新疆西天山备战铁矿花岗岩锆石U-Pb年龄及其构造成因

Zircon U-Pb age and tectonic genesis of the granite in the Beizhan iron ore deposit of western Tianshan,Xinjiang

西天山阿吾拉勒成矿带是中国重要的铁成矿带之一,但目前对该成矿带上的铁矿成因类型和成矿构造环境仍存在争议,主要原因是对赋铁矿火山岩和侵入其中的花岗岩缺乏深入研究,特别是对可能与成矿有关的花岗岩研究更少。通过对该成矿带中的备战铁矿区花岗岩进行锆石U-Pb测年和岩石地球化学研究,探讨其成因及构造环境,为西天山地区铁矿成因提供新信息。对备战矿区花岗岩进行LA-MC-ICP-MS锆石U-Pb测年,获得正长花岗岩年龄为321.5±3.6 Ma,花岗斑岩年龄为308±1.2 Ma,均形成于晚石炭世。岩石地球化学特征显示,该区花岗岩具有高钾钙碱性、准铝质特征;稀土元素球粒陨石标准化分布图表现为左陡右缓,Eu强烈负异常;微量元素分析结果显示,富集Rb、K、Th、Sr、U等,亏损Ba、Nb、Ta、Ti、P等元素,结合构造环境判别图解,认为其形成于后碰撞环境。εHf(t)值分别介于5.7~13.3和7.5~12.5之间,平均值为10.91和10.06,为较高的正值,Hf二阶段模式年龄分别介于479~947 Ma和526~843 Ma之间,平均值为700 Ma和679 Ma。综合前人研究成果,认为晚石炭世西天山阿吾拉勒铁成矿带主要形成于由汇聚向伸展演化的后碰撞造山的构造转折期,此时由于岩石圈拆沉导致地幔物质底侵下地壳发生部分熔融作用,最终形成了晚石炭世备战铁矿花岗岩。

The Awulale Mineralization Belt in the Western Tianshan Mountains is one of the important iron mineralization belts in China,but there is still controversy over the genetic types and tectonic environment of iron deposits in this mineralization belt.The main reason for the inconsistency in understanding is the lack of in-depth research on the iron-bearing volcanic rocks and granites intruded into them,especially on the granites that may be related to mineralization.By studying the zircon U-Pb age and rock geochemical data of the granites in the Beizhan iron mining area of the Awulale Metallogenic Belt,this paper aims to explore their genesis and tectonic environment and provide new information on the genesis of iron deposits in the Western Tianshan.LA-MC-ICP-MS zircon U-Pb dating of the granites in the Beizhan iron mining area represent that the age of the K-feldspar granite is 321.5±3.6 Ma,and the age of the granite porphyry is 308±1.2 Ma,which was formed in Late Carboniferous.The geochemical characteristics of the rock samples show that they have high K 2O+CaO contents and are characterized by quasi-aluminous features;the chondrite normalized distribution pattern of rare earth elements shows steep left and gentle right,andδEu value is strongly negative;trace elements analysis show enrichment in Rb,K,Th,Sr,U,etc.,and depletion in Ba,Nb,Ta,Ti,P and other elements.Combined with the tectonic environment discrimination diagrams,it is believed that they were formed in a post-collisional environment.TheεHf(t)values range between 5.7~13.3 and 7.5~12.5,with average values of 10.91 and 10.06 respectively,which are higher positive values.The Hf two-stage mode ages t MD2 are between 479~947 Ma and 526~843 Ma,with average ages of 700 Ma and 679 Ma,respeetively.Based on comprehensive regional research results,it is believed that the Late Carboniferous Awulale Iron Metallogenic Belt of Western Tianshan was formed in a period of tectonic transition,and may be post-collision stage from convergence to extension evolution.During this period,granite in Beizhan region was formed.