Petrogenesis of Granodiorite-Porphyry in the Qiaoqiaoshang Area, Eastern Margin of the Songpan-Garze Orogenic Belt, Sichuan——Constraints from Monazite Geochronology and Geochemistry

Granodiorite porphyry is exposed in the Qiaoqiaoshang area of Songpan County, Sichuan Province, located at the eastern margin of the Songpan-Garze Orogenic Belt. To investigate its formation age, petrogenesis and tectonic setting, this paper determines the crystallization age of the granodiorite porphyry and discusses its petrogenesis and tectonic background by means of petrography, monazite U-Pb geochronology and whole-rock geochemistry analyses.The results show that the monazite U-Pb age of the Qiaoqiaoshang granodiorite porphyry is 215.6 Ma, indicating that its diagenetic age is the Late Triassic. Whole-rock geochemical data reveal that the granodiorite porphyry is characterized by high silica (SiO2=70.50%~73.09%), peraluminous (A/CNK=1.09~1.19), low K2O/Na2O ratio (0.22~1.08) and high differentiation index (DI=83.1~90.3). With normative corundum contents ranging from 1.51% to 2.85%, the rock belongs to the peraluminous calc-alkaline S-type granite.In terms of rare earth elements, the rock is enriched in light rare earth elements (LREE) and depleted in heavy rare earth elements, with moderate negative Eu anomalies (δEu=0.61~0.79). For trace elements, it is enriched in Rb, U, Hf and Zr, but depleted in Ba, K, Nb and Ti. The characteristics of trace and rare earth elements indicate that the primary magma was derived from the partial melting of clay-poor metagreywacke in the upper crust under high-temperature conditions.Combined with the regional tectonic setting, geochronological and geochemical characteristics, it is concluded that the Qiaoqiaoshang granodiorite porphyry was a product of lithospheric delamination in a post-collisional setting during the Late Triassic.