LP/HT metamorphic rocks in Wulan County, Qinghai Province: An Early Paleozoic paired metamorphic belt on the northern Qaidam Basin?

Abstract

Low-pressure/high-temperature (LP/HT) sillimanite-garnet-biotite-plagioclase gneisses occur south of Erlangdong area in Wulan County on the northern margin of the Qaidam Basin. They consist of garnet, biotite, plagioclase and sillimanite, which have experienced high amphibolite-facies metamorphism with peak metamorphic conditions of <italic>T</italic>=677-696℃ and <italic>P</italic>=0.349-0.419 GPa. Twenty-nine spot analyses on zircons using LA-ICP-MS method yield a concordant ²⁰⁶Pb/²³⁸U age of 483-450 Ma representing the time of the peak LP/HT metamorphism. We suggest that the LP/HT high amphibolite-facies rocks were formed by the northward subduction of the Proto-Tethyan Ocean beneath the Qilian Block but before final continental collision with the Qaidam Block. A strong thermal anomaly associated with arc magmatism and migmatization affected the base of the Qilian Block resulting in formation of the LP/HT high amphibolite-facies rocks in Erlongdong area. The age of these LP/HT rocks are the same as nearby high-pressure assemblages and the spatial relationships between the two metamorphic facies indicate this to be a paired metamorphic belt. We interpret this to have developed along the northern margin of the Qaidam Basin during the Ordovician.