Progressive metamorphism of the Taitao ophiolite ; Evidence for axial and off-axis hydrothermal alterations
Progressive metamorphism of the Taitao ophiolite ; Evidence for axial and off-axis hydrothermal alterations
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DOI：
10.1016/j.lithos.2007.04.003
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发表时间：
2007-10
期刊：
影响因子：
3.5
通讯作者：
T. Shibuya;T. Komiya;R. Anma;T. Ota;S. Omori;Y. Kon;S. Yamamoto;S. Maruyama
中科院分区：
文献类型：
--
作者：
T. Shibuya;T. Komiya;R. Anma;T. Ota;S. Omori;Y. Kon;S. Yamamoto;S. Maruyama
我们估计了与智利三重结合部有关的∼6-Ma Taitao蛇绿岩的变质条件。根据次生基质矿物估算的蛇绿岩变质程度，在地层上从沸石相向下变化，经历了前驱体-阳起岩相、绿片岩相和绿片岩-角闪岩过渡阶段，再到角闪岩相。变质相系对应于低压型。变质带边界与蛇绿岩内部岩性边界近平行，表明变质作用是大洋中脊轴向热液蚀变作用的结果。随着深度的增加，矿脉的矿物组合和成分从石英为主，到绿帘石为主，再到前驱石为主。从脉状组合估计的温度范围从火山单元的∼230°C到辉长岩单元底部的∼380°C，系统地比相邻的基质矿物估计的∼200°C低。矿脉发育较晚，系统温度较低，表明成矿蚀变是离轴热液蚀变作用的结果。将具有大洋中脊(MOR)亲和性的太陶蛇绿岩与其他蛇绿岩和MOR结壳进行对比，发现太陶蛇绿岩具有许多与MOR结壳相似的热液蚀变特征。这与太陶蛇绿岩形成于南美洲大陆附近的南智利海脊系(Anma，R.，Armstrong，R.，Danhara，T.，Orihashi，Y.和Iwano，H.，2006)的构造史一致。智利南部太陶蛇绿岩中辉长岩和席状岩的锆石高质量分辨离子探针U-Pb和裂变径迹年龄及其构造意义岛弧，15(1)：130-142)。
We estimated metamorphic conditions for the ∼6 Ma Taitao ophiolite, associated with the Chile triple junction. The metamorphic grade of the ophiolite, estimated from secondary matrix minerals, changes stratigraphically downwards from the zeolite facies, through the prehnite–actinolite facies, greenschist facies and the greenschist–amphibolite transition, to the amphibolite facies. The metamorphic facies series corresponds to the low-pressure type. The metamorphic zone boundaries are subparallel to the internal lithological boundaries of the ophiolite, indicating that the metamorphism was due to axial hydrothermal alteration at a mid-ocean ridge. Mineral assemblages and the compositions of veins systematically change from quartz-dominated, through epidote-dominated, to prehnite-dominated with increasing depth. Temperatures estimated from the vein assemblages range from ∼230 °C in the volcanic unit to ∼380 °C at the bottom of the gabbro unit, systematically ∼200 °C lower than estimates from the adjoining matrix minerals. The late development of veins and the systematically lower temperatures suggest that the vein-forming alteration was due to off-axis hydrothermal alteration. Comparison between the Taitao ophiolite with its mid-ocean ridge (MOR) affinity, and other ophiolites and MOR crusts, suggests that the Taitao ophiolite has many hydrothermal alteration features similar to those of MOR crusts. This is consistent with the tectonic history that the Taitao ophiolite was formed at the South Chile ridge system near the South American continent (Anma, R., Armstrong, R., Danhara, T., Orihashi, Y. and Iwano, H., 2006. Zircon sensitive high mass-resolution ion microprobe U–Pb and fission-track ages for gabbros and sheeted dykes of the Taitao ophiolite, Southern Chile, and their tectonic implications. The Island Arc, 15(1): 130–142).