东天山印支期闪长质岩墙岩石成因及构造意义

Petrogenesis and tectonic implications of the Indosinian dioritic dykes in Eastern Tianshan

东天山地区发育大量的印支期岩浆岩,其岩石成因及其动力学背景对于理解中亚造山带晚期构造演化过程具有重要意义。本文对东天山尾亚和天湖地区新发现的闪长质岩墙开展了岩石学、地球化学及锆石U-Pb年代学和Hf同位素综合研究。锆石U-Pb测年结果表明,天湖地区闪长质岩墙形成时代为246.6±1.9Ma和248.4±3.8Ma,为早三叠世岩浆活动的产物;尾亚地区闪长质岩墙形成时代为237.7±1.3Ma、234.4±2.1Ma、238.3±1.6Ma和233.9±1.6Ma,为中三叠世岩浆活动的产物。岩石地球化学特征显示研究区闪长质岩墙总体呈现出较高硅(SiO_(2)为60.51%~67.56%)、富碱(K_(2)O+Na_(2)O为6.39%~8.23%)和准铝质(A/CNK为0.91~0.98)特征,属于高钾钙碱性系列。岩石相对富集K、Rb、Ba等大离子亲石元素(LILE),亏损Nb、Ti、Ta等高场强元素(HFSE),具有微弱Eu负异常(δEu=0.83~1.03)。锆石Hf同位素分析显示样品具有正的ε_(Hf)(t)值(+1.26~+6.32)和较年轻的亏损地幔模式年龄(tDM1=638~828Ma)。结合岩石地球化学特征,表明闪长质岩墙可能为交代地幔楔部分熔融形成的幔源岩浆底侵诱发新生下地壳熔融,随后由底侵的玄武质岩浆与壳源长英质岩浆混合形成。结合区域地质和相关构造环境分析,指示闪长质岩墙形成于板内伸展环境。结合前人研究,本文认为东天山发育的闪长质岩墙群可作为区域伸展作用的重要标志,表明东天山在印支期为陆内伸展的构造环境。

There are a large number of Indosinian magmatic rocks in Eastern Tianshan,the geodynamic settings that they formed are of great significance for understanding the late-stage tectonic evolution of the Central Asian Orogenic Belt.In this contribution,we present new field and petrological observations,zircon U-Pb ages,in situ Hf isotopic compositions,and whole-rock geochemical data for the newly discovered dioritic dykes in the Weiya and Tianhu areas of the Eastern Tianshan.Zircon U-Pb dating reveals that the dioritic dykes in Tianhu area formed at 246.6±1.9Ma and 248.4±3.8Ma,which might be the products of Early Triassic magmatic event.The dioritic dykes in Weiya area have crystallization ages of 237.7±1.3Ma,234.4±2.1Ma,238.3±1.6Ma and 233.9±1.6Ma,which would be the products of Middle Triassic magmatic event.The whole-rock geochemical characteristics show that the dioritic dykes in the study area have high SiO_(2)(60.51%~67.56%),total alkali(K_(2)O+Na_(2)O=6.39%~8.23%)contents,and showing the characteristics of metaluminous(A/CNK=0.91~0.98)and high-K calc-alkaline series.These rocks are relatively enriched in K,Rb,and Ba while depleted in Nb,Ti,and Ta,exhibiting slight negative Eu anomalies(δEu=0.83~1.03).The zircon Hf isotope analysis of the samples shows positiveε_(Hf)(t)values of+1.26~+6.32 and young one-stage Hf model ages(t DM1=638~828Ma).Combined with all the above characteristics,it is suggested that the dioritic dykes were probably generated by partial melting of the juvenile basic lower crust induced by the underplated mantle magma and magma mixing between mafic and felsic magmas.Integrating with previous studies,this study proposes that the dioritic dykes in the Eastern Tianshan are important indicators for regional extension,indicating that the Eastern Tianshan was in an intracontinental extensional tectonic setting during the Indosinian Period.