Geochemistry And Magma Nature Of Tulasu Pluton In Northwest Tianshan And Its Metallogenic Significance

Tulasu pluton is located in the northeast of Tulasu basin,North Tianshan,which is composed of three lithofacies including quartzdiorite porphyry,pyroxene diorite porphyry and granite.Zircon U-Pb dating indicates that the quartz diorite porphyry and pyroxene diorite porphyry were crystallized in Late Devonian?with ages at 360.3±7.8 Ma and 375.4±6.2 Ma,respectively?,while the granite was formed in Late Carboniferous?at 305.9±5.9 Ma?,suggesting that the Tulasu pluton was the product of two magmatic episodes.Late Devonian Quartz diorite and pyroxene diorite porphyry belong tocalc-alkaline to high-K calc-alkaline series and appear metaluminous.Quartz diorite porphyry and pyroxene diorite porphyry are characterized by the prominent enrichment in LILEs,such as Rb,Th,U and K,and depletion in HFSEs,such as Nb,Ta,P and Ti,which is similar to typical subduction-related arc magma.The?Hf?t?values of quartz diorite porphyry and pyroxene diorite porphyry are+4.5⁺8.8 and+2.0⁺6.4,respectively,indicating that they are derived from depleted mantle wedge.Their T_DM2 ages are 814¹087 Ma and 955¹227 Ma,which shows that Mesoproterozoic to Neoproterozoic crustal materials were involved.Their high Ba/La?10.26¹7.92?,low Th/Yb?1.96⁵.80?,and relatively low Ce/Th?3.16⁷.34?and Ba/Th?18.29³8.92?ratios indicate that they were probably produced by partial melting of depleted mantle wedge metasomatized by slab-derived aqueous fluids,with involvement of subducted sediment.Late Late Carboniferous granite was composed of granodiorite porphyry and granite porphyry.Granodiorite porphyry are high-K calc-alkalineand peraluminous,while granite porphyry are in the boundary zones of shoshonite and high-K calc-alkaline,as well as metaluminous and weakly peraluminous.Granodiorite porphyry shows features of I-type granite,while granite porphyry has the characteristics of A-type granite.From granodiorite porphyry to granite porphyry,the degree of fractional crystallization and the content of crust material increased gradually.High ¹⁷⁶Hf/¹⁷⁷Hf values?0.282600⁰.282833?and wide range?Hf?t?values?+0.4⁺8.7?indicate granite has affinities with both depleted mantle and crust.It was formed in post-collisional setting.The mean Ce⁴⁺/Ce³⁺ratio,magmatic oxygen fugacity and water content of Late Devonian quartz diorite porphyry are 291.33,?FMQ=1.0-1.8 and 0.9-2.2 wt.%,respectively,while those of pyroxene diorite porphyry are 96.20,?FMQ=2.6-3.3 and 0.2-1.3 wt.%.Late Carboniferous granodiorite porphyry has mean Ce⁴⁺/Ce³⁺ratio of 138.49.The crustal thicknesses of Late Devonian dioritoids and late Late Carboniferous granitoids in Tulasu pluton are 30-35km and⁴0km,respectively,which are similar to porphyry Cu-Au and Cu deposits in the CAOB?30-35km-<40km?.Therefore,Tulasu pluton has favorable magmatic conditions for the evolution of porphyry Cu-Au metallogenic system.