青龙山榴辉岩的退变质显微结构及相关的物质迁移——南苏鲁榴辉岩退变质过程中流体活动的证据
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P588.3 P571

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国家重点基础研究(973)资助项目(2003CB716506);国家自然科学基金资助项目(40399141)


Mass transfer and microtexture of retrograded Qinglongshan eclogite:evidence of fluid activity
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    摘要:

    对南苏鲁青龙山榴辉岩的扫描电子显微镜及光学显微镜的详细观察表明,其峰期矿物组合为石榴石 绿辉石Ⅰ 蓝晶石 金红石( 柯石英),基本无含水矿物,并被第二期矿物(或组合)逐步替代,形成后成合晶或冠状体假像,即:绿辉石Ⅰ→绿辉石Ⅱ 钠长石 铁氧化物后成合晶,金红石→金红石 钛铁矿,蓝晶石→钠云母。第二期矿物又被第三期矿物(组合)替代:绿辉石Ⅱ( 钠长石 铁氧化物)→角闪石( 斜长石 铁氧化物),石榴石→韭闪石 铁氧化物。绿帘石与石英是最晚期流体沿微裂隙活动的产物。详细的矿物成分分析及成分迁移估算结果显示,早期后成合晶的形成过程产生多余的Fe、Mg、Na,但消耗部分Ca、Si。产生的Mg、Fe迁移到石榴石边部,引发石榴石内部的(Mg,Fe)/Ca交换,在石榴石颗粒中形成向边部Ca降低、Mg和Fe升高的成分环带。交代产生的Ca被后成合晶消耗,而后成合晶产生的Na被替代蓝晶石的钠云母消耗。在退变质作用晚期,更多流体进入榴辉岩,引发流体渗滤交代反应,无水的早期后成合晶被含水后成合晶(角闪石 斜长石 铁氧化物)替代,同时在石榴石边部形成角闪石 少量铁氧化物反应边。成分迁移估算显示,上述两个过程的成分变化具有一定程度的耦合性。矿物反应曲线、THERMOCALC计算确定的P-T轨迹显示,青龙山榴辉岩的退变质过程主要发生在高压条件(低地温梯度)下,明显不同于北苏鲁地区榴辉岩。这种热演化特点的差异暗示南北苏鲁超高压变质块体可能具有不同的回返历史。

    Abstract:

    The eclogites in Southern Sulu ultrahigh pressure metamorphic (UHPM) terrene show very different retrograded microtextures from the corresponding rocks in Northern Sulu, implying different thermal processes. Scanning electron and optical microscope observations indicate that the primary peak assemblage is anhydrous, composed of Grt + OmpⅠ + Ky + Rt (+ Coe). These peak minerals were replaced by the second and third stage minerals, resulting in pseudomorphic symplectite or coronas through the process of OmpⅠ→OmpⅡ + Ab + Fe-oxide symplectite (type Ⅰ), Rt→Rt+Ilm, Ky→Pg; OmpⅡ (+Ab+Fe-oxide)→Hb (+Pl+Fe-oxide) (type Ⅱ), and Grt→Prg+Fe-oxide. In contrast, epidote and quartz were formed at a very late stage as a result of fluid activity along micro-fractures. Mineral chemistry and mass-balance estimation demonstrate that the pseudomorphic textures were developed by metasomatism involving dissolution and precipitation that were intensified by fluids along the grain boundary. The form ation of symplectite by replacing primary omphacite would evidently produce extra Fe, Mg and Na but consume Ca and Si. The extra Mg and Fe (diffused to garnet grains) triggered an interexchange of (Mg, Fe)-Ca of the garnet, resulting in compositional zonation with Ca decreasing towards the edge of garnet grains where the expelled Ca was consumed during symplectite formation. The replacement of kyanite by paragonite consumed the extra Na. At the later stage when more aqueous fluid intruded into the eclogite, fluid infiltration partially converted Symplectite Ⅰ to Symplectite Ⅱ, and a film of pargasite resorbed a garnet from its boundary. Mass balance estimation suggests that the transformation and resorption might have been coupled qualitatively. p-T conditions based on mineral reactions and THERMOCALC software suggest that the retrogression of the eclogite occurred under HP conditions. The difference in p-T path between the Southern and Northern Sulu eclogites suggests the existence of different exhumation historis within the Sulu terrene.

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杨天南 徐文华 陈方远,2004,青龙山榴辉岩的退变质显微结构及相关的物质迁移——南苏鲁榴辉岩退变质过程中流体活动的证据[J].岩石矿物学杂志,23(3):236~252. YANG Tian-nan~,XU Wen-hua~,CHEN Fang-yuan~,2004,Mass transfer and microtexture of retrograded Qinglongshan eclogite:evidence of fluid activity[J]. Acta Petrologica et Mineralogica,23(3):236~252.

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