陕西老牛山印支期高Ba-Sr花岗岩成因及其构造指示意义

摘要

The Laoniushan complex granitic batholith occurs in Xiaoqinling orogen along the southern margin of the North China Craton. Zircon LA-ICP-MS U-Pb dating yields an age of 207.9 ± 0.72 Ma for the Kangping pluton, which is the early-stage granites in Laoniushan batholith, indicating that they were generated from the Early Mesozoic magmatism. These granites are characterized by metaluminous, high silica and total alkalinity with high FeO*(FeO*+MgO) ratios, LILEs (especially Sr and Ba) and LREEs, low HREEs and Y contents, and insignificant Eu anomalies, similar to the high Ba-Sr granites. In comparison to the typical slab-derived adakites, the Kangping granites have low compatible elements (such as Cr, Ni) and high K2O contents with high-K calc-alkaline features, and display insignificant fractionation of HREEs. During the diagenetic processes, the geochemicalbehaviors of these granites were mainly constrained by the fractional crystallization of amphibole (clinopyroxene), plagioclase (K-feldspar), and minor garnet. Element and isotope compositions suggest that the sources of the Kangping granites in Laoniushan complex batholith are the mixture of the ancient basement materials of Taihua Group and small amounts of enriched lithospheric mantle metasomatised by fluid/melt that was derived from pelagic sediments-bearing subducted slab. These granites were generated by partial melting of the lowest part of the crust due to the subducted slab break-off during the post-collision extensional stage of the continental collision and orogenic processes.%老牛山复式花岗岩基位于华北克拉通南缘小秦岭地区,锆石LA-ICP-MS U-Pb定年结果表明,该岩基早期花岗岩——康坪岩体形成于晚三叠世[(207.9±0.72)]Ma,是早中生代岩浆作用的产物.该岩体具有高硅、富碱、准铝的地球化学特征,铁镁比值较高,富集LILE(尤其是Sr和Ba)和LREE,Eu无明显异常,相对亏损HREE和Y,可归属为高Ba-Sr花岗岩.但与典型埃达克岩相比,康坪岩体Cr,Ni等相容元素含量较低,而K20含量明显偏高,属高钾钙碱性系列,重稀土元素分馏不显著.在成岩过程中,康坪岩体的地球化学行为主要受角闪石(单斜辉石)、斜长石(钾长石)以及极少量石榴子石的分离结晶控制.元素和同位素地球化学研究表明,老牛山复式花岗岩基康坪岩体成岩物质主要来自含远洋沉积物俯冲板片析出流体/熔体交代的富集地幔以及古老太华群基底物质所组成的混合源区,是大陆碰撞造山晚期后碰撞“松弛”阶段,俯冲板片断离后下地壳底部物质发生部分熔融形成的岩浆产物.