Origin of zircon in jadeitite from the Nishisonogi metamorphic rocks, Kyushu, Japan
Origin of zircon in jadeitite from the Nishisonogi metamorphic rocks, Kyushu, Japan
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DOI：
10.1111/j.1525-1314.2011.00935.x
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发表时间：
2011-08
影响因子：
3.4
通讯作者：
Y. Mori;Y. Orihashi;T. Miyamoto;K. Shimada;M. Shigeno;T. Nishiyama
中科院分区：
文献类型：
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作者：
Y. Mori;Y. Orihashi;T. Miyamoto;K. Shimada;M. Shigeno;T. Nishiyama
根据内部构造、激光烧蚀U-Pb年龄和微量元素组成，对西松诺基变质岩中翡翠岩中锆石的成因进行了探讨。锆石由被自面体边缘覆盖的自面体分带核组成。岩芯中含有白云母、石英、钠长石和钾长石包裹体，238U-206Pb年龄为126±6 Ma(±2SD，n=45，MSWD=1.0)，Th/U比值为0.48~1.64。其边缘含翡翠包裹体，~(238)U-~(206)Pb年龄为84±6 Ma(±2SD，n=14，MSWD=1.1)，Th/U比值为0.06。岩芯富含Y、Th、Ti和稀土元素，但富含Hf和U。球粒陨石标准化的稀土配分模式显示，与边缘相比，岩芯的SMN/LAN比值较高，YBN/GDN比值较低，Ce正异常较大。因此，岩芯和边缘具有不同的~(238)U-~(206)Pb年龄和微量元素组成，反映了锆石的两个生长阶段。虽然岩芯的~(238)U-~(206)Pb年龄与已报道的翡翠岩中基质白云母的40Ar/~(39)Ar斑点熔融年龄一致，但岩芯的~(238)U-~(206)Pb年龄偏老。岩芯中的矿物包裹体和高的Th/U比值最好地解释了长英质岩浆的结晶作用。因此，这些岩心被认为是火成岩前体岩石的残留物。继承核心周围的边缘可能是在翡翠岩形成过程中从水溶液中沉淀出来的。边缘铀浓度的升高表明，外部流体的渗透是降水的原因。这项研究提供了一个原岩交代交代形成翡翠的例子。
On the basis of internal structures, laser ablation U–Pb ages and trace element compositions, the origin of zircon in jadeitite in the Nishisonogi metamorphic rocks was examined. The zircon comprises euhedral zoned cores overgrown by euhedral rims. The cores contain inclusions of muscovite, quartz, albite and possibly K‐feldspar, yield 238U–206Pb ages of 126 ± 6 Ma (±2 SD, n = 45, MSWD = 1.0), and have Th/U ratios of 0.48–1.64. The rims contain inclusions of jadeite, yield 238U–206Pb ages of 84 ± 6 Ma (±2 SD, n = 14, MSWD = 1.1), and have Th/U ratios of <0.06. The cores are richer in Y, Th, Ti and rare earth elements (REEs), but the rims are richer in Hf and U. Chondrite‐normalized REE patterns of the cores indicate higher SmN/LaN ratios, lower YbN/GdN ratios and larger positive Ce anomalies compared with those of the rims. Thus, the cores and rims have different 238U–206Pb ages and trace element compositions, suggesting two stages of zircon growth. Although the 238U–206Pb ages of the rims are consistent with the reported 40Ar/39Ar spot‐fusion ages of matrix muscovite in the jadeitite, the 238U–206Pb ages of the cores are older. The mineral inclusions and high Th/U ratios in the cores are best explained by crystallization from felsic magma. Therefore, the cores are considered relicts from igneous precursor rocks. The rims surrounding the inherited cores possibly precipitated from aqueous fluids during jadeitite formation. The elevated U concentrations in the rims suggest that infiltration of external fluids was responsible for the precipitation. This study provides an example of jadeitite formation by metasomatic replacement of a protolith.