Abstract
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, CO2 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.
Highlights
Equilibrium omphacite-garnet-bearing mafic rocks have been classified as eclogites, either pristine or retrogressed, that were formed at great depths in the lithosphere
Experimental investigations suggest that gabbro/basalt to eclogite transformation is characterized by the formation of garnet-omphacite assemblage, where there is a gradual increase in the pyrope content of the garnet, and plagioclase is a bsent[21]
For the first time, based on the textural evidence, mineral chemistry, and phase diagram modeling, we have provided an excellent natural example of eclogite resembling disequilibrium garnet-omphacite-bearing mafic metamorphic rock, which might have formed under amphibolite to granulite facies P–T conditions due to fluidrock interactions
Summary
Electron microprobe analyses were conducted on a JEOL JXA-8100. Superprobe, Electron Probe Micro Analyzer (EPMA), housed at the Department of Earth System Sciences, Yonsei University, Seoul, South Korea. Peak position was adjusted to 129.9 for measuring Na content in the omphacite. In the standard position 129.27, Na content and total composition were low. Slight adjustment has done to measure Mg and Al in garnet, amphibole, diopside and omphacite to obtain maximum count by scanning for peaks before quantitative analysis. Phase diagram modeling using bulk-chemistry of the amphibolite provide a good opportunity to examine the stability of different mineral phases and their compositional changes during the evolution of this particular rock in a pressure temperature space. PERPLE_X 6.7.9 version available at http://www.perplex.ethz.ch[59] and inbuilt thermodynamic data file[60] were used for phase diagram calculations. The two-dimensional P–T pseudosection was calculated in a window where temperature changes from 660 to 715 oC on the X-axis and pressure varies from 0.72 to 1 GPa on the Y-axis
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