Bangong Nujiang Tethyan Research Articles

Early Cretaceous Na-rich granitoids and their enclaves in the Tengchong Block, SW China: Magmatism in relation to subduction of the Bangong–Nujiang Tethys ocean

The Na-rich intermediate-to-felsic granitic rocks provide insights into the generation of magmas in subduction zones. This paper presents zircon LA-ICP-MS U–Pb ages as well as whole-rock geochemical, mineral chemical, and in situ zircon Hf isotopic data on Na-rich granitic rocks from the Tengchong Block, SW China. The granodiorites and associated mafic magmatic enclaves (MMEs) from the Menglian batholith yield zircon U–Pb ages of 116.1±0.8 to 117.8±0.6Ma and 117.7±0.7Ma, respectively. Both host granodiorites and enclaves show calc-alkaline and sodium-rich nature, enrichment in large-ion lithophile elements (LILEs), and variable depletion in zircon Hf isotopic compositions. Euhedral amphiboles in both granodiorites and associated enclaves are magnesian-hornblende with high Mg and Ca and contain euhedral plagioclase inclusions of labradorite to andesine (An36–57) composition. The granodiorite was most likely derived through the mixing of partial melts derived from juvenile basaltic lower crust and a minor evolved component of ancient crustal sources. The quartz monzodiorite–granodiorites and associated MMEs from the Xiaotang-Mangdong batholith yield zircon U–Pb ages of 120.3±1.3 to 122.6±0.8Ma and 120.7±1.5Ma. These rocks are also sodium-rich and show calc-alkaline trend with negative zircon Hf isotopic compositions (−5.55 to +0.58). The MMEs in the host intrusions are monzogabbro with variable and depleted zircon Hf isotopic compositions. The amphiboles in the both host intrusions and the enclaves show Al-rich ferro-tschermakite composition. We infer that the quartz monzodiorite–granodiorites were derived from magmas generated by the melting of ancient basaltic rocks in the lower arc crust induced by the underplating of mantle-derived mafic magmas. The formation of the different types of Na-rich granitic rocks is correlated to the subduction of Bangong–Nujiang Tethyan ocean. A comparison with magmatism in the northern magmatic belt suggests that mantle-derived magmas played an important role in the genesis of Early Cretaceous intrusions from Tengchong to Lhasa Blocks, although crustal melting is the dominant contributor.

Reconstructing in space and time the closure of the middle and western segments of the Bangong–Nujiang Tethyan Ocean in the Tibetan Plateau

When and how the Bangong–Nujiang Tethyan Ocean closed is a highly controversial subject. In this paper, we present a detailed study and review of the Cretaceous ophiolites, ocean islands, and flysch deposits in the middle and western segments of the Bangong–Nujiang suture zone (BNSZ), and the Cretaceous volcanic rocks, late Mesozoic sediments, and unconformities within the BNSZ and surrounding areas. Our aim was to reconstruct the spatial–temporal patterns of the closing of the middle and western segments of the Bangong–Nujiang Tethyan Ocean. Our conclusion is that the closure of the ocean started during the Late Jurassic and was mainly complete by the end of the Early Cretaceous. The closure of the ocean involved both “longitudinal diachronous closure” from north to south and “transverse diachronous closure” from east to west. The spatial–temporal patterns of the closure process can be summarized as follows: the development of the Bangong–Nujiang Tethyan oceanic lithosphere and its subduction started before the Late Jurassic; after the Late Jurassic, the ocean began to close because of the compressional regime surrounding the BNSZ; along the northern margin of the Bangong–Nujiang Tethyan Ocean, collisions involving the arcs, back-arc basins, and marginal basins of a multi-arc basin system first took place during the Late Jurassic–early Early Cretaceous, resulting in regional uplift and the regional unconformity along the northern margin of the ocean and in the Southern Qiangtang Terrane on the northern side of the ocean. However, the closure of the Bangong–Nujiang Tethyan Ocean cannot be attributed to these arc–arc and arc–continent collisions, because subduction and the development of the Bangong–Nujiang Tethyan oceanic lithosphere continued until the late Early Cretaceous. The gradual closure of the middle and western segments of Bangong–Nujiang Tethyan Ocean was diachronous from east to west, starting in the east in the middle Early Cretaceous, and being mainly complete by the end of the Early Cretaceous. The BNSZ and its surrounding areas underwent orogenic uplift during the Late Cretaceous.

