Zircon U-Pb ages, geochemistry and geologi-cal significance of gneissic granodiorite in northern Lenghu, Qinghai Province

The gneissic granodiorite in northern Lenghu of Qinghai Province is characterized by low SiO₂ and MgO, and high Al₂O₃, Na₂O and CaO, with alumina saturation index A/CNK of 0.92~0.797. These features suggest that this gneissic granodiorite belongs to the high-K calci-alkaline peraluminous rocks and metaluminous rocks. The chondrite-normalized REE values show right-in-clined patterns with slightly negative Eu anomaly. The primitive mantle normalized trace element spidergrams are characterized by en-riched LILE(such as Rb, Th and U) and depleted HFSE(such as Nb, Ta and Zr), which is comparable with the characteristics of oro-genic magmatic rocks, suggesting a volcanic arc tectonic setting associated with subduction. Moreover, the gneissic granodiorite is characterized by rich Na₂O and A1₂O₃, high Sr, Sr/Yb and LREE, and inconspicuous differentiation of HREE. These features imply that the rocks in northern Lenghu of Qinghai were probably formed by the melting of basaltic rocks with amphibole as a residua phase. With the U-Pb isotopic age of 470.2±1.2Ma(n=19, MSWD=0.68) of granodiorite, the authors believe that north Al-tun oceanic crust subducted toward Oulongbuluke block in Early Ordovician.