Early Indosinian high-Mg# and high-Sr/Y ratio granodiorites in the Xiahe area, West Qinling, Central China: Petrogenesis and geodynamic implications

Abstract

Indosinian granitic rocks are widespread in the West Qinling Orogen (WQO) in central China. Some of the early Indosinian granitoids have high Sr and low Y contents, and therefore high Sr/Y ratios, and they have been interpreted as adakitic rocks that formed by partial melting of thickened continental lower crust. However, several studies have revealed that granitoids with high Sr/Y ratios can be formed by various geological processes. Here, we report new zircon U-Pb ages, whole–rock geochemical data, and Sr–Nd–Hf isotope data for the Ganjiagongma pluton in the Xiahe area of the WQO, and we discuss whether the WQO underwent crustal thickening during the early Indosinian. The zircon U-Pb dating results indicate that the Ganjiagongma granodiorite and its mafic microgranular enclaves (MMEs) crystallized at ca 242 Ma. The MMEs are characterized by low SiO2 contents and Sr/Y ratios, and high MgO, Cr, and Ni contents, as well as high Mg# values, and they have evolved zircon Hf and whole–rock Sr-Nd isotopes, indicating they were produced by the partial melting of metasomatized continental lithospheric mantle. The granodiorite samples have some adakitic characteristics with SiO2 = 65.22 to 68.01 wt%, MgO = 1.07 to 1.71 wt%, Mg# = 59 to 62, Sr = 357 to 620 ppm, Y = 9.7 to 17.4 ppm, Sr/Y = 30.26 to 47.46, and no significant Eu anomalies. However, the granodiorites have relatively flat HREE patterns, indicating that amphibole, rather than garnet was the main residue in the source area, and this proposal is consistent with the relatively low but constant Nb/Ta values and varied Zr/Sm ratios. Simple two-end member modeling results indicate that the granodiorite could have been formed in normal or slightly thickened crust by mixing of 20%–30% mantle-derived melt with 70%–80% melt derived from the partial melting of basement rocks in the continental lower crust of the WQO. Taking into account other geological evidence, we propose that the formation of the Ganjiagongma pluton was related to the subduction of the A'nimaque-Mianlue oceanic crust, and that no significant thickening of the continental crust occurred in the WQO during the early Indosinian.