Early Paleozoic magmatic event in the Sanjiang orogenic belt in southwest China and its response to the evolution of the Proto-Tethyan Ocean

The Sanjiang orogenic belt in southwest China is composed of several sutures and many intermediate massifs of different sizes.It is tectonically located in the eastern part of Tethys structural domain where Eurasia plate and India plate joint.Zircon U-Pb dating of samples collected from Gudanghe gneissic granite, Nujiangba Town gneiss, Yongde metamorphic rhyolite, Caojian granite, Ximeng granitic gneiss and Lancang basic medium basic volcanic rocks in Maji Town of Fugong County yielded 465±5 Ma, 512±7 Ma, 510±16 Ma, 441~457 Ma, 454~456 Ma, 456~459 Ma respectively.Combined with the age data of the early Paleozoic magmatic rocks in thSanjiang area, Southwest China, middle Late Ordovician intrusives with island arc characteristics were emplaced on the east and west sides of the Changlian-menglian Tethys Ocean.The Cambrian volcanic rocks with two peaks in the interior and east of Baoshan massif in the western margin of Changlian-Menglian Tethys ocean revealing tension environment indicate that the Tethys ocean existed in Cambrian and is still expanding.In the Middle Ordovician, the tectonic system began to change, and the oceanic plates began to subduct toward east and west, forming island arcs on the continental margin.