Geochronology, magmatic genesis and tectonic setting of the Musidanjiao tufflava in the Asa area, Tibet

In order to further explore the evolution history of Cretaceous magmatism in the Asa area, central Tibet, samples were collected from Early Cretaceous rhyolitic tufflava near Musidanjiao, south of the Asa area, to carry out the geochronological and geochemical studies.The mean zircon ²⁰⁶Pb/²³⁸U age of Musidanjiao tufflava yields 129.3±1.9 Ma(n=17, mswd=0.039).The geochemical test results show that the tufflava has extremely high SiO₂ content(76.27%~78.70%), moderate Al₂O₃(11.23~12.21%), total alkali content of 5.60%~6.30%, very low total iron(1.16%~1.50%), CaO(0.62%~1.40%), MgO(0.19%~0.24%), TiO₂(0.09%~0.15%), and P₂O₅(0.02%~0.02%) contents.Moreover, they show enrichment of large ion lithophile elements such as U, Th and Pb, and depletion of high field strength elements such as Nb, Ta and Ti, as well as enrichment of LREE, flat HREE and obvious negative Eu anomaly.These characteristics indicate that they might be originated from the partial melting of middle crust and experienced significant crystallization differentiation in the later period, which is geochemically similar to the typical Ⅰ-type granitoids developed in the continental margin arc region.In combination with the reported continental margin island arc magmatism of~120 Ma and the extensional magmatism of~105 Ma in the Asa area, it is suggested that the Early Cretaceous rhyolitic taffflava magmatism of~130 Ma in the Asa area might be formed in the background of the southward subduction of Bangong-Nujiang Ocean.