Age and geochemical characteristics of Late Cretaceous leucogranites pluton and dykes in Xieqiong area, Tibet: constraints on the post-collisional setting of Bangong Co-Nujiang belt

The Late Cretaceous intrusive rocks in the Zuogong Block,eastern Tibet are rarely reported. This article reports on the granitic pluton and dykes invaded in the Neoproterozoic Youxi Group in the Xieqiong area,through petrology,geochemistry and zircon U-Pb chronology,to reveal its petrogenesis and formation environment. The granitic pluton type is tourmaline-bearing muscovite monzonitic granite,and the dike rock type is fine-grained granitic dike. The LA-ICP-MS zircon U-Pb ages of the two are 88.5±0.7 Ma and 85.8±2.3 Ma,respectively,both invaded in the Late Cretaceous. They contains minor muscovite,no hornblende.The granitic pluton and dike are characterized by high SiO₂(SiO₂=74.17%~78.88%),rich alkali(K₂O+Na₂O=5.92%~9.00%)and rich Al(Al₂O₃=12.61%~15.13%),A/CNK =1.11~1.40,Ritman index δ < 3.3,indicating that they belongs to the peraluminous calc-alkaline series; all are enriched in Rb,U,K,P and Pb elements and depleted in Nb,Ti and Ba elements; and strong negative Eu anomalies(δEu=0.18~0.56),which are indicative of peraluminous sub-alkaline S-type granite,and characteristics of a tourmaline-type leucogranite. It may be the product of dehydration and partial melting of water-bearing minerals(muscovite)in the upper crustal metamorphic mudstone caused by the transition from shortening and thickening extrusion environment to crustal extension environment. This result provides new constraints for the post-collision evolution process of the Bangong Co -Nujiang suture zone in the east.