秦祁结合部晚奥陶—早志留世埃达克质片麻状花岗闪长岩体年代学、地球化学及其构造意义OA

The Longshan area is located at the junction of Qinling and Qilian,and the Qijiawan adakitic gneissose granodiorite is located at the eastern end of the North Qilian orogenic belt in the Longshan area,intruding into the Longshan rock group.Zircon and monazite U-Pb geochronological studies conducted on this pluton yielded weighted mean ages of 445.3±2.3 Ma and 435.9±2.2 Ma,respectively.Zircon U-Pb dating of the mafic enclaves within it provided a weighted mean age of 445.0±3.8 Ma.These results indicate that the Qijiawan rock mass formed during the Late Ordovician and that this phase of magmatic activity persisted over an extended period(445～436 Ma).The gneissose granodiorite exhibits high SiO2(64.97％～69.70％),Al2O3(15.66％～17.00％),and Na2O(4.40％～5.99％)contents,with K2O/Na2O ratios of 0.22～0.35,and A/CNK values of 0.88～1.06,classifying it as metaluminous to weakly peraluminous and calc-alkaline.The pluton is characterized by low Y(3.95×10-6～9.21×10-6),low Yb(0.37×10-6～0.97×10-6),high Sr(604×10-6～762×10-6),high Sr/Y ratios(73～181),and δEu values of 0.69～1.38.Analyses of major and trace elements,along with U-Pb isotopes,reveal that the pluton exhibits characteristic features of adakitic rocks.It originated from the partial melting of a subducted oceanic slab,followed by interaction with mantle materials to a certain extent.Based on the regional geological context,we propose that the Qijiawan gneissose granodiorite formed in a back-arc basin setting induced by northward subduction of the eastern North Qilian oceanic crust.This magmatic event persisted over an extended duration,spanning from the Late Ordovician to the Early Silurian.