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Abstract: 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. [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 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: 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: 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. [Copyright &y& Elsevier]