SHRIMP ZIRCON U-PB DATING OF THE ERTAIBEI GRANODIORITE AND ITS ENCLAVES FROM EASTERN JUNGGAR, XINJIANG, AND GEOLOGICAL IMPLICATIONS

Abstract

High-precision SHRIMP zircon U-Pb dating has revealed that the Ertaibei granodiorite in eastern Junggar, northern Xinjiang, was formed at 299.3±8.7Ma, which represents the age of magma emplacement into upper crust, and that microgranular enclaves in the granodiorite were formed at 301.5±6.6Ma. Both ages are very consistent within errors, implying that the granodiorite and its enclaves were formed at the same time. In combination with fieldwork, petrography, and geochemical data, it is suggested that microgranular enclaves were not crystallized from granodioritic magma. They could result from the globules of more mafic magma, which mingled with the host granodioritic magma, while it was in a rapid cooling state. Such a process would result in the same zircon U-Pb ages for the granodiorite and its enclaves. It is possible that the Ertaibei granodiorite and other granitoids to the north of Ertis-Zaisan fault have the same formation ages as the peralkaline granites and other granitoids iv the south of the fault. All the granitoids were produced by postcollisional magmatism in the late Paleozoic. Low initial ~(87)Sr/~(86)Sr ratios and positive e_(Nd)(t) values from these granitoids indicate that they would be differentiates of mantle-derived magma, being the evidence for vertical growth of the continental crust resulting from the underplating of mantle-derived magmas in a postcollisional setting.