WHAT CONTROLS THE CHEMICO-MINERALOGY OF CORONAL ASSEMBLAGE FORMED AT OLIVINE – PLAGIOCLASE CONTACTS?

A common micro-structure in mafic rocks are coronae between olivine (Ol) and plagioclase (Pl). Mineral(s) and their composition in such a corona is determined by a variety of factors like, pressure, temperature, mineral chemistry, bulk chemistry, etc. In this study, chemical data from three published studies on genesis of corona texture between Ol and Pl were examined and a number of calculations (petrogenetic grid, chemical potential diagram, pseudosection) were performed using uniform parameters, to evaluate the effect of these factors on the final coronal chemistry. The comparison was made between coronae from, mafic intrusives of Chotanagpur Granite Gneissic Complex, India (Adak & Dutta, 2020); mafic dykes of the Southern Granulite Terrain, India (Banerjee et al. 2019); and troctolitic gabbros of the Valle Fértil and La Huerta range, Argentina (Gallien et al. 2012). Although in all cases, corona formed at the contact of Ol and Pl, the final coronal phase assemblage was different for each scenario. Petrogenetic grids showed that reactions considering same reactant assemblages in the different samples are actually comparable in terms of the summation of the product chemistry in reaction. But the final product phases are also determined by factors like behaviour of system (open or closed), the order in which P-T-t path (related to the tectonic evolution of the terrain) intersected the univariant curves in a P-T space, etc. Reaction topology was examined in μ_MgO-μ_CaO diagrams comparing the stability of reactants Ol and Pl between the three samples. Results 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. Such variability in reaction kinetics can finally yield completely different final corona assemblages. Pseudosection analysis of these samples were done considering an ‘effective reaction bulk’, estimated using a uniform logic that all the layers in these coronas represent a single stage equilibrium assemblage, formed by reaction between Ol and Pl. This was adopted to understand and compare with the actual scenario in each case, the influence of bulk chemistry, pressure and temperature on the coronal mineralogy.