阿尔金奇克山斜长角闪岩同位素年龄及其地球化学特征

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1.05~2.771,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。

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

The amphibolite distributes in the northwestern part of the Qikeshan hill in Altyn Tagh. Field geological investigation reveals that the amphibolite is a magmatic intrusion rather than part of an ophiolite suite. The rock have high SiO2 and Al2O3contents of 49.29%?51.03% and 12.27%?14.49%, low Na2O+K2O content of 2.35%?4.37%, MgO and Mg#values of 6.48%?10.12% and 48.7?59.6. The rocks are enriched in LREE with negative Eu anomalies, and show linear trends on diagrams of MgO-SiO2, MgO-Al2O3, MgO-TiO2 and Rb-Cr and Rb/Zr-Rb/Nb. These show amphibolite rocks are formed by the differentiation and evolution of mantle magma. Trace elements are characterized by Rb, Th and U enrichments and Nb, Sr, P depletion. The Th/Ta (1.05?2.77) and Nb/La (0.88?1.02) values demonstrate that continental contamination is minor. Geochemical characteristics of the rocks suggest an intraplate extensional environment. LA-ICP-MS U-Pb isotopic dating of zircons yielded206Pb/238U-207Pb/235U concordia age and206Pb/238U weighted mean age of 503±5 Ma and 479±6 Ma, respectively. According to the Th/U ratio of zircons, we believe these ages belong to the peak metamorphic stage. In other word, it may indicate that the rocks formed before 479 Ma.