摘要
通过主微量元素特征研究,MgO、Fe O含量分别介于5.78%~12.3%和5.16%~10.4%,Al_2O_3含量介于14.2%~21.56%,整体含量较高。TiO_2含量相对较高,介于0.18%~1.32%,平均为0.77%,与岛弧拉斑玄武岩的0.84%较为接近,K_2O含量偏低,介于0.05%~0.53%,平均为0.2%,与洋中脊拉斑玄武岩0.2%一致;轻稀土元素富集,重稀土元素亏损;富集大离子亲石元素（K除外）,高场强元素Ti、P等轻微富集,而Nb、Zr亏损,结合构造环境判别认为形成环境为弧后盆地。利用锆石U-Pb测年,获得两组年龄,其中上交点206Pb/238U表面年龄加权平均值（501±1）Ma（MSWD=0.25）,为寒武世晚期,代表原岩结晶年龄;下交点206Pb/238U表面年龄加权平均值（451±2）Ma（MSWD=0.084）,为晚奥陶世早期,代表变质年龄。结合主量和微量元素特征、构造环境以及同位素年龄,认为洋壳俯冲始于晚寒武世之前,且在晚奥陶世已开始发生碰撞。
Major and trace element analysis were conducted in this study. The rocks have MgO contents between 5. 78% and 12. 3%,Fe O contents between 5. 16% and 10. 4%,Al_2O_3 contents between 14. 2% and21. 56%,which is relatively high in all. The TiO_2 contents of the samples range from 0. 18% to 1. 32%（ average = 0. 77%）,which is relatively high and similar to those of the island arc tholeiitic basalts（ TiO_2=0. 84%）. However,the rocks show relatively low contents of K_2O（ 0. 05%- 0. 53% with an average of0. 2%）,consistent with those of the ocean ridge tholeiitic basalt（ K_2O = 0. 2%）. The REE patterns are characterized by an enrichment of LREEs and a depletion of HREEs. The trace element spidergram shows enrichments for LILEs（ except K）,Ti and P,and depletions for Nb and Zr. It is suggested that the rocks formed in island Back-arc basin. Zircon U-Pb dating was taken and two groups of age were obtained. The upper intercept age of（ 501 ± 1） Ma（ MSWD = 0. 25） indicates the crystallization age of the original rock during the Late Cambrian. The lower intercept age of（ 451 ± 2） Ma（ MSWD = 0. 084） suggests the age of the metamorphism during the early stage of Late Ordovician. Combined with the result of major and trace element and zircon U-Pb dating,it is concluded that the subduction of oceanic crust began from the Late Cambrian and the subsequent collision happened no later than the Late Ordovician.
出处
《现代地质》
CAS
CSCD
北大核心
2016年第5期1004-1013,共10页
Geoscience
基金
新疆维吾尔自治区地质勘查中央专项资金项目(ZYZXKC2009-5)
