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The continental crust in the overriding plate of the India-Asia collision zone in southern Tibet is characterized by an overthickened layer of felsic composition with an underlying granulite-eclogite layer. A large data set indicates that this crust experienced magmatism from 245 to 10 Ma, as recorded by the Gangdese Batholith. Magmatism was punctuated by flare-ups at 185−170, 90−75, and 55−45 Ma caused by a combination of external and internal factors. The growth of this crust starts with a period dominated by fractional crystallization and the formation of voluminous (ultra)mafic arc cumulates in the lower crust during subduction, followed by their melting during late-subduction and collision, due to changes in convergence rate. This combined accumulation-melting process resulted in the vertical stratification and density sorting of the Gangdese crust. Comparisons with other similarly thickened collision zones suggests that this is a general process that leads to the stabilization of continental crust. The Gangdese Batholith records the time-integrated development of the world's thickest crust, reaching greater than 50 km at 55–45 Ma and greater than 70 km after 32 Ma. The Gangdese Batholith records three magmatic flare-ups in response to distinct drivers; the last one at 55−45 Ma marks the arrival of India. Magmatism was first dominated by fractional crystallization (accumulation) followed by crustal melting: the accumulation-melting process. Accumulation-melting in other collision zones provides a general process for vertical stratification and stabilization of continental crust. [ABSTRACT FROM AUTHOR]

The temporal and spatial distribution of enriched source components at sites of continent–continent collision provides critical insights into mantle dynamic processes associated with subduction- and collision-related events. However, determining the origin of such enriched components remains a significant challenge. We report a comprehensive dataset of the Linzizong volcanic succession (LVS) from four locations with varying distance across-strike to the Indus–Yarlung suture in southern Tibet, which marks the exposed surface expression of the India–Asia collision zone. The LVS rocks in this study can be divided into two groups: a calc-alkaline Group 1 (69–55 Ma), mainly including basaltic–andesitic varieties, and a shoshonitic Group 2 (52–50 Ma), consisting predominantly of silicic rocks with minor mafic compositions. Group 1 samples are likely derived from the fractional crystallization of primitive basaltic melts as a result of the partial melting of a metasomatized mantle wedge. These samples are decoupled in Nd–Hf isotopic compositions, suggesting an incorporation of subducting sediment-derived melts into the mantle wedge. The influence of sediment-derived melt on the mantle source increases away from the suture zone toward Asia (i.e. from the south to the north) as indicated by the more enriched Sr, Nd, Pb, and Hf isotopic compositions, as well as elevated Th/La and La/Sm ratios. The heavy δ26Mg values, and high Ba/Th and Sr/Th ratios of samples close to the suture coincide with the dehydration of the subducting Neo-Tethyan slab. Group 2 mafic samples have depleted and coupled εNd–εHf isotopic compositions and are characterized by elevated (La/Yb)N and Dy/Yb ratios as well as low Zr/Nb ratios, indicating an origin of enriched garnet-bearing lithospheric mantle with contributions from asthenosphere-derived materials. Group 2 silicic samples are isotopically enriched both near and far away from the suture, which can be attributed to the involvement of ancient lower crust-derived melt from Tethyan Himalaya and central Lhasa subterrane basement, respectively. Our results show that the LVS are the magmatic response to late subduction (Group 1), and to initial India–Asia collision and slab breakoff (Group 2). Negative trends in the whole-rock Nd and zircon Hf isotopic compositions at ~51 Ma should be treated with caution for geodynamic interpretations, depending on the distance between the samples and the India–Asia suture. [ABSTRACT FROM AUTHOR]

