Geochemical characteristics and geological significance of Late Hercynian volcanic rocks in Tahe area

Abstract：Core observation shows that mafic volcanic and intermediate-acidic volcanic rocks were developed successively in Tahe area in Late Hercynian. These two types of volcanic rocks have different geochemical compositions. In mafic volcanic rock, K2O+Na2O=3.49%~4.25%，K2O/Na2O=0.18~0.38<1, large-ion lithophile elements (LILE) K, Rb, Ba, Th and high-field strength elements (HFSE) Nb, Ta, Ti, Zr, P are high, and δEu≈1, with no significant anomaly, suggesting that this type of volcanic rock belongs to the sodic series basalt. As for intermediate-acidic volcanic rock, K2O+Na2O= 6.86%~8.82%，K2O/Na2O=1.25~1.59>1, HFSE Nb, Ta, Sr, P, Ti are poor, LILE K, Rb, Ba, Th are high, and δEu=0.43~0.66<1, with negative anomaly, implying that this kind of volcanic rock is potash basaltic or high potassium-calcium alkaline-dacite. The HFSE ratios Th/Ta and Nb/U imply that basalt and dacite don’t have obvious affinity, the basalt originated from the mantle, whereas the dacite originated from the crust. A comparison with the igneous rocks of adjacent areas shows that the basalt and dacite are similar respectively to the basalt in Tazhong and the granite in South Tianshan in geochemical characteristics. An analysis of tectonic sub-diagram reveals that the basalt was formed in the intraplate rift, whereas the dacite and granite were formed on the edge of the island arc plate tectonic setting. As a special paleo-tectonic location, Tahe area has well-developed basalt and dacite, which are the coupling results of the intraplate main rift basin and the volcanic-arc edge.