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湘东南印支期花岗岩成因及构造背景
Genesis and tectonic setting of Indosinian granites in southeast Hunan
  修订日期:2006-09-14
中文关键词:印支期,花岗岩,地壳加厚与重熔,岩浆底侵,后碰撞环境,湘东南
英文关键词:Indosinian,granites,thickening and melting of the crust,underplating of magma,post-collisional tectonic setting,southeast Hunan
基金项目:国土资源部地质大调查资助项目(200213000035)
柏道远  周亮  马铁球  王先辉
湖南省地质调查研究院 湖南湘潭411100
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中文摘要:
      湘东南地区以印支期主俯冲断裂——茶陵-郴州大断裂为界,分为东部炎陵-汝城隆起区和西部衡阳-桂阳坳陷区。区内印支期花岗岩总体属高钾钙碱性过铝质花岗岩,并可根据岩石特征分为2组。第1组产于衡阳-桂阳坳陷区NW向深断裂中,由角闪石黑云母花岗闪长岩、角闪石黑云母二长花岗岩和黑(二)云母二长花岗岩等组成,SiO2平均含量总体较低,为69.68%;大多为弱过铝,ASI值0.96~1.29,平均为1.08;具有相对较低的ISr值(0.7080~0.7168)和t2DM值(1.48~1.72Ga)以及相对较高的εNd(t)值(-8.90~-5.83)。第2组位于炎陵-汝城隆起区及衡阳-桂阳坳陷区将军庙次级隆起带中,由黑云母二长花岗岩和二云母二长花岗岩组成,SiO2平均含量相对较高,为72.23%;总体为ASI值大于1.1的强过铝花岗岩,ASI值1.01~2.60,平均达1.30;具有相对较高的ISr值(0.7159~0.7345)和t2DM值(1.81~1.97Ga)以及相对较低的εNd(t)值(-12.0~-9.85)。根据上述特征,结合岩石圈结构和A/MF-C/MF图解分析,第2组花岗岩源岩主要为中地壳结晶片岩、片麻岩等,第1组花岗岩源岩除中地壳岩石外,可能还有少量下地壳基性麻粒岩成分以及幔源组分的加入,说明存在幔源岩浆的底侵作用。根据多种氧化物与微量元素构造环境判别图解,结合地质特征、构造演化背景等,认为印支期花岗岩形成于同造山阶段的后碰撞构造环境,地壳加厚及其后的减压导致中地壳重熔是岩浆形成的主导因素,幔源岩浆底侵使坳陷区与深大断裂带内的岩体中混入下地壳成分及少量地幔物质。已有研究表明湘东南燕山早期花岗岩形成于后造山构造环境,因此印支期与燕山早期两阶段花岗岩的构造环境彼此之间提供了制约。
英文摘要:
      Southeast Hunan is divided into two tectonic units by the Indosinian primary subduction fault (Chaling_Chinzhou fault), i.e., Yanling-Rucheng upwarping region to the east of this fault, and Hengyang-Guiyang downwarping region to the west. The Indosinian granitoids are high-K calc-alkaline peraluminous on the whole, and can be divided into two groups. The first group occurs in the NW-trending faults of Hengyang-Guiyang downwarping region, and is composed of hornblende biotite granodiorite, hornblende biotite monzogranite, biotite monzogranite and two-mica monzogranite; the rocks have relatively low contents of SiO2 with an average of 69.68%, and are mostly weak peraluminous with an average ASI value of 1.08 (0.96~1.29); they have comparatively low ISr values (0.708?0~0.716?8 ) and t2DM ages (1.48~1.72 Ga), high εNd(t) values of -8.90~-5.83. The second group lies in the Yanling-Rucheng upwarping region and Jiangjunmiao sub-upwarping region of Hengyang-Guiyang downwarping region, and is composed of fairly acidic biotite monzogranite and twomica monzogranite, with an average SiO2 content of 72.23%; the rocks are strong peraluminous with ASI valuesof 1.01~2.60 (averaging 1.30), and have rather high ISr values (0.715?9~0.734?5) and t2DMages (1.81~1.97 Ga) and low εNd(t) values (-12.0~-9.85). According to these characteristics as well as lithospheric structure and A/MF-C/MF diagram, it is thought that the second group granites were derived from schist and gneiss of the middle crust, whereas the first group granites came mainly from rocks of the middle crust, with the probable participation of minor basic granulite from the lower crust and mantle, suggesting the underplatingof mantle magma. Based on multiple oxide and trace element-diagrams for discrimination of structural environments, geological characteristics and tectonic evolutional settings, the authors consider that the Indosinian granites were formedin a post_collisional tectonic setting of the syn_orogenic stage, and the thickening of the crust and the subsequent weakening of the stress caused the melting of the middle crust, with magma formed sequentially. The underplating of mantle magma caused lower crust and mantle materials to enter the granitic plutons in the downwarping region and deep faults. Researches indicate that the Early Yanshanian granitoids were formed in a post-orogenic tectonic setting, and the Indosinian and Early Yanshanian tectonic settings restricted each other.
柏道远,周亮,马铁球,王先辉,2007,湘东南印支期花岗岩成因及构造背景[J].岩石矿物学杂志,26(3):197~212.
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