Remnants of a Late Triassic ocean island in the Gufeng area, northern Tibet: Implications for the opening and early evolution of the Bangong–Nujiang Tethyan Ocean

In this paper we present new major and trace element compositions of basaltic rocks in the Gufeng ocean island (GFOI) area in the western segment of the Bangong–Nujiang Suture Zone, northern Tibet. Our aim was to assess the genesis of these rocks and discuss the implications of this new dataset for the evolution of the Bangong–Nujiang Tethyan Ocean. An ocean-island-type double-layer structure comprising a basaltic basement and an oceanic sedimentary cover sequence found within the GFOI provides direct evidence for the interpretation that the assemblage is a typical ocean island. The basalts in the GFOI can be divided into three types (named G1, G2 and G3 basalts), and these basalts range in composition from MORB to OIB types, which is typical of ocean islands. The G1 basalts have MORB-type affinities, possibly indicating the existence of MORB oceanic crust under the GFOI. The G2 basalts represent the early stage of formation of the GFOI, and are produced by the interaction of rising OIB-type basaltic magma and the existing MORB oceanic crust. The G3 basalts are typical OIB basalts and they are the products of the direct eruption of OIB-type basaltic magmas. The G3 basalts have high (La/Yb)N (12.3–14.4), (Ce/Yb)N (10.8–11.8), (La/Sm)N (2.39–2.76), and (Sm/Yb)N (4.89–5.23) ratios, indicating the presence of oceanic lithosphere below the GFOI with a thickness of 50–60km. Geochemical analyses of the GFOI cherts show that they contain terrigenous material, indicating the GFOI formed close to a continental margin. Norian conodont fossils within the GFOI limestones indicate the GFOI formed during the Late Triassic. These data, combined with geological evidence and a half-space model of lithosphere cooling, where the thickness of the oceanic lithosphere is determined from the age of the lithosphere, indicate that the western segment of the Bangong–Nujiang Tethyan Ocean opened initially in the late Permian, expanded rapidly during the Early–Middle Triassic, and was a mature ocean during the Late Triassic.

Middle Triassic ultrapotassic rhyolites from the Tanggula Pass, southern Qiangtang, China: A previously unrecognized stage of silicic magmatism

There is a Mid–Late Mesozoic (Jurassic–Cretaceous) arc magmatic belt in southern Qiangtang subterrane associated with the north-dipping subduction of the Bangong–Nujiang Tethyan Ocean. Early Mesozoic (Triassic) magmatism, tectonic evolution of the southern Qiangtang subterrane, and the mechanism of its crustal growth, however, remain unclear. This paper reports zircon U–Pb age, whole-rock major, trace elemental, and Sr-Nd-Hf isotopic components for the Mesozoic Tanggula Pass rhyolites in southern Qiangtang subterrane. Our new data reveal a significant, previously unrecognized stage of magmatism in southern Qiangtang subterrane. The Tanggula Pass rhyolites are moderately-strongly peraluminous (A/CNK=1.09–1.54), highly fractionated (SiO2>73%) and ultrapotassic (K2O/Na2O>44%). Zircon U–Pb dating by LA-ICP-MS yields a concordant age with a weighted mean 206Pb/238U age of 235.9±0.86Ma (n=18, MSWD=0.25). The large variations of major and trace element concentrations are mainly attributed to fractional crystallization of plagioclase, hornblende, Fe-Ti oxides, and accessory minerals such as apatite, allanite, monazite, and zircon in the shallow level. K-feldspar mainly occurs as cumulated mineral, resulting in high K2O contents and K2O/Na2O ratios. The rhyolites were derived by partial melting of mixed source rocks including older sedimentary rocks in the upper crust and juvenile materials, triggered by north-dipping subduction of the Bangong-Nujiang Tethyan oceanic lithosphere. The eruption age of Middle Triassic suggests that the initiation of northward subduction of the Bangong-Nujiang Tethyan ocean took place earlier than is currently assumed. Whole-rock Nd (ca. 1.55 to 2.34Ga) and zircon Hf (ca. 1.77 to 2.33Ga) data reflect the presence of the Proterozoic basement in the southern Qiangtang subterrane. Basaltic magma underplating and accretion of intra-oceanic arc complexes and oceanic plateau contributed to vertical crustal growth and lateral crustal growth, respectively.

