Zircon Chronology and Hf Isotope Study of Sanyanlong-Galazi Granite in the Southern Songpan-Garze Orogenic Belt

The Sanyanlong-Galazi granite complex is located in the southern part of the Songpan-Garze orogenic belt, a region with unique structural characteristics. Detailed studies of its chronology and isotopic features are crucial for understanding the complex tectonic evolution and basement properties of the orogenic belt. Through detailed field investigations and microscopic observations, using LA-ICP-MS zircon U–Pb dating and Lu–Hf isotopic analysis, we found that this complex granite body was emplaced in the Triassic Xikang Group strata. It exhibits a rock evolution sequence of quartz diorite, granodiorite, and monzogranite. The formation ages of monzogranite and granodiorite are 211.5 ± 1.2 and 215.6 ± 1.1 Ma, respectively. Zircon Hf isotopic 176Hf/177Hf values range from 0.282347 to 0.282640, with εHf(t) values ranging from –10.38 to –0.50, and two-stage Hf model ages from 1173 to 1896 Ma. Comprehensive analysis suggests that the Sanyanlong-Galazi pluton formed in the late Indosinian period within a post-collisional tectonic setting. This indicates that the Garze-Litang ocean had closed by the Late Triassic (215 Ma). The material source is newly formed crust from the early to middle Proterozoic, confirming the possible existence of a Precambrian basement similar to the Yangtze Block in the southern margin of the Songpan-Garze orogenic belt.