Late Ordovician amphibolites in the Taoxinghu area of central Qiangtang, northern Tibet, and their tectonic significance

Abstract

The early Paleozoic tectonic evolution of the Qinghai-Tibetan Plateau remains not fully understood. The Longmu Co-Shuanghu Suture Zone within the Qiangtang block of northern Qinghai-Tibetan Plateau, records important evidence of the early Paleozoic evolutionary history of the plateau. In this study, we investigated early Paleozoic amphibolites exposed in the Taoxinghu area of central Qiangtang, using zircon U–Pb dating, mineral chemistry, whole-rock geochemistry, and zircon Hf–O isotopes. The objective was to identify the characteristics of the protoliths and the metamorphic processes. Two types of zircons with different characteristics occur in the amphibolites, yielding zircon U–Pb ages of 490 and 440 Ma attained using kernel density estimation, which represent the ages of the protolith and subsequent metamorphism, respectively. The amphibolites have SiO₂ (42.4–55.7 wt.%), and Al₂O₃ (14.4–18.7 wt.%) and MgO (3.62–8.80 wt.%) contents. Their geochemical characteristics show large ion lithophile elements enriched and high field strength elements depleted. The zircon U–Pb age and rock geochemical data suggest the protoliths of the amphibolites were late Cambrian supra-subduction zone-type ophiolites that organized in association with Proto-Tethyan subduction. The subsequent Late Ordovician metamorphic event might be associated with the collision between continental blocks after oceanic closure. We favor that amphibolites provide essential constraints on the early Paleozoic tectonic evolutionary history of the Qinghai-Tibetan Plateau.