Evolution of the Bangong–Nujiang Tethyan ocean: Insights from the geochronology and geochemistry of mafic rocks within ophiolites

The Bangong–Nujiang suture zone (BNSZ) is located in the central Tibetan Plateau, and represents the relict of the Bangong–Nujiang Tethyan Ocean (BNTO). This paper presents zircon U–Pb ages and whole-rock geochemical and Sr–Nd isotope data for the ophiolitic rocks from the Rutog, Dongco, Dongqiao, Amdo, and Dengqen areas (from west to east) in the BNSZ. Zircon U–Pb ages obtained from five gabbros and one leucogabbro from the five areas are 169±2, 167±2, 187±2, 184±2, 177±3, and 164±2Ma, respectively. Mafic rocks (gabbros and basalts) in the BNSZ can be divided into MORB-like and OIB-type, based on geochemical data. The MORB-like rocks are tholeiitic and can be further divided into N- and E-MORB subtypes, marked by depleted and flat REE patterns, respectively. All MORB-like rocks show supra-subduction zone (SSZ) geochemical affinities such as mild enrichment in the large ion lithophile elements and depletion in the high field strength elements, coupled with high and positive whole-rock εNd(t) values (+8.1 to +5.4). The OIB-type rocks, which formed in the Early Cretaceous (~132–108Ma), are exposed in the Dongco, Dongqiao, and Tarenben areas, and they consist mostly of alkali basalts with within-plate geochemical characteristics and positive εNd(t) values (+5.9 to +4.5). On the basis of these new data, combined with information from the literature, we further support that the BNTO was an intra-oceanic arc–backarc basin that existed between the North Lhasa subterrane (NLT) and the South Qiangtang subterrane (SQT) during the Early–Middle Jurassic. This basin may have been active until the Early Cretaceous, when the OIB-type basalts erupted.

Late Triassic island-arc–back-arc basin development along the Bangong–Nujiang suture zone (central Tibet): Geological, geochemical and chronological evidence from volcanic rocks

A major debate related to the evolution of the Tibetan Plateau is centered on whether or not an island arc–back-arc basin system occurred along the Bangong–Nujiang suture zone, central Tibet. Here we present new zircon U–Pb geochronology, rare earth elements (REEs) and bulk-rock geochemistry of these magmatic rocks in the Amdo area, the middle Bangong–Nujiang suture zone, central Tibet, to identify significant and new records of Mesozoic tectonomagmatic processes. Zircon U–Pb dating using LA-ICP-MS techniques yields a concordant age with a weighted mean 206Pb/238U age of 228.6±1.6Ma (n=7, MSWD=1.19) for the Quehala basalts, and a mean age of 220.0±2.1 (n=8, MSWD=1.5) for the Amdo pillow lavas. On the normalized REE patterns of zircon, significant Ce enrichment indicates that the magma sources of these magmatic rocks have been subjected to modification of slab-derived fluid. Geochemical features suggest that the Quehala basalts (ca. 228Ma), displaying an island arc tholeiites (IATs) affinity, resulted from partial melting of an relatively enriched mantle wedge in the subduction zone, whereas the Amdo pillow lavas (ca. 220Ma) characterized by both arc-like and N-MORB-like geochemical characteristics occurred as associated back-arc basin basalts (BABBs) at the spreading center of back-arc basin after the formation of island arc tholeiites. In conclusion, the volcanic rocks in the Amdo area have documented the magmatic processes from early-stage subduction to development of associated back-arc basin, confirming the occurrence of intra-oceanic subduction within the Bangong–Nujiang Tethys during the late Triassic. Furthermore, the spatial relationships among the Quehala formation, Tumengela formation and Amdo pillow lavas indicate likely northward subduction of the Bangong–Nujiang Tethyan Ocean during the Late Triassic to middle Jurassic.

