PARAMETERS CHARACTERIZING INFLUENCE ON PRODUCT COMPOSITION IN METAMORPHIC REACTION: AN APPROACH USING CHEMICAL POTENTIAL DIAGRAM

Corona texture between olivine (Ol)-plagioclase (Pl) is a common phenomenon and has been reported from different geological terrane of the world. Although in all cases the corona develops along Ol and/or Pl, the resultant coronal assemblage varies. In this study, we have tried to explore the effect of different parameters like pressure, temperature, reactant mineral chemistry, behaviour of the system (Open/Close), chemical potential gradient, P-T-t path etc. on the genesis of such distinct coronal minerals. To address this, we have taken three samples from published literature. The samples are #CGGC, a mafic intrusive from Chotanagpur Granite Gneissic Complex, India (Adak & Dutta, 2020); #GTSI, an olivine bearing mafic dyke from the Granulite Terrane of South India (Banerjee et al. 2019); #VFH, a troctolitic gabbro from Valle Fértil and La Huerta range, Argentina (Gallien et al. 2012). The layers in coronae of #CGGC and #GTSI are defined by three phases of separate composition; orthopyroxene and amphibole are common, but #CGGC contains spinel and #GTSI contains magnetite. #VFH contains clinopyroxene in addition to other phases. P-T grid shows the behaviour of the system (open/closed) and the P-T-t path (related to the tectonic evolution of the terrain) followed by the rock samples is important in deciding the final coronal assemblage. Topological relations in calculated μCaO/μMgO–μH₂O show though H₂O participation in the reaction was definitely necessary to produce Amp from the anhydrous minerals but difference in chemical potential of H₂O in reaction site does not seem to have any significant role in development of the hydrous and anhydrous coronae assemblage in any of the sample. T–μCaO/μMgO diagrams show variability in the level of stability of any one reactant between the three samples, can initiate breakdown of one reactant before another in a sample, whereas in other samples reaction may involve simultaneous breakdown of both the reactants. In pseudosection modelling, this study has considered only the modal volume proportion of product phases in calculating effective reaction bulk considering that using a uniform parameter that all the layers in these coronae form in a single stage. This helps to understand if overall P-T condition of metamorphism had any major role in development of the coronal phases.