Kwon Sanghoon, Samuel Vinod O, Song Yungoo, Kim Sung Won, Park Seung-Ik, Jang Yirang, Santosh M
Department of Earth System Sciences, Yonsei University, Seoul, 03722, Republic of Korea.
Geology Division, Korea Institute of Geoscience and Mineral Resources, Daejeon, 34132, Republic of Korea.
Sci Rep. 2020 Nov 16;10(1):19869. doi: 10.1038/s41598-020-76999-y.
Equilibrium omphacite-garnet-bearing mafic rocks have been classified as eclogites, either pristine or retrogressed, that were formed at great depths in the lithosphere. Here we report a unique natural example of eclogite resembling assemblage in disequilibrium formed through fluid-induced metasomatic reactions under the amphibolite to granulite facies. Primarily, the amphibolized protolith experienced a garnet-amphibolite facies metamorphism at ~ 500-700 °C and ~ 0.8-1 GPa. Subsequently, CO fluid induced fracturing and dissolution-reprecipitation reactions occurred at peak metamorphic conditions of ~ 700 °C and ~ 1 GPa. Occasional omphacite-albite assemblage, which gradually replace diopside-oligoclase symplectite adjacent to albite veins along fractures, indicates fluid-induced coupled dissolution-reprecipitation disequilibrium reactions. Here the albite-omphacite assemblage is in local equilibrium at least on 1 mm length scale, during cooling, below ~ 600 ºC and ~ 1 GPa, within the amphibolite facies conditions. The results from this study clearly suggest that disequilibrium garnet-omphacite assemblage in mafic rocks could be formed by crustal reworking processes below granulite facies conditions, and their textural equilibrium is an important criterion while defining eclogite facies.
含绿辉石-石榴石的镁铁质平衡岩石已被归类为榴辉岩，无论是原生的还是退变的，它们形成于岩石圈的深处。在此，我们报告了一个独特的天然榴辉岩实例，其类似的组合是在角闪岩相到麻粒岩相条件下通过流体诱导的交代反应形成的不平衡状态。最初，角闪石化的原岩在约500-700°C和约0.8-1吉帕的条件下经历了石榴石-角闪岩相变质作用。随后，CO流体在约700°C和约1吉帕的峰值变质条件下引发了破裂以及溶解-再沉淀反应。偶尔出现的绿辉石-钠长石组合，沿着裂缝逐渐取代与钠长石脉相邻的透辉石-奥长石交生体，表明了流体诱导的耦合溶解-再沉淀不平衡反应。在此，绿辉石-钠长石组合至少在1毫米的长度尺度上处于局部平衡状态，在冷却过程中，低于约600ºC和约1吉帕，处于角闪岩相条件下。这项研究的结果清楚地表明，镁铁质岩石中不平衡的石榴石-绿辉石组合可能是在麻粒岩相条件以下的地壳改造过程中形成的，并且它们的结构平衡是定义榴辉岩相时的一个重要标准。
