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Previous studies of the Southern Qiangtang (SQ) Terrane in Tibet have shown significant growth of continental crust during the late Mesozoic (ca. 170–120 Ma) through underplating of mantle-derived melts beneath ancient lower crust. However, the mechanism of crustal growth remains defective during the Late Jurassic. In this contribution, we report new zircon U–Pb ages, whole-rock major and trace element concentrations, and zircon Hf isotopic compositions of Late Jurassic rhyolites from the Duobuza area in the SQ Terrane. Zircon LA–ICP–MS U–Pb dating yields concordant ages and weighted mean 206Pb/238U ages of 151–149 Ma, indicating that the rhyolites were erupted during the Late Jurassic. The rhyolite samples yield high SiO2 (69.1–73.4 wt.%) and Na2O (3.82–6.49 wt.%) concentrations, high values of the differentiation index (95.4–98.5), moderate Al2O3 (13.1–15.5 wt.%) and K2O (2.55–3.26 wt.%) concentrations, and relatively low TiO2 (0.34–0.44 wt.%), MgO (0.14–0.67 wt.%), and MnO (0.01–0.10 wt.%) concentrations and Mg# values (8.9–33.5). They are enriched in Rb, Th, Ba, and Pb, depleted in Nb, Ta, Sr, Ti, and P, and yield negative Eu anomalies. These characteristics suggest that the Duobuza rhyolites represent medium- to high-K calc-alkaline, highly fractionated I-type granitoids. Variations in major and trace element concentrations indicate that the rhyolites underwent significant fractionation of Fe–Ti oxides, plagioclase, K-feldspar, hornblende, biotite, and apatite. The samples yield positive zircon εHf(t) values (+0.08 to +10.9) and young TCDM model ages (816–386 Ma), indicating that they were generated through partial melting of juvenile lower crust of the SQ Terrane. These data are distinct from those of Late Jurassic intermediate–felsic plutons in the SQ Terrane that generally yield negative εHf(t) values and ancient TCDM model ages. We suggest that the occurrence of the Duobuza rhyolites is the significant witness. The presence of juvenile lower crust beneath the SQ Terrane during the Late Jurassic (ca. 150 Ma) rather than during the Early Cretaceous (ca. 120Ma). The transition from ancient to juvenile lower crust has already began in the SQ Terrane during the Late Jurassic. [ABSTRACT FROM AUTHOR]

Abstract The Gangdese magmatic belt is oriented E–W and located between the Indus–Yarlung Tsangpo and Bangong–Nujiang suture zones in Tibet. It records the subduction of the Neo-Tethyan oceanic crust, Indian–Eurasian collision, and uplift of the Tibetan Plateau. Here we present new zircon U–Pb geochronology, Hf isotopic compositions, and whole-rock geochemistry of the granodiorites in the Sumdo area of the Gangdese Belt. Our aim was to identify the petrogenesis of granitoid rcoks and to clarify the early-stage evolution of the Neo-Tethyan Ocean during the Late Triassic to Early Jurassic. The granodiorites yield zircon U–Pb ages of 193–201 Ma, indicating an Early Jurassic magmatic event. They are metaluminous, with medium- to high-K calc-alkaline geochemistry, and show characteristics of I-type granitoids. They show enrichment in light rare earth elements (LREEs) and depletion in high-field-strength elements (HFSEs) in primitive-mantle-normalized spider diagrams, indicating that their initial magma was generated in a subduction-related active continental margin. They have negative ε Hf (t) values (−11.3 to −4.1) and crustal model ages (TC DM) of 1.50–1.95 Ga. Hf isotopes and whole-rock geochemistry indicate that the granodiorites were derived from partial melting of ancient lower-crust material mixed with mantle material. Our data, in combination with published geochronological and geochemical data, show that the Late Triassic–Early Jurassic arc magmatism in the Gangdese magmatic belt was formed by the northward subduction of the Neo-Tethyan oceanic crust beneath the Lhasa Terrane. Highlights • Ca. 201–193 Ma granitoids occur in the Sumdo area of the Gangdese belt, Tibet • The rocks were derived from crustal melting and subsequent crust-mantle magma mixing. • Coeval magmatism in the Gangdese Belt is related to the Neo-Tethyan Ocean. [ABSTRACT FROM AUTHOR]

• The Banduo Pb-Zn deposit formed at the end of the Late Triassic, which is the oldest deposit in the Gangdese Metallogenic Belt. • The metal-rich magma was a mixture of mantle and crustal melts that experienced fractional crystallization. • The Banduo Pb–Zn deposit formed as a result of the northward subduction of the Neo-Tethys oceanic crust in the Late Triassic. The newly discovered Banduo Pb–Zn deposit in the Somdo area of Tibet is hosted by a Late Triassic granodiorite. A combination of zircon U–Pb dating, Hf isotopic compositions, sulfide Rb–Sr dating, and whole-rock geochemistry was used to determine the age of the mineralization and characterize the metallogenesis of the deposit. The weighted-mean U–Pb ages of zircon from the granodiorite (203.8 ± 1.7 Ma, MSWD = 0.53; 206.1 ± 0.4 Ma, MSWD = 0.098) and the weighted-mean Rb–Sr isochron age of sulfides from the Pb–Zn mineralization (202.7 ± 2.3 Ma; MSWD = 1.3) suggest that the timing of mineralization was close to that of granodiorite crystallization during the Late Triassic. The average (87Sr/86Sr) i value of 0.71026 implies that the ore-forming metals were sourced from a mixture of mantle and crustal material. The weakly altered granodiorite of the deposit is characterized by SiO 2 = 66.78–67.85 wt%, K 2 O = 2.57–3.37 wt%, Al 2 O 3 = 13.53–15.15 wt%, and Mg# = 45.5–51.3. It is enriched in LREEs and LILEs and depleted in HREEs and HFSEs, and it exhibits a negative Eu anomaly. The granodiorite has positive and negative εHf(t) values (−2.5 to 1.7) and crustal model ages (TC DM) of 1.14–1.40 Ga. Hf isotopic analyses and whole-rock geochemistry indicate that the Late Triassic granodiorite was likely sourced from of a mixture of mantle and ancient lower-crust material that had undergone variable degrees of fractional crystallization. Based on regional data, the Banduo deposit is the oldest Pb–Zn deposit in the Gangdese belt and likely formed as a result of the northward subduction of the Neo-Tethys Ocean. The metallogenic epoch indicates that deposits related to the northward subduction of the Neo-Tethys oceanic crust exist in this area. The Late Triassic age of granitoid-hosted mineralization provides additional prospects for exploration in the Gangdese Metallogenic Belt. [ABSTRACT FROM AUTHOR]