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Abstract: Mesozoic post-collisional andesites occur in the Gerze area within the Qiangtang Terrane of northern Tibet, China. Geochronological, geochemical, and whole-rock Sr–Nd isotopic analyses were performed on this suite of volcanic rocks to characterize their petrogenesis. Laser ablation-inductively coupled plasma-mass spectrometry U–Pb zircon analyses yielded consistent ages ranging from 122.4±0.4Ma to 124.4±0.4Ma for two andesite samples (JC01 and GZ01). The andesites belong to the alkaline and sub-alkaline magma series in terms of K2O+Na2O contents (4.7–10wt.%), and belong to the high-K calc-alkaline and calc-alkaline series on the basis of their high K2O contents (1.0–3.3 wt.%). The volcanic rocks are further characterized by the enrichment of light rare earth elements [(La/Yb)N =7.4–11.5] and large-ion lithophile elements (e.g., Rb), slightly negative Eu anomalies (Eu/Eu* =0.79–0.94), and negative anomalies in high field strength elements (e.g., Nb, Ta, and Ti) relative to primitive mantle. The samples show slightly elevated (87Sr/86Sr)i values that range from 0.7049 to 0.7057, and low ε Nd(t) values of −3.4 to 0.7. These results suggest that the rocks were derived from a compositionally heterogeneous mantle source and that their parent magmas are basalt magma. The parent mafic magmas may have undergone fractional crystallization of olivine, pyroxene, hornblende, and to a lesser extent, plagioclase during ascent through the thickened crustal pile, with minor or no crustal contamination, before the finally erupting as andesites. [Copyright &y& Elsevier]

Abstract: High-pressure (HP)/low-temperature (LT) metamorphic rocks, such as eclogite and blueschist, are generally regarded as an indicator of subduction-zone metamorphism. Eclogites have recently been discovered in the central Qiangtang Block. Their occurrence is highly significant to the understanding of the closure of the Paleo-Tethys and tectonic evolution of northern Tibet. We report the results of petrological, mineralogical and geochronological investigations of these rocks, and discuss their tectonic implications. The Qiangtang eclogite occurs as blocks and lenses in Grt–Phn schist and marble, and is composed of garnet, omphacite, phengite and rutile. Eclogitic garnet contains numerous inclusions, such as glaucophane and phengite in the core, and omphacite in the mantle or inner rim. In strongly retrograded eclogite, the omphacite is replaced by glaucophane, barroisite and albite. Four stages of metamorphic evolution can be determined: (1) prograde blueschist facies; (2) peak eclogite facies; (3) decompression blueschist facies and (4) retrograde greenschist facies. Using the Grt–Omp–Phn geothermobarometer, a peak eclogite facies metamorphic condition of 410–460°C and 2.0–2.5GPa was determined. Zircon U–Pb dating gave ages of 230±3Ma and 237±4Ma for two eclogite samples. The ages are interpreted as the time of eclogite facies metamorphism. Moreover, 40Ar/39Ar dating of phengite from the eclogite and Grt–Phn schist yielded ages about 220Ma, which are probably indicative of the time of exhumation to the middle crust. We conclude that the HP/LT metamorphic rocks were formed by northward subduction of the Paleo-Tethys Ocean and they marked a Triassic suture zone between the Gondwana-derived block and Laurasia. [Copyright &y& Elsevier]