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The Lhasa terrane of the southern Tibetan Plateau participated in a Mesozoic Andean-type orogeny caused by the northward subduction of the Neo-Tethyan oceanic lithosphere. However, metamorphic rocks, which can unravel details of the geodynamic evolution, are rare and only exposed in the south-eastern part of the Lhasa terrane. Therefore, we conducted a detailed petrological, geochemical and U–Pb zircon geochronological study of the late Cretaceous metamorphic rocks and associated gabbros from the Nyemo inlier of the southern Lhasa terrane. The Nyemo metamorphic rocks including gneisses, schists, marbles and calc-silicate rocks, experienced peak amphibolite-facies contact metamorphism under P–T conditions of 3.5–4.0 kbar and 642–657 °C with a very high geothermal gradient of 45–50 °C/km, revealing a distinct deflection from the steady-state geotherm during low-pressure metamorphism. Inherited magmatic zircon cores from the metamorphic rocks yielded protolith ages of 197–194 Ma, while overgrowth zircon rims yielded metamorphic ages of ca. 86 Ma. Whole-rock chemistry and zircon Hf isotopes suggest that the protoliths of the gneisses and schists are andesites and tuffs of the early Jurassic Sangri Group, which were derived from a depleted mantle source of a continental arc affinity. The coeval intimately-associated gabbro (ca. 86 Ma) crystallized under P–T conditions of 3.5–5.3 kbar and 914–970 °C, supplying the heat flux high enough to cause the contact metamorphism of the Sangri Group rock types. We propose that the intrusion of the gabbro and a simultaneous pressure increase of up to 4.0 kbar, which is related to crustal thickening due to crustal overthrusting and the intrusion of mafic material, resulted in the late Cretaceous metamorphism of the early Jurassic Sangri Group during an Andean-type orogeny. Furthermore the Nyemo metamorphic rocks, which have previously been considered to represent slivers of the Precambrian metamorphic basement of the Lhasa terrane, are late Cretaceous metamorphic supracrustal rocks. [ABSTRACT FROM AUTHOR]

Abstract: The Lhasa terrane in southern Tibet experienced Late Paleozoic and Mesozoic–Cenozoic composite orogenesis. This work reports a study on the petrology, geochemistry, zircon U–Pb chronology and Hf isotopes of Late Paleozoic and Late Mesozoic intrusive rocks from the southeastern Lhasa terrane. The Late Paleozoic intrusive rocks crystallized in the Late Devonian–Early Carboniferous of 371 to 355Ma, representing a bimodal igneous association formed in the back-arc extensional setting. The mafic end-member originated from the enriched mantle and experienced contamination of crustal materials, characterized by a slight enrichment of LREE, positive anomalies of U, K and Pb and negative anomalies of Th, Nb, Ta and Ti. The felsic end-member was derived from the partial melting of the ancient continental crust, characterized by metaluminous, positive anomalies of Th, Zr and Hf, negative anomalies of Ba, Sr, Nb, Ta and Ti and negative εHf(t) values of zircon with TDM2 ages from 1.90 to 1.40Ga. The Late Cretaceous (ca. 107Ma) mafic intrusions, along with the Late Paleozoic intrusive rocks, underwent nearly syn-intrusion amphibolite-facies metamorphism under P–T conditions of 0.56 to 0.69GPa and 692 to 735°C during the Andean-type orogeny correlated with the subduction of the Neo-Tethyan oceanic slab beneath the Lhasa terrane. This study provides a new insight into the pre-Cenozoic tectonic evolution of the Lhasa terrane. [Copyright &y& Elsevier]

