High-Mg# Adakitic Tonalite from the Xichahe Area, South Qinling Orogenic Belt (Central China): Petrogenesis and Geological Implications

Abstract

The Xichahe tonalite emplaced into the Proterozoic Foping metamorphic complex, south Qinling Mountains, central China, has been dated at 213.6 ± 2.2 Ma using the LA-ICP MS zircon U-Pb method, indicating that it is post-orogenic (≤242 ± 21 Ma). With the exception of higher Mg# (52.94-67.59) values, the tonalite has compositional similarities to high-silica adakites from suprasubduction-zone tectonic settings. Chondrite-normalized rare-earth element patterns of the tonalite are characterized by high (La/Yb)N ratios and concave-upward shapes of HREE, and without a significant Eu anomaly (δEu = 0.65-1.10). In conjunction with high Ba (877.13-1963.52 ppm) and Sr (820.76-1252.75 ppm), low Y (10.58-18.30 ppm) and HREE, e.g., Yb = 0.79-1.51 ppm), trace elements, and REE patterns suggest that the tonalite magma protolith was most likely a feldsparpoor, garnet ± amphibole-rich eclogite assemblage. Isotopic values of 87Sr/86Sr = 0.7065-0.7069, I sr = 0.7054-0.7064, 143Nd/144Nd = 0.5123-0.5124, ∊Nd(t) = -1.52 to -3.62, 206Pb/204Pb = 18.294-18.349, 207Pb/204Pb = 15.993-15.997, and 208Pb/204Pb =39.502-39.526, are similar to those of the Proterozoic Yaolinghe Group mafic volcanics in the south Qinling Mountains; the Nd isotopic model age of 1.15-1.38 Ga is close to the formation age (~1.1 Ga) for the Yaolinghe metabasalts. High Mg# and Nb/Ta ratios, and Cr and Ni contents of the Xichahe adakitic tonalitie indicate that the parental magma mixed with a mantle melt. Based on the geological setting, it is concluded that the Xichahe tonalite was unlikely to have been produced by partial melting of recycling lower continental crust, and that mixing of the parental adakite magma with mantle melts resulted from Triassic asthenospheric upwelling caused by breakoff of the subducting Mianlue oceanic slab during the postorogenic stage of the South Qinling orogenic belt.