Geochemistry, Hf Isotopes and Petrogenesis of Biotite Granodiorites in the Mohe Area

Li Liang; Sun Fengyue; Li Bile; Chen Guangjun; Xu Qinglin; Zhang Yajing; Qian Ye; Wang Linlin

Abstract：Previous studies are mostly concentrated on the biotite granodiorites exposed during the Indosinian period in southern Mohe county,while the research on the Early Yanshanian tectonic evolution in this area is relatively weak.This paper presents new zircon U-Pb dating,Hf isotope,major and trace elements for biotite granodiorites in southern Mohe County for the discussion in their geochronology,magma sources and tectonic setting.Zircon dating results demonstrate that biotite granodiorites were formed in Early Jurassic (185 ±-2 Ma and 182± 2 Ma).Geochemically,these rocks have A/CNK =0.90 ～ 1.03,Mg# =42～48,belonging to the high-K calc-alkaline series.The pluton has features similar to adakitic rock with high Sr (489 × 10-6 ～ 653× 10 6) and low Yb (1.33× 10-6 ～1.99 × 10-6).They have the geochemistry of arc magmatic rocks,which are enriched LREE and LILE (e.g.,Rb and K),depleted HREE and HFSE (e.g.,Nb,Ta and Ti),as well as very weak negative Eu anomalies (Eu/Eu* =0.79～0.95),indicating that the magma source was derived from the partial melting of lower crust.Zircon eHf(t) values and two-stage Hf model ages (tDM2) of doitite granodoirites range from-3.26 to-1.46 and 1.25 Ga to 1.59 Ga,respectively,indicative of formation from primary magmas generated by partial melting of thickened low crust that formed from the depleted mantle during the Meso-to Neoproterozoic.Considering the regional tectonic revolution,we suggest that the diotite granodiorite was formed in the setting of continuous southward subduction of Mongol-Okhotsk oceanic plate during the Early Mesozoic.