U-Pb Zircon Geochronology and Geochemistry of Granitoids in the Douling Group in the Eastern Qinling

LA-ICPMS U-Pb zircon dating of the Sanpinggou, Gangou and Fengzishan granitoids in the Douling Group of the Eastern Qinling yields ages of 760-685 Ma, which represents a strong tectono-magmatic event in the southern Qinling during the late Neoproterozoic. Geochemical data show that these intrusions have wide compositions ranging from minor gabbros through diorites to granodiorites. They are relatively enriched in LILE, poor in HFSE and strongly depleted in Nb and Ta, displaying affinities of I-type granites formed in an active continental margin with oceanic subduction. In contrast to granitoids, gabbros and enclaves in the granitoids have higher REE abundances, relatively flat REE patterns, lower LILE, slightly higher HFSE and more depletion in Nb and Ta. All these suggest that the gabbros were formed by partial melting of the upper mantle above the subduction zone and the granitoids by the partial melting of the lower crust. Combined with regional geological data, the subduction-related granitoids in the Douling Group, together with the Tuwushan A-type granite with an age of 725 Ma and contemporaneous basic dikes in the Wudang Block, provide evidence for local subduction of oceanic basins between different blocks during the rifling in the Southern Qinling in the Neoproterozoic. Thus, the coexistences of various magmatic rocks formed in different tectonic environments indicate a complicated tectonic evolution and variety of tectonic frameworks in the Qinling area in the Neoproterozoic.