阿尔泰南缘克朗盆地铁木尔特花岗岩体年龄、地球化学特征及成因

Age,geochemistry and petrogenesis of Tiemierte granites in the Kelang basin at the southern margin of Altay,Xinjiang

铁木尔特黑云母花岗岩出露于阿尔泰造山带南缘克朗盆地北西侧,岩体主要为黑云母花岗岩。锆石的SHRIMPU-Pb定年结果表明该岩体结晶年龄为459±4.9Ma。岩石具有高的SiO_2(71.92%～76.58%)和Al_2O_3(12.92%～14.55%)含量,富碱(K_2O+Na_2O=7.09%～8.11%)、高钾(K_2O/Na_2O=1.1～1.62)、低P_2O_5(<0.14%)和MgO+FeO(<5%)含量,强过铝质(ACNK=1.13～1.34);富集LREE和Th、U、Pb,贫Ba、P、Sr、Ti、Nb,呈现明显的Eu负异常(δEu=0.33～0.53);并具有低的Al_2O_3/TiO_2(<100)比值和高的CaO/Na_2O(>0.3)比值。以上特征表明该岩体属高钾钙碱性高温型强过铝花岗岩。同时岩石具有低的Sr初始值(^(87)Sr/^(86)Sr=0.701727～0.709951)和相对高的ε_(Nd)(459Ma)(-1.43～-0.98)值,两阶段Nd模式年龄较为均一(t_(2DM)=1.2～1.3Ga)。岩体的原始岩浆可能是富含白云母和黑云母的变泥质岩在大于875℃的条件下脱水熔融的产物,在部分熔融过程中有磷灰石、斜长石、钛铁矿等矿物的残留,并混入少量的幔源物质。结合阿尔泰南缘的区域地质背景综合分析,推测岩体形成于俯冲作用的初期,可能是在挤压背景下由于局部伸展减压以及异常地幔和深部热流不均匀上升提供的热影响发生的部分熔融产生。

SHRIMP U-Pb zircon ages and geochemical data of whole rocks are reported for Tiemierte biotite granite in the Kelang basin at the southern margin of Altay orogenic belt.The zircons with well-defined oscillatory zoning from granite samples yielded mean～(206)Pb/～(238)U ages of 459±4.9Ma,which is a good evidence to explain the magmatic event during the Early Paleozoic at the southern margin of Altay.The granites are high in the concentrations of SiO_2(71.92%to 76.58%),total alkali(Na_2O + K_2O)(7.09%to 8.11%) and A1_2O_3(13.05%to 14.74%),and low in the concentrations of P_2O_5(<0.14%) and MgO + FeO(<5%).In addition,they are enriched in potassic(K_2O/Na_2O = 1.1 to 1.62) with high A/CNK values(1.13 to 1.34),and low Al_2O_3/TiO_2 (<100) ratios and high CaO/Na_2O(>0.3) ratios,which are defined as high-K calalkaline high-temperature strongly peraluminious granites.They display noticeable negative anomaly of Ti,P,Sr,Ba andδEu(0.33 to 0.53),obvious enrichment of LREE and Th, U,Pb characteristics.They have near-zero or negativeε_(Nd)(t) values(- 1.43 to -0.98) and high Sr isotopic compositions(initial～(87)Sr/～(86)Sr values =0.7018～0.7099) with corresponding to Nd model ages of 1.2～1.3Ga,indicating a mixture of old components and juvenile mantle material.The granitic melts were probably derived from dehydration melting of a biotite-bearing pelite within middle to upper crust(above the granet stability field) upper 875℃temperature and water-undersaturated condition,with limited assimilation of mantle material.Conbined with the tectonic evolution of the southern magin of the Altay,we suggest that the Tiemierte gratites in the Kelang basin probably have been generated in initial continental arc environment by increasing heat from decreasing pressure in local extentional,ascending of anomalous mantle and heat flow under compressional environment.