 900–950°C/10.5–11kbar and 800–830°C/8.5–9kbar, respectively, and clockwise P–T paths as inferred from decompression textures and mineral compositional isopleths. These conditions are significantly lower than the high-pressure (>17kbar) and ultrahigh-temperature (>1000°C) conditions obtained previously for similar metagabbros from the Highland Complex, whereas our results are consistent with those of similar rocks from the Palghat-Cauvery Suture Zone in India and the Lützow-Holm Complex in East Antarctica. The latest Neoproterozoic to Cambrian metamorphic ages (574±28Ma and 534±14Ma) as well as the available petrological data suggest that the metagabbros in the Highland Complex could be correlated with those in the Lützow-Holm Complex in East Antarctica. The results of this study confirm that the Highland Complex is an extension of the Lützow-Holm Complex, and that both these terranes involve a package of accreted trench sediments and remnant oceanic lithosphere metamorphosed during the final collision related to the Gondwana assembly.

Late Paleoproterozoic medium-P high grade metamorphism of basement rocks beneath the northern margin of the Ordos Basin, NW China: Petrology, phase equilibrium modelling and U–Pb geochronology

Basement rocks from drill cores at the northern margin of the Ordos Basin were investigated by integrating petrography, geochronological analysis and mineral equilibria modelling in the Na2OCaOK2OFeOMgOAl2O3SiO2H2OTiO2O system. The P–T conditions constrained by mineral compositional isopleths on calculated pseudosections indicate that the rocks underwent medium-pressure high grade metamorphism. The pelitic rocks reached granulite facies with peak P–T conditions of 780–810°C at c. 6.3–7.8kbar. SHRIMP U–Pb analyses of detrital zircons indicate Paleoproterozoic protoliths for the basement rocks. NanoSIMS U–Pb analyses of metamorphic overgrowths on zircon grains yield upper intercept ages of 1964±21 and 1960±21Ma for pelitic (QT1-2) and psammitic (LT1-2) samples, respectively. The basement rocks are petrologically, geochronologically and metamorphically comparable with rocks from the Khondalite Belt, which posits the existence of khondalite series rocks beneath the northern margin of the Ordos Basin. Therefore, the Paleoproterozoic mobile belt between the Yinshan and Ordos blocks is likely to be wider than previously thought.