湘西南苗儿山岩体北段新元古代晋宁期花岗岩岩石学、地球化学特征及其形成构造背景

发布时间：2018-03-28 11:56

本文选题：岩石学　切入点：地球化学　出处：《地质科技情报》2017年06期

【摘要】：苗儿山岩体位于湘西南与桂北交界处,其北段处于湖南省境内,南段主要位于广西省境内,岩体总出露面积约1 600km2。该岩体大地构造位置上处于扬子地块东南缘江南造山带与华夏地块的结合部位,为晋宁期、加里东期、印支期和燕山期花岗岩组成的复式岩体。其中晋宁期花岗岩可划分为第一、第二和第三侵入期次,分别对应猫儿界岩体群、谭家坳岩体群和报木坪岩体群,其成岩年龄分别为811.3,(807±11),(806±9)Ma,不同侵入期次间隔时间短,岩浆活动连续,并在空间上具有从北东往南西依次侵入的特点。晋宁期花岗岩经加里东早期动力变质作用后,已经全部变为糜棱岩化花岗岩或花岗质糜棱岩,按化学成分主要属花岗闪长岩。晋宁期花岗岩属铝过饱和的钙碱性酸性花岗岩类,其Al_2O_3含量高,铝饱和度(A/CNK)和铝饱和指数(ASI)均大于1.1,富集Rb、K、Th、U和稀土元素,强烈亏损Ba、Nb、Sr、P、Ti元素,Nb/Ta比值(9.36)较小,显示出地壳物质熔融形成的强过铝质S型花岗岩的特征,但其SiO_2质量分数(69.38%)较低,K/Na比值(0.85)小于1,且铁镁质成分TFeO和MgO质量分数(4.04%和2.34%)偏高,与典型的华南壳源花岗岩略有不同,暗示可能有少量幔源物质混入。Rb/Sr-Rb/Ba图解表明晋宁期花岗岩的源岩主要是难熔的低成熟度的砂屑岩。多组微量元素构造环境判别图解表明晋宁期花岗岩为与洋壳俯冲有关的岛弧型花岗岩,暗示华南洋俯冲板片的折断和拆沉引发深部地幔上涌致使基底地壳的部分熔融而形成花岗质岩浆活动,同时有少量幔源成分的加入。晋宁期花岗闪长岩糜棱岩化形成于郁南运动和北流(崇余)运动之中,反映在加里东早、中期的造山运动中,除了强烈的碰撞挤压作用外,还兼具水平方向的韧性剪切作用。
[Abstract]:The Miaoershan rock body is located at the junction of southwest Hunan and northern Guangxi, and the northern part is in the territory of Hunan Province, and the southern segment is mainly located in the territory of Guangxi Province. The total exposed area of the rock mass is about 1600km2. The tectonic position of the rock body is located in the combination of the Jiangnan orogenic belt and the Huaxia block in the southeast margin of the Yangtze block, which are Jinning and Caledonian periods. The Jinning granites can be divided into the first, the second and the third intrusive stages, corresponding to the Maoerjie rock mass group, the Tan Jia ao rock mass group and the Shimuping rock mass group, respectively. The diagenetic age is 811.3 卤11.7 卤110.806 卤9Ma. the interval between different intrusions is short, the magmatic activity is continuous, and it has the characteristics of invading from north to east to south west in space. After the early Caledonian dynamic metamorphism, the Jinning granites were metamorphosed in the early Caledonian period. All of them have become mylonitic granite or granitic mylonite, mainly belong to granodiorite according to their chemical composition. Jinning period granite belongs to calc-alkaline acid granites with aluminum supersaturation, and its Al_2O_3 content is high. Both Al saturation ratio (A / CNK) and aluminum saturation index (ASI) are greater than 1.1, and they are rich in RbPK, ThU and rare earth elements, and strongly depleted in BaNbSn-Sr-PnTi / Ti (9.36), indicating the characteristics of strongly peraluminous S-type granite formed by crustal material melting, and the results show that the high peraluminous S-type granites formed by the melting of crustal materials are characterized by strong peraluminous S-type granites. However, the SiO_2 mass fraction (69.38%) is lower than 0.85), and the TFeO and MgO mass fractions (4.04% and 2.34%) of ferromagnesite are higher than those of typical southern China crust-derived granites, which are slightly different from those of typical southern China crust-derived granites. It is suggested that there may be a small amount of mantle source material mixed in. RB / Sr-Rb / Ba diagram shows that the source rocks of Jinning period granite are mainly refractory and low maturity sandclastic rocks. Multiple trace element tectonic environment discriminant diagrams show that Jinning period granite is subducted with oceanic crust. Related island arc granites, It is suggested that the breakage and delamination of the subduction plate in the South China Ocean lead to the upwelling of the deep mantle and the partial melting of the basement crust to form granitic magmatism. In addition, the granodiorite mylonitization in Jinning period was formed in the Yu Nan Movement and the Northern current (Chongyu) movement, which was reflected in the early and middle Caledonian orogeny, except for the strong collision and compression. It also has horizontal ductile shear.
【作者单位】：湖南省地质调查院;
【基金】：中国地质调查局地质调查子项目“湖南苗儿山地区矿产地质调查”(12120114024201)所属的二级项目“南岭成矿带地质矿产调查”(121201009000150002)
【分类号】：P588.121

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