Granulite Terrane Research Articles

The Paleoproterozoic magmatic arc of Trivandrum Block, southern India: From Columbia to Gondwana

The southern part of Peninsular India is designated as the ‘Southern Granulite Terrane’ (SGT), the southern domain of which is composed of three Paleoproterozoic crustal segments – the Madras, Trivandrum and Nagercoil blocks. The Trivandrum block is mostly covered by metasupracrustals, termed as khondalites, representing granulite facies metapelites with the basement rocks occurring as NW-SE trending ribbon-like exposures and previously termed as massive charnockites. Here we investigate these basement rocks and identify that they are dominantly composed of charnockites (felsic) and enderbites (intermediate) corresponding to orthopyroxene bearing granulite facies granitoids, together with (meta) gabbro and diorite. We present petrologic, geochemical and zircon U-Pb-Hf and REE data on these rocks. The gabbros are mostly ferroan and calc-alkaline to calcic affinity with tholeiitic parentage. Their REE patterns are relatively flat and both REE and trace element plots indicate an E-MBORB source. The diorites are ferroan and tholeiitic with calcic to calc-alkaline nature and are slightly metaluminous The enderbite corresponds to granodioritic composition with ferroan signature and tholeiitic lineage. The charnockites are high silica granitoids with a transitional feature between magnesian and ferroan. Marked LREE enrichment and steep LREE to HREE slope, HFSE depletion, and other geochemical features suggest arc magmatic affinity in a subduction-related setting. Trace element discrimination plots also suggest subduction-related origin for all the rock types. Zircon U-Pb data show that the gabbro, charnockite and enderbite are broadly coeval with magma emplacement during the Paleoproterozoic at 1990–1962 Ma, and the dominantly positive εHf(t) values (up to + 7.0) indicate formation of new crust from juvenile magmas. The diorites represent a slightly younger phase of magmatism at 1808–1794 Ma involving crustal recycling as indicated by the negative εHf(t) values (-5.8 to −10.5). The construction of the Trivandrum block correlates with the timing of assembly of Columbia. Zircon U-Pb data also date the timing of metamorphism as 590–496 Ma, suggesting long-lived heat input and metamorphism, coinciding with Gondwana amalgamation. A compilation of the zircon U-Pb-Hf data from this block shows a major pulse of magmatism at ca. 2.1 to 1.9 Ga that involved building of new crust from juvenile magmatic sources. In contrast, the younger pulse which lasted up to 1.7 Ga dominantly involved the reworking of Paleoproterozoic as well as the vestiges of ancient Meso-Neoarchean crust. Thus, the Trivandrum Block records the gradual maturity of a subduction-related magmatic arc within a Paleoproterozoic convergent margin regime. The lithologic assemblages from the basement of Trivandrum block trace the history from the Paleoproterozoic Columbia to the late Neoproterozoic – Cambrian Gondwana supercontinents.

Assembly and Tectonic Evolution of Continental Lower Crust: Monazite Petrochronology of the Ivrea‐Verbano Zone (Val Strona di Omegna)

AbstractMetasediments are common constituents of exhumed lower‐to‐mid‐crustal granulite terranes; understanding their emplacement is significant for the assembly and tectonic evolution of deep continental crust. Here, we report a monazite U‐Th‐Pb petrochronological investigation of the Variscan Ivrea‐Verbano Zone (IVZ) (Val Strona di Omegna section)—an archetypal section of lower crust. Monazite Th‐Pb dates from 11 metapelitic samples decrease with structural depth from 310 to 285 Ma for amphibolite‐facies samples to <290 Ma for granulite‐facies samples. These dates exhibit a time‐resolved variation in monazite trace‐element composition, dominated by the effects of plagioclase and garnet partitioning. Monazite growth under prograde to peak metamorphic conditions began as early as 316 ± 2 Ma. Amphibolite‐facies monazite defines a trend consistent with progressively decreasing garnet modal abundances during decompression and cooling starting at ∼310 Ma; the timing of the onset of exhumation decreases to ∼290 Ma at the base of the amphibolite‐facies portion of the section. Structurally lower, granulite‐facies monazite equilibrated under garnet‐present pressure‐temperature conditions at <290 Ma, with monazite (re)crystallization persisting until at least ∼260 Ma. Combined with existing detrital zircon U‐Pb dates, the monazite data define a <30 Myr duration between deposition of clastic sediments and their burial and heating, potentially to peak amphibolite‐to‐granulite‐facies conditions. Similarly brief timescales for deposition, burial and prograde metamorphism of lower crustal sediments have been reported from continental magmatic arc terranes—supporting the interpretation that the IVZ represents sediments accreted to the base of a Variscan arc magmatic system >5 Myr prior to the onset of regional extension and mafic magmatism.

Timing of UHT metamorphism and cooling in south Indian granulites: New P-T-t results from a sapphirine granulite

The Precambrian granulite terrane of south India is known for the preservation of regional-scale high to ultrahigh-temperature (HT-UHT) metamorphic rocks. In the Madurai block, the largest crustal block in the terrane, a significant proportion of these HT-UHT rocks occur along a narrow belt referred to as the Kambam ultrahigh-temperature belt. Understanding the P-T-t evolution of these HT-UHT granulites is vital in decoding the spatial and temporal evolution of the Madurai block in relation to other crustal blocks. Here we present the petrology, mineral chemistry, phase equilibrium modelling, and accessory mineral (zircon, monazite, rutile and apatite) geochronological and geochemical data for a hitherto unreported sapphirine granulite from the Kambam ultrahigh-temperature belt. Combined mineral reaction and phase equilibrium modelling indicate extreme P-T conditions up to 1130 °C at 11 kbar. The geometry of the P-T path is clockwise with initial isothermal decompression followed by near isobaric cooling. LA-ICPMS U-Pb geochronological studies on accessory phases report two distinct thermal events: (i) Paleoproterozoic high-T event and associated crustal anatexis at ∼2.5 Ga from zircons and (ii) Neoproterozoic UHT event at ∼550 Ma from monazites. The monazite ages suggest a 50–60 Ma prolonged thermal event, attributed to Ediacaran-Cambrian collisional orogenesis. Additional metamorphic pulses are also identified, demonstrating the polydeformed crustal evolution history of the terrane. U-Pb age dating of rutile and apatite in the sample provide ages of ∼458 Ma and ∼392 Ma and cooling rates of ∼6 °C/Ma and ∼0.3–2 °C/Ma respectively. This suggests a complex history of slow cooling followed by ultraslow cooling. The results bring forth the significance of the Kambam ultrahigh-temperature belt in understanding the tectonothermal evolution of Madurai block and provide additional evidence for long-lived Neoproterozoic UHT orogenesis in south India.

Deformation mechanisms and characteristics of the meta-BIFs from an early Proterozoic shear system of the Southern Granulite Terrane (SGT), India

We studied three samples of Banded Iron Formation (BIF) rocks from Southern Granulite Terrane (SGT) of India, which was metamorphosed at high-pressure granulite facies and exhumed from 40 to 50 km depth. We examined the microstructure, texture, and deformation mechanisms of the primary minerals (quartz, magnetite, and hematite). Microstructures, variations in size and shape of quartz grains suggest that the three samples deformed to different intensities and temperatures. The EBSD derived CPOs for quartz and magnetite grains are strong, implying dislocation creep as the dominant mechanism. The presence of distinct subgrains in magnetite, rare in natural samples, attests to the same. Furthermore, twinning on {111}, as evident from the misorientation angle and axis distributions, indicates ductile deformation of magnetite below 400 °C. Subgrain boundary trace analysis in selected magnetite grains reveals that multiple-slip planes {111}, {110}, and {100} were operational. Topotaxial replacement, via redox reaction, of magnetite grains along their octahedral planes by hematite is evident. Well-developed CPOs but low (<1°) GOS values of both magnetite and hematite grains suggest that recovery processes in these were rapid. Post-replacement deformation was weak and did not affect the hematite CPOs that were inherited from the relatively more deformed host magnetite grains.

Соотношения петрологического коро-мантийного перехода и сейсмического раздела Мохо под гранулитовыми террейнами: признаки преобразований корневой части Восточно-Тункинского блока в глубинных нодулях из позднекайнозойских вулканических пород

From compositions of garnet-bearing and garnet-free assemblages of deep-seated nodules brought to a surface by Cenozoic and older (Phanerozoic) magmatic melts, petrological zones of crust-mantle transition (PZСMT) are defined under granulite terranes of craton and non-craton regions. Present-day Moho discontinuities only partially coincide with petrological estimates of a change from felsic-basic rocks (that belong to the continental crust) by predominantly ultramafic rocks (that represent the continental mantle lithosphere) and often lie much deeper than the PZСMT. Depths of such zones change over time. For a garnet-free assemblage of deep-seated nodules ejected by basaltic melts about 13 Ma ago from the root of a granulite terrane exposed in the eastern part of the Tunka Valley, two PT trends were obtained, one of which corresponds to a high (up to 120 mW/m2) rift conductive geotherm, another one crosses low conductive geotherms (drops below 60 mW/m2 one). The PZСMT shows here a temperature approximately 200°C lower than the PZСMT of granulite terranes in Eastern Australia, China, and Svalbard. Deep-seated nodules characterize the development of hot transtension under the rift valley in the cold root part of the East Tunka block with the accumulation and release of elastic stresses accompanied by significant synkinematic (metasomatic and magmatic) processes in the time interval 18–12 Ma ago. The transtension was followed by a crustal transpression with inversional uplift of an area and a probable relative increase in the depth of the Moho discontinuity, determined from the both P- and S-wave velocities for the modern crust and lithospheric part of the mantle.

Formation and evolution of lower crustal granulite terrane in the southern North China Craton

研究前寒武纪地体的构造-岩浆事件和变质作用过程是揭示早期陆壳的生长与演化等热点问题的重要途径。目前，尽管已经开展了大量的相关研究，但关于华北克拉通南缘前寒武纪变质地体的成因、构造背景仍存在争议，限定了我们对于华北克拉通南缘前寒武纪形成和演化的认识。本文根据前人研究成果，包括华北南缘太华杂岩的岩相学、地球化学、年代学以及变质<i>P-T-t</i>轨迹等，结合对鲁山地区太华杂岩的全岩主微量、长石矿物原位Pb同位素组成特征以及基性麻粒岩的年代学等研究成果，系统梳理并探讨了太华杂岩代表的新太古代-古元古代下地壳物质组成、岩石成因以及多阶段构造-岩浆事件。结果显示华北南缘记录了2.92~2.71Ga和2.53~2.45Ga两期重要的地壳生长事件，2.35~2.30Ga和2.19~2.06Ga的陆壳再造，以及1.96~1.84Ga古元古代的变质事件。再结合华北克拉通中部造山带中、北部的变质程度、变质<i>P-T</i>轨迹和变质年龄，我们认为华北南缘前寒武纪基底与中部造山带的中、北部相似，构造归属于中部造山带，发生了一个较长的俯冲碰撞造山过程（1.96~1.86Ga）。

Neoarchaean crustal evolution along the eastern flank of Nallamalai Shear Zone, southern India

ABSTRACT The Southern Granulite Terrane (SGT) in India is composed of Archaean to Proterozoic crustal blocks and intervening shear/suture zones. The Nallamalai shear zone lies along the eastern flank of the Shevaroy Block. This study is focused on the eastern part of this shear zone where the basement rocks are dominantly composed of charnockite, quartzo-feldspathic gneiss and metagabbro, all of which preserve original magmatic texture. Zircon U-Pb LA-ICPMS analysis on the charnockite samples yielded an age of 2680 to 2500 Ma for the protolith emplacement followed by metamorphism at 2520 to 2450 Ma. Magmatic zircon grains in the quartzo-feldspathic gneiss and metagabbro yielded weighted mean ages of ca. 2557 Ma and ca. 2583 Ma with metamorphism at ca. 2518 Ma. Zircon Lu-Hf data suggest that the protolith of metagabbro was sourced mainly from the recycled Mesoarchaean crustal component. The ƐHf (t) of charnockite (−0.4 to 4.1) and quartzo-feldspathic gneiss (0.5 to 6.0) represent a combined source of the mantle as well as recycling crust. All the rocks underwent amphibolite to granulite facies metamorphism and petrological studies and pseudosection modelling shows a similar P-T range of 7–8 kbar and 650–830°C. Results from the present study suggest that crustal evolution along the eastern flank of the Nallamalai Shear Zone involved melting of older crustal components (Mesoarchaean age) with significant juvenile input (ƐHf (t) = ~+6.0) within a continental arc setting.

A Gondwanan micro-fragment adjacent to southern granulite terrane of India: Evidence from satellite gravity studies

The Southern Granulite Terrane (SGT) of Peninsular India is a complex collage of Mesoarchean to Late Neoproterozoic – Cambrian tectonic units, which were part of Earth's major supercontinents such as Ur, Columbia, Rodinia, Gondwana and Pangea. We analyzed high-resolution satellite gravity field with a view to demarcate the underlying crustal and lithospheric mantle structure of the SGT. The western part of SGT is characterized by a high-order elongated negative residual gravity anomaly of −80 mGal together with a thicker Moho (43 km) and the lithosphere (~130 km). In comparison, in the central and eastern parts of SGT, the gravity anomalies are distinctly positive, Moho is shallow (~36 km) and the lithosphere is thin (~100 km). An important finding of the present study is the delineation of a fragmented micro-continental block of the SGT, submerged under Indian Ocean, and located about 30 km west of the rifted continental margin of India, which is characterized by moderate crustal seismicity, and high-order gravity low of −80 mGal, identical to the features of western SGT. This distinct micro-block might represent a vestige of the rifting between India and Madagascar at around 90 Ma due to upwelling of the Marion mantle plume and marking the final breakup of the Gondwana assembly.

Insights into the crustal architecture from the analysis of gravity and magnetic data across Salem-Attur Shear Zone (SASZ), Southern Granulite Terrane (SGT), India: an evidence of accretional tectonics

Insights into the crustal architecture from the analysis of gravity and magnetic data across Salem-Attur Shear Zone (SASZ), Southern Granulite Terrane (SGT), India: an evidence of accretional tectonics

Combined U-Pb/Hf isotopic studies and phase equilibrium modelling of HT-UHT metapelites from Kambam ultrahigh-temperature belt, south India: Constraints on tectonothermal history of the terrane

The Madurai block which forms the central crustal block in the Precambrian Southern Granulite Terrane, south India preserves crustal evolution history with record of multiple magmatic and metamorphic episodes ranging from early Archaean to late Neoproterozoic. Documenting the precise chronology and tectonothermal evolution of these events are of paramount importance to correlate with other crustal blocks in Southern Granulite Terrane as well as to link with other continental fragments of East Gondwana. The present study focuses on hitherto unreported location within a key section of Madurai block along the transcrustal Suruli shear zone known as the Kambam ultrahigh-temperature belt. This belt is noticeable for the proliferous occurrence of HT-UHT metapelites with an overall clockwise P-T trajectory. Samples of garnet-spinel granulite, cordierite-spinel granulite and associated granite gneiss were subjected to detailed petrographic, thermobarometric and geochronologic analysis. Combined petrography, mineral reaction, geothermobarometry and pseudosection modelling of the granulite samples reveal HT-UHT metamorphic conditions with clockwise P-T path demonstrating significant decompression and cooling. LA-ICPMS U-Pb dating of zircon, monazite and rutile phases within these samples constrain the timing of metamorphism as well as cooling. U-Pb zircon ages of ~2500 Ma from the granulites points to new evidences for Paleoproterozoic high-grade metamorphism in the area where monazite U-Pb ages at ~593 and ~557 Ma throw light into the imprints on ~36 Ma prolonged Neoproterozoic thermal event attributed to Pan African collisional orogeny. LA-MC-ICPMS Hf isotopic record from zircon cores point to the role of significant juvenile crustal melting/anatexis during Paleoproterozoic. In addition, several cryptic metamorphic pulses are also identified evincing the polydeformed evolutionary history of these rocks. The ca. ~432 Ma cooling age record from rutile estimates an average cooling rate of ~3.0 °C/Ma over a time period of ~125 Ma, classifying Madurai block as a classic slow cooled granulite terrane with long lived UHT orogenic history. The results also bring to the fore the importance of Kambam ultrahigh-temperature belt and Suruli shear zone which can be projected as a major terrane boundary within the Southern Granulite Terrane.

Neoarchean ultrahigh-temperature sapphirine granulites from the Karimnagar Granulite Terrane, Southern India: Implications for hot orogen along the Eastern Dharwar Craton margin

We have uncovered evidence of Neoarchean ultrahigh-temperature (UHT) metamorphism in the Karimnagar Granulite Terrane (KGT), southern India, based on new petrological and mineralogical data of sapphirine-bearing and related granulites. We investigated their petrogenesis and implications of a hot orogen along the north-eastern margin of the Eastern Dharwar Craton (EDC). We found various reaction textures, such as spinel mantled by sapphirine and hydration of orthopyroxene, indicating a peak mineral assemblage of cordierite + sapphirine + spinel + orthopyroxene + biotite + rutile + K-feldspar. Phase-equilibria modelling in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 system indicates peak metamorphic conditions of 905 °C–910 °C and 4.5–4.6 kbar. The peak stage was followed by a retrograde metamorphic stage represented by increasing modal abundance of biotite and consumption of rutile and spinel at ~730 °C and ~ 4 kbar, suggesting near-isobaric cooling. Using the chemical Th–U-Pbtotal isochron method, we dated monazites in textural association with sapphirines and cordierites in a sapphirine–spinel–orthopyroxene–cordierite granulite. The resulting Neoarchean age (2638 ± 44 Ma) probably represents the timing of peak UHT metamorphism. We infer that Neoarchean UHT metamorphism and post-peak near-isobaric cooling took place in the terrane during the terminal stage of complex accretion–collision tectonic processes related to amalgamation of microcontinents and/or magmatic arcs that occurred during 2.70–2.62 Ga, one of the major episodic crust-forming events in the EDC. Our findings and regional distribution of lithologies overturn the existing tectonic model of the KGT as a collisional suture zone between the EDC and the Bastar Craton. Moreover, we propose that the KGT is a discrete tectonic block composed of ~2.7–2.6-Ga granulites and younger granite representing the mid-crustal segment of the EDC, which was uplifted as a tilted tectonic block along the southern flank of the Kadam outlier.

Beach placers of southwestern India: An archive of Precambrian supercontinent growth histories

The southwestern coast of India is known for its rich beach placer deposits. So far, there have been no substantial attempts to determine the provenance of these deposits on a regional scale. Here we present for the first time geochronology, trace element chemistry and Hf isotope systematics of zircon and monazite from representative beach placer deposits along the southwestern coast of India to constrain provenance. Detrital zircon grains display a wide variety of ages, with modes in the ranges of 650–450 Ma, 1100–650 Ma, 2300–1600 Ma, 2800–2300 Ma and 3500–2800 Ma. A high proportion of ages fall within the Neoproterozoic–Late Cambrian range (1066–490 Ma). Growth zoning in zircon grains shows magmatic growth of many generations with ages affected by Neoproterozoic Pb loss, along with overgrowths typical of those formed during high-T metamorphism. Grains of metamorphic zircon, along with grains of monazite from the same samples of sediment, have strong modes between 570 and 470 Ma, indicating derivation from sources in the Southern Granulite Terrane affected by a major Late Neoproterozoic to Cambrian tectonothermal event. Mineral geochemistry of zircon grains indicates continental derivation, especially with older zircon from TTG-type sources. The geochemistry of late Neoproterozoic–Cambrian zircon and monazite are consistent with derivation from amphibolite to granulite-facies source rocks. Zircon ages and Hf isotope systematics can be ascribed to provenance from various source terranes exposed along the Western Ghats. This young rift-flank escarpment formed along the coast at a high angle to the distribution of different Precambrian terranes in the Indian Peninsula. Erosion promoted by a tropical climate and a steep coastal geomorphology, in combination with strong southward along-shore ocean currents, led to the concentration and distribution of heavy minerals from multiple sources along southwestern coast of India. The relative intensity of age modes in detrital zircon differs between localities, demonstrating differing inputs from proximal sources (from rivers dissecting steep terrain) versus distal sources from more northerly cratons and terranes brought to deposition sites by along-shore drift.

The Tectonic and Metamorphic Perspective of Southern Granulite Terrane, India

The Tectonic and Metamorphic Perspective of Southern Granulite Terrane, India

Causal relationship between mafic magma underplating and migmatization of arc crust: Evidence from the Madras block of Southern Granulite terrane, India

Causal relationship between mafic magma underplating and migmatization of arc crust: Evidence from the Madras block of Southern Granulite terrane, India

Petrology and geochronology of a suite of meta-supracrustal rocks from Madukkarai, Tamil Nadu: Implications for the Ediacaran-Cambrian orogenesis of the Granulite Terrane of South India

The Granulite Terrane of South India (GTSI) has two major components. An orthogneiss dominated northern block of Archaean/early Palaeoproterozoic age and an Ediacaran-Cambrian, high to ultra-high temperature meta-supracrustal dominated southern block (the Madurai Block, MB). It is commonly perceived that the two blocks of GTSI came into juxtaposition along the Palghat-Cauvery Shear/Suture Zone (PCSZ) during a continent-continent collision. However, the timing and the nature of the contact relations between the two major components remains controversial. In this work, detail petrological and geochronological study of two metapelites along with a meta-psammite (only for geochronological study), from the Madukkarai Supracrustal Unit (MSU) within the PCSZ, are presented. The studied metapelites show two distinct assemblages:A.Garnet + biotite + sillimanite (both idioblasts and pseudomorphs after kyanite) + K-feldspar + plagioclase + quartz + ilmenite (rutile, zircon and monazite as accessory phases).B.Mineralogy is similar to A but contains cordierite and minor spinel, but no zircon.Both the metapelites develop a gneissic fabric that is defined by centimetre thick laterally discontinuous garnet-bearing leucosomes. ‘Frozen in’ reaction textures, phase equilibria modelling and mineralogical thermobarometry yielded “peak” temperature and pressure of metamorphism in the range of ~730–790 °C, ~5.5–7 kbar (considering sillimanite and ilmenite to be stable). Subsequently, the rocks followed a decompressive retrograde P-T path recorded till ~630 °C and ~4 kbar. The geometry of the construed retrograde P-T path is similar to that commonly found in rocks of a continent-continent collision zone. LA-ICP-MS UPb zircon dating and U-Th-total Pb monazite dating constrain the major high-grade metamorphism and deformation during ~550–520 Ma with vestiges of ~600 Ma old tectonothermal/ thermal event. The geological history of the MSU shares many characteristics with the Ediacaran-Cambrian meta-supracrustal rocks lying on both sides of the PCSZ, in terms of lithological association, timing of protolith formation, deformation and metamorphism. The variation in intensity of metamorphism in the meta-supracrustal rocks of the GTSI could be linked to differential upliftment of the GTSI during the neotectonic activities, followed by erosion. Integrating all the geological, geochronological and geophysical information, it has been argued that the GTSI behaved as a coherent continental block that was involved in continent-continent collision during the formation of the Ediacaran-Cambrian supercontinent Gondwanaland. The findings of this study do not corroborate the view that PCSZ is an Ediacaran-Cambrian terrane boundary.

Paleoproterozoic emplacement and Cambrian ultrahigh-temperature metamorphism of a layered magmatic intrusion from the Central Madurai Block, southern India: From Columbia to Gondwana

The Madurai Block in the Southern Granulite Terrane (SGT) of Peninsular India is one of the largest crustal blocks within the Neoproterozoic Gondwana assembly. This block is composed of three sub-blocks: the Neoarchean Northern Madurai block, Paleoproterozoic Central Madurai block and the dominantly Neoproterozoic Southern Madurai Block. The margins of these blocks are well-known for the occurrence of ultrahigh-temperature (UHT) granulite facies rocks mostly represented by Mg-Al metasediments. Here we report a dismembered layered mafic–ultramafic intrusion occurring in association with Mg-Al granulites from the classic locality of Ganguvarpatti in the Central Madurai Block. The major rock types of the layered intrusion include spinel orthopyroxenite, garnet-bearing gabbro, gabbro and gabbroic anorthosite showing rhythmic stratification and cumulate texture. The orthopyroxene-cordierite granulite from the associated Mg-Al layer is composed of spinel, cordierite and orthopyroxene. The pyroxene in both rock units is high-Al orthopyroxene formed under UHT metamorphic conditions. Conventional thermobarometry yields near-peak metamorphic conditions of 9.5–10 kbar pressure and a minimum temperature of 980 °C. We computed P–T pseudosections and contoured for the compositional as well as modal isopleths of the major mineral phases, which yield temperature above 1000 °C. FMAS petrogenetic grid, Al-in-orthopyroxene isopleth, conventional thermobarometry and calculated pseudosection reveal a clockwise pressure–temperature (P–T) path and near isothermal decompression. The U–Pb data on zircon grains from the layered magmatic suite indicate emplacement of the protolith at ca. 2.0 Ga and the metamorphic overgrowths yield weighted 206Pb/238U mean ages ca. 520 Ma. Monazite from the garnet-bearing gabbro and Opx-Crd granulite yielded 206Pb/238U weighted mean ages of ca. 532 Ma and 523 Ma marking the timing of metamorphism. We correlate the layered intrusion to a Paleoproterozoic suprasubduction zone setting, defining the Ganguvarpatti area as part of a collisional suture assembling the Northern and Central Madurai Blocks. The Paleoproterozoic magmatism and late Neoproterozoic-Cambrian UHT metamorphism can be linked to the tectonics of the Columbia and Gondwana supercontinents.

Phase equilibria modeling of anatexis during ultra-high temperature metamorphism of the crust

Ultra-high temperature (UHT) granulite-facies metamorphism is commonly identified in Mg–Al-rich rocks. Many such lithologies are thought to be metasedimentary rocks based on the presence of detrital zircon. However, whether a metasedimentary protolith is required to form diagnostic mineral assemblages and how much melt is lost during burial and heating to reach UHT conditions varies significantly according to the prograde pressure–temperature path. In this study, integrated thermodynamic modeling, accessory mineral modeling, and trace element modeling has been conducted on an average metapelite composition and average mid-ocean ridge basalt (MORB) composition under open-system conditions. The results show at least three and two melt loss events occur before reaching UHT conditions in initially water-saturated pelitic and basaltic rocks, respectively. Around 22–27 vol% S-type granite and 12–17 vol% I-type granite would be produced during these evolutions, respectively. Melt extraction leads to an increase of Al and Mg in metapelite residua. Mineral assemblage sapphirine + quartz is then allowed to form at higher grades; however, metabasalt does not form these UHT diagnostic assemblages, regardless of the amount of melt lost. During equilibrium melting, crustal differentiation induced by UHT metamorphism would significantly reduce the amount of heat-producing elements (HPEs) in the melt-depleted residuum, which progressively decreases a rock's heat production capacity. In reality, however, new monazite grains likely form during peak metamorphism and some monazite grains would be shielded by porphyroblasts, resulting in an increase of Th in the residua during heating. Even so, we suggest the efficiency of heat production would still decrease, given the heat production rate of Th is much lower than U. Considering some granitic rocks with relatively high heat production are emplaced into the granulite terrane, we suggest that radioactive heat production may be a contributing driving force for UHT metamorphism; however, it is not sufficient. Most heat required to generate UHT granulites must come from hybrid sources, such as advected heat from the mantle, conducted heat from nearby magmatic intrusions, radioactive heat production, and ductile shear deformation.

