Paleo-Pacific Subduction in the Interior of Eastern China: Evidence from Adakitic Rocks in the Edong-Jiurui District

Abstract

The Edong-Jiurui district is located more than 1000 km from the current Pacific subduction zone. It is part of the well-known middle and lower Yangtze River Cu-Fe-Au belt in central eastern China. Cu mineralization in this area is spatially and temporally associated with Late Mesozoic magmatic rocks. These rocks exhibit geochemical features of adakites, but their origin is not yet fully understood. To explore the relationship between Cu mineralization and Mesozoic magmatism, we report geochemical, Sr-Nd-Pb isotopic, and zircon U-Pb age data from adakitic rocks in the Edong-Jiurui area. Zircon U-Pb ages point to a protracted period of magmatic activity from 151 to 139 Ma. This time span coincides with the Cu mineralization (146–137 Ma) in the middle and lower Yangtze River belt. Adakitic features of the rocks are displayed by high contents of SiO2, Al2O3, Na2O, and Sr; enrichment of light rare earth elements (REEs) and large-ion lithophile elements; depletion of heavy REEs; positive Sr and negative Nb, Ta, and Ti anomalies; and high Sr/Y and La/Yb ratios. We favor a model of melt segregation from a plagioclase-free and garnet-bearing residue. Compared to non-Cu-bearing Mesozoic adakitic rocks in the Dabie terrane, adakitic rocks in the Edong-Jiurui area have higher initial εNd values (−3.4 to −6.3), Pb isotopic ratios, and Th contents and lower Pb/Ce values. Altogether, these features indicate that the melts were probably derived from subducted ocean mixed with marine sediment.