Geochemistry Of Late Paleozoic Igneous Rocks From Yabulaishan Area In Alxa And Its Tectonic Implications

Yablaishan batholith, witch including intrusive rocks(granite, granodiorite, and K-feldspar granite) and volcanic rocks(dacite), located in the northern Alxa League. Zircon U-Pb dating indicates the Yabulaishan granodiorlte(280±1 Ma), K-feldspar granite(286±1 Ma) and dacite(275±1 Ma) are formed in the Early Permian, tonalite(272±1 Ma) is formed in the Middle Permain. The measurement of zircon Hf isotopic in most of the Yabulaishan batholith exhibits wide range of εHf(t) values(-13~+2), and the relatively old mantle model ages(2.1~1.2 Ga); part εHf(t) of K-feldspar granite are positive. The Hf isotopic compositons suggest that the igneous rocks were derived from the partial melting of ancient crust probably with little additions of mantle material, which is similar to the igneous rocks formed on the ancient landmass. According to their geochemistry, granodiorlte and K-feldspar granite have similar composition of trace and REE, both are high in LREEs and low in HREEs with week to moderate Eu negative anomaly; compared with former, tonalite has relatively high Sr/Y(>300) with moderate Eu positive anomaly in some degree. The feature of tonalite’s trace and REE probably resulting from assimilation-contamination of some surrounding rocks during the emplacement without melt fractionation. Takeing the effects of assimilation-contamination into consideration, the parental magma of Yabulaishan tonalite should have high Sr/Y and Sm/Dy ratios, which might imply the presence of garnet in the house. The Permian igneous rocks in northern Alxa block are perfectly comparable with that in the northern NCC. Permian igneous rocks may form a nearly east-westward magmatic rock belt in the northern margin of NCC and Alax Block. The magmatic rock belt may related by the closure of the Paleo-Asian Ocean, subduction or an extensional regime in a post-collision setting.