胶东半岛三佛山高Ba-Sr花岗岩成因

Petrogenesis of the Sanfoshan High-Ba-Sr Granite,Jiaodong Peninsula,Eastern China

胶东半岛三佛山岩体是昆嵛山杂岩体的重要组成部分，其岩性主要由二长花岗岩组成，位于苏鲁超高压碰撞带与胶东陆块之间的缝合带中。岩石化学特点具高钾钙碱性岩石系列特征，岩体为准铝Ⅰ型花岗岩，并具有高Ba—Sr花岗岩的岩石地球化学特征，即高Ba、Sr含量，高Sr／Y、La／Yb、K／Rb值，低Y（〈13μg／g）、Yb（1．8μg／g）、Rb／Sr比值（平均为0．33），弱的Eu负异常，亏损Nb、P、Ti等高场强元素。根据该岩体岩石地球化学特征、包体岩石学特征，并结合前人对高Ba—sr花岗岩成因研究成果，笔者认为该岩体可能是幔源基性岩浆与地壳熔融的酸性端元混合而成。混合后的岩浆没有明显的长石和云母类矿物的结晶分异作用，混合岩浆最大温度在750-800℃左右。酸性岩浆的源区以石榴子石＋辉石＋角闪石＋斜长石的残留为特征。残留相物质组成特征暗示源区应位于壳幔边界，深度30km土，结合早期形成的昆嵛山二长花岗岩源区深度大于40km这一现象，表明胶东地区中生代岩石圈减薄作用在110Ma达到最大，地壳厚度恢复至正常厚度。

The late Mesozoic Sanfoshan granite formed during the Yanshanian in the eastern Jiaodong Peninsula, between the Sulu Ultra-high-Pressure-Metamorphic Belt and the Jiaodong Terrane of the North China Craton. The granite that belongs to the Kunyushan Complex mainly consists of monzogranite. In this article, we mainly discuss the geochemistry and petrogenesis of the intrusion. In major elements, the granite shows features of the high-K calca-alkaline series. In trace elements, it shows enrichment of LILE, such as Rb, Th, Ba, Sr and K, and LREE, and depleting of HFS, such as Nb, Ta, Hf, Ti and P, high St/Y, Lan/Ybn（〉20） ratios, low HREE, Y （〈18×10^-6） and Yb （〈2×10^-6）, with little negative anomalies of Eu. These features indicate that the granite is similar to the so-called high-Ba-Sr granite. Compared with the petrogenesis of high-Ba-Sr granites, microgranular enclaves in this granite imply that it is a production of mixture between mantle mafic magma and crustal partial melting magma. Garnet, clinopyroxene, amphibole and plagioclase are remanents in the source of the crustal end member and no mass of feldspar fractionated from magma before the hybrid magma solidified, according to the high abundance of Ba, Sr and low Rb/Sr ratios with little negative Eu anomaly. Remanents of the source indicate that the depth of the crustal end member is of about 30 km, which is shallower than that of the earlier formed Kunyushan granite （〉40 km）, which was formed before 140 Ma. The depth of the two types of granites in the Kunyushan Complex generated from the lowermost crust indicates that an extensive lithospheric thinning event, therefore, peaked around 110 Ma in the Jiaodong peninsula and the crust was thinned to a normal thickness of ca. 30 km.