[SMP34-P08] The formation process of ultra-high pressure eclogite constrained from the origin mm-scale layering structure (Nové Dvory, the Moldanubian Zone of the Bohemian Massif)
Keywords:eclogite, UHP, garnet
The study samples are bi-mineralic eclogite composed of garnet-rich matrix and pyroxene-rich layer. Within each sample, garnet grains vary core compositions by layers, but show identical rim compositions regardless of their locations. For example, ND0207 contains a 3-mm thick pyroxene-rich layer. In ND0207, garnet grains in the garnet-rich matrix more than 10mm apart from pyroxene-rich layer has Mg-Cr-poor core (Fe:Mg:Ca=35:32:33 and Cr2O3<0.1wt%), those in the garnet-rich matrix near pyroxene-rich layer has Mg-rich Cr-poor core (Fe:Mg:Ca=23:55:22 and Cr2O3<0.1wt%), and those in pyroxene-rich layer has Mg-Cr-rich core (Fe:Mg:Ca=30:49:21, Cr2O3=1wt%). These garnet grains show similar rim compositions (Fe:Mg:Ca ≈ 28:42:30, Cr2O3 < 0.3 wt%), and contain omphacite only in their rim. Compositions of the omphacite inclusions are Na-rich in the garnet-rich matrix (Na2O=4-5wt%, Cr2O3<0.1wt%, XMg=Mg/(Mg+Fe) = 0.83-0.87), and Cr-Mg-rich in pyroxene-rich layer (Na2O=3-4wt%, Cr2O3<0.4wt%, XMg=0.85–0.90).
Chemical compositions of the layering structure are determined by a quantitative mapping technique using electron probe micro analyzer (Yasumoto et al., under review). The result revealed that garnet-rich matrix increases Cr2O3 (0.0 to 0.3wt%) and XMg (0.5 to 0.8) from the relatively homogeneous part of the garnet-rich matrix to pyroxene-rich layer. This trend is concordant to the variation of chemical compositions of the minerals.
The significant chemical variation of minerals suggests that the ND eclogite or its protolith was not produced only by accumulation. A comparison of (local) bulk compositions reveal that the garnet-rich matrix corresponds to the Gföhl eclogite (Beard et al., 1992; Medaris et al., 1995; Obata et al., 2006) and the gabbroic rocks from South Indian Ridge (Niu et al., 2002), whereas the pyroxene-rich layer corresponds to the Gföhl pyroxenites (Medaris et al., 1995). In addition, prograde relict amphibole is identified in the ND eclogite (Yasumoto et al., 2016), and the ND pyroxenite is considered to be accumulated from melt (Svojtka et al., 2016). These facts suggest that the ND eclogite was metagabbro that was partially infiltrated and metasomatised by pyroxenitic melt under high-pressure conditions. Chemical variation of garnet cores and lack of omphacite in the garnet cores indicate that the layering structure was present before the omphacite growth. In other words, the melt infiltrated before eclogite-facies metamorphism. Driving force of the changes in local bulk compositions can be prograde heating (Nakamura et al., 2013) or dehydration of amphibole during eclogitization.