印支地块晚二叠世弧后伸展:来自老挝川圹高原A型花岗岩的证据  

A back-arc extensional environment of the Late Permian within the Indochina plate: Evidences from A-type granites in Xieng Khuoang Plateau,North Laos

作　　者：王新雨 王世锋[1] 江万[1] 

机构地区：[1]中国地质科学院地质力学研究所,北京100081

出　　处：《岩石学报》2017年第11期3675-3690,共16页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41172192);中国地质调查局项目(121201101000150014)联合资助

摘　　要：印支地块老挝地区的地质构造特征由于历史原因很少公开报道,基于最近的野外考察和室内测试,本文报道了老挝北部川圹高原A型花岗岩的岩石学、年代学及地球化学的特征,并进一步揭示古特提斯板块汇聚初期印支地块内部的伸展特性。Nuna花岗岩体3个花岗岩样品的U-Pb年龄为258.7±1.9 Ma、259.0±1.7Ma及251.0±0.9Ma。花岗岩富硅(SiO_2平均含量71.94%),富FeOT(2.91%),富碱(Na_2O+K_2O平均含量9.2%,且K_2O含量高于Na_2O),贫镁(Mg O平均含量0.11%),富集轻稀土元素和Zr、Hf、Rb、Th和U等元素,亏损Sr、Eu和Ti、Ba等元素,以及较高的锆石饱和温度(平均927℃),显示A型花岗岩的性质特征。花岗岩全岩εNd(t)值和二阶Nd模式年龄分别位于-2.4^-2.1和1.20~1.22Ga之间,表明其岩浆源区主要为中元古代古老地壳。通过与印支地块陆壳增生事件的对比,作者认为Nuna A型花岗岩岩浆源区为中元古代古老地壳与少量幔源底侵形成的新生地壳的混合。Nuna花岗岩微量元素图解投点全部落入板内环境,结合其形成时代与区域构造岩浆演化历史,作者认为Nuna花岗岩形成于华南地块向西南俯冲于印支地块之下由于板片回撤引起的弧后伸展环境。Due to the inaccessible of the Laos, there are few papers about the Indochina plate in Laos. This paper reports new zircon U-Pb ages, Nd isotope and whole-rock major, trace element data for granites from the North Laos. Zircon LA-ICP-MS dating for the Nuna granite pluton yields weighted mean ages of 258.7±1.9Ma, 259.0±1.7Ma and 251.0±0.9Ma, suggesting that the crystallization age of the Nuna pluton is Late Permian. Petrological and geochemical data show that the pluton is characterized by high silicon （the average content of SiO2 is 72.94%）, abundant ferrous （the average content of FeOT is 2.91%） and alkali （（Na2O＋K2O） of granites with an average of 9.2%）, depleted magnesium （the average content of MgO is 0.11%）. Moreover, the granite is enriched in Zr, Hf, Rb, Th, U and LREE, depleted of Sr, Eu, Ti, Ba, and high magmatic crystallization temperature （with average temperature of 927℃）, indicating A-type granite geochemical characters. The granites have negative εNd（t） ratios （-2.4^-2.1） and old second stage Nd model ages （1.20~1.22Ga）, indicating they were mainly derived from Proterozoic crust. Compared with the two stages of the crustal growth in Indochina plates, we infer that the source of the Nuna A-type granites are mainly the Mesoproterozoic crust with a small volume of juvenile crust which derived from underplating of mantle material. Most plots of the Nuna granites in tectonic discrimination indicate a within-plate tectonic setting, in combination with their ages and the evolutionary history of tectno-magma in this region, we suggest that the Nuna granites are generated in a back-arc extensional environment within the Simao-Indochina plate during the subduction of the South China plate beneath the Indochina plate.

关 键 词：老挝 A型花岗岩 锆石U—Pb年龄 全岩主量元素 稀土元素 钕同位素 弧后伸展环境 

分 类 号：P588.121[天文地球—岩石学] P597.3[天文地球—地质学]