Early-Middle Triassic Adakitic and A-type Granite in Middle Segment of Central Asian Orogenic Belt: Petrogenesis and Tectonic Implications

Typical igneous rocks during the accretionary orogeny process in the Central Asian Orogenic Belt (CAOB) play a key role in understanding its tectonic evolution. We present new LA-ICP-MS in-situ zircon U-Pb and bulk geochemical data for the Dalunwusu monzogranite and granite porphyry suites which are located in Southernmost CAOB. The Dalunwusu monzogranites, have a crystallization age of (249.0±2.3) Ma, exhibit adakite-like geochemical characteristics, such as high Sr content and low Yb and Y contents, coupled with high Sr/Y values (88.55～140.34) and show a weakly negative Eu anomalies (δEu=0.68～0.98). Geochemical compositions indicate the Dalunwusu monzogranites derived from partial melting of mafic granulite in the lower thickened crust. The Dalunwusu granite porphyrys, have a crystallization age of (241.0±2.8) Ma, show typical geochemical features of A-type granites, which are characterized by having high SiO₂, low CaO and MgO content, high FeO^_T/(FeO^_T+MgO) and 10000×Ga/Al values. Moreover, the granite porphyrys show trace element features of A-type granites including rich in Zr, Nb, Ta abundances, and high values for Pb, Hf, Rb, K and Th, and low values for Ba, Sr, P, and Ti. Taking into account available data of the regional geological background, we may suggest that the adakites were products through partial melting of the thickened lower crust after the closure of the Paleo-Asian Ocean, and the A-type granite porphyry were likely produced at the tectonic setting of post-collisional phase with crustal extension and thinning. These two typical igneous rocks reflect the shift of geodynamic setting, from an earlier accretionary orogen environment to a later extensional setting during early- middle Triassic.