武当隆起西段牌楼新元古代A1型花岗岩的发现及其地质意义

牌楼似斑状二长花岗岩出露于武当隆起的西部，是南秦岭地区新元古代岩浆活动的典型代表.锆石LA-ICP-MS年代学研究获得其成岩年龄为667.2±3.5 Ma.岩体Na₂O+K₂O含量和A/CNK分别介于8.31%~8.47%和0.89~0.94，显示出亚碱性、准铝质特征；同时样品富集大离子亲石元素Rb、K、U、Pb及高场强元素Nb、Ta，亏损Sr、P、Ti和重稀土元素，具有较高的Zr+Nb+Y+Ce含量以及10 000 Ga/Al、FeOT/（FeOT+MgO）比值，出现特征矿物铁韭闪石和富铁钠闪石，属于A₁型花岗岩.牌楼岩体ε_Hf（t）主要集中于-1.8~+1.5，来源于软流圈交代岩石圈地幔类似于OIB熔体，并伴随有一定的地壳混染作用.微量元素模拟显示，母岩浆在成岩之前发生了大规模的斜长石和辉石的分离结晶.综合区域研究资料、地质年代学、地球化学及同位素特征，结果表明武当隆起在新元古代成冰纪处于板片断裂的弧后伸展环境. 

The Discovery and Geological Significance of the Neoproterozoic A1-Type Granite in the Pailou Area,Wudang Uplift

As a typical case of the Neoproterozoic magmatism in South Qinling, the Pailou porphyritic monzogranite exposed in the western margin of Wudang uplift. Zircon U-Pb dating on the granite yields an age of 667.2±3.5 Ma. The granite shows sub-alkaline and metaluminous affinities, with Na₂O+K₂O and A/CNK values ranging 8.31%-8.47% and 0.89-0.94, respectively. The monzogranite enriched in LILEs (i.e. Rb, K, U and Pb) and HFSEs (i.e. Nb and Ti), and depleted in Sr, P, Ti and HREE. Due to the high content of Zr+Nb+Y+Ce and ratios of 10 000 Ga/Al and FeOT/(FeOT+MgO), as well as characteristic minerals of hastingsite and ferropargasite, the Pailou monzogranite displaying A₁-type geochemical features. The zircon ε_Hf(t) values of the study rocks mainly vary from -1.8 to +1.5, indicating that the magma likely to have been sourced from mantle metasomatized by asthenosphere, OIB-like melts, with minor crustal assimilation. Based on modal calculations of trace elements, the fractional crystallization of plagioclase and pyroxene has been proposed during magmatic evolution. Integrating the data obtained from studies on geology, geochronology, geochemistry and isotopic compositions, we propose that the Pailou magmatism was the product of norogenic magmatism and formed in tectonic setting of a back-arc rift setting that probably developed in relation to slab tearing during continued slab rollback.