大兴安岭牙克石地区新元古代与晚古生代两类岩石组合的构造属性及其地质意义

Tectonic nature and geological significance for two types of Neoproterozoic and Late Paleozoic rock assemblages in Yakeshi region,Great Hinggan Range

在内蒙牙克石地区发育两种不同构造属性的岩石组合:一类为乌奴耳-头道桥蛇绿混杂岩,另一类为晚古生代弧属性侵入岩。乌奴耳-头道桥蛇绿混杂岩由辉长岩、辉长辉绿岩、辉绿玢岩(岩墙?)、变玄武岩和放射虫硅质岩组成;地球化学特征显示基性岩类属于拉斑玄武岩系列,具有相似的稀土和原始地幔标准化配分模式,与N-MORB的特征类似,不具有Nb-Ta负异常,Nb/Nb~*值平均>1;构造环境判别图显示该蛇绿混杂岩可能形成于扩张脊。晚古生代弧属性侵入岩出露于白井山、乌尔其汗,由中基性单元(辉长岩、辉长闪长岩和石英闪长岩)和酸性单元(花岗闪长岩和二长花岗岩)组成;地球化学特征显示中基性单元和酸性单元属于钙碱性岩系列,富集LREEs和LILEs,具有Nb-Ta负异常,高Sr、Sr/Y值,低HREEs和Y,Eu异常不明显;酸性单元显示埃达克质岩的地球化学特征。乌奴耳-头道桥蛇绿混杂岩可能形成于新元古代,与头道桥蓝片岩、吉峰蛇绿混杂岩、新林蛇绿岩构成一条重要的缝合带,暗示本区古洋盆的存在。晚古生代弧属性侵入岩中,辉长岩的结晶年龄为326±1.9Ma,花岗闪长岩的结晶年龄为323.7±1.9Ma;微量元素的组成特征显示,中基性单元和酸性单元的形成与早石炭世洋壳板片的俯冲作用有关,暗示兴安地块和松嫩地块之间洋盆的萎缩。

Two different types of rock assemblages exist in Yakeshi region, Inner Mongolia：one is Wunuer-Toudaoqiao ophiolitic mélange; the other is Late Paleozoic arc-type intrusion. The Wunuer-Toudaoqiao ophiolitic mélange comprises gabbro, gabbro-diabase, allgovite （dyke？）, metabasalt and radiolarian siliceous rocks. The geochemical data show that the basic rocks from the Wunuer-Toudaoqiao ophiolitic mélange are tholeiitic series, and have consistent distributed patterns of REE and trace elements, similar to the features of N-MORB. They don＇t display negative Nb-Ta anomalies with ratios of Nb/Nb＊〉1. Additionally, tectonic discrimination shows the Wunuer-Toudaoqiao ophiolitic mélange might have been formed in a spreading ridge. The Late Paleozoic arc-type intrusions outcrop in Baijingshan and Wuerqihan, and comprise a medium-basic unit （gabbro, gabbro diorite and quartz diorite） and an acidic unit （granodiorite and monzogranite）. The geochemical data show that the medium-basic and acidic units are calc-alkali series, and enriched in LREEs and LILEs; they have negative Nb-Ta anomalies with high Sr concentrations and Sr/Y ratios; they show low HREE contents and Y concentrations without Eu anomalies. In addition, the acidic unit has geochemical features of adakitic rocks. The Wunuer-Toudaoqiao ophiolitic mélange might form in Neoproterozoic, which, together with Toudaoqiao blueschist, Jifeng ophiolite mélange and Xinlin ophiolite, consists of a vital suture zone, suggesting a Paleo-ocean once existed in Yakeshi region. The formation age of the gabbro and granodiorite from the Late Paleozoic arc intrusions is 326±1.9Ma and 323.7±1.9Ma, respectively. In terms of trace elements, we confirmed that the formation of the Late Paleozoic arc-type intrusions was related to Early Carboniferous oceanic slab subduction, suggesting the oceanic basin wither between Xing＇an block and Songnen block.