Ion microprobe zircon U-Pb age and geochemistry of the Myanmar jadeitite
Ion microprobe zircon U-Pb age and geochemistry of the Myanmar jadeitite
复制标题
缅甸硬玉的离子微探针锆石U-Pb年龄及地球化学
DOI：
10.1144/0016-76492006-119
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发表时间：
2008-01-01
影响因子：
2.7
通讯作者：
Chu, Bingbing
中科院分区：
文献类型：
--
作者：
Shi, Guanghai;Cui, Wenyuan;Chu, Bingbing
综合测定了缅甸翡翠岩的地球化学和年代学特征。块状岩石微量元素组成呈U型稀土配分模式，Eu正异常明显。稀土元素总丰度很低，不到球粒陨石的一半，高场强元素和一些大离子亲石元素呈中等富集度。这些特征表明是交代起源。有三组不同内部特征的锆石，分别是阴极发光、矿物包裹体、化学成分和灵敏的高分辨率离子探针U-Pb年龄。I组锆石平均年龄为163.2 Ma±3.3 Ma，大多具有明显的振荡分带、最高的U、Th含量和无钠富镁矿物包裹体，指示中侏罗世发生了火成岩(洋壳形成)或热液(蛇纹岩化和/或橄榄岩化)事件。第二类锆石发光强度大，无振荡分带，平均年龄146.5±3.4M a，包裹体中含有硬玉和硬玉辉石包裹体，表明缅甸硬玉岩的形成和东印度洋板块的俯冲作用发生在晚侏罗世。第三类锆石的年龄为122.2±4.8 Ma，代表了一次较晚的未知热事件。中侏罗世锆石的发现为东印缅山脉的构造演化提供了年代学上的制约。
Combined geochemistry and geochronology of the Myanmar jadeitite were determined. Bulk-rock trace element compositions display U-shaped REE patterns with pronounced positive Eu anomalies. The total REE abundances are very low, less than half chondritic, and the high field strength elements and some large ion lithophile elements are moderately enriched. These features indicate a metasomatic origin. There are three groups of zircons with different interior characteristics, cathodoluminescence, mineral inclusions, chemical compositions and sensitive high-resolution ion microprobe U–Pb ages. Group-I zircons, with a mean age of 163.2 ± 3.3 Ma, mostly have distinct oscillatory zoning, highest U and Th contents, and Na-free, Mg-rich mineral inclusions, and thus indicate an igneous (formation of oceanic crust) or hydrothermal (serpentinization and/or rodingitization) event in the Middle Jurassic. Group-II zircons, with a mean age of 146.5 ± 3.4 Ma, have bright luminescence without oscillatory zoning and include jadeite and jadeitic pyroxene inclusions, suggesting that formation of the Myanmar jadeitites, as well as subduction of the eastern Indian oceanic plate, occurred in the Late Jurassic. Group-III zircons have an age of 122.2 ± 4.8 Ma, which represents a later unknown thermal event. Discovery of the Middle Jurassic zircons provides geochronological constraint on the tectonic evolution of the eastern Indo-Burman Range.