Phase equilibria and metamorphic evolution of glaucophane‐bearing UHP eclogites from the Western Dabieshan Terrane, Central China
Phase equilibria and metamorphic evolution of glaucophane‐bearing UHP eclogites from the Western Dabieshan Terrane, Central China
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DOI：
10.1111/j.1525-1314.2010.00884.x
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发表时间：
2010-08
影响因子：
3.4
通讯作者：
C. Wei;Y. Li;Y. Yu;J. S. Zhang
中科院分区：
文献类型：
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作者：
C. Wei;Y. Li;Y. Yu;J. S. Zhang
大别山地体西部含绿闪石超高压榴辉岩由石榴石、绿辉石、蓝晶石、绿帘石、多硅白云母、石英/柯石英、金红石等组成。部分石榴石成斑岩呈核幔分带，粗粒陨石含量略有增加，粗粒陨石含量略有减少；而地幔边缘分带剖面，粗粒榴石含量明显增加，粗粒陨石迅速减少。绿辉石通常呈核缘减少的带状分布，j(O)[=Na/(Ca+Na)]。蓝闪石在某些样品中以闪斑的形式出现，并含有石榴石、绿辉石和绿帘石包裹体。NCKMnFMASHO系统对5个有代表性的样品进行了假点位计算，结合岩相学观察表明，超高压榴辉岩记录了4期变质作用。(I)在石榴石分带和包裹体模式的基础上，进行性阶段P-T矢量以加热为主，压力略有增加，表明早期为缓慢俯冲过程，P-T矢量以压力显著增加和轻微加热为主，指示晚期快速俯冲过程。进变质作用以蓝闪石脱水为主，其次为绿泥石、绿帘石和共晶石，释放出结合在水合矿物中的∼27wt%的水。(2)峰期以石榴石边缘成分最多，粗粒含量最小，P-T条件为28.2~31.8kbar，605~613°C为代表，模拟的峰期矿物组合主要为石榴石+绿辉石+硬钠铝石+滑石+多硅白云母+柯石英±蓝晶石。(3)早期降压阶段以硬钠铝石脱水为特征，释放峰矿物组合中结合水∼70-90wt%，导致蓝闪石生长，绿辉石j(O)减少，形成绿帘石。在5-10kbar和500-580°C的温压条件下，榴辉岩块的边缘或强烈的叶片化区域发育角闪角闪石+绿帘石+钠长石/渐长石+石英矿物组合。超高压榴辉岩的变质作用阶段与岩石学观察结果一致，但与前人的研究结果有较大差异。
Glaucophane‐bearing ultrahigh pressure (UHP) eclogites from the western Dabieshan terrane consist of garnet, omphacite, glaucophane, kyanite, epidote, phengite, quartz/coesite and rutile with or without talc and paragonite. Some garnet porphyroblasts exhibit a core–mantle zoning profile with slight increase in pyrope content and minor or slight decrease in grossular and a mantle–rim zoning profile characterized by a pronounced increase in pyrope and rapid decrease in grossular. Omphacite is usually zoned with a core–rim decrease in j(o) [=Na/(Ca + Na)]. Glaucophane occurs as porphyroblasts in some samples and contains inclusions of garnet, omphacite and epidote. Pseudosections calculated in the NCKMnFMASHO system for five representative samples, combined with petrographic observations suggest that the UHP eclogites record four stages of metamorphism. (i) The prograde stage, on the basis of modelling of garnet zoning and inclusions in garnet, involves P–T vectors dominated by heating with a slight increase in pressure, suggesting an early slow subduction process, and P–T vectors dominated by a pronounced increase in pressure and slight heating, pointing to a late fast subduction process. The prograde metamorphism is predominated by dehydration of glaucophane and, to a lesser extent, chlorite, epidote and paragonite, releasing ∼27 wt% water that was bound in the hydrous minerals. (ii) The peak stage is represented by garnet rim compositions with maximum pyrope and minimum grossular contents, and P–T conditions of 28.2–31.8 kbar and 605–613 °C, with the modelled peak‐stage mineral assemblage mostly involving garnet + omphacite + lawsonite + talc + phengite + coesite ± glaucophane ± kyanite. (iii) The early decompression stage is characterized by dehydration of lawsonite, releasing ∼70–90 wt% water bound in the peak mineral assemblages, which results in the growth of glaucophane, j(o) decrease in omphacite and formation of epidote. And, (iv) The late retrograde stage is characterized by the mineral assemblage of hornblendic amphibole + epidote + albite/oligoclase + quartz developed in the margins or strongly foliated domains of eclogite blocks due to fluid infiltration at P–T conditions of 5–10 kbar and 500–580 °C. The proposed metamorphic stages for the UHP eclogites are consistent with the petrological observations, but considerably different from those presented in the previous studies.