中天山天湖东铁钼矿含矿片麻状花岗岩年代学、地球化学和锆石 Hf 同位素对于中天山早古生代构造演化的启示

摘要

We present a case study of the Early Paleozoic ore-bearing gneiss granite in the Tianhudong iron-molybdenum ore deposit in the central Tianshan in an attempt to provide new insights for the early Paleozoic tectonic setting and tectonic evolution of the central Tianshan.LA ICP MS zircon U Pb dating yields (445.3±4.6)Ma for the studied granite.The granite is composed mainly of biotite,amphibole,quartz,plagioclase and K-feldspar.Geochemical analyses show the granite belongs to subaluminous calc-alkaline granite.The granite is generally enriched in large ion lithophile elements (LILEs)such as Rb,Ba and LREEs,but depleted in typical high field strength elements (HFSEs)such as Nb,Ta,Ti,Y and HREEs,consistent with the geochemical characteristics of typical arc igneous rock.The zircon Hf isotope compositions (εHf (445 Ma))of the studied granite vary from - 6.30 to-1.77,and the T DM2 ranges from 1.538 to 1.825 Ga,indicating the source rocks of the granite contain significant crustal materials.Integrating all the available data,we suggest that the granite is produced by melting of the mainly crustal protolith during a subduction process.Combined with the previous studies and our recent study results,it is suggested that the central Tianshan zone was a magmatic arc tectonic environment during the Early Paleozoic.The mechanism of the Early Paleozoic central Tianshan arc can be ascribed to the southern subduction of the ancient Tianshan ocean located between the Tuha block and Tarim block.%以中天山东段的天湖东铁钼矿含矿花岗岩为例，在 LA ICP MS 锆石 U Pb 年代学测定其为早古生代花岗岩（（445．3±4．6）Ma）基础上，通过岩相学、地球化学及锆石原位 Hf 同位素组成等多方面研究，探讨该岩体的成岩作用及其构造背景。天湖东含矿片麻状花岗岩的主要矿物为斜长石、石英、钾长石，并含少量黑云母和角闪石等。全岩地球化学分析结果表明，该片麻状花岗岩高硅、弱富铝、富钙、富钠而贫钾，ASI 值为0．68～0．82，属于准铝质钙碱性花岗岩，总体上富集大离子亲石元素（LILE）Rb、Ba 等和轻稀土元素 La、Ce、Nd 等，而亏损高场强元素 Nb、Ta、Ti、Yb 等，轻重稀土分异明显，轻稀土分异较为明显，而重稀土分异不明显，表现出典型岛弧岩浆岩的地球化学特征。锆石的εHf （445 Ma）值为-6．31～-1．77，二阶段 Hf 模式年龄（T DM2）为1．538～1．825 Ga，表明该花岗岩的源区主要为壳源物质。综合分析上述资料，认为天湖东铁钼矿片麻状花岗岩是由俯冲过程中地壳物质重熔的产物。结合前人的研究和本课题组的新近研究成果认为，在早古生代时，中天山为岩浆弧构造环境，形成一系列的钙碱性岩浆岩，而该岩浆弧的形成可能是受到介于吐哈陆块和塔里木板块之间的古天山大洋在早古生代时期向南俯冲而形成的。