Metamorphism And Deformation Characteristics Of Metamorphic Rocks In Shangyun-huimin Section Of Changning-Menglian Suture, Yunnan, China

Shangyun–Huimin belt in the southern section of Changning-Menglian suturezone,which located between Baoshan and Simao blochs. It has been the researchfocus in recent years for Chinese and foreign geologists. This Paper took theMetamorphic rocks as the main research object. According to the studying ofmetamorphism and deformation characteristics, it explored the subduction andexhumation mechanism of metamorphic rocks and discusses the tectonic evolution ofthe Paleo-Tethys subduction.Geological Survey and Comprehensive Studies in recent years have revealed thatthe southern section metamorphic complex of Changning-Menglian suture is actuallya tectonic melange belt, including phyllite、 Metamorphic quartz sandstone、Lenticular marble、Muscovite-quartz schist、garnet–phengite quartz schist andglaucophane-phengite schist,etc. The metamorphic rocks distributed along the suturezone with generally north west-south east direction.On the basis of macroscopic and microscopic theory of structural analysis,combined with previous findings,it infers that the metamorphic rocks in Shangyun-Huimin area has been experienced three deformed and two metamorphism: Phaseone deformation and metamorphism (D1M1) occurred during the W–E directionsubduction period of Paleo-Tethys, which takes the whole shape of the tectonicmelange belt under the force of plastic rheology with dextral shear nearly dextralshear. The first phase of deformation accompanies with strong metamorphism andpresumably reaches to the epidote-blue schist facies. Phase two metamorphism anddeformation (D2M2) occurred in the reduction progress of Paleo-Tethys with thecollision between Baoshan and Simao blocks. Mainly horizontal shrinkagedeformation, forms the regional fold axial plane cleavages and slip cleavages S2,which completely replaced S1.During the Phaze two, the accretionary complex isstripped up to the shallower levels with the weak metamorphism. Phase III (D3) stilloccurred between the continent-continent collision, continuing deformation on thebasis of Phase Ⅱ. Development of small superimposed folds and wrinkles lineation, which is characterized by shallow-surface structure level.S3foliation has anon-penetrative characteristics and no newborn metamorphic minerals.P-T-t path reveals that rock occurred to exhumation in some extent, since theend of subduction.Each rock is bounded by the faults. Phengite in muscovite-quartzschist has yielded Ar-Ar ages around255.3Ma, glaucophane has yielded Ar-Ar agesaround248.5Ma; Combined with previous40Ar-39Ar dating data of phengite andglaucophane, It is believed that Paleo-Tethys has experienced a long period ofsubductin,the time is in close proximity to293-248.5Ma.