SHRIMP zircon U-Pb ages of garnet pyroxenite and Fushui gabbroic complex in Song- shugou region and constraints on tectonic evolution of Qinling Orogenic Belt

摘要

"Qinling Group (?)" in Songshugou region in the north of the Shangdan Fault consists predominantly of amphibolite with or without garnet and amphibole-plagioclase gneiss and intercalated with lensiod blocks of garnet pyroxenite, where the famous Songshugou dunite body occurs (Fig. 1). Petrographic study reveals that garnet pyroxenite is the representative rock of early high-pressure granulite-facies metamorphism, whereas amphibolite with or without garnet is the retrograde product Much attention hasbeen paid to the tectonic environment and forming ages of these rocks for a long time due to the importance of their tectonic position Li et al. reported a Sm-Nd isochron age of 983+-140 Ma for the garnet pyroxenite, and Zhou et al. also reported an Sm-Nd isochron age of 1030+-46 Ma for the amphibolite. Based on these ages, most researchers regarded these rocks as typical Proterozoic ophiolite components and therefore suggested existence of a Neoproterozoic ocean in this region (see summaries by Zhang et al. and Zhang and Zhou). The Fushui complex body is located in the north of this garnet-pyroxenite + amphibolite sequence. It consists of weakly metamorphosed gabbro and diorite, occupies an area of about 40 km~2, and extends along the tectonic lines of Qinling Group. Petrological and geo-chemical studies suggested that this complex body formed in the island-arc environment. However, its forming age is not clear, nor the relationship with the Songshugou ophiolite sequence. This contribution reports the zircon ion microprobe (SHRIMP) dating results for Songshugou garnet pyroxenite and Fushui gabbro. These data play an important role in understanding tectonic evolution of the Qinling Orogenic Belt.