Polyphase growth of garnet in eclogite from the Hong'an orogen: Constraints from garnet zoning and phase equilibrium
Polyphase growth of garnet in eclogite from the Hong'an orogen: Constraints from garnet zoning and phase equilibrium
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DOI：
10.1016/j.lithos.2014.06.020
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发表时间：
2014-10
期刊：
影响因子：
3.5
通讯作者：
Ligang Zhou;Qiong-Xia Xia-Qiong-Xia-Xia-104886927;Yong‐Fei Zheng;Zhaochu Hu
中科院分区：
文献类型：
--
作者：
Ligang Zhou;Qiong-Xia Xia-Qiong-Xia-Xia-104886927;Yong‐Fei Zheng;Zhaochu Hu
对红安造山带低T/高P榴辉岩相带中块状和片理化榴辉岩中石榴石的主、微量元素和矿物包裹体进行了分析。两类榴辉岩中的石榴石具有不同的核边分带和矿物包裹体。对于大多数石榴石晶粒，石榴石生长至少有两个阶段是明显的。榴辉岩中的石榴石在核部含有大量的石英、金红石、角闪石、金红石、多硅白云母等矿物包裹体，而在边缘则含有少量的石英、金红石、钛铁矿等矿物包裹体。这些石榴石颗粒表现出增加MgO，但减少CaO和稀土元素含量从核心到边缘。连同假截面计算，它建议，含水矿物，如镁铝榴石，绿帘石和角闪石可能是主要的反应物的石榴石核的生长过程中俯冲，而石榴石边缘的生长涉及角闪石分解和/或溶解以前形成的石榴石。在另一方面，石榴子石的叶理榴辉岩表现出几乎恒定的MgO，减少MnO，但增加CaO和重稀土元素含量（HREE）从核心到边缘。沿着假截面计算，推测绿帘石的分解将持续促进石榴石核和边的生长。基质中绿帘石分带也支持这一观点，在核心中出现低硬玉绿辉石包裹体，在边缘出现少量金红石包裹体。绿帘石边缘轻稀土元素相对于核部富集，重稀土元素相对于核部亏损，表明绿帘石边缘与石榴石边缘处于平衡重结晶状态。在原岩成分方面，片理化榴辉岩的SiO2、Na 2 O、K2 O和MgO含量高于块状榴辉岩，而Al 2 O3、Fe 2 O3和TiO 2含量低于块状榴辉岩。沿着相平衡模拟结果表明，榴辉岩原岩成分的差异主要决定了石榴子石分带模式的差异以及基质和包裹体矿物组合的差异。俯冲带变质作用中石榴石的生长需要水化矿物的分解，如方解石、角闪石和绿帘石。
Major and trace element profiles as well as mineral inclusions were analyzed in garnets from massive and foliated eclogites from the low-T/high-P eclogite-facies zone in the Hong'an orogen, China. Garnets in the two types of eclogites show different core–rim zoning and mineral inclusions. At least two stages of garnet growth are evident for most garnet grains. Garnets in the massive eclogite contain abundant mineral inclusions such as quartz, chlorite, amphibole, rutile and phengite in cores, but only a few mineral inclusions such as quartz, rutile and titanite in rims. These garnet grains exhibit increasing MgO but decreasing CaO and REE contents from core to rim. Together with pseudosection calculations it is suggested that hydrous minerals such as chlorite, epidote and amphibole were probably the major reactants for the growth of garnet cores during prograde subduction, whereas the growth of garnet rims involves amphibole breakdown and/or dissolution of previously formed garnet. On the other hand, garnets in the foliated eclogite exhibit nearly constant MgO, decreasing MnO but increasing CaO and heavy REE contents (HREE) from core to rim. Along with pseudosection calculations it is inferred that the decomposition of epidote would consistently contribute to the growth of garnet core and rim. This is also supported by epidote zoning in matrix, with the occurrence of low-jadeite omphacite inclusions in the core and a few rutile inclusions in the rim. The enrichment of light REE (LREE) and depletion of HREE in the epidote rim relative to the core indicate that the epidote rim was recrystallized in equilibrium with the garnet rims. For protolith compositions, the foliated eclogite shows higher SiO2, Na2O, K2O and MgO contents but lower Al2O3, Fe2O3and TiO2contents than the massive eclogite. Along with phase equilibrium modeling, it is concluded that the differences in the protolith compositions of eclogites primarily dictate the differences in the garnet zoning patterns and the mineral assemblages of matrix and inclusions. The decomposition of hydrous minerals such as chlorite, amphibole and epidote is necessary for garnet growth during subduction-zone metamorphism.