The Nyingchi complex, forming the eastern segment of the Gangdese magmatic arc, occurs within the southern Lhasa terrane in south Tibet, and is composed dominantly of plutons and their metamorphosed equivalents. Together with some metasedimentary units these rocks record multiple Mesozoic and Cenozoic magmatic and metamorphic events during northward subduction of Neo-Tethyan oceanic lithosphere beneath the Eurasian continent. Petrological and geochronological studies reveal that the Nyingchi complex experienced intense Paleocene subduction-related magmatism and almost synchronous granulite-facies metamorphism accompanied by the formation of S-type granites. Subduction-related I-type granitoids show geochemical features typical of continental magmatic arcs; zircon separates from them yield 206Pb/238U ages from c. 65 to c. 56 Ma, and commonly display positive ϵHf(t) values ranging from −1·7 to +13·0. The occurrence of magmatic epidote, as well as the syn-intrusion high-grade metamorphism, indicates that the plutons were emplaced at middle to lower crustal depths within the Lhasa terrane. Associated S-type granitoids are peraluminous and contain garnet and muscovite; their zircons yield 206Pb/238U ages ranging from c. 66 to c. 55 Ma, and these have distinct but mostly negative ϵHf(t) values from −18·4 to +2·1. The zircons from the associated metasedimentary rocks include both detrital and metamorphic types; the detrital zircons yielded variable inherited 206Pb/238U ages ranging from c. 2910 to c. 235 Ma, constraining the maximum depositional age to the Triassic. The metamorphic zircons from the metaplutonic and metasedimentary rocks yielded ages from c. 67 to 52 Ma. Phase equilibria modeling shows that the Nyingchi complex experienced peak granulite-facies metamorphism and partial melting under conditions of 800–830°C and 9–10·5 kbar, and then cooled isobarically to c. 700°C in the lower crust at depths of >30 km. We argue that rollback of the flat-subducted Neo-Tethyan oceanic slab during Early Paleogene times resulted in a contractional orogeny and intrusion of voluminous mantle-derived magmas, which caused large-scale crustal heating, partial melting and granulite-facies metamorphism within the deep crust of the Gangdese arc. The Nyingchi complex represents the exposed lower crust of the Gangdese magmatic arc, and links the granulite-facies metamorphism with silicic magmatism and crustal growth during Paleocene arc accretion. [ABSTRACT FROM PUBLISHER]

• Protoliths of Milin gneisses are ∼1556–1517 Ma continental arc-like granitoids. • Bulk-rock Nd and zircon Hf isotopes show their Paleoproterozoic material derivation. • The gneiss metamorphosed at ∼1200 Ma represents the Precambrian metamorphic basement. • Milin gneisses suggest the Lhasa terrane origins from the supercontinent Columbia. The Precambrian basement of the Lhasa terrane provides important information for understanding the formation and evolution of the Tibetan Plateau. However, due to the paucity of exposure, the properties of the Precambrian basement of the Lhasa terrane remain poorly known. Here we report zircon U–Pb ages, bulk-rock geochemical and zircon Hf isotopic data on the orthogneisses in the Milin area of the southeastern Lhasa terrane, southern Tibet. Inherited magmatic zircon cores of these gneisses yield protolith crystalline ages of 1556–1517 Ma, whereas the zircon rims give metamorphic ages of 1197–1192 Ma, both could be local provenance for the Mesoproterozoic detrital zircons in the Neoproterozoic–Paleozoic strata in the Lhasa terrane. Geochemical data indicate that the protoliths of the Mesoproterozoic rocks are continental arc-like granitoids with bulk-rock ε Nd (t) values of −3.7 to +0.7 (T DM2 = 1.99–2.34 Ga) and zircon core ε Hf (t) values of +1.8 to +8.6 (T DM2 = 1.71–2.17 Ga), suggesting their most likely derivation from partial melting of Paleoproterozoic crustal materials. We propose that the Mesoproterozoic gneisses in the Milin area represent the Precambrian metamorphic basement of the Lhasa terrane, likely related to the growth of Columbia supercontinent. Local exposure of the Precambrian metamorphic basement may be caused by crustal rapid uplift and erosion in the late Cenozoic and their inhomogeneous distribution. [ABSTRACT FROM AUTHOR]

