The Geochemistry, Petrogenesis And Tectonic Setting Of The Rhyolite In Early, South Of Yili Basin,china

Ancient South Tianshan Ocean and Tarim Plate subducted and stitched toKazakhstan-Yili Plate in early Late Paleozoic, rhyolite of the southern margin of Yilibasin in Kazakhstan-Yili Plate are edge response and rock record of tectonicproperties in the subduction and collision process. The early carboniferous volcanicsedimentary rock of Dahalajunshan Formation is widely distributed in south of Yilibasin, a small amount of basalt and andesite, rhyolite lava, rhyolitic lithic-crystalwelded tuff and volcanic clastic rocks are also included. Our preliminary studies findthat Dahalajunshan Formation rhyolite has a close relationship to depleted mantle,which does a good deal to their petrogenesis and tectonic settings. In the article weselect Dahalajunshan Formation rhyolite as our research object, throughing theanalysis of petrochemical-geochemical and isotope of Sr-Nd-Pb, combining with theresults of previous studies, geochemical characteristics, petrogenesis and tectonicsettings of the rocks are discussed systematically in this paper.(1) The geochemical studies show that the early carboniferous rhyolites insouthern margin of Yili basin are syncollisional volcanic arc rocks,which have acharacteristic of high aluminum and potassium, and are devided to calc-alkaline rocks,moderate total amount of Rare Earth Elements (REE), The rocks show light LREEenriched patterns in chondrite-normalized REE diagram and primitivemantle-normalized trace elements diagram with high LILE(Rb, Ba, Th, U) abundance,depletion of HFSE and Eu.(2) With characteristics of low ISr, high Nd (t)(positive), the source of rhyolitesin this area has a characteristic of depleted mantle. All samples have the samefSm/Nd(fractionation factors)(fSm/Nd=-0.43~-0.48). Basides, constrainted by the Plot ofNb/Zr-Nb/Ba, the rhyolitic magma source area should give priority to with thelithosphere mantle components; There is a high-abnormally isotope data of Pbusually in the sduty aera.(3) Combined the geochemical evidence of this paper, we think the earlycarboniferous rhyolite belongs to syn-collision with volcanic arc volcanic rocks. In the Plot of La-La/Sm, They has the characteristics of consanguineous with this groupof basalt magma, that samples from partial melting of basalt, most samples also has atendency to mantle source region crystallization differentiation.(4) Dahalajunshan Formation rhyolite comes from the fact that it is located insouth of Kazakhstan-Yili Plate, combining the analysis of the restricting of age andtectonic settings, we think that the main dynamic background is a tensionenvironment the of back-arc of the active continental margin, we can speculate thatwhich is a partial tension basin in edge of continental, formed by Tarim Plate andAncient South Tianshan Ocean subducted and stitched to Kazakhstan-Yili Plate.Dahalajunshan Formation rhyolite should show the early stage of the back-arcextension.