Geochronology and geochemistry of Blatter mineralized sub-volcanic rocks in Xiemisitai area, Xinjiang: Implications for tectonic setting and Cu-mineralization

The Blatter mineralized sub-volcanic rocks occur in the middle part of Xiemisitai area. They are mainly composed of dacite porphyry and rhyolite porphyry which with strong copper mineralization. We carried out combined studies of geology, geochronology, geochemistry and whole rock Sr-Nd and zircon Hf isotopes of the Blatter mineralized sub-volcanic rocks, to discussing their formation tectonic environment, genesis and metallogenic significance. The result shows that the dacite porphyry belongs to high K calc-alkaline and shoshonite series metaluminous-highly peraluminous rock and formed in Early Silurian (434. 9 +/- 2. 3 Ma). The samples have moderate concentrations of total REE (Sigma REE = 97. 9 x 10(-6) similar to 107 x 10(-6)), relatively enriched in LREE with (La/Yb)(N) =7. 93 similar to 9. 95 and weakly-significant Eu negative anomaly (delta Eu = 0. 78 similar to 0. 90), and relatively enriched in Rb, Th, U, K etc., infertile in Nb, Ta, P, Ti etc.. Sr-Nd isotopic compositions indicate that the dacite porphyry is characterized by higher positive ENd(t) (+2. 64- +5. 01), lower (Sr-87/Sr-86), (0. 7036 similar to 0. 7043) and relatively younger one-stage model ages (t(DM1)) ranging from 750Ma to 930Ma. The rhyolite porphyry belongs to high-K calc-alkaline and shoshonite series metaluminous-highly peraluminous rock and formed in Late Silurian (423.2 +/- 0. 9Ma). The samples have moderate concentrations of total REE (Sigma REE =65.3 x 10(-6) similar to 127 x 10(-6)), relatively enriched in LREE with (La/Yb)(N) = 6. 82 similar to 8. 24 and clearly-significant Eu negative anomaly (delta Eu = 0. 51 similar to 0. 71), and relatively enriched in Rb, Th, U, K etc., infertile in Nb, Ta, P, Ti etc.. The samples also have higher positive epsilon(Nd) (t) (+2. 86 similar to +5. 78), lower (Sr-87/Sr-86), (0. 7022 similar to 0. 7038) and relatively younger one-stage model ages (t(DM1)) ranging from 680Ma to 940Ma. The epsilon(Hf) (t) values of zircon vary from +9. 8 to +14. 9, and Hf model ages (t(DM2)) are 456 similar to 783Ma. In conclusion, the Blatter mineralized dacite porphyry and rhyolite porphyry are products of different magmatic evolution and crystallization. They are probably the products of the granitic magma formed by partial melting of the juvenile lower crust and mixed with the basaltic magma derived from the mantle wedge, upwelling, chilling, and crystallizing, in the environment of continental margin arc. In the course of formation of the magma of rhyolite porphyry formed later, the rate of partial melting of the juvenile lower crust is lower, and the magma is more acidic. The magmatic rock belt in Xiemisitai area, which is represented by Silurian to Early Devonian volcanic rock, sub volcanic rocks and the intermediate-acidic intrusive mass, is a continental margin arc magmatic belt formed mainly in the Early Palaeozoic, and is a favourable area for formation and looking for porphyry copper deposit.