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Jurassic continental arc magmatism occurred widely across the southern Qiangtang Terrane, and provides an opportunity to study how continental arc magmas evolve towards intermediate–felsic compositions. We present new LA–ICP–MS zircon U–Pb ages, whole-rock major and trace element compositions, and Sr–Nd and in-situ zircon Hf isotope data for late Jurassic intermediate–felsic intrusive rocks from the Rena Co area of southern Qiangtang, central Tibet. Jurassic intrusive rocks in this area consist mainly of granodiorite, diorite, and related gabbro–diorite mafic enclaves, which have been dated at 153.1–147.6 Ma. Combining our data with previous results shows that continental arc magmatism in the southern Qiangtang Terrane has relatively homogeneous Sr–Nd isotopic compositions, with (87Sr/86Sr) i ratios of 0.7069–0.7092 and ε Nd (t) values of −6.3 to −2.2. Assuming that the hot, hydrous, basaltic melt generated by the subduction of the oceanic slab has a composition similar to island arc basalts, mixing ~50% basaltic magma with ~50% lower crustal material can produce the Sr–Nd isotopic compositions of the Rena Co intrusive rocks. The southern Qiangtang arc magmatism also has similar Hf isotopic compositions, albeit with a large range of ε Hf (t) values (−10 to +3). The different Rena Co intrusive rocks have different geochemical characteristics, including major element and rare earth element (REE) contents, Mg#, and evolutionary trends on Harker diagrams. This suggests that these arc rocks were derived from a common parent magma, but experienced different magmatic processes. The diorites were derived from primitive arc magma through amphibole-dominated low-pressure fractional crystallization, whereas the granodiorites underwent garnet-dominated, high-pressure fractional crystallization in secondary magma chambers. Fractionation of igneous garnet and amphibole played a significant role in the evolution of the continental arc, and was likely an important mechanism in driving the continental arc towards silica-rich compositions. The fractionation of igneous garnet and amphibole possibly changes the Mg# and Nb/Ta ratios of the derivative magmas. • Amphibole-bearing schlieren (MEs) are the direct evidence of the fractional crystallization. • The diorites were derived from initial arc magma through an amphibole dominated LPFC. • The granodiorites were derived from initial magma through an a garnet dominated HPFC. • Igneous garnet and amphibole fractionation play a significant role in continental arc. [ABSTRACT FROM AUTHOR]

Abstract: Triassic Paleo-Tethyan subduction-related volcanic rocks are extensively distributed in the Xiangcheng and Changtai regions, northern Yidun Terrane, eastern Tibet. The ~228Ma volcanic rocks in the Xiangcheng region have intermediate-felsic composition (60.1 to 64.7wt.% SiO2) with relatively low Y (14–18ppm) and high Sr (630–1830ppm) yielding high Sr/Y ratios (43–94). They have LREE-enriched and HREE-depleted REE patterns (La/Yb=19–26), typical of adakites. Sr–Nd isotopic compositions of these volcanic rocks are similar to those of the contemporaneous high silica adakitic rocks in the southern Yidun Terrane. The ~231–230Ma volcanic rocks in the Changtai region are composed of basalts, andesites, dacites and rhyolites with SiO2 ranging from 47.7 to 81.0wt.%. Basalts in the Changtai region have initial 87Sr/86Sr ratios from 0.7052 to 0.7058 and εNd(t) from +1.1 to +1.5. They display OIB-like geochemical signatures, and were likely derived from low degrees of partial melting of an OIB-like mantle source with subordinate input of subduction components indicated by increasing Th content. Andesites and dacites show elemental and isotopic compositions that evolved from the basalts, indicative of derivation via crustal contamination and fractional crystallization (AFC) of basaltic magmas. Rhyolites have lower initial 87Sr/86Sr ratios and higher εNd(t) values than andesites/dacites, inconsistent with an AFC process and, are suggested to have been derived from anatexis of crustal materials. Emplacement of volcanic rocks in the Changtai and Xiangcheng regions could be attributed to the subduction of the Ganzi–Litang Ocean, a branch of the Paleo-Tethys Ocean. Volcanic rocks in the Changtai and Xiangchang regions have zircon U–Pb ages about 4–6myr older than that of arc granites in the eastern Yidun Terrane. The spatio-temporal distribution of the volcanic rocks indicates that the subduction was initiated under the southern Yidun Terrane in the Middle Triassic and extended to the northern Yidun Terrane at the beginning of the Late Triassic. [Copyright &y& Elsevier]

The processes and related rock types in post-collision setting are important to fully understand the geodynamic evolution of a collision zone. Bangong-Nujiang collision zone is the oldest collision zone in the Tibet Plateau. Slab break-off is one of the primary processes during post-collision, and is considered to have occured in the Bangong-Nujiang collision zone. Here, we report a newly identified rhyolite-biotite monzogranite (A2-type granite)-norite belt from this collision zone, which trend E-W for nearly 1000 km, and formed 115–104 Ma. The felsic rocks include rhyolite and biotite monzogranite, who share whole-rock geochemical similarities with A2-type granites. These felsic rocks were likely generated from a juvenile mafic crust melting at low pressure and high-temperature. The juvenile crust was derived from melting of metasomatized mantle wedge. The norites were generated from low degree (1–5%) melting of remnant enriched lithospheric mantle mixed with upwelling asthenospheric mantle at low pressure (~1.0 GPa) and high melting temperature (up to 1244 °C). A slab break-off beneath the Lhasa Terrane accounts for this post-collisional high-temperature extensional environment. The results provide important implications for the evolution of the Bangong-Nujiang collisional zone, including the subducting polarity, and timing of the Bangong-Nujiang ocean closure. • ~1000 km long rhyolite-A2-granite-norite belt in Bangong-Nujiang collision zone, Tibet • Felsic rocks formed from melting of a juvenile mafic crust. • Norites formed by asthenospheric and metasomatized lithospheric mantle interaction • The belt formed in a low-P/high-T post-collisional setting associated with slab break-off. [ABSTRACT FROM AUTHOR]