Low-temperature hydrothermal alteration of natural metamict zircons from the Eastern Desert, Egypt
Low-temperature hydrothermal alteration of natural metamict zircons from the Eastern Desert, Egypt
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DOI：
10.1180/0026461036730112
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发表时间：
2003-06
影响因子：
2.7
通讯作者：
T. Geisler;A. A. Rashwan-A.;M. Rahn;U. Poller;H. Zwingmann;R. Pidgeon;H. Schleicher;F. Tomaschek
中科院分区：
文献类型：
--
作者：
T. Geisler;A. A. Rashwan-A.;M. Rahn;U. Poller;H. Zwingmann;R. Pidgeon;H. Schleicher;F. Tomaschek
摘要研究了埃及东部沙漠南部619±17 (2σ) Ma古构造后花岗岩变质锆石晶体的化学和结构蚀变。晶体呈现出简单的振荡生长带和变质作用引起的裂缝，为流体渗透提供了途径。电子探针和离子探针分析表明，富U和富th的变质岩区大量富集Ca、Al、Fe、Mn、LREE和a水种，并损失了Zr、Si和放射性成因Pb。这些化学变化是与低温（120-200°C）水溶液剧烈反应的结果。化学反应可能发生在元晶网络的无定形区域内。微拉曼和红外测量表明，在锆石-流体相互作用过程中，变质岩结构部分恢复。变质作用的阈值是锆石发生热液蚀变的必要条件，由α-衰变剂量Dc经验定义。我们提出Dc标志着第一个渗透点，在这里，非晶畴开始在元网络中形成渗透簇，并且在这里，散装化学扩散被认为是急剧增加的。热液蚀变的时间由U-Pb SHRIMP不协调的较低截距年龄（17.9+6.9 ~ 7.4 (2σ) Ma）确定，与磷灰石裂变径迹年龄（22.2+5.4 ~ 4.8 (2σ) Ma）吻合较好。热液蚀变事件与红海主裂陷期和红海沿岸广泛的低温矿化同时发生。
Abstract The chemical and structural alteration of metamict zircon crystals from a 619±17 (2σ) Ma old, post-tectonic granite in the southern part of the Eastern Desert, Egypt was studied. The crystals show simple oscillatory growth zones with metamictization-induced fractures, which provided pathways for fluid infiltration. Electron and ion microprobe analyses reveal that metamict, i.e. U and Th-rich, areas are heavily enriched in Ca, Al, Fe, Mn, LREE, and a water species, and have lost Zr and Si as well as radiogenic Pb. These chemical changes are the result of an intensive reaction with a low-temperature (120-200°C) aqueous solution. The chemical reactions probably occurred within the amorphous regions of the metamict network. During the zircon-fluid interactions the metamict structure was partially recovered, as demonstrated by micro-Raman and -infrared measurements. A threshold degree of metamictization, as defined empirically by an α-decay dose, Dc, was necessary for zircons to undergo hydrothermal alteration. It is proposed that Dc marks the first percolation point, where the amorphous domains start to form percolating clusters in the metamict network and where bulk chemical diffusion is believed to increase dramatically. The time of the hydrothermal alteration is determined by a lower intercept age of a U-Pb SHRIMP discordia of 17.9+6.9-7.4 (2σ) Ma, which is in good agreement with an apatite fission track age of 22.2+5.4-4.8 (2σ) Ma. The hydrothermal alteration event occurred contemporaneously with the main rifting phase of the Red Sea and widespread low-temperature mineralizations along the Red Sea coast.