桐柏山地区低温榴辉岩变质作用的P-T轨迹
P-T path of low-temperature eclogites from the Tongbaishan area
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摘要: 桐柏山地区发育南、北两条低温高压榴辉岩带，中间被桐柏山高级变质杂岩所分隔。本文对两条带内的4个保存完好的榴辉岩体及其退变产物进行了详细的岩石学研究和P—T条件估算。结果表明，北带榴辉岩的形成温压条件较高，为530～6100C、1．7～2．0GPa，南带榴辉岩较低，为470～5200C、1．3～1．7GPa，说明两条榴辉岩带不属于同一个高压岩片。实际上，二者分别对应于西大别地区的浒湾榴辉岩带（Ⅰ带）和七角山榴辉岩带（Ⅴ带）。电子探针分析发现，所有榴辉岩中的石榴石都展示了进变质环带，而退变榴辉岩的成分环带更为复杂，即保留了早期的进变环带，也显示了晚期的低温取代以及后期相对高温条件下的再生长，由此为榴辉岩的变质作用推导出一个“发卡式”演化轨迹。这种轨迹表明，榴辉岩先是在降温条件下缓慢向上回返的，这与目前已建立的多数高压、超高压岩石的快速抬升模式有所差异，而后在其进入到中地壳后又在相对较高的地温梯度下遭受退变质作用的改造。
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Key words:
- Eclogites /
- P-T path /
- Tongbaishan
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