Fluid-induced dissolution breakdown of monazite from Tso Morari complex, NW Himalayas: evidence for immobility of trace elements
Fluid-induced dissolution breakdown of monazite from Tso Morari complex, NW Himalayas: evidence for immobility of trace elements
复制标题
喜马拉雅山西北部措莫拉里杂岩中独居石的流体诱导溶解分解：微量元素固定性的证据
DOI：
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发表时间：
2012
影响因子：
3.5
通讯作者：
K. Pruseth
中科院分区：
文献类型：
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作者：
D. Upadhyay;K. Pruseth
喜马拉雅西部左莫拉日杂岩Polokongka La花岗岩中的原生火成岩独居岩已部分被变质氟磷灰石、尿囊石、含U和Th相(石榴石)和绿帘石组成的三层日冕所取代。这种蚀变与高压角闪岩相(10-11kbar和587-695℃)流体引起的超高压花岗岩在印度-亚洲碰撞后折返过程中的退退有关。这种日冕结构可以通过流体介导的溶解-再沉淀耦合作用，假象地将原始独居石部分置换为磷灰石和尿晶石来解释。利用冠状矿物的体积分数和组成进行质量平衡计算表明，REE、U、Th、Pb、BA和P等元素在蚀变日冕外不迁移，而是保守的。独居石形成的氟磷灰石、尿安石、绿柱石和石英以及REE、U和Th的静止，需要高Ca/Na的含碱、富氟、富Ca流体进入日冕。我们只知道另外两个这种蚀变结构的产状，它们在地球动力学背景和寄主岩石的P-T历史方面有几个相似之处。我们认为，碰撞造山带花岗岩类岩石在退变过程中可能存在一种共同的折返方式、流体来源和组成机制，这种破裂结构可以用来识别大陆碰撞带经历了高P变质的变质花岗岩，否则由于这些岩石中矿物组合的高度变化，很难对其进行约束。
Primary igneous monazite from the Polokongka La granite of the Tso Morari complex in the western Himalayas has been partially replaced by a three-layered corona of metamorphic fluor-apatite, allanite + U- and Th-bearing phases (huttonite + brabantite), and epidote. The alteration is related to high-pressure amphibolite-facies (10–11 kbar and 587–695 °C) fluid-induced retrogression of the ultra-high-pressure granite during exhumation after India–Asia collision. The corona textures can be explained by pseudomorphic partial replacement of the original monazite to apatite and allanite via a fluid-mediated coupled dissolution–reprecipitation process. Mass balance calculations using the volume proportions and compositions of coronal minerals show that the REE, U, Th, Pb, Ba and P were conserved and not transported outside the alteration corona. The formation of fluor-apatite, allanite, huttonite and coffinite from monazite and the immobility of REE, U and Th require an influx of alkali- and F-bearing, Ca-rich fluid having high Ca/Na into the corona. We are aware of only two other occurrences of such alteration textures, and these have several similarities in terms of geodynamic setting and P–T histories of the host rocks. We suggest that there may be a common mechanism of exhumation style, and source and composition of fluids during retrogression of granitoid rocks in collisional orogens and that such breakdown textures can be used to identify metagranites that have experienced high-P metamorphism in continental collision zones, which is otherwise difficult to constrain due to the high variance of the mineral assemblages in these rocks.