阿尔金清水泉斜长角闪岩同位素定年及其地球化学特征

阿尔金南缘清水泉地区与超基性岩及斜长花岗岩伴生的斜长角闪岩岩石地球化学组成显示:SiO_2和Al_2O_3含量较高,分别为47.63%~55.26%和16.42%~18.97%,Na_2O+K_2O含量低为4.91%~5.60%,MgO和Mg~#值较低为4.41%~5.72%和44.9~57.9,具有负Eu异常,其Mg#与Ti O2、Fe OT及Cr与Rb呈明显的相关性,表明岩石为地幔岩浆经分异演化形成。岩石富集大离子亲石元素Rb、Ba、Sr及Sm和Th,亏损高场强元素Nb、Ta、Zr、Hf、Yb、Y、Lu及Ti,且Th/Ta为2.61~18.121,Nb/La为0.21~0.661,指示岩石在演化过程中受到了陆壳的混染,推断岩石形成于裂谷环境。利用LA-ICP-MS微区原位定年获得斜长角闪岩中锆石206Pb/238U加权平均年龄为461±4 Ma,指示阿尔金南缘在中奥陶世早期处于伸展的构造动力学背景之下,为研究阿尔金构造带的形成及演化提供了新的证据。

Isotopic Age and Geochemical Characteristics of Qingshuiquan Amphibolite in South Altyn Tagh

TheQingshuiquan amphibolite distributes in the middle part of South Altyn Tagh associated with plagiogranite and ultramafic rocks. This amphibolite is characterized by relatively high SiO2(47.63%~55.26%), Al2O3 (16.42%~18.97%)contents,low K2O+Na2O (4.91%~5.60%), MgO (4.41%~5.72%) contents and low Mg# values of 44.9~57.9, and negative Eu anomalies. The negative correlations of Mg#-TiO2, Mg#-FeOT and the positive correlations of Cr-Rb also suggest that the amphibolite was derived from mantle source and experienced differentiation. The rocks are enriched in large ion lithophile elements (Rb, Ba, Sr) and Sm, Th, depleted in high field strength elements (Nb, Ta, Zr, Hf, Yb, Y, Lu) and Ti, with Th/Ta>1 (2.61~18.12), Nb/La<1 (0.21~0.66). Thus, we suggest that the rocks were produced in rift environment and were contaminated by the crust. LA-ICP-MS zircon U-Pb dating yielded206Pb/238U weighted mean age of 461±4 Ma. Through comprehensive analysis, the south Altyn Tagh in the early Middle Ordovician was under extensional background.