Age and geochemical features of the Early Devonian Xiangquan A-type syenogranites from Baoji area at the conjunction of Qinling and Qilian Orogen and their tectonic significance

Xiangquan syengranites are located in Baoji area at the conjunction of Qinling and Qilian Orogen, and their LA-ICPMS zircon U-Pb geochronological and petrogeochemical studies were conducted in this paper. The results show that the zircon ²⁰⁶Pb/²³⁸Pb weighted average age of Xiangquan syengranites is 410±5Ma (MSWD=0.20, n=18), indicating that the crystallization age of the syengranites is Early Devonian. Xiangquan syengranites are high in SiO₂ (SiO₂=69.63%~73.94%), K₂O (K₂O=4.24%~4.88%, K₂O/Na₂O=1.23~1.44), and TFe₂O₃ (TFe₂O₃=2.10%~3.70%, TFe₂O₃/MgO=3.88~6.84), but low in MgO (MgO=0.31%~0.94%) and P₂O₅(P₂O₅=0.08%~0.21%), belonging to high-K calc-alkaline series with metaluminous features. In addition, they are enriched in total REE (318×10^-6~499×10^-6), with obvious negative Eu anomaly (δEu=0.37~0.46). The trace elements show enrichement of Rb, Th, Zr, Sm and Ga (10000×Ga/Al=2.59~2.93), depletion of Ba, Nb, Ta and Sr, with the characteristics of A-type granites on the whole. Combined with regional data, the authors hold that the Xiangquan syenogranites were formed in a post-orogenic tectonic setting and derived from partially melted felsic crust under low pressures.