Zircon U–Pb age and Hf isotopic compositions of Mesozoic granitoids in southern Qiangtang, Tibet: Implications for the subduction of the Bangong–Nujiang Tethyan Ocean

The southern Qiangtang magmatic belt was formed by the north-dipping subduction of the Bangong–Nujiang Tethyan Ocean during Mesozoic. To better understand the petrogenesis, time–space distribution along the length of this belt, 21 samples of several granitoid bodies, from west to east, in the Bangong Co, Gaize, Dongqiao and Amdo areas were selected for in-situ zircon U–Pb dating, Hf isotopic and whole-rock chemical analyses. The results suggest a prolonged period of magmatic activity (185–84Ma) with two major stages during the Jurassic (185–150Ma) and the Early Cretaceous (126–100Ma). Both the Jurassic and Cretaceous granitoids are high-K calc-alkaline I-type rocks, except the Cretaceous two-mica granite from Amdo in the east, which belongs to S-type. The granitoids are generated from different source materials as indicated by zircon Hf isotopic compositions. The Bangong Co and Dongqiao granitoids show high zircon εHf(t) values of −1.3–13.6 with younger TDMC ages of 293–1263Ma, suggesting a relatively juvenile source; whereas the Gaize and Amdo granitoids have low εHf(t) values of −16.1–2.9 with older TDMC ages of 999–2024Ma, indicating an old crustal contribution. These source rocks melt at different P–T conditions as suggested by Sr/Y ratio and TZr. The Sr/Y ratio of both stage granitoids increases with decreasing age. However, the TZr of the Jurassic granitoids decreases, whereas the TZr of the Cretaceous granitoids increases with decreasing age. The contrasting geochemical signatures of these granitoids may be controlled by the varying contribution of slab-derived fluids involved in the generation of the Jurassic and Cretaceous granitic magmas; i.e. increasing amount of fluids in the Jurassic, whereas decreasing amount of fluids in the Cretaceous. Therefore, it is proposed that the Jurassic and Cretaceous magmatism may be related to subduction and closure of the Bangong–Nujiang Tethyan Ocean, respectively. The age pattern of the Jurassic and Cretaceous granitoids suggests an oblique subduction of the Bangong–Nujiang Tethyan Ocean and a diachronous collision between the Lhasa and Qiangtang blocks.

Re-Os isotopic constraints on the evolution of the Bangong-Nujiang Tethyan oceanic mantle, Central Tibet

