Genesis of the Indosinian Guangtoushan adakitic biotite plagiogranite in the Mianxian-Lueyang (Mianlue) suture, South Qinling, China, and its tectonic implications

Geochemical study of the Indosinian Guangtoushan adakitic biotite plagiogranite at the northern side of the Mianlue suture zone indicates that the rocks are characterized by SiO2≥56%, Al2O315% and Na2OK2O, enrichment in LILE and LREE, an unpronounced negative Eu anomaly, strong enrichment in Sr (Sr=704.61-1052.81 μg/g with an average of 828.45 μg/g) and depletion in Y (Y=7.17-9.68 μg/g with an average of 7.66 μg/g) and high Sr/Y ratios (80.74-281.41), exhibiting the marked geochemical properties of adakite. According to those properties, combined with the temporal-spatial location of occurrence of the intrusion, it is thought that the Guangtoushan intrusion formed in a typical post-collisional tectonic environment. It is possible that the collision between the North China and Yangtze plates during the Triassic resulted in break-off of the subducting slab, upwelling and underplating of asthenospheric materials and thickening and fusion of the lower crust, thus generating melts of adakitic affinity and garnet-bearing residual phases.