Abstract: The Lhasa terrane has been traditionally considered as the northern boundary of the Indian continent. Here we report the discovery of high-pressure granulites occurring as blocks within the Paleozoic sedimentary rocks of the North Lhasa terrane. The mafic HP granulite is composed of garnet, clinopyroxene, plagioclase, rutile and quartz, whereas the pelitic HP granulite has an assemblage of garnet, kyanite, biotite, muscovite, plagioclase and quartz. The mafic rocks show whole-rock compositional features of MORB. Our thermobarometric computations yield P–T conditions of ca. 1.7–1.8GPa and 690–730°C, corresponding to metamorphism under eclogite-facies, with a peak metamorphic gradient of ∼14°C/km, typical of collision-zone metamorphism. In situ zircon U–Pb dating places the timing of the early HP metamorphism at ca. 650Ma, with the late amphibolite-facies retrogression lasting up to ca. 480Ma. Our study reports the first evidence for Late Neoproterozoic HP metamorphism from Tibet, suggesting that the North Lhasa terrane might have been derived from the northern segment of the East African Orogen formed during the assembly of crustal blocks within the Eastern and Western Gondwana. [Copyright &y& Elsevier]

Abstract: The high-grade metamorphic rocks from the Lhasa terrane, south Tibet, have been traditionally grouped as the Precambrian metamorphic basement. However, a number of recent studies show that these metamorphic rocks from the southern part of Lhasa terrane, including granulite- and eclogite-facies rocks, were metamorphosed during the Mesozoic and Cenozoic. Based on the LA–ICP–MS in situ zircon U–Pb chronology, we report for the first time late Neoproterozoic metamorphic and magmatic events from the Lhasa terrane. The rocks analyzed in this study occur in the northern part of Lhasa terrane, and include amphibolite, garnet-bearing muscovite schist and marble, with the mineral assemblages of hornblende+andesine+quartz, muscovite+garnet+plagioclase+quartz and calcite+diopside+quartz, indicating an amphibolite-facies metamorphic event. Zircons from a marble and an amphibolite sample show typical features of metamorphic origin, and yield peak-metamorphic ages of ca. 676Ma and ca. 661Ma, respectively. Zircons from another amphibolite sample possess inherited magmatic cores and metamorphic overgrown rims, yielding a protolith age of ca. 856Ma and a metamorphic age of ca. 683Ma. Zircons from the schist include detrital magmatic cores showing a wide spectrum of Proterozoic ages from ca. 1747 to 789Ma, representing derivation from multiple sources. However, the metamorphic rims of these zircons yield an age population of ca. 690Ma. Based on the geochemical and geochronological data presented in this work and from a synthesis of data from previous studies, we propose the location of the Lhasa terrain in a reconstruction of the Neoproterozoic Rodinia supercontinent. We correlate the late Neoproterozoic metamorphic and magmatic events with an Andean-type orogeny that resulted from oceanic subduction beneath the northwestern margin of the Rodinia supercontinent. We correlate the orogeny with a global-scale late Proterozoic arc magmatism widely represented in the Seychelles, Madagascar, northwestern India and South China. [Copyright &y& Elsevier]

Phanerozoic growth of continental crust on our planet is one of the important research themes in Earth Science. Here, we present the results of a systematic study of newly found and previously reported Mesozoic igneous rocks, including diorite cumulate, granodiorite cumulate, mafic magmatic enclaves (MME) and host granitoids in the central Lhasa terrane, southern Tibet. These igneous rocks give zircon U Pb crystallization ages of 199–189 Ma. Based on constituent mineral and bulk-rock compositions, the cumulates are best understood as resulting from amphibole, plagioclase and titanite crystallization from a mafic andesitic magma. The host granitoids also show compositional systematics consistent with amphibole-plagioclase fractional crystallization from andesitic magma. The MMEs share many characteristics with their host granitoids in common, including identical crystallization age, similar mineralogy, mineral chemistry and zircon isotopic compositions, representing earlier cumulate derived from the same magmatic system as their host rocks. The magma parental to the studied Early Jurassic igneous rocks is best explained as resulting from partial melting of hydrated ocean crust together with varying continental material. The increasing zircon ε Hf (t) values of multiple plutons in the central Lhasa terrane with time during ~215–170 Ma indicate its gradual increase in mantle contribution. We present the Early Mesozoic crustal evolution in the Lhasa terrane. • The igneous rocks include ca. 200 Ma cumulates, MME-bearing granitoid in the CLT. • The igneous rocks are formed by AFC from andesitic magma. • Parental magma is from melting of the hydrated ocean crust with continental material. • Mesozoic plutons record the increase in mantle contribution of crust in the CLT. [ABSTRACT FROM AUTHOR]