Geochemical (including Re-Os isotopic) studies of the mantle rocks of ophiolites in the Bangong-Nujiang suture zone in central Tibet have provided a coherent picture of the evolution of the Bangong-Nujiang Tethyan oceanic mantle from mid-ocean ridge (MOR) to subduction-zone (SSZ) settings.Clinopyroxene (cpx)-harzburgites and lherzolites in the Bangong Lake ophiolite were formed in a MOR setting, as demonstrated by the Cr# of spinels (<0.60) and whole-rock LREE-depleted patterns. Suprachondritic 187Re/188Os ratios (up to 1.833) of cpx-harzburgites and their spinels can be explained by interaction with melts derived from high Re/Os sources. Re-depletion (TRD) model ages (0.48-0.55Ga) suggest these rocks may represent a Pan-African domain beneath the Gondwana continent. High TiO2 contents of spinels and whole-rock samples imply that the lherzolites were formed through a refertilization process. Similarly, Re-Os isotopic systematics of sulfides in the lherzolites (187Re/188Os: 0.173-1.717, 187Os/188Os: 0.12646-0.17340) demonstrate that they are mixtures of primary and secondary sulfides. 187Os/188Os ratios (0.1211-0.1226) of whole-rock lherzolites give TRD ages of 0.73-0.97Ga, indicating the presence of Neoproterozoic lithospheric mantle under the spreading ridges.Mantle rocks in the SSZ-type ophiolites from Bangong Lake, Dongqiao and Nagqu reflect the complex evolution of the Bangong-Nujiang oceanic mantle during the SSZ stage. Most harzburgites from the Bangong Lake ophiolite give TRD ages of 1.0-1.5Ga, possibly representing relics of a Mesoproterozoic lithospheric mantle. However, three samples have both high Os contents (1.32-4.45ppb) and near-chondritic 187Os/188Os (0.1260-0.1297), and may represent Mesozoic oceanic lithospheric mantle. 187Os/188Os ratios of dunites and harzburgites from the Dongqiao and Nagqu ophiolites vary from 0.1174 to 0.1316 and give TRD ages up to 1.43Ga, also suggesting the existence of a Mesoproterozoic lithospheric mantle which has experienced mantle refertilization and later metasomatism caused by percolation of several generations of melts and/or fluids.We interpret the old mantle domains in the Bangong-Nujiang ophiolite belt as relics of ancient sub-continental lithospheric mantle (SCLM), which may have survived during the opening of the Bangong-Nujiang Tethyan ocean basin, and subsequently were incorporated into Mesozoic oceanic lithospheric mantle.

Petrogenesis and tectonic setting of the Queershan composite granitic pluton, eastern Tibetan Plateau: Constraints from geochronology, geochemistry and Hf isotope data

The Queershan composite granitic pluton is located in the north of the late Paleozoic Yidun arc collision-orogenic belt, eastern Tibetan Plateau. The main rock types are coarse-grained porphyritic alkalic-monzonite granite with minor fine-grained porphyritic monzogranite and granodiorite distributed in the eastern and southwestern regions. Here we report their zircon U-Pb ages and geochemical data. The intrusive contact relations indicate that granodiorite was formed earlier than the alkalic-monzonite granite (105.9±1.3 Ma) and monzogranite (102.6±1.1 Ma). These suggest that the Queershan composite granitic pluton was formed through three-stage magmatic events. The alkalic-monzonite granite (105.9±1.3 Ma) and monzogranite (102.6±1.1 Ma) are characterized by high SiO2 (73.5%–77.7%), K2O+Na2O (6.9%–8.5%), Ga/Al ratios (2.6–3.4) and low Al2O3 (11.8%–14.5%), CaO (0.25%–1.5%), MgO (0.18%–0.69%), negative Ba, Sr and Eu anomalies, showing A-type granite affinities. The granodiorite exhibits lower SiO2, P2O5 and K2O+Na2O contents, but higher Al2O3, CaO and MgO contents than alkalic-monzonite granite and monzogranite, showing I-type granite affinity. 176Hf/177Hf ratios of the alkalic-monzonite granite and the monzogranite are 0.282692–0.282749 and 0.282685–0.282765, respectively, and with similar ɛHf(t) values (−0.56 to 1.43 and −0.87 to 1.90 respectively). They also present similar TDM2 model ages (1.04–1.22 and 1.07–1.2 Ga respectively), indicating they may be sourced from a similar rock source, mostly like Kangding Complex. The homogeneity of the Hf isotopic compositions and the absence of the MMEs demonstrate that little depleted mantle materials have contributed to the source. We propose that the Mesoproterozoic crust materials of the Yangtze Craton exist beneath the Yidun arc terrane and support it was a dismembered part of the Yangtze Craton. The A-type granites of Queershan composite granitic pluton are most probably related to the closure of the Bangong-Nujiang Tethys ocean.

