Genesis of the high Sr/Y rocks in Qinling orogenic belt, central China

Abstract

The genesis of high-Sr/Y rocks and their connection with specific geodynamic settings are not yet resolved. Triassic granitic intrusions with high Sr/Y ratios are widespread in the Qinling orogenic belt of China. Three intrusions of granodiorite, granite porphyry, and dacite porphyry in the western Qinling orogenic belt have zircon LA–ICP–MS U–Pb ages of 237–216 Ma. The rocks are metaluminous to weakly peraluminous and are characterized by enrichment in large-ion lithophile elements (e.g., Rb, Ba, Th, and U), depletion in high-field-strength elements (e.g., Nb, Ta, and Ti), high Sr/Y (31.8–77.5) and (La/Yb)N (13.6–72.6) ratios, very low Y (5.3–16.3 ppm) and Yb (0.35–1.53 ppm) contents, and negligible Eu anomalies. Their (87Sr/86Sr)i ratios (0.70715–0.70884), εNd(t) (−5.93 to −3.07), zircon εHf(t) (−2.34 to +0.85) values and ancient two-stage model ages (TDM2Nd = 1.26–1.47 Ga; TDM2Hf = 1.20–1.39 Ga), and characteristic of low-temperature granitoid indicate that they are partial melts of Neoproterozoic lower crust, with input of enriched-mantle-derived mafic magma. There is no evidence of formation through high-pressure magmatic processes, and a proposed partial melting model indicates that the high-Sr/Y signature of these intrusive rocks may have been inherited from source rocks (lower crustal rocks of the South Qinling terrane). Studies of regional tectonic evolution have shown that the rocks were formed in a subduction regime and/or a subduction-to-initial-collision transitional setting. A high Sr/Y ratio in the magma source may therefore have imparted the high-Sr/Y signature, rather than it being associated with a specific geodynamic setting.