Paleozoic Tectonic Evolution In The Northern Margin Of East Junggar,NW China

The East Junggar region of Xinjiang is an important tectonic unit in the southern part of the Central Asian Orogenic Belt,which records complicated accretionary orogenic processes of the Paleozoic Asian Ocean.Based on field geological investigations in the East Junggar region,this thesis carried out petrological,geochemical,geochronological,and Lu-Hf and Sr-Nd isotopic studies on the Paleozoic volcanic successions in the region.This study has revealed the source nature and evolutionary processes of magmas,as well as tectonic environments during their formation,and tentatively determined the tectonic evolution of the Paleozoic palaeo-oceanic basins in the region,with main conclusions listed below.(1)Early Paleozoic andesite(466～422 Ma)was identified from the Devonian Pingdingshan Formation,which was probably formed by partial melting of the mantle wedge within the overlying spinel-garnet transition zone during the subduction of the Karameri Ocean plate.It is inferred that the initial subduction of the oceanic crust occurred in the early Paleozoic prior to 466 Ma.(2)The Devonian volcanic rocks in the Qiakuertu area of the northern margin of East Junggar are mainly composed of high aluminous basalt,high Sr/Y andesite,normal andesite,andesite and rhyolite,which were formed at about 392～362 Ma.The high aluminous basalt may be formed by partial melting of the mantle wedge due to the subduction of oceanic slab.The high Sr/Y andesite was the product of the interaction between slab-derived fluids and the overlying mantle peridotites under amphibolite-facies conditions.The normal andesite was formed by the varying-degree partial melting of the depleted mantle wedge metasomatized by slab-derived fluids.The andesite and rhyolite were formed by the partial melting of the young mafic lower crust under amphibolite-facies conditions.The study area was in an oceanic subduction environment during Devonian time.(3)The Carboniferous volcanic rocks in the Qiakuertu area of the northern margin of East Junggar are mainly composed of high Sr/Y andesite,normal andesite and rhyolite,which were formed at about 354～326 Ma.The high Sr/Y andesite was the product of the interaction between the oceanic slab melt and the overlying mantle peridotites under garnet-bearing amphibolite-facies conditions.The normal andesite was formed by different degrees of partial melting of the depleted mantle wedge;the rhyolite was formed by partial melting of the young lower crust under amphibolite-facies conditions.The area was inferred in a subduction environment in the Early Carboniferous and probably in a collisional environment at the end of the Early Carboniferous.(4)The Early Permian volcanic rocks in the Qiakuertu area of the northern margin of East Junggar are mainly composed of tholeiitic basalt,andesite and rhyolite,which were formed at about 277 Ma.The tholeiitic basalt was probably originated from the partial melting of the mantle wedge metasomatized by subducted-slab-derived fluids;the andesite was formed by different degrees of partial melting of the depleted mantle wedge metasomatized by a large amount of slab-derived fluids and a small amount of sediment melts.The rhyolite was formed by the partial melting of young and mafic lower crust under the condition of amphibolite-facies.The study area was in a post-collisional extensional setting in the Early Permian.(5)By comparing the rock assemblages,ages,Lu-Hf and Sr-Nd isotopes of the Paleozoic volcanic rocks,this study has demonstrated that the East Junggar region was in an intra-oceanic island-arc system during Paleozoic time,and proposed that there possibly existed tectonic transitions between advancing and retreating regimes during the subduction and accretion of the Karameri Ocean plate.(6)Based on a comprehensive study and regional comparison,it is proposed that the East Junction and the Altai blocks were likely amalgamated at the end of the Early Carboniferous.