• The Mesoproterozoic rocks (1520–1506 Ma) are recognized in the southern Lhasa subterrane (SLT). • Bulk-rock Nd and zircon Hf isotopes show their derivation from the Paleoproterozoic material. • The Mesoproterozoic rocks metamorphosed at 605–590 Ma represent the Precambrian metamorphic basement of the Lhasa terrane. • Titanite in situ U-Pb dating gives more recent metamorphic age of ~26 Ma. • The SLT basement has been strongly reworked by mantle-derived magmatism and metamorphism since the Mesozoic. Due to the paucity of exposure, the formation and evolution of the Precambrian basement of the Lhasa terrane remain poorly known. Here we report zircon and titanite in situ U–Pb ages, bulk-rock geochemical and zircon Hf isotopic data on the orthogneisses from the Dongjiu area of the southern Lhasa subterrane (SLT), southern Tibet. Geochemical data suggest that the protoliths of the biotite-amphibole gneiss and biotite gneiss are granodiorite and granite, respectively. Inherited magmatic zircon cores from these orthogneisses give protolith crystalline ages of 1520–1506 Ma, whereas the overgrown zircon rims give metamorphic ages of 605–590 Ma. The Mesoproterozoic granitic rocks have bulk-rock ε Nd (t) values of −3.6 to +0.1 and zircon core ε Hf (t) values of −4.5 to +2.6, which give similar T DM2 ages of 2.35–2.05 Ga and 2.54–2.10 Ga respectively, suggesting their derivation from partial melting of Paleoproterozoic crustal material. The granitic rocks are also local provenance for the Mesoproterozoic detrital zircons in the Paleozoic strata in the Lhasa terrane. Titanite in situ U–Pb ages further indicate that the Dongjiu orthogneiss experienced more recent metamorphism at ~26 Ma. The mineral assemblage and thermobarometry calculations indicate that the Oligocene metamorphism occurred under medium-pressure (MP) amphibolite-facies conditions (5.4–7.2 kbar, 691–765 °C). We propose that the Dongjiu gneisses represent the Precambrian metamorphic basement of the Lhasa terrane, but have been intensively reworked by metamorphism in the SLT in response to the continued India-Asia convergence since the collision. [ABSTRACT FROM AUTHOR]

The latest Cretaceous magmatic activity in the eastern segment of the Lhasa terrane provides important insights for tracking the magma source and geodynamic setting of the eastern Gangdese batholith, eastward of eastern Himalayan Syntaxis. Detailed petrological, geochemical and geochronological studies of the intrusive rocks (monzodiorites and granodiorites) of the eastern Gangdese batholith are presented with monzodiorites and granodiorites giving zircon U–Pb crystallization dates of 70–66 Ma and 71–66 Ma with εHf(t) values of –4.8 to +6.2 and –1.9 to +5.3, respectively. These rocks are metaluminous to weakly peraluminous I‐type granites showing geochemically arc‐related features of enrichment in LREEs and some LILEs, e.g., Rb, Th, and U, and depletion in HREEs and some HFSEs, e.g., Nb, Ta, and Ti. The rocks are interpreted to be derived from partial melting of mantle material and juvenile crust, respectively, which are proposed to be triggered by Neo‐Tethyan slab rollback during northward subduction, with both experiencing ancient crustal contamination. The studied intrusive rocks formed in a transitional geodynamic setting caused by Neo‐Tethyan oceanic flat subduction to slab rollback beneath the eastern Gangdese belt during the latest Cretaceous. [ABSTRACT FROM AUTHOR]