Northward subduction of Bangong–Nujiang Tethys: Insight from Late Jurassic intrusive rocks from Bangong Tso in western Tibet

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.

Slab breakoff triggered ca. 113 Ma magmatism around Xainza area of the Lhasa Terrane, Tibet

The Lhasa Terrane in southern Tibet is an ideal region to investigate the geodynamic processes of magmatism in relation to slab breakoff. Here we report zircon SHRIMP U–Pb ages and Hf-isotopes, and whole-rock geochemical data from the late Early Cretaceous magmatic rocks around Xainza. The rocks analyzed in this study include basalts, andesites, dacites, and rhyolites from the south of Xainza in the central Lhasa subterrane and the coeval rhyolites in Daguo from the north of Xainza in the northern Lhasa subterrane. The zircon U–Pb data indicate that the Xainza dacites and the Daguo rhyolites erupted contemporaneously at ca. 116Ma, approximately coeval with the Xainza basalts as judged from an adjacent volcanic sequence with systematic compositional change with time. The Xainza basalts are calc-alkaline and display geochemical signatures comparable to those of within-plate basalts rather than typical arc-type basalts. The Xainza andesites and dacites are high-K calc-alkaline and metaluminous to slightly peraluminous. The Daguo rhyolites belong to high-K calc-alkaline–shoshonite and are mostly strongly peraluminous. The whole-rock εNd(t) of the Xainza basalts (−3.5 to 0.6) is markedly different from those of the Xainza andesites (−13.6 to −13.0) and dacites (−9.4 to −9.1). The Daguo rhyolites are high-K calc-alkaline–shoshonite and peraluminous and display geochemical signatures similar to highly fractionated A2-type silicic rocks. Zircons from a rhyolite from Daguo yield εHf(t) values of −1.1 to +4.6, significantly different from those of the Xainza dacites (−8.2 to −4.4). The Xainza basalts are most likely derived from partial melting of an enriched mantle wedge that was metasomatized by subduction-related components, accompanied with significant contributions from subslab asthenospheric mantle. The andesites are generated from partial melting of an ancient lower crust, whereas the dacites are interpreted as the consequences of varying degrees of interaction between the mature Lhasa basement (Al-rich) and mantle-derived melts. The Daguo rhyolites are derived from partial melting of a juvenile crust and experienced significant fractional crystallization. In combination with the other existing data, we suggest that the generation of the ca. 113Ma magmatic rocks around Xainza is most likely triggered by the slab breakoff of the southward subducting Bangong–Nujiang Tethyan Ocean lithosphere. The geological records of the Lhasa–Qiangtang collision zone, particularly the presence of mafic rocks showing within-plate basalt geochemistry, A2-type silicic rocks, and coeval bimodal volcanic rocks in a continent–continent collision zone suggest a petrological association diagnostic of slab breakoff setting.

