Research On Orogenesis And Teconic Evolution Of The Southern Jinshajiang Orogenic Belt,Eastern Tibetan Plateau

The southern Jinshajiang orogenic belt in the eastern Tibetan Plateau,located in the Paleo-Tethys domain,preserved petrologic,stratigraphic and structural records of subduction-collision of the southern Jinshajiang orogeny.Therefore,this orogenic belt,is an ideal natural laboratory for research on orogenesis and tectonic evolution for Paleo-Tethys.Previous study focused on the geochemical and isotopic data of orogeny-related magmatism and the discussion for tectonic settings based on these data.However,weak constraints of the structural style and kinematics lead to variable tectonic intepretations and hamper the resconstruction of the tectonic evolution.Thus,we choose the tectonites,plutonic rocks and an angular unconformity in southern Jiashajiang suture zone?a key part of this orogenic belt?to conduct study on deformational sequences formed in different tectonic settings.Combined with previous studies,we reconstructed the tectonic evolution of the southern Jinshajiang orogenic belt,and furtherly promote the understanding of the tectonic evolution of the Paleo-Tethys domain.By detailed structural analysis,two episodes of deformation are distinguished.The first stage?D₁?is recorded by high-angle west-dipping penetrative foliations formed during early stage of the compressive regime of continental collision.With the help of kinematic marks in deformed rocks,we recognized a top-to-east thrust.Due to strain calculation on pyrite-fringe-complexes,the suture zone has been shortened to 15.5%comparing to its original width.Based on the distinct occurrences and superposition to D₁ deformation,the second stage?D2?deformation was identified.The D2 deformation can be divided into two subdivisions due to their different characteristics:first type(D_2-1)was produced by lower-middle crustal extrusion or exhumation,and second type(D_2-2)by emplacement of plutons.D_2-1 structure preserved in middle-to high-grade metamorphic rocks,recorded by low-angle NNW-dipping penetrative foliations with locally developed L tectonite that parallel to the orogen,representing the late stage of the declining compressive tectonic regime.The D_2-2 structure was restricted to the wall rock of the plutons that emplaced during Middle to Late Triassic.The post-tectonic magmatism is account for the D_2-2 deformation.Due to the results of isotopic dating on syn-D₁ and post-D_2-1 plutons,the compressive tectonic regime was constrained to last during 269-259 Ma.On the other hand,the angular conformity suggest that the tectonic regime already shifted to extension was constrained to form during 254-249 Ma and the compressive regime already ended before this time.Trace element analysis of magmatic oxygen fugacity,petrogenesis and tectonic implication conducted on zircons extracted from plutons that crystalized during different deformational stage also supports our deduction of the tectonic regimes.Thus,combined with previous studies on orogeny-related magmatism,we propose a three-stage history of tectonic evolution:?1?Subduction stage?285-270 Ma?,including early stage of intra-oceanic subduction and late stage of ocean-continental subduction,giving birth to the Early-Middle Permian subduction-related magmatism;?2?Collisional stage?269-259 Ma?,the regional penetrative structure and shortening of the suture zone?D₁?took place during the early stage in which the tectonic regime was intensely compressive;the orogeny-parallel L tectonite came into being(D_2-1)during the tectonic force declining late stage;the magmatism was produced by the synchronous reworking of thickened orogenic lithosphere and partial melting of subduction-related mantle wedge.?3?Post-collisional stage?258-220 Ma?,the tectonic regime shifted to extension,as the angular disconformity took place during Late Permian?254-249Ma?;massive magmatism formed by the detachment,roll-back and break-off of the subducted oceanic slab during this stage;emplacement of plutons during this stage have imposed localized deformation on wallrocks(D_2-2).