The collapse of Tengchong-Gangdise Cenozoic orogenic belt and the formation of intercontinental rift: Petrological records of Pliocene-Pleistocene dacite in Tengchong

The study of Cenozoic intermediate-acidic volcanic rocks in Tengchong has been very insufficient in the past. Based on geochemical studies of intermediate-acidic volcanic rocks, the authors divided the Cenozoic volcanic rocks into Sr-high, Yb-low Adak-type dacite and Sr-low, Yb-medium Zhe-Min (Zhejiang-Fujian) type dacite, with the time limit being N2（2.78~4.04Ma）and Q1（0.73~1.19Ma）respectively. These intermediate-acidic volcanic rocks were formed during the transform process from stableness to collapse after the orogenic uplift of the Tengchong- Gangdise belt. In Miocene, the stress of Tengchong-Gangdise plateau was transformed from the domination of EW-trending compression to NS-trending strike-slip, and the orogenic belt began breakup. Blocked by the Luxi-Baoshan land mass, the direction of the strike-slip was deflected and resulted in the detachment and formation of rift basin among axial fault slices at the turning end in the early period of the formation of rift, the mantle was decompressed and intruded upward, and the high-pressure part of the lower crust was partly melted to form Adak-ype granitic magma. In early Pleistocene, with the intensification of the rifting, the crust became further thinner, and the lower crust was partly melted under the condition of low pressure to form Zhe-Min type granitic magma.