Petrogenesis Of Triassic Igneous Rocks From Cikatong Region, Northwest Yunan Province, And Its Tectonic Evolution Characteristics

In this study, we conducted a research of petrology, chronology and geochemistry on the Triassic igneous rocks generated in the suture of Yangtze Block and Gondwana Continents, where now geographically located in Deqin County, northwest of Yunnan Province.The rocks range from basalt, andesite, dacite to rhyolite, and as well as tuff in the study area, all of which are distributed in the Triassic layers of Pantiange and Cuiyibi Groups.Using U-Pb dating by LA-ICP-MS method, we obtained the zircon ages of rhyolites both from Cuiyibi and Pantiange Groups which are respectively(240.8±1.6) Ma and(241.2±1.2) Ma. The chronological results suggest they were formed at early Indo-china epoch.The basaltic rocks are subalkaline and sodium-enriched, pertaining to tholeiite series; while the intermediate rocks are subalkaline but potassium-enriched. They all show an enriched light rare earth element(LREE) but a depleted HREE pattern and negative Eu anomaly, with intermediate rocks having higher total REE concentration relative to basaltic rocks.The volcanic rocks generally show similar geochemical feature to the arc lavas from Jiangda-Weixi arc. The acidic rocks of Pantiange Group show a character of the crustal origin and may be generated by deep-crust melting during the period of the continental blocks collision. In contrast, the volcanic rocks of Cuiyibi Group with bimodal magmatism feature show a character of the mantle origin. The basaltic lavas were formed by mantle partial melting on post-collisional extension condition, subsequently experienced assimilation and factional crystallization during magma ascent in the crust that eventually lead to rhyolite generation. The interactive eruption of both basaltic and rhyolitic lavas explains the bimodal volcanism.