Petrogenesis Of Xiangtaohu Amphibolites From North Tibet: Implications For The Tectonic Evolution Of Qiangtang

The absence of petrographical, metamorphic and geochemical studies on the Early Paleozoic amphibolite-facies metamorphic rocks from Qiangtang, Northern Tibet, constraints the discussion of their petrogenesis and tectonic significance. Herein, on the basis of geological survey, we report petrology, mineralogy and whole-rock geochemistry for Xiangtaohu amphibolites, and combine with geochronological and geochemical limited data available in order to investigate their petrogenesis and perfect the tectonic evolution of Qiangtang in Cambrian-Carboniferous period.Xiangtaohu amphibolites occur in the west part of Longmu Co-ShuanghuLancangjiang Suture(LSSZ), they are primarily unconformably overlain by the Paleogene Niubao Group, and are partly in structural contact with late Paleozoic ophiolite, meanwhile, a little plagiogranite intrude into them and high-pressure(HP) basic granulites occur as lens-shaped bodies of different sizes in them. Detailed petrography and metamorphism reveal that they are mainly composed of amphibole and plagioclase, with minor diopside in some very local areas, and they have high amphibolite facies metamorphic p-t condition: T=626-672 ℃, P=0.55-0.66 Gpa. According to the geochemistry, Xiangtaohu amphibolites have features of Ocean island basalts(OIB) and Enriched mid-ocean ridge basalts(E-MORB) with enrichment of high field strength elements such as Nb and Ta, and their protolith were formed in a mid-ocean range with the superposition of mid-ocean ridge axis type mantle plume, with no crustal contamimation.Xiangtaohu amphibolites have similar metamorphic ages, zircon morphology, characteristic of Th and U and different mineralogy, geochemistry, metamorphism with HP granulites, suggesting that they were not the products during retrogression of HP granulites. Based on previous research data and our new results, Xiangtaohu amphibolites and HP granulites both are the products of northward subduction of Longmu Co-Shuanghu-Lancangjiang Ocean, and we conclude that this ocean should have subducted in the Late Silurian rather than the Early Carboniferous.Finally, we propose that Xiangtaohu amphibolites have been subjected to an Andean-type orogenic event, and that “flat-slab subduction” and “slab rollback” played important roles in this event.