阿尔金奇克山斜长角闪岩同位素年龄及其地球化学特征

LA-ICP-MS Zircon U-Pb Age and Geochemical Characteristics of Amphibolite at Qikeshan in South Altyn Tagh

阿尔金奇克山地区侵入于地层中的斜长角闪岩原岩为辉长岩,岩石地球化学组成显示斜长角闪岩的SiO_2和Al_2O_3含量较高为49.29%~51.03%和12.27%~14.49%,Na_2O+K_2O含量低为2.35%~4.37%,MgO和Mg~#值为6.48%~10.12%和48.7~59.6,具Eu负异常,样品的MgO对SiO_2、Al_2O_3、TiO_2及Cr-Rb和Rb/Nb-Rb/Zr关系图显示明显相关性,表明其为地幔岩浆经分异演化形成。岩石富集Rb、Th和U,亏损Nb、Sr和P,且Th/Ta为1.05~2.77>1,Nb/La为0.88~1.02,多小于1,显示岩石在演化过程中受到了地壳轻微混染,推断岩石形成于板内伸展环境。利用LA-ICP-MS微区原位定年获得斜长角闪岩^(206)Pb/^(238)U-^(207)Pb/^(235)U谐和年龄503±5 Ma和^(206)Pb/^(238)U加权平均年龄为479±6 Ma,该年龄段为阿尔金峰期变质阶段,指示岩石形成于479 Ma以前。

The amphibolite distributes in the northwestern part of the Qikeshan hill in Altyn Tagh.Field geologicalinvestigation reveals that the amphibolite is a magmatic intrusion rather than part of an ophiolite suite.The rock havehigh SiO2and Al2O3contents of49.29%-51.03%and12.27%-14.49%,low Na2O+K2O content of2.35%-4.37%,MgOand Mg#values of6.48%-10.12%and48.7-59.6.The rocks are enriched in LREE with negative Eu anomalies,andshow linear trends on diagrams of MgO-SiO2,MgO-Al2O3,MgO-TiO2and Rb-Cr and Rb/Zr-Rb/Nb.These showamphibolite rocks are formed by the differentiation and evolution of mantle magma.Trace elements are characterized byRb,Th and U enrichments and Nb,Sr,P depletion.The Th/Ta(1.05-2.77)and Nb/La(0.88-1.02)values demonstratethat continental contamination is minor.Geochemical characteristics of the rocks suggest an intraplate extensionalenvironment.LA-ICP-MS U-Pb isotopic dating of zircons yielded206Pb/238U-207Pb/235U concordia age and206Pb/238Uweighted mean age of503±5Ma and479±6Ma,respectively.According to the Th/U ratio of zircons,we believe theseages belong to the peak metamorphic stage.In other word,it may indicate that the rocks formed before479Ma.