GENESIS OF TWO DISTINCT TYPES OF CORONAE ASSEMBLAGES AT CLINOPYROXENE-PLAGIOCLASE CONTACT IN MAFIC DYKE FROM SOUTHERN GRANULITE TERRANE (SGT), INDIA

Mafic dykes from Southern Granulite Terrane (SGT) exhibit spectacular coronae development at the clinopyroxene (Cpx)-plagioclase-I (Pl I) interface. The coronae are composed of two distinct assemblages -i) garnet (Grt)-quartz (Qtz) and ii) amphibole (Am)-plagioclase-II (Pl II). Pl II are compositionally distinