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The Cenozoic collision of India and Eurasia clearly built the high-elevation Tibetan Plateau, but how this collision was accommodated, and the Tibetan Plateau uplifted, remains an area of study. The widespread occurrence of Cenozoic potassic–ultrapotassic lavas provides a valuable opportunity to constrain the relationship between surface deformation and underlying geodynamic processes. In this study, we report sanidine Ar–Ar dating, whole-rock geochemical and clinopyroxene Sr–Nd isotope analyses on pseudoleucite phonolites from the Yulinshan area of south Qiangtang (central Tibet) to determine their petrogenesis and regional tectonic significance. Sanidine Ar–Ar dating yields an Early Oligocene age of ca. 30 Ma, in agreement with previous studies. A high modal abundance of pseudoleucite in the samples indicates their ultrapotassic and silica-undersaturated composition. All of the rocks have typical arc-like geochemical signatures and enriched whole rock and clinopyroxene Sr–Nd isotope signatures. Based on thermodynamic phase equilibria modeling and our detailed study of the textural and in-situ compositional information of clinopyroxene, the intermediate to felsic potassic magmas are interpreted to have been formed through differentiation of primitive, mantle-derived, potassic magmas at shallow crustal levels. The inferred enrichment of the lithospheric mantle is probably related to subduction of the Songpan–Ganze continental lithosphere beneath Qiangtang after India-Asia collision. Voluminous Late Eocene–Early Oligocene magmatism in central Tibet may reflect partial removal of the mantle lithosphere, perhaps triggered by instabilities associated with previous lithospheric thickening. Our work provides additional constraints on the timing of mantle root loss that likely contributed to the surface uplift of central Tibet, which post-dated the cessation of upper crustal shortening and deformation. [ABSTRACT FROM AUTHOR]

The prograde metamorphic P–T conditions of eclogites correspond to the subduction processes of oceanic plates. The conditions may be recorded in mineral inclusions and the compositional zoning of garnets, and they play significant roles in revealing the dehydration depth, geothermal structure of the subduction plate and in understanding arc magmatism. The P–T conditions of the Baqing eclogite have been determined previously using traditional geothermobarometers and Ti-in-zircon geothermometers. To obtain more precise prograde metamorphic P–T conditions and details of the Paleo-Tethyan subduction process, we carried out phase equilibrium modeling for a newly collected Baqing eclogite sample. The sample is characterized by well-developed prograde zonings in metamorphic garnets with increasing Mg and decreasing Mn contents from cores to rims, a peak metamorphic mineral assemblage of garnet + omphacite + phengite + rutile + quartz, and a retrograde metamorphic mineral assemblage of amphibole + plagioclase that replaced omphacite. We calculated the peak metamorphic conditions of the Baqing eclogite using the Garnet–Omphacite–Phengite–Quartz thermobarometers to be 26.5 ± 1 kbar and 730 ± 60°C, consistent with the results (23.5–27.5 kbar, 730–750°C) calculated using a P–T pseudosection. The prograde metamorphic P–T conditions were constrained to be ~21.4 kbar and ~580°C using pyrope and grossular isopleths. These results are consistent with previous calculations for the Baqing eclogite. Eclogites of the Central Qiangtang metamorphic belt, the Baqing metamorphic complex, and Changning–Menglian metamorphic belt show parallel P–T paths with different geothermal gradients, that relate to their different protoliths but similar heating and burial processes during Triassic subduction of the Shuanghu Paleo-Tethyan oceanic plate. [ABSTRACT FROM AUTHOR]

The metamorphism of the Central Qilian Block in the northeastern Tibetan Plateau records a complete tectonic history of the Qilian Orogen. Here, results of structural measurements, geochronological data, and thermobarometry for metamorphic rocks in the Central Qilian Block were presented, which trace the tectonic evolution of the Qilian Orogen. New U–Pb dating of detrital zircon from one paragneiss shows a main age population between c. 1500 and c. 1250 Ma, with the youngest age of 1085 Ma. This unit was intruded by an orthogneiss which has a U–Pb weighted mean age of 920 ± 18 Ma. Together with c. 1200–1000 Ma ages from inherited zircon cores in amphibolites, these results indicate that the protoliths of the Huangyuan Group were formed during the Mesoproterozoic and Neoproterozoic. Rims of these zircons obtain tightly constrained and concordant ages ranging from c. 459 to c. 427 Ma, with a weighted mean age of c. 450 Ma. Phase equilibrium modelling and conventional thermobarometry jointly indicate high‐temperature/medium‐pressure HT–MP metamorphism along a clockwise pressure–temperature (P–T) path at c. 450 Ma, passing through prograde conditions of 7.8–8.0 kbar and 620–650°C to peak conditions of ~7 kbar and ~780–800°C. Together with documented widespread metamorphism, magmatism, and ductile shear belts, these new results reassert that the Central Qilian Block experienced a three‐stage tectono‐metamorphic evolution during the Early Palaeozoic and the HT–MP metamorphism of Huangyuan Group was developed by a continental collision geodynamic setting with coeval mafic and granitic magmatism. [ABSTRACT FROM AUTHOR]

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The thermal histories of Himalayan leucogranites provide critical information for unravelling the post‐collisional geodynamics of the Himalayas. The Ramba Dome is located at the intersection of the Tethyan Himalayan leucogranite belt with the Yadong–Gulu Rift and hosts several generations of granitic intrusions. Of these intrusions, the 8‐Ma two‐mica granites and garnet leucogranite dykes are the youngest of Himalayan leucogranites. In this study, we focus on the carbonaceous staurolite schist located ~1.3 km from the intrusion to constrain the thermal history of the aureole that marked the cessation of leucogranite magmatism. The schist contains euhedral garnet and staurolite porphyroblasts in a foliated matrix of muscovite + biotite + chlorite + plagioclase + quartz + graphite. The staurolite shows minor compositional variations from the inclusion‐free core to the inclusion‐rich rim. By contrast, the garnet features a distinctive bell‐shaped Mn profile and increasing Mg# from the garnet core to rims. In a graphite‐bearing equilibrium phase diagram for a modified bulk composition with garnet cores removed, the garnet rim composition suggests a peak temperature of ~550°C, consistent with an independent thermometer based on the Raman spectra of carbonaceous materials (RSCM; 548 ± 9°C). The P–T condition lies within the narrow low‐variance field bracketed by the staurolite‐in and chlorite‐out boundaries, indicating minimal overstepping of staurolite nucleation and growth. On the other hand, the garnet core composition indicates 520°C at 2.5 kbar, about 40°C higher than the predicted garnet‐in boundary (~480°C). This apparent temperature overstep corresponds to a small chemical affinity (<5 kJ/mol 12 O) for garnet nucleation, comparable to previous estimates. The sharp boundaries of the high‐Ca sector zoning in the core indicate limited diffusion modification (~1.5 Ma if at the peak temperature). The short thermal pulse involves advective heat transfer by leucogranite emplacement, followed by rapid cooling toward the end of Himalayan magmatism and rapid exhumation likely facilitated by the Yadong–Gulu Rift. [ABSTRACT FROM AUTHOR]

Potassium-rich adakitic rocks have been used to infer high-pressure (HP) melting of thickened or foundered mafic lower crust in post-collisional settings. However, their origin remains debated because of their potassic rather than sodic nature. We address this debate by investigating the source of post-collisional Oligocene high-K adakitic granites in southern Tibet, which are widely attributed to melting of overthickened mafic lower crust. Our new data provide evidence for the generation of these high-K adakitic granites by anatexis of exposed migmatitic intermediate to felsic arc orthogneisses. These granites contain high-Th/U Oligocene magmatic zircons (30-22 Ma), and inherited zircons (66-48 Ma) with the same εHf(t) (0 to +12) as the coeval migmatites. The migmatites have in turn low-Th/U metamorphic Oligocene zircon rims around 66-48 Ma magmatic zircon cores recording the anatectic event at 29-25 Ma. Phase equilibrium modeling yields an anatectic temperature of ∼740 °C and pressure (P) of ∼0.9 GPa and shows that garnet is stable at P > 0.7 GPa in the melt-present field. The results indicate that high-K adakitic magmas are derived from melting of older intermediate to felsic arc rocks at intermediate pressures, in the garnet stability field, without any involvement of HP melting of metabasaltic rocks. We propose that hybridization between such purely crustal magmas and subcontinental lithospheric mantle-derived shoshonitic magmas results in the hybridized post-collisional Oligocene-Miocene mafic microgranular enclave-bearing potassic adakitic granitic rocks in southern Tibet. [ABSTRACT FROM AUTHOR]

• UHP eclogites are overprinted by two phases of (U)HT metamorphism in Kunlun-Qaidam Massif. • The overprinting reveals thermal evolution from very high to intermediate and to low P/T type metamorphism. • It records tectonic transition from continental subduction to orogenic collapse. • The Kunlun-Qaidam Massif is a unique diffusive cryptic suture zone. Multiple high-temperature (HT) to ultrahigh-temperature (UHT) overprinting events on continental ultrahigh-pressure (UHP) terranes offer valuable insights into the tectonic evolution from subduction to orogenic collapse. However, the tectonic mechanisms driving the metamorphic evolution remain poorly understood. Herein, we present a detailed study integrating petrography, phase equilibria modeling, and zircon dating of a suite of eclogite, granulized eclogite, and mafic granulite samples from the Eastern Kunlun domain in the Kunlun-Qaidam Massif, North Tibet. Our work reveals three episodes of metamorphism: (1) peak-pressure UHP eclogite-facies metamorphism (>2.7–2.9 GPa, 700–740 °C) at ca. 450–430 Ma; (2) post-peak isothermal decompression under fluid/melt-absent conditions with the formation of symplectites, followed by decompression-heating with evolving P – T conditions from 1.3 to 1.2 GPa/750–780 °C to ∼0.9 GPa/840–870 °C at 423–410 Ma; and (3) continued decompressional heating culminating in UHT conditions of 0.7–0.4 GPa/∼980 °C at ca. 410–400 Ma. Combined with regional magmatic data, a four-stage tectonic model is proposed to explain our results: (a) the Proto-Tethys Ocean subduction during 537–450 Ma; (b) continental subduction and crustal thickening during 450–430 Ma; (c) tectonic underplating and crustal thinning during 430–410 Ma resulting from exhumation and lateral flow of the subducted material beneath a broad arc-backarc region after slab breakoff; and (d) post-collisional extension and orogenic collapse during 410–380 Ma triggered by lithosphere delamination and mantle upwelling. Our model implies that the transition from continental subduction to orogenic collapse occurred within 20 Myr, and also portrays that the formation of a unique diffusive cryptic suture zone characterized by tectonic mixtures of subducted materials and upper-plate sequences across the Kunlun-Qaidam Massif. [Display omitted] [ABSTRACT FROM AUTHOR]

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

The Himalayan-Tibetan orogen represents one of the major Cenozoic tectonic features on Earth, and yet considerable debate continues over the timing and sequence of collisional events leading to its formation. In this contribution, we present new field relations, petrology, geochemistry, geochronology, and phase equilibria modeling in the Gangdese belt of southern Tibet in an effort to address Indo-Asian collisional events in the region. These investigations reveal that the dominantly dioritic Nymo intrusive complex was formed at ca. 50-47 Ma. We establish that the Jurassic-aged Bima volcano-sedimentary sequence underwent early Eocene (50-47 Ma) high-temperature (HT) amphibolite-facies metamorphism. Petrology and phase equilibria modeling of garnet-biotite schists in the Bima rocks reveals mineral assemblages of melt + plagioclase + garnet + biotite + magnetite + ilmenite + sillimanite formed under conditions of 5.3-7.5 kbar and 700-800 °C. We contend that the early Eocene Nymo intrusive complex represents part of the ca. 50 Ma high-flux magmatic "flare-up" that triggered the HT amphibolite-facies metamorphism within the overlying plate during Indo-Asian collision. The synchroneity of high-flux magmatism and HT metamorphism in the Gangdese belt roughly coincided with the continuing Indo-Asian collisional process, implying the early Eocene closure of the Neotethys Ocean along the southern margin of the Lhasa terrane. [ABSTRACT FROM AUTHOR]

Eclogites in the high‐pressure (HP) and ultrahigh‐pressure (UHP) belt record important information about the subduction process and evolution history of the orogenic belt. The Luliangshan eclogites surrounded by granitic gneiss or paragneiss as lenses are exposed in the western segment of the North Qaidam UHP metamorphic belt, northwestern China. Petrology, mineral chemistry, and P–T pseudosection modelling show that the eclogites have experienced a multi‐stage metamorphic process. The peak eclogite‐facies metamorphic stage, is characterized by omphacite in matrix and as inclusion in garnet, with the peak mineral assemblages of garnet + omphacite + rutile + quartz at T > 790°C and P > 25.5 kbar. The initial HP granulite‐facies retrogression is characterized by the symplectite of diopside + plagioclase around omphacite, with P–T conditions of 911°C and 16.9 kbar. The subsequent amphibolite‐facies stage is characterized by amphibole + plagioclase symplectite around the clinopyroxene, with the metamorphic conditions of 674–686°C and 6.4–6.9 kbar. Zircon U–Pb analyses yielded two metamorphic age clusters: (a) HP granulite‐facies metamorphic age of 422–425 Ma, and (b) amphibolites‐facies retrograde age of 397–420 Ma. The protolith of eclogite have geochemical characteristics similar to those of normal mid‐ocean ridge basalt (N‐MORB); and the varying εNd(t) values (−6.3 to 2.1) indicate that the Luliangshan eclogites were derived from a mantle source with rare crustal contamination. Combining these data with previous studies, a multi‐stage tectonic model can be proposed: In the Early Neoproterozoic, the protolith of the Luliangshan eclogites were emplaced into ancient continental crust; during 460–430 Ma, following the oceanic subduction, the subduction of continental crust was dragged by the oceanic slab and continue to subduct towards the Qilian Block, and metamorphosed at the depth of at least 75 km. After a long and slow exhumation process, it returned to the shallow crust. [ABSTRACT FROM AUTHOR]

Amphibolite- and granulite-facies metamorphic rocks are common in the eastern Himalayan syntaxis of southeastern Tibet. These rocks are composed mainly of gneiss, amphibolite and schist that underwent various degrees of migmatization to produce leucogranites, pegmatites and felsic veins. Zircon U–Pb dating of biotite gneiss, leucocratic vein and vein granite from the syntaxis yields consistent ages of ∼49 Ma, indicating crustal anatexis during continental collision between India and Asia. Garnets in these rocks are categorized into peritecitc and anatectic varieties based on their mode of occurrence, mineral inclusions and major- and trace-element zoning. The peritectic garnets mainly occur in the biotite gneiss (mesosome layer) and leucocratic veins. They are anhedral and contain abundant mineral inclusions such as high-Ti biotites and quartz, and show almost homogeneous major-element compositions (except Ca) and decreasing HREE contents from core to rim, indicating growth during the P - and T -increasing anatexis. Peak anatectic conditions at 760–800°C and 9–10·5 kbar are well constrained by phase equilibrium calculations, mineral assemblages, and garnet isopleths. In contrast, anatectic garnets only occur in the vein granite. They are round or subhedral, contain quartz inclusions, and exhibit increasing spessartine and trace-element contents from core to rim. The garnet–biotite geothermometry and the garnet–biotite–plagioclase–quartz geobarometry suggest that the anatectic garnets crystallized at ∼620–650°C and 4–5 kbar. Some garnet grains show two-stage zoning in major and trace elements, with the core similar to the peritectic garnet but the rim similar to the anatectic garnet. Mineralogy, whole-rock major- and trace-element compositions and zircon O isotopes indicate that the two types of leucosomes were produced by hydration (water-present) melting and dehydration (water-absent) melting, respectively. The leucocratic veins contain peritectic garnet but no K-feldspar, have lower whole-rock K2O contents and Rb/Sr ratios, higher whole-rock CaO contents and Sr/Ba ratios, and show homogeneous δ18O values that are lower than those of relict zircons, indicating that such veins were produced by the hydration melting. In contrast, the vein granite contains peritectic garnet and K-feldspar, has higher whole-rock K2O contents and Rb/Sr ratios, lower whole-rock CaO contents and Sr/Ba ratios, and shows comparable δ18O values with those of relict zircons, suggesting that this granite were generated by the dehydration melting. Accordingly, both hydration and dehydration melting mechanisms have occurred in the eastern Himalayan syntaxis. [ABSTRACT FROM AUTHOR]

Despite several decades of investigations, the nature and timing of the India–Asia collision remain debated. In the western Himalaya, the leading edge of the Indian continent was deeply subducted to mantle depths and experienced ultrahigh-pressure metamorphism in the Eocene at c . 50 Ma. In this paper, however, we demonstrate that the North Himalayan metamorphic rocks in the eastern Himalaya underwent Early Eocene (48–45 Ma) medium-pressure (MP) metamorphism due to shallow subduction of the Indian continent beneath southeastern Tibet. The studied garnet–kyanite–staurolite schists occur in the core of the Yardoi gneiss dome, the easternmost North Himalayan Gneiss Dome, and represent the upper structural level of the Higher Himalayan Crystallines (HHC). Petrology and phase equilibria modeling show that these rocks have mineral assemblages of Grt + Pl + Bt + Qz ± Ky ± St ± Ms that were formed under conditions of 7–8 kbar and 630–660 °C. Zircon U–Pb chronology shows that these rocks have peak-metamorphic ages of 48–45 Ma and protracted zircon growth, indicating that the collision between Indian and Asian continents must have occurred at c . 50 Ma in southeastern Tibet. Combining with available data, we suggest that the HHC represents a crustal section of the subducted and subsequently exhumed Indian continent. Due to shallow subduction of the continent during the Eocene, the middle to lower crust of the continent was subducted into depths of 40–60 km and underwent high-pressure (HP) and high-temperature (HT) granulite-facies metamorphism and intense anatexis, whereas the upper crust was buried to shallower depths of 20–30 km and witnessed MP metamorphism and intrusion of leucogranites derived from the lower structural level of the HHC. [ABSTRACT FROM AUTHOR]

Post-collisional granites are widely distributed in continental collision orogenic belts, and deciphering their petrogenesis has great bearings for understanding the deep crustal compositional and tectono-thermal evolution of collisional orogens. Here we carried out a combined study of whole-rock major-trace elements and Sr–Nd–Hf isotopes, and zircon U Pb ages and Lu Hf isotopes for late Paleozoic granites from the North Qaidam orogen, northeastern Tibet. The results indicate that they were derived from partial melting of subducted continental crust at high thermal gradients due to post-collisional rifting orogeny. Zircon U Pb dating of the granites yields weighted mean 206Pb/238U ages between 377 ± 3 Ma and 384 ± 5 Ma, which postdate the ultrahigh-pressure (UHP) metamorphic ages (ca. 438–420 Ma) of deeply subducted continental crust in the North Qaidam orogen. Therefore, they are products of post-collisional magmatism. Most relict zircons in the granites have concordant Neoproterozoic (793–972 Ma) and early Paleozoic (405–442 Ma) U Pb ages, generally consistent with the protolith and metamorphic ages of the UHP metaigneous rocks in the orogen, respectively. The granites are commonly calc-alkaline to high-K calc-alkaline with total alkali contents of 6.89–8.56 wt%, and weakly to moderately peraluminous with A/CNK values of 1.00–1.18 [A/CNK = Al 2 O 3 /(CaO + Na 2 O + K 2 O), in molar]. They exhibit arc-like trace element distribution patterns and enriched Sr–Nd–Hf isotope compositions with whole-rock (87Sr/86Sr) i ratios of 0.7074–0.7104, ɛ Nd (t) values of −9.0 to −4.4 and ɛ Hf (t) values of −4.2 to +2.9, and syn -magmatic zircon ɛ Hf (t) values of −7.6 to +7.7 (except one analysis of −15.0). These granites mostly have comparable Sr Nd isotope compositions to the UHP orthogneiss and continental eclogite in this collisional orogen, indicating their derivation from the subducted continental crust. Using the average compositions of the orthogneiss and continental eclogite in the orogen as the starting materials, phase equilibrium modelling results suggest that the whole-rock major elements of the post-collisional granites can be explained by the mixed melts mainly derived from these two lithologies, which were partially melted at high thermobaric ratios of 1156–1867 °C/GPa associated with different water contents (1–5 wt% for continental eclogite, 1–3 wt% for orthogneiss). Therefore, this study provides integrated geochemical constraints on the source rocks of the post-collisional granites and physicochemical conditions for their genesis. • Post-collisional granites formed at 384–377 Ma are distributed along the North Qaidam orogen • Derived from partial melting of the subducted continental crust at variable crustal levels under different water contents • These granitic rocks have recorded high thermobaric ratios due to post-collisional rifting orogeny [ABSTRACT FROM AUTHOR]

Abstract: Rare kyanite-bearing anatectic paragneisses are found as boudins within sillimanite-bearing paragneisses of the core of the Namche Barwa Antiform, Tibet. In the present study, we document an occurrence from the NW side of the Yarlung Zangbo River. These rocks mainly consist of the assemblage garnet+K-feldspar+kyanite±biotite+quartz+rutile±plagioclase with kyanite locally pseudomorphed by sillimanite. The documented textures are consistent with the rocks having undergone biotite-dehydration melting in the kyanite stability field, under high-P granulite facies conditions, and having experienced melt extraction. However textures related to melt crystallization are ubiquitous both in polymineralic inclusions in garnet and in the matrix, suggesting that a melt fraction had remained in these rocks. Phase equilibria modelling was undertaken in the NCKFMASTHO system with THERMOCALC. P–T pseudosections built with the bulk compositions of one aluminous and one sub-aluminous paragneiss samples predict a biotite–kyanite–garnet–quartz–plagioclase–K-feldspar–liquid–rutile±ilmenite field, in which biotite-dehydration melting occurs, located in the P–T range of ~800–875°C and ~10–17kbar. In addition, the topologies of these pseudosections are consistent with substantial melt loss during prograde metamorphism. A second set of P–T pseudosections with melt-reintegrated model bulk compositions were thus constructed to evaluate the effect of melt loss. The integration of textural information, precise mineral modes, mineral chemistry, and phase equilibria modelling allowed to constrain a P–T path where the rocks are buried to lower crustal depths at peak P–T conditions higher than 14kbar and 825°C, possibly in the order of 15–16kbar and 850°C, followed by decompression and cooling to P–T conditions of around 9kbar and 810°C, under which the remaining melt was solidified. The implications for granite production at the NBA and for Himalayan tectonic models are discussed. [Copyright &y& Elsevier]