Abstract: The petrogenesis and geodynamic implications of the Cenozoic adakites in southern Tibet remain topics of debate. Here we report geochronological and geochemical data for host granites and mafic enclaves from Wolong in the eastern Gangdese Batholith, southern Tibet. Zircon LA-ICP-MS dating indicates that the Wolong host granites and enclaves were synchronously emplaced at ca. 38Ma. The host granites are medium- to high-K calc-alkaline, metaluminous (A/CNK=0.93–0.96), with high Al2O3 (15.47–17.68%), low MgO (0.67–1.18%), very low abundances of compatible elements (e.g., Cr=3.87–8.36ppm, Ni=3.04–5.71ppm), and high Sr/Y ratios (127–217), similar to those typical of adakite. The mafic enclaves (SiO2 =51.08–56.29%) have 3.83–5.02% MgO and an Mg# of 48–50, with negative Eu anomalies (δEu=0.59–0.79). The Wolong host granites and enclaves have similar Sr–Nd isotopic compositions (initial 87Sr/86Sr=0.7053–0.7055, ε Nd(t)=−2.7 to −1.4), with varying zircon ε Hf(t) values, ranging from +6.0 to +12.6. A comprehensive study of the data available for adakitic rocks from the Gangdese Batholith indicates that the Wolong adakitic host granites were derived from partial melting of a thickened lower crust, while the parental magmas of the mafic enclaves were most likely derived from lithospheric mantle beneath southern Tibet. The Wolong granitoids are interpreted as the result of mixing between the thickened lower crust-derived melts and lithospheric mantle-derived mafic melts, which are likely the protracted magmatic response to the break-off of the Neo-Tethyan oceanic slab at about 50Ma. Our results suggest that the crustal thickening in southern Tibet occurred prior to ~38Ma, and support the general view that the India–Asia collision must have occurred before 40Ma. [Copyright &y& Elsevier]

The geographical location of the Lhasa terrane in the Permian remains a subject of debate. The recognition of the Permian basalts in the Tethyan Himalaya and the Permian volcanic rocks in the Lhasa terrane in southern Tibet together with the geochemistry of these rocks offer some new insights. The Permian basalts in the Tethyan Himalaya show a geochemical affinity with tholeiitic continental fl ood basalts, and are interpreted to have formed in an extensional setting. The new geochemical data and the geographical distribution of these basalts indicate that they probably represent the easternmost extent of the Panjal continental flood basalt province. All of the Permian basalts in the Lhasa terrane show a calcalkaline , high-alumina basalt affinity, with significant negative Nb-Ta-Ti anomalies. These geochemical features, combined with the recent documentation of the Permian Songdo eclogite and sedimentological observations, indicate the existence of a subduction system beneath the central Lhasa subterrane in the Permian. The presence of both extension- and arc-type magmatism of Permian age in present-day southern Tibet is inconsistent with the general view that the Lhasa terrane did not rift away from the northern margin of the Greater India until the Late Permian or Triassic. Instead, we suggest that the central Lhasa subterrane may have been a microcontinent isolated in the Paleo-Tethyan Ocean basin, at least during the Carboniferous--Middle Permian time. [ABSTRACT FROM AUTHOR]

Abstract: Bulk-rock major and trace element, Sr–Nd isotope, zircon U–Pb and Hf-isotope data are reported for Early Cretaceous igneous rocks sampled along an east–west traverse through the central Lhasa subterrane (E80°00′–E89°) in southern Tibet. These results offer new insights into the pre-Cenozoic geology and tectonomagmatic evolution of the Lhasa Terrane, and provide boundary conditions for the origin and evolution of the Tibetan Plateau. Our new data, together with data from the literature indicate that the Early Cretaceous magmatism in the central Lhasa subterrane lasted for a long period (~143–102Ma) with a magmatic flare-up at ~110 Ma. The latter is accompanied by mafic magmatism in this terrane. The silicic rocks are metaluminous to peraluminous, and enriched in Rb, Th, and U, and depleted in Ba, Nb, Ta, Sr, P, and Ti, with varying initial 87Sr/86Sr (0.7073–0.7209), negative ε Nd(t) (−13.7 to −4.6), and negative to positive zircon ε Hf(t). The andesitic and dioritic enclaves are characterized by initial 87Sr/86Sr of 0.7088–0.7148, ε Nd(t) of −9.9 to −7.8, and zircon ε Hf(t) of −9.5 to −0.2, similar to those of the 110±3Ma silicic rocks. The bulk-rock Sr and Nd isotopic data and zircon ε Hf(t) values indicate an increased contribution of a mantle component in the generation of the Early Cretaceous igneous rocks in the central Lhasa subterrane at ~110Ma. Available bulk-rock Nd- and zircon Hf-isotope data indicate that the crust of the central Lhasa subterrane, at least its oldest elements, was emplaced during the Archean. This further indicates that the central Lhasa subterrane with ancient basement must have been a micro-continental block (i.e., the Lhasa micro-continental block) extending in an east–west direction for >700km (E82°–E89°) with a width of ~100km. The contemporaneous presence of S-type (more crustal source) and I-type (significant mantle input) melts of the Early Cretaceous igneous rocks in this subterrane are interpreted as the consequences of varying extents of interactions between the existing continental crust and mantle-derived melts (including crustal anatexis). We suggest that the mantle-derived melts resulted from southward subduction of the Bangong–Nujiang Ocean seafloor in a syncollisional setting related to the Lhasa–Qiangtang collision, and that the magmatic flare-up with strong mantle input at ~110Ma was genetically associated with the slab break-off of this subducting seafloor. [Copyright &y& Elsevier]