Were South India, the North China Craton, and the Korean Peninsula contiguous in a Neoarchaean supercontinent? New geochemical and isotopic constraints

The composition and configuration of possible Archaean supercontinents remain unresolved. Kenorland, a Neoarchaean supercontinent containing the Southern Granulite Terrane (SGT) in South India, the eastern block of the North China Craton (E-NCC), and the north-central Korean Peninsula, was probably assembled at ca. 2.5 Ga. A detailed comparison of meta-granitoid samples from the Madras Block (SGT), the Yishui Terrane (Shandong Peninsula, E-NCC), and Daeijak Island (NW-Gyeonggi Massif, Korean Peninsula) demonstrates their close similarities in geological setting, age, petrochemistry, isotopic composition and metamorphic history. They were all formed at 2.6–2.5 Ga and metamorphosed at a high grade soon after ca. 2.5 Ga. All are LREE-enriched and HREE-depleted, have low 87Sr/86Sri (0.70201–0.70375) and similar near-chondritic ƐNd(T) (+1.2 to −1.9). These factors, and their close match of geological features, suggest that the three terranes were once contiguous as part of a Neoarchaean supercontinent.

Metamorphic and geochronological evolution of Paleoproterozoic high-pressure ultra-high-temperature pelitic granulite, Chicheng, northern Trans-North China Orogen

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

HP–UHT granulites in the East Kunlun Orogen, NW China: Constraints on the transition from compression to extension in an arc setting of the Proto‐Tethys Ocean

AbstractThe East Kunlun Orogen (EKO) of the northwest part of the Central China Orogenic belt records long‐lived orogenic events from the Cambrian to the Triassic, involving the tectonic evolution from Proto‐Tethys to Palaeo‐Tethys. Although high‐ and ultra high pressure (HP‒UHP) metamorphism has been recognized in the EKO, other metamorphic events associated with the orogenic history are little known. The metamorphic history of high‐ and ultra high temperature granulites and their pressure–temperature–time (P‒T–t) path is keys to reconstructing tectonic events associated with a subduction‐to‐collision transition. In this contribution, we present P‒T–t results for the mafic and pelitic granulite facies metamorphic rocks in the Qingshuiquan region of the EKO, determined using phase equilibrium modelling and zircon geochronology. The Jingshuikou granulite terrane records a clockwise P–T path with high‐P and ultra high temperature (HP–UHT) conditions at Pmax ~900 to 950°C at 13 kbar, at Tmax of ~950°C at 9‒12 kbar, and a final stage of 800‒895°C at 6‒10 kbar. LA‐ICP‐MS U–Pb dating of zircons provides systemic constraints on a long‐lived metamorphic age span of 530‒465 Ma, with a peak of c. 503 Ma. The HP–UHT metamorphism of the Qingshuiquan granulite terrane reveals a complete process from arc compression at the early stage of subduction to extension on the active continental margin related to rollback of the subducted slab in the Early Palaeozoic, driven by heat transfer from upwelling of the sub‐arc hot asthenosphere. The timing and P‒T conditions of the high geothermal gradient metamorphism can be used to understand the subduction and collision process of the Proto‐Tethys Ocean, including the relationship between high‐P and high‐ to ultra‐temperature metamorphism in the East Kunlun and other orogenic belts.