TIMS U-Pb Isotopic Dating and Geochemical Characteristics of Paleozoic Granitic Rocks from the Middle-Western Section of Tianshan

The results of TIMS U-Pb isotopic dating for the Paleozoic granitic rocks from the middle-western section of Tianshan show that, the ages of four granitic rock bodies, plagiogranite, alkali feldspar granite, granodiorite and quartz diorite, intruded into Ordovician Kekenaike Group distributing between Tuokeshun and Kumishi, Central Tianshan, are 424.1±1.1 Ma, 395.1±0.9 Ma, 393.2±1.4 Ma and 327.3±0.9 Ma, respectively, which reveal three stages of structure-magma events in middle section of Central Tianshan, occurred in Early Silurian, Middle Devonian and Early Carboniferous, respectively; the biotite granite from Haxilegendaban, Central Tiangshan, formed in 286.8±0.8 Ma; the granodiorite from Guozigou, Huocheng County, granodiorite from Zhaoshu coal mine, and bojite from Zheketai, Xinyuan County, all of which distribute in the Carboniferous to Permian rift belt of Yili, formed in 351.9±1.6 Ma, 348.4±0.8 Ma and 308.3±1.2 Ma, respectively. The hornblende-plagiogranite and biotite granite from the southern section of Du-ku Road, Kuergan, formed in 426.3±1.9 Ma and 425.1±1.8 Ma, respectively. The Silurian to Devonian granitic rocks distributing between Tuokeshun and Kumishi, Central Tianshan, are calc-alkaline series, enriched in LREE, Rb, Th and Ba, depleted in Ta and Nb on the Ocean ridge granite-normalized trace elements diagram, and should form in volcanic arc setting; the Carboniferous to Permian granitic rocks are calc-alkaline or tholeiitic series and metaluminous-peraluminous rocks, characterized by slight enrichment in LREE, intensive enrichment in Rb and Th relative to HFSE and Ba, should form in post-collision setting. The Carboniferous to Permian granitic rocks from Carboniferous to Permian rift belt are peraluminous, characterized by enrichment in LREE, Rb and Th, intensive depletion in Ba and HFSE, and also formed in post-collision setting. The granites from Kuergan, southern Tianshan, whose formation time correspond with the subduction time of Paleozoic ocean of Tianshan, are calc-alkalic peraluminous rocks with high Al_2O_3, distinct Eu negative abnormlity, middle ΣREE contents (157.89×10~ -6 ~81.81×10~ -6 ), and characterized by enrichment in LILE (K, Rb, Th and Ba) relatively to HFSE (Nb,Ta, Zr and Hf) on the ORG-normalized trace elements diagram, which suggest they should form in island arc setting. According to the Sr-Nd-Pb isotope studies, high (~ 87 Sr/~ 86 Sr) _t values (0.70435~0.70982), lower e_ Nd (t) values (lower than 0 or very lower positive values), high T_ 2DM or T_ DM (distinctly higher than the formation ages of rock bodies), high δ~ 18 O‰(mostly higher than 10, exceptional 6.7), of all these Paleozoic granitic rock from the middle-western section of Tianshan in this study,indicate that the age-old continental crust take an important role for the formation of granitic magma. Meanwhile, considering the varying of e_ Nd (t) values of these granitic rocks around 0, the juvenile crust play an important role ,too, for the granitic magma formation. The comparing studies on the e_ Nd (t) values of these granite and Precambrian rocks suggest that, the Silurian plagiogranite body from the north of Kumishi, central Tianshan, is likely to the partial melting products of the Mesoproterozoic to Neoproterozoic amphibolite; the Carboniferous-Permian granites probably derive from the partial melting of Mesoproterozoic-Neoproterozoic gneiss or schist, as well as from the magma originated from the juvenile crust partial melting contaminated by age-old crust. The Carboniferous-Permian granites from Yili and the Silurian granites from Kuergan, southern Tianshan, show the Nb isotopic signatures of magma from juvenile crust partial melting contaminated by age-old crust.