湘南西山铝质A型花岗质火山-侵入杂岩的地球化学及其形成环境

Geochemistry and tectonic setting of Xishan aluminous A-type granitic volcanic-intrusive complex, Southern Hunan

初步研究表明,以往被认为是典型的S型花岗质岩石的湘南西山火山侵入杂岩应为铝质A型花岗质岩石。该杂岩富硅碱贫钙镁、K2O/Na2O>1、准铝过铝质(0.94～1.26);FeO /MgO比值大(平均12.05),高于M型,S型和I型花岗岩。在微量元素和稀土元素组成上,岩石富LREE,Ga,Y,Nb,Zr等大离子高场强元素及亏损Ni,Cr,Eu,Ti,V,P,Sr等,与国内外A型花岗岩相似。与一般A型花岗岩来源于I型花岗岩或大量抽提了S型花岗岩物质后的地壳源区不同,西山火山侵入杂岩的I(Sr)(0.71612～0.71823)较高,εNd(-7.0～-8.0)较低,钕模式年龄(1 517～1 600Ma)略小于区域上变质基底和中国东南部中生代花岗岩类的钕模式年龄,显示其直接来源于地壳物质的部分熔融,但可能有少量新生地幔物质的加入。西山铝质A型花岗质火山侵入杂岩是中侏罗世晚期挤压造山作用趋于结束,造山带崩塌,岩石圈强烈伸展减薄、热流值上升的构造背景下形成的造山后花岗质岩石。

Xishan volcanic-intrusive complex, with exposure area of about 705 km^2 in Southern Hunan occurs in the western part of Nanling E-W tectonic magmatic belt. Petrochemically, the complex is metaluminous to peraluminous( 0.94～1.26) , rich in SiO2, K2O+Na2O, poor in CaO, MgO, and is characterized by high FeO~*/MgO ratio ( with an average of 12.05), which is similar to those of A-type granite and different from the ordinary S- and I-type granites. Xishan complex is rich in Ga and Nb, Ta, Zr and other high field strength elements, and depeleted in Sr, P, Eu, Ti, which can well be analogued to the known A-type granites in the world. Isotopically, the Complex is higher in initial Sr ratios (Isr=0.716 12 ～(0.718 23) , ) lower in εNd ( -7.0～ -8.0) ; their T(2DM)(1 517～1 600 Ma) is slightly lower than those of the metamorphic basement and Mesozoic granitoids in the southern-eastern China, indicating that the complex was mainly derived from crustal material, but had a little influence of mantle-derived materials. Based on the major and trace element data as well as isotopic features, it is suggested that Xishan volcanic-intrusive complex belong to the post-collisional aluminous A-type granite . It is commonly accepted that A-type granite was derived from the I-type sources or crustal sources from which S-type melt has been extracted. The preliminary study on Xishan volcanic-intrusive complex shows that A-type granite can be directly derived from typical S-type crustal sources, its petrogenesis mechanism need further investigation. According to the geological characteristics and combining with the regional tectonic evolution,it is reasonable that the formation of Xishan complex should be related to the asthenospheric upwelling and lithospheric extension and thinning during the Mesozoic.