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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]

The Gangdese batholith of the southern Tibet is regarded as a principal component of the pre-Himalayan collision, Andean-type continental margin. However, the Gangdese arc root remains poorly understood because most of previous studies concentrated on granitoids from the middle and upper crust of the arc. In this paper, we report new petrochemical and geochronological characteristics of metagabbros occurred as a part of the Late Cretaceous Lilong batholith in the eastern Gangdese arc. The gabbros with protolith ages of ∼82–95 Ma, underwent slightly late (68–77 Ma) granulite-facies metamorphism and nearly coeval anatexis, typical of lower arc crusts. The metagabbros and diorites of the Lilong batholith show geochemical features of juvenile crust in continental magmatic arcs, and an original magmatic differentiation trend, with gabbros formed as the cumulates, whereas diorites as the crystalline products of evolved magmas. The Lilong batholith forms an arc crustal section with a possible thickness of 20–30 km, and may have an overall andesitic composition, providing a support for the “andesite model” of continental crust growth. [ABSTRACT FROM AUTHOR]

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 studied amphibolite-facies metamorphic rocks, typical of the Nyainqentanglha Group, occur as a tectonic block in the central Lhasa terrane, south Tibet, and consist mainly of gneiss, amphibolite, quartz schist and marble. The amphibolite has a similar mineral paragenesis of plagioclase+garnet+biotite+amphibole+quartz±muscovite, and were metamorphosed under P–T conditions of 0.63–0.66GPa and 588–622°C. Zircons from three dated samples include two types. First type of zircons has an inherited magmatic core and a metamorphic rim; whereas the other type of zircons is of metamorphic origin. LA-ICP-MS in situ U–Pb analysis indicates that the inherited magmatic cores of zircons yield widely variable 206Pb/238U ages ranging from 1857Ma to 743Ma, recording the magmatic-thermal events of the material source of the metamorphic rock protoliths. In contrast, the metamorphic zircons yield concordant 206Pb/238U ages of 225–213Ma. The results indicate that the Nyainqentanglha Group has been subjected to Triassic metamorphism, instead of the Precambrian metamorphic basement of the Lhasa terrane as considered traditionally. These rocks, together with the coeval Sumdo eclogite and Pana blueschist, form a Permian to Triassic metamorphic belt with an E–W extension of up to 300km. Therefore, this work provides new constraints on the late Paleozoic to early Mesozoic orogeny in the central Lhasa terrane, south Tibet. [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]

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]

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]

Abstract: The Namche Barwa complex in the eastern Himalayan syntaxis has experienced high-, medium-pressure granulite-facies and amphibolite-facies metamorphism. The high-pressure granulites contain complex fluid inclusion assemblages, including H2O–CO2 ±CH4, carbonic (CO2 ±CH4 ±N2), CO2-solid, brine, medium- to low-salinity aqueous inclusions and low-density gaseous inclusions. Textural evidence suggests that the isolated H2O–CO2 ±CH4 inclusions occurring in quartz blebs enclosed in garnet porphyroblasts were trapped in the peak metamorphic stage whereas isolated and trail-bound CO2 inclusions (along intragranular fractures) in quartz, plagioclase and garnet were mainly trapped during the early decompression stage of the high-pressure granulites although some CO2-rich inclusions have formed by selective leakage of H2O out of H2O–CO2 inclusions. This together with wide variations in homogenization temperatures indicates modifications and density resetting of the fluid inclusions during exhumation. In contrast to the common presence of high-density CO2 fluid inclusions in granulite terrains worldwide the high-pressure granulites in the Namche Barwa complex contain dominantly medium- to low-density H2O–CO2 ±CH4 and CO2 inclusions. This may be explained as that the high-pressure granulites occur in the continental subduction-zone tectonic setting. [Copyright &y& Elsevier]

Abstract: Charnockites, occurring in the well-studied Gangdese batholith in the southern Lhasa terrane, eastern Himalayan orogen, consist of orthopyroxene, clinopyroxene, plagioclase, and minor quartz and K-feldspar, and show adakitic affinity. Here we present a systematic study on the fluid inclusions in Gangdese charnockites which reveals that the primary fluid inclusions occur isolated or randomly distributed and as trails along intragranular fractures in quartz, and that the secondary ones occur along healed mircofractures and coexist commonly with mineral inclusions of calcite, magnetite and hematite within the host plagioclase and quartz. The fluid inclusions contain dominantly near-pure CO2 with traces of N2. Most of the primary fluid inclusions have low CO2 homogenization temperatures and with densities of 1.138–1.013g/cm3 suggesting trapping pressures of 0.7–1.0GPa at temperatures of 850–950°C. We propose a model in which the southward subduction of Neo-Tethyan mid-oceanic ridge beneath the Lhasa terrane resulted in the release of heat and CO2 from the upwelling asthenosphere through a slab window, providing high-temperatures (HT) and dry CO2-rich fluids for the formation and stabilization of the adakitic charnockite as well as the associated HT granulite-facies metamorphic rocks in the Late Cretaceous, prior to the final collision of the Indian plate with the Asian continent. Our study provides new insights into the geodynamics of Andean-type orogeny in the southern Tibetan Plateau during the Late Mesozoic. [Copyright &y& Elsevier]