Highlights: [•] The protoliths of the Basongco metamorphic rocks consist mainly of early Paleozoic sedimentary rocks. [•] These rocks experienced the early Mesozoic metamorphism, and associated with the coeval magmatism. [•] The metamorphic conditions are T =640–705°C and P =6.0–8.0kbar. [ABSTRACT FROM AUTHOR]

Highlights: [•] The metamorphic rocks from the Lhasa terrane recorded multistages of Precambrian tectonic processes. [•] The Lhasa terrane witnessed the Late Paleoproterozoic (∼1780Ma) magmatism. [•] The Lhasa terrane experienced two stages of metamorphism at ∼1120Ma and ∼610Ma. [•] The Lhasa terrane has similar Precambrian evolution to the eastern margin of India. [Copyright &y& Elsevier]

The early Mesozoic evolution of the Lhasa terrane, which represents a major component of the Himalayan–Tibetan orogen, remains highly controversial. In particular, geological units and events documented either side of the eastern Himalayan syntaxis (EHS) are poorly correlated. Here, we report new petrological, geochemical and geochronological data for cogenetic peraluminous S-type granites and metamorphic rocks (gneiss and schist) from the Motuo–Bomi–Chayu region of the eastern Lhasa terrane, located on the eastern flank of the EHS. Zircon U–Pb dating indicates that these units record both Late Triassic magmatic (216–206 Ma) and metamorphic (209–198 Ma) episodes. The granites were derived from a Paleoproterozoic crustal source with negative zircon εHf(t) values (–5·5 to –16·6) and TDM2 model ages of 1·51–1·99 Ga, and are interpreted to have formed by crustal anatexis of nearby metasediments during collisional orogeny and crustal thickening. The gneisses and schists experienced similar upper amphibolite-facies peak metamorphism and associated partial melting, followed by decompressional cooling and retrograde metamorphism. These rocks were buried to lower-crustal depths and then exhumed to the surface in a collisional orogenic setting during plate convergence. From comparison of these data with those for other metamorphic belts with similar grades and ages, and association of coeval granitic magmatism widespread in the central–east Lhasa terrane, we propose that the studied cogenetic magmatism and metamorphism in the Motuo–Bomi–Chayu region records Late Triassic accretion of the North Lhasa and South Lhasa terranes, which represents the first evidence of the Paleo-Tethys Ocean closure in this part of Asia. These data provide new constraints on the spatial and temporal evolution of the Paleo-Tethyan Wilson Cycle and provide a 'missing link' to correlate the geology and tectonic history of the Lhasa terrane continental crust on either side of the EHS. [ABSTRACT FROM AUTHOR]

The metamorphic belt in the Dongjiu area is located in the eastern segment of the Lhasa terrane in South Tibet. The Dongjiu metamorphic rocks are primarily composed of schist and gneiss, with minor amounts of marble, and the protoliths are sedimentary rocks with Precambrian and early Palaeozoic zircons probably deposited during the Palaeozoic or late Neoproterozoic. On the basis of petrology and phase equilibria modelling, this study shows that the Dongjiu metamorphic belt has experienced a kyanite-grade metamorphism, which is characterized by a decompressional vector with slight cooling from a peak of 9.6 kbar and 745°C to medium-pressure amphibolite-facies metamorphic overprinting at 5–6 kbar and 600–630°C. This P–T path was well recorded and recovered by garnet zoning profiles. Laser ablation inductively coupled plasma mass spectrometry in situ U–Pb analyses on metamorphic zircons and zircon rims yielded concordant 206Pb/238U ages of c. 194–192 Ma, suggesting that the Dongjiu metamorphic rocks were formed during the Early Jurassic. Therefore, the Dongjiu metamorphic belt, together with the western Nyainqentanglha, Basongco, and Zhala metamorphic belts, constitutes a nearly continuous tectonic unit with an E–W extension of at least 500 km between the northern and southern Lhasa terranes. The metamorphic ages of these belts, ranging from 230 to 192 Ma, show a younger trend from west to east, indicating that the central segment of the Lhasa terrane experienced an eastward asynchronous collisional orogeny during the Late Triassic to Early Jurassic. [ABSTRACT FROM AUTHOR]