LA-ICP-MS zircon U-Pb dating for high-pressure basic granulite from North Qinling and its geological significance

Abstract

A combined study of CL imaging, LA-ICP-MS U-Pb dating and trace element analysis were carried out for zircon grains from high-pressure basic granulite at Songshugou in North Qinling. The zircons display core-rim internal texture. The cores are made up of sector or planar domains, partially displaying weak zoning or misty texture, and have the feature of low content of Th, U, REE, HREE, low Th/U ratio and high (Nd/Yb)n ratio, and the REE pattern typical for zircon coexisting with garnet. These features indicate the metamorphic genesis for the zircon cores. The rims have narrow light margins (<30im in width) around the cores and boundaries between core and rim are rounded or lough-like, indicating that the rims were formed during retrograde metamorphism. U-Pb isotopic data of 24 spots from the cores of 11 zircon grains yield a 206Pb/238U weighted average age of 485±3.3 Ma, which is interpreted as the peak metamorphic age of high-pressure (HP) basic granulite. This age is well consistent within error with that of ultrahigh-pressure (UHP) eclogite and its country rock gneiss (507±37 Ma) that occur at the north side of the Qinling Group, possibly indicating the occurrence of two contemporaneous Early-Paleozoic HP to UHP metamorphic rock belts on the north and south sides of the Qinling Group. Combining with the regional geological studies and the distribution of Early-Paleozoic granites related to subduction and collision, the two HP to UHP metamorphic belts may be the products of continental crust bi-directional deep subduction toward Qinling micro-block caused by the subduction and collision of Paleozoic Qinling ocean and Erlangping ocean, respectively.