Discovery of middle- and high-grade metamorphic rocks in the central Qiangtang basin, Tibet, China

Up to now, no gneiss has been reported in the Qiangtang basin. Divergent views have long existed as to whether the pre-Paleozoic crystalline basement occurs in the basin. Recently gneiss with the medium- and high-grade metamorphic features has been found in the vicinity of Ejiumai near Lanxin Mountain on the northern margin the Central uplift zone in the Qiangtang basin. Through tracing, the authors found a gneiss outcrop, which is 50-200 m wide and 4 km long, extending discontinuously along a thrust. Petrographic study shows that the gneiss contains the typical regional metamorphic minerals sillimanite and kyanite, and in addition, it has been found that the nearby Ordovician only undergone low-grade metamorphism and recent geophysical data have revealed that there might be crystalline basement in the Qiangtang basin. Based on these findings, the authors preliminarily conclude that the Ejiumai gneiss is a slice of pre-Ordovician crystalline basement in the Qiangtang basin and that a stable crystalline basement might occur at the depth of the Qiangtang basin.