The origin and pre-Cenozoic evolution of the Tibetan Plateau

Different hypotheses have been proposed for the origin and pre-Cenozoic evolution of the Tibetan Plateau as a result of several collision events between a series of Gondwana-derived terranes (e.g., Qiangtang, Lhasa and India) and Asian continent since the early Paleozoic. This paper reviews and reevaluates these hypotheses in light of new data from Tibet including (1) the distribution of major tectonic boundaries and suture zones, (2) basement rocks and their sedimentary covers, (3) magmatic suites, and (4) detrital zircon constraints from Paleozoic metasedimentary rocks. The Western Qiangtang, Amdo, and Tethyan Himalaya terranes have the Indian Gondwana origin, whereas the Lhasa Terrane shows an Australian Gondwana affinity. The Cambrian magmatic record in the Lhasa Terrane resulted from the subduction of the proto-Tethyan Ocean lithosphere beneath the Australian Gondwana. The newly identified late Devonian granitoids in the southern margin of the Lhasa Terrane may represent an extensional magmatic event associated with its rifting, which ultimately resulted in the opening of the Songdo Tethyan Ocean. The Lhasa−northern Australia collision at ~263Ma was likely responsible for the initiation of a southward-dipping subduction of the Bangong-Nujiang Tethyan Oceanic lithosphere. The Yarlung-Zangbo Tethyan Ocean opened as a back-arc basin in the late Triassic, leading to the separation of the Lhasa Terrane from northern Australia. The subsequent northward subduction of the Yarlung-Zangbo Tethyan Ocean lithosphere beneath the Lhasa Terrane may have been triggered by the Qiangtang–Lhasa collision in the earliest Cretaceous. The mafic dike swarms (ca. 284Ma) in the Western Qiangtang originated from the Panjal plume activity that resulted in continental rifting and its separation from the northern Indian continent. The subsequent collision of the Western Qiangtang with the Eastern Qiangtang in the middle Triassic was followed by slab breakoff that led to the exhumation of the Qiangtang metamorphic rocks. This collision may have caused the northward subduction initiation of the Bangong-Nujiang Ocean lithosphere beneath the Western Qiangtang. Collision-related coeval igneous rocks occurring on both sides of the suture zone and the within-plate basalt affinity of associated mafic lithologies suggest slab breakoff-induced magmatism in a continent−continent collision zone. This zone may be the site of net continental crust growth, as exemplified by the Tibetan Plateau.

The Lhasa Terrane: Record of a microcontinent and its histories of drift and growth

The Lhasa Terrane in southern Tibet has long been accepted as the last geological block accreted to Eurasia before its collision with the northward drifting Indian continent in the Cenozoic, but its lithospheric architecture, drift and growth histories and the nature of its northern suture with Eurasia via the Qiangtang Terrane remain enigmatic. Using zircon in situ U–Pb and Lu–Hf isotopic and bulk-rock geochemical data of Mesozoic–Early Tertiary magmatic rocks sampled along four north–south traverses across the Lhasa Terrane, we show that the Lhasa Terrane has ancient basement rocks of Proterozoic and Archean ages (up to 2870Ma) in its centre with younger and juvenile crust (Phanerozoic) accreted towards its both northern and southern edges. This finding proves that the central Lhasa subterrane was once a microcontinent. This continent has survived from its long journey across the Paleo-Tethyan Ocean basins and has grown at the edges through magmatism resulting from oceanic lithosphere subduction towards beneath it during its journey and subsequent collisions with the Qiangtang Terrane to the north and with the Indian continent to the south. Zircon Hf isotope data indicate significant mantle source contributions to the generation of these granitoid rocks (e.g., ~50–90%, 0–70%, and 30–100% to the Mesozoic magmatism in the southern, central, and northern Lhasa subterranes, respectively). We suggest that much of the Mesozoic magmatism in the Lhasa Terrane may be associated with the southward Bangong–Nujiang Tethyan seafloor subduction beneath the Lhasa Terrane, which likely began in the Middle Permian (or earlier) and ceased in the late Early Cretaceous, and that the significant changes of zircon εHf(t) at ~113 and ~52Ma record tectonomagmatic activities as a result of slab break-off and related mantle melting events following the Qiangtang–Lhasa amalgamation and India–Lhasa amalgamation, respectively. These results manifest the efficacy of zircons as a chronometer (U–Pb dating) and a geochemical tracer (Hf isotopes) in understanding the origin and histories of lithospheric plates and in revealing the tectonic evolution of old orogenies in the context of plate tectonics.