Early Carboniferous subduction of Lungmu Co-Shuanghu Paleo-Tethys Ocean: Evidence from island arc volcanic rocks in Riwanchaka, Central Qiangtang

Though Lungmu Co-Shuanghu suture experienced the evolution process from early Paleozoic to Triassic, the existing data only show that there are numerous collision-related volcanic rocks along this suture in central Qiangtang, whereas information about subduction-related volcanic rocks is very insufficient. To tackle this problem, the authors report the zircon LA-ICPMS U-Pb dating results as well as petrological and whole-rcok geochemical data for andesites from the east side of Riwanchaka in the west part of the Lungmu Co-Shuanghu suture. One sample was dated at 330.6±2.8Ma, suggesting that the Riwanchaka andesites were formed in Early Carboniferous. Volcanic rocks were replaced by silicic fluid and display unstable SiO2 content (54.84%~72.84%) , but all the samples fall in the andesite field in the Nb/Y-Zr/TiO2×0.00001 diagram. Riwanchaka andesites belong to the low to medium-K calc-alkaline series in SiO2-K2O diagram. They are enriched in large-ion lithophile elements such as Rb, Th, U and K, and depleted in high field strength elements such as Nb, Ta and Ti, which are comparable with the characteristics of volcanic arc-related rocks. The Riwanchaka Early Carboniferous andesites in central Qiangtang were eventually emplaced at an island arc setting. The results obtained by the authors provide geochronologic and petrological evidence for early stage subduction process of Lungmu Co-Shuanghu paleo-Tethys Ocean.