Abstract: The Lhasa terrane has long been interpreted as a simple tectonic block rifted from Gondwana during the late Paleozoic and then drifted northward before finally amalgamating with the Qiangtang terrane during the Early Cretaceous. In this paper we document Permian peraluminous granites near Pikang in the southern margin of the central Lhasa terrane, close to the recently documented Songdo eclogite of Permian age. Zircon SHRIMP and LA-ICPMS U–Pb dating for a Pikang granite sample gives an identical crystallization age of about 263 Ma and a wide age range of inherited zircons (283–2141 Ma). In situ Hf isotopic analyses for 20 zircons of 263 Ma yielded ε Hf(t) values of −4.5 to +1.9. The Pikang granites have high A/CNK values (≥1.08) and high normative corundum (1.3–2.0%), indicative of peraluminous S-type granite. They are characterized by moderately negative Eu anomalies (Eu/Eu⁎=0.48–0.61), and strongly negative Ba, Nb, Sr, P and Ti anomalies. The granites have high ε Nd(t) values (−6.4 to −6.0) and low initial 87Sr/86Sr ratios (0.7082–0.7096) relative to melts derived from mature continental crust. These rocks are interpreted to have been generated by mixing between mantle melts and their induced melting of mature crustal materials. We interpret the Pikang peraluminous granite magmatism, the regional angular unconformity between the Middle and the Upper Permian and the eclogite of the same metamorphic age (∼262 Ma) from the same geotectonic location to represent different products of a common event in time and space. We tentatively term this common event as syncollisional orogeny, i.e., “the Permian Gangdese Orogeny”. We further hypothesize that the orogeny may be genetically associated with the collision between the Lhasa terrane and the northern margin of Australia, following the closure of the Paleo-Tethyan Ocean south of the Lhasa terrane. [Copyright &y& Elsevier]

The East Kunlun Orogenic Belt (EKOB), on the northern margin of the Tibetan Plateau, forms a part of the western segment of the orogenic system in Central China. The Paleozoic tectonic evolution of the EKOB, along the western margin of Gondwana, is closely related to Paleozoic subduction and closure of the Proto-Tethys and is key to the palinspastic reconstruction of the Central Asia and Tethys domains. Numerous studies have been conducted on the Early Paleozoic granitoids in the EKOB. However, a convincing magmatic proxy to indicate the timing of oceanic subduction during the Early Paleozoic is yet to be well constrained, partially because the arc geochemical signature of the calc-alkaline granitoids is not a reliable indicator of oceanic subduction. Herein, we present new zircon U–Pb ages, whole-rock major and trace element analyses, and Nd–Pb and zircon Hf isotopic data for Early Ordovician appinite (mafic), diorite (intermediate), and granodiorite (felsic) intrusive rocks in the Dundegou (DDG) area of the EKOB, northern Tibet Plateau. The zircon U–Pb ages suggest this complex was emplaced at approximately 477–472 Ma. Abundant 557–510 Ma zircon xenocrysts and/or inherited zircons were found in the granodiorite and appinites, suggesting two earlier magmatic episodes during the late Ediacaran–Cambrian. Appinites and diorites are characterized by depletion in high field strength elements, with negative initial ε Nd (t) values (appinites: −6.5 to −9.4; diorites: −8.2 to −9.1) and negative zircon ε Hf (t) values (appinites: −2.5 to −10.3; diorites: −1.0 to −9.1). These features suggest that appinites and diorites were most likely derived from the partial melting of enriched mantle wedge peridotites with the addition of sediment-derived melts from the sedimentary veneer of the subducting slab, along with variable degrees of fractional crystallization and a limited extent of crustal contamination. The granodioritic magma was contributed by (a) the ancient mafic lower crust-derived magma, (b) the derivative felsic magma from a dioritic parent, and (c) a minor mantle-derived magma component, resulted in the formation of granodiorites with highly unradiogenic initial Nd and Hf isotopic compositions (ε Nd (t) values of −8.5 and ε Hf (t) values from −0.6 to −7.3). Our study suggests that the subcontinental lithospheric mantle beneath the EKOB is likely heterogeneous in terms of its isotopic composition. The DDG complex is the oldest appinite-bearing complex found so far in the EKOB and a good indicator of proto-Tethys oceanic subduction. • The appinite–diorite–granodiorite complex was emplaced at 477–472 Ma. • Mixing/mingling between magmas is critical to form appinite-granite complexes. • The oldest appinite complex associated with oceanic subduction is within the EKOB. [ABSTRACT FROM AUTHOR]

We report 11 new U-Pb zircon ages obtained by sensitive high-resolution ion microprobe (SHRIMP) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) for a large province of Early Cretaceous Comei igneous rocks consisting of basaltic lavas, mafi c sills and dikes, and gabbroic intrusions together with subordinate layered ultramafi c intrusions and silicic volcanic rocks exposed in the Tethyan Himalaya, southeastern Tibet. Available zircon U-Pb ages obtained from various rocks in this province, which has an areal extent of ∼40,000 km2 (∼270 km × 150 km), indicate that the magmatism occurred ca. 132 Ma ago, coeval with the Bunbury Basalt in southwestern Australia. Such a striking similarity in emplacement age, in combination with the tectonic reconstruction of eastern Gondwana ca. 132 Ma ago, allows us to propose that the extensive Comei igneous rocks in southeastern Tibet and the Bunbury Basalts in southwestern Australia may represent the erosional and/or deformational remnants of a large igneous province, which we call the Comei-Bunbury LIP. We argue that this newly identifi ed LIP was likely caused by the Kerguelen mantle plume, which started in the Early Cretaceous and may have played a role in the breakup of eastern Gondwana and the development of the 132 Ma old Weissert oceanic anoxic event. [ABSTRACT FROM AUTHOR]

Continental magmatic arcs are characterized by the accretion of voluminous mantle-derived magmatic rocks and the growth of juvenile crust. However, significant volumes of meta-sedimentary rocks occur in the middle and lower arc crust, and the contributions of these rocks to the evolution of arc crust remain unclear. In this paper, we conduct a systematic study of petrology, geochronology, and geochemistry of migmatitic paragneisses from the eastern Gangdese magmatic arc, southern Tibet. The results show that the paragneisses were derived from late Carboniferous greywacke, and underwent an early Cenozoic (69-41 Ma) upper amphibolite-facies metamorphism and partial melting at pressure-temperature conditions of ~11 kbar and ~740 °C, and generated granitic melts with enriched Hf isotopic compositions (anatectic zircon εHf(t) = -10.57 to +0.78). Combined with the existing results, we conclude that the widely distributed meta-sedimentary rocks in the eastern Gangdese arc deep crust have the same protolith ages of late Carboniferous, and record northwestward-decreasing metamorphic conditions. We consider that the deeply buried sedimentary rocks resulted in the compositional change of juvenile lower crust from mafic to felsic and the formation of syn-collisional S-type granitoids. The mixing of melts derived from mantle, juvenile lower crust, and ancient crustal materials resulted in the isotopic enrichment of the syn-collisional arc-type magmatic rocks of the Gangdese arc. We suggest that crustal shortening and underthrusting, and the accretion of mantle-derived magma during the Indo-Asian collision transported the supracrustal rocks to the deep crust of the Gangdese arc. [ABSTRACT FROM AUTHOR]

New zircon LA-ICPMS U-Pb age data, whole-rock major and trace elements, and zircon Hf isotopic data of intrusive rocks from Larelaxin and Caima plutons in the southern margin of the Western Qiangtang subterrane provide important evidence of northward subduction of the Bangong-Nujiang Tethyan Ocean lithosphere. Host granitoids including quartz diorites, granodiorites, and syenogranites and associated mafic varieties including gabbroic enclaves, dioritic dikes and enclaves, and quartz dioritic enclaves are investigated in this study. Five host granitoid samples are dated at 163-160 Ma, and one dioritic dike and three dioritic enclave samples are dated at 162-158 Ma, which indicates that these rocks were contemporaneous. The quartz diorites and granodiorites are normal calc-alkaline I-type granitoids. The syenogranites are characterized by high SiO2 (74-77 wt.%) and differentiation index (DI = 92-97) and marked depletion in Ba, Nb, Ta, Sr, P, Ti, and Eu, thus indicating that they are highly fractionated I-type granitoids. The host granitoids exhibit uniform zircon εHf(t) values (- 1.4 to + 1.9). Considering the current data and their negative whole-rock εNd(t) values (- 4.7 to - 3.5) reported in recent studies, these rocks can be interpreted to have resulted from the partial melting of ancient mafic lower crust with varying contributions from mantle-derived or juvenile crust-derived components. One gabbroic enclave sample exhibits high Al2O3 (18.3 wt.%), low MgO (3.3 wt.%), high TiO2 (1.4 wt.%), and high Nb (24 ppm) and is geochemically similar to high-alumina basalt (HAB) and high-Nb basalt (HNB). This indicates that the gabbroic enclave originated from the partial melting of mantle wedge peridotite that was metasomatized by slab melting and subsequently experienced significant fractional crystallization of olivine and clinopyroxene. The dioritic dike and enclave samples yield εHf(t) values (- 1.6 to + 3.1) similar to those of the host granitoids. They most likely originated from magma mixing between mantle-derived and crust-derived melts at or close to the Moho. The intrusive rocks with geochemical diversity of the Larelaxin and Caima plutons from Bangong Tso in western Tibet can be interpreted to have resulted from melting, assimilation, storage, and homogenization (a MASH process) above subduction zones, thus indicating that the Bangong-Nujiang Tethyan Ocean lithosphere was subducted northward beneath the Western Qiangtang subterrane during the Late Jurassic. [ABSTRACT FROM AUTHOR]

Abstract: Bulk-rock major and trace element, Sr–Nd–Hf isotope, zircon U–Pb age, and zircon Hf isotopic data of the Late Cretaceous Zhuogapu volcanic rocks in the northern Lhasa subterrane provide a new insight into tectonic processes following the collision of the terrane with the Qiangtang zone. SHRIMP zircon U–Pb dating reveals that the Zhuogapu volcanic rocks crystallized at ca. 91Ma, postdating the development of a regional angular unconformity between the Upper Cretaceous and the underlying strata in the Lhasa–Qiangtang collision zone. Compared to the Andean arc-type andesites and dacites, the Zhuogapu volcanic rocks are characterized by higher MgO of 2.78–5.86wt.% and Mg# of 54–64 for andesites and MgO of 2.30–2.61wt.% and Mg# of 55–58 for dacites. Eight andesite samples have whole-rock (87Sr/86Sr)i of 0.7054–0.7065, εNd(t) of −3.2 to −1.7, and εHf(t) of +3.8–+6.4, similar to those of the three dacite samples with (87Sr/86Sr)i =0.7056–0.7060, εNd(t) of −2.7 to −2.2, and εHf(t) of +5.6–+7.0. Thirteen analyses from a dacite sample give positive zircon εHf(t) of +5.6 to +8.7. These signatures indicate that the Zhuogapu Mg-rich andesites were most likely derived from partial melting of a delaminated mafic lower crust (including the lowermost crust straddling the northern and central Lhasa subterranes) that led to the generation of the Zhuogapu primary melts with adakitic signatures and small negative εNd(t). Such melts subsequently experienced interaction of melt-asthenospheric mantle peridotite followed by the modification of highly fractionated magmas in shallow crustal magma chamber. Hornblende-controlled fractionation results in the change of geochemical composition from Mg-rich andesitic to Mg-rich dacitic magmas. Field observations, together with geochronological and geochemical data, indicate that the Zhuogapu Mg-rich volcanic rocks and coeval magmatism in the northern Lhasa subterrane may be the result of thickened lithospheric delamination following the final Lhasa–Qiangtang amalgamation. [Copyright &y& Elsevier]

Ophiolite plays a key role in identifying paleo-ocean and paleo-plate and rebuilding the evolutionary history of ancient orogen. Mafic–ultramafic rocks are distributed in a broadly E–W direction in the middle of the Qiangtang terrane, northern Tibetan plateau. However, interpretation of these rocks as ophiolite and subsequent tectonic implications have been much disputed, and they were regarded by some to have an origin in a continental rift setting. A detailed zircon dating and geochemical and Sr–Nd–Hf isotopic study of this suite of rocks has been undertaken in order to clarify this important issue. The mafic–ultramafic suite was collected from the Gangma Co and Guoganjianian localities and is composed of cumulate and isotropic gabbro, basalt, actinolitite and plagiogranite. All basaltic rocks are tholeiitic and have low rare earth element (REE) abundances with variable REE patterns and slight negative Nb and Ti anomalies. These features are comparable with those of normal mid-ocean ridge basalts (N-MORB) and/or enriched mid-ocean ridge basalts (E-MORB). The positive whole-rock εNd(t) and zircon εHf(t) values indicate that these rocks were derived from a long-term depleted mantle source. Zircon U–Pb dating using a sensitive high-resolution ion microprobe (SHRIMP) on two cumulate gabbros and two plagiogranites samples yielded Carboniferous ages of 357±2.5Ma, 356.1±3.0Ma, 354.7±4.7Ma and 345.4±4.6Ma. The ophiolite marks a Paleo-Tethys Ocean basin in middle of the Qiangtang terrane, and it is interpreted as the western extension of the Changning–Menglian Paleo-Tethys ophiolite in the eastern margin of the Tibetan plateau. [ABSTRACT FROM AUTHOR]

The Yanhu area in the northern Lhasa subterrane exposes diverse rock types including basalts, rhyolites, quartz dioritic porphyries, and associated dioritic enclaves. The basalts and rhyolites occur as a bimodal volcanic suite, and the quartz dioritic porphyries intrude into the older basalts as a small apophysis. In this paper, we report for the first time the zircon LA-ICP-MS U–Pb age and Hf isotopic composition data, whole-rock major and trace element composition data, and Sr–Nd isotopic data from the diverse Early Cretaceous magmatic rocks from Yanhu. The three basalt samples that we dated yielded zircon U–Pb ages of 110±0.7Ma, 108.9±1.1Ma, and 111.8±3.2Ma. The zircons from one quartz diorite porphyry yielded an age of 109.7±0.8Ma, which is coeval with the dioritic enclave (110.4±1.4Ma). The basalts show a high-K calc-alkaline signature, enriched in Rb, Th, U, and light rare earth elements (REEs) and depleted in Nb, Ta, Ti, Zr, and Hf. These rocks possess varying whole-rock εNd(t) (−0.6 to +2.1) and zircon εHf(t) (+0.6 to +8.9) values. The rhyolite samples are high-K calc-alkaline and are metaluminous to slightly peraluminous. These rocks yielded whole-rock εNd(t) of +0.1 to +0.9 and zircon εHf(t) of +5.1 to +12.4. The quartz dioritic porphyries are characterized by high Al2O3 content (15.9–16.1wt.%), high Sr (466–556ppm), low Yb (1.36–1.41ppm), and low Y (13.8ppm) abundances. Similar geochemical signatures are also present in the dioritic enclaves, revealing that both the quartz dioritic porphyries and the dioritic enclaves have an affinity with adakitic rocks. Moreover, the host rocks and the enclaves display homogeneous εNd(t) (+3.5 to +3.6 and +3.4 to +4.0, respectively) and zircon εHf(t) values (+12.4 to +16.9 and +11.5 to +15.7, respectively). Our geochemical data indicate that the magmatic rocks from Yanhu were derived from the partial melting of distinct source regions, i.e., a heterogeneous metasomatized mantle wedge source for basalts, a juvenile crust source for rhyolites, and a thickened mafic lower crust source that mixed with basaltic magmas for the adakitic rocks (including quartz dioritic porphyries and dioritic enclaves). Compared to typical arc basalts, the basalts from Yanhu are relatively enriched with high field strength elements (HFSEs) (e.g., Zr, Nb), resembling those of within-plate basalts elsewhere. In combination with the presence of a coeval bimodal volcanic rock suite, the ca. 110Ma magmatism in Yanhu is inferred to have occurred in an extensional setting. Our new data, together with recently published data, enable us to correlate the generation of the compositional diversity of the ca. 110Ma Yanhu magmatic rocks that formed in an extensional setting to the slab break-off during the southward subduction of the Bangong–Nujiang Ocean lithosphere. The presence of basaltic magmatism and coeval silicic magmatic rocks with positive zircon εHf(t) indicate that the extensive magmatism at ca. 110Ma have contributed significantly to the crustal growth of the northern Lhasa subterrane. [ABSTRACT FROM AUTHOR]

Abstract: Felsic magmatic rocks of Triassic ages occur on the north side of the Qiangtang metamorphic belt. Zircon dating using the SHRIMP and LA-ICP-MS techniques reveals that these rocks were emplaced between 223 and 205Ma. The emplacement time is similar to the Ar–Ar ages of phengites and sodic amphiboles of the eclogites and blueschist from the Qiangtang metamorphic belt. The felsic magmatic rocks can be divided into two groups with distinct geochemical and Sr–Nd–Hf isotopic compositions. The first and older group (223–219Ma) includes diorite from the Baohu area and some volcanic rocks from the Juhuashan area. These rocks have high Sr (291–1367ppm) and low Y (5.5–13.9ppm) contents with high Sr/Y ratios (28–117). They are characterized by slightly negative ε Nd(t) values (−1.9 to −5.7), positive ε Hf(t) values (+3.2 to +5.4) and young zircon Hf model ages (644–729Ma). The second and younger group (215–210Ma) comprises volcanic rocks from the Jiangai and Guoganjianian areas. They show high initial 87Sr/86Sr ratios (0.708–0.714), distinct negative ε Nd(t) (−9.6 to −7.9) and ε Hf(t) values (−15.3 to −10.1) and much older bulk-rock Nd model ages (1658–2178Ma) and zircon Hf model ages (1251–2012Ma). Geochemical arguments led us to suggest that the rocks of the first group were generated by the melting of subducted oceanic crust and associated sediments, and the second group rocks were derived from anatexis of the Proterozoic continent crust. In conclusion, the Late Triassic magmatic rocks from the northern Qiangtang have recorded the process from subduction to break-off of an oceanic slab. The Qiangtang metamorphic belt to the south represents a northward oceanic subduction zone. [Copyright &y& Elsevier]

Abstract: This paper reports new zircon LA-ICP-MS U–Pb age and Hf-isotope, whole-rock major and trace element, and Sr–Nd isotope data from Cambrian metavolcanic rocks in the central Lhasa subterrane of southern Tibet. These rocks form a bimodal volcanic suite consisting mainly of silicic metavolcanic rocks with subordinate metabasalts. Five silicic metavolcanic samples dated at ca. 492Ma and one metabasalt sample yielding a near-concordant 206Pb/238U age of 492±4Ma indicate that mafic and silicic eruptions were contemporaneous. The metabasalts are mostly high-K calc-alkaline, enriched in Th, U, and light rare earth elements (LREEs), and depleted in Nb, Ta, Ti, Zr, and Hf, geochemically resembling the Andean arc basalts. The silicic metavolcanic rocks are high-K calc-alkaline and low in Nb and Zr. The metabasaltic rocks have negative whole-rock εNd(t) values (−4.7 to −3.5) and varying zircon εHf(t) values (−0.7 to +7.5), differing significantly from those of the silicic metavolcanic rocks, which yield negative whole-rock εNd(t) values of −8.4 to −7.2 and varying zircon εHf(t) values (−13.9 to −4.6). The metabasaltic rocks are interpreted as resulting from partial melting of an enriched lithospheric mantle source that was metasomatized by subduction-related components, whereas the silicic metavolcanic rocks were derived from basaltic melt-induced anatexis of the ancient Lhasa basement with inherited mantle melt signatures. The Western Qiangtang–Amdo–Tethyan Himalaya situated in the Indian proto-Tethyan margin and the Lhasa and other possible microcontinents or terranes (e.g., Gongshan, Baoshan, Tengchong, Burma, and Sibumasu) paleographically located in the Australian proto-Tethyan margin represent an early Paleozoic Andean-type magmatic arc facing the proto-Tethyan Ocean. The emplacement of the bimodal volcanic rocks and the development of the Cambro–Ordovician angular unconformity in the central Lhasa subterrane can be attributed to slab break-off of the proto-Tethyan Ocean lithosphere following the collisional accretion of microcontinents or terranes located outboard of the magmatic arc (possibly Eastern Qiangtang and South China). [Copyright &y& Elsevier]

Abstract: This study reports a dataset of 21 trace elements of magmatic zircons from typical I-type (93 analyses) and S-type (73 analyses) granitoids from the Lhasa and Himalayan terranes, southern Tibet, and A-type granitoids (21 analyses) from the Songpan–Ganzi terrane, eastern Tibet. Our results indicate that magmatic zircons from the I-type granitoids are characterized by relatively lower Pb concentrations and higher (Nb/Pb) N ratios, distinct from those of the S-type granitoids that reveal higher Pb, lower (Nb/Pb) N and significant Eu negative anomalies (Eu/Eu* ⩽0.3); while these values are transitional in zircons from the A-type granitoids. Such differences, most likely governed by compositional variations in the host magmas, are considered as a useful tool, in addition to U–Pb and Hf isotopes, for tracing the source provenance of detrital zircons. Consequently, the trace element compositions of detrital zircons with a diagnostic age peak (∼1170Ma) from the Lhasa Terrane indicate the presence of coeval S-type magmatism in their source region. This study exemplifies the usefulness of zircon trace element geochemistry in investigating sedimentary source provenance and paleogeographic reconstruction. [Copyright &y& Elsevier]

The U-Pb age and Hf isotope data on detrital zircons from Paleozoic metasedimentary rocks in the Lhasa terrane (Tibet) define a distinctive age population of ca. 1170 Ma with εHf (t) values identical to the coeval detrital zircons from Western Australia, but those from the western Qiangtang and Tethyan Himalaya terranes define an age population of ca. 950 Ma with a similar εHf (t) range. The ca. 1170 Ma detrital zircons in the Lhasa terrane were most likely derived from the Albany-Fraser belt in southwest Australia, whereas the ca. 950 Ma detrital zircons from both the western Qiangtang and Tethyan Himalaya terranes might have been sourced from the High Himalaya to the south. Such detrital zircon connections enable us to propose that the Lhasa terrane is exotic to the Tibetan Plateau system, and should no longer be considered as part of the Qiangtang-Greater India-Tethyan Himalaya continental margin system in the Paleozoic reconstruction of the Indian plate, as current models show; rather, it should be placed at the northwestern margin of Australia. These results provide new constraints on the paleogeographic reconstruction and tectonic evolution of southern Tibet, and indicate that the Lhasa terrane evolved as part of the late Precambrian-early Paleozoic evolution as part of Australia in a different paleogeographical setting than that of the Qiangtang-Greater India-Tethyan Himalaya system. [ABSTRACT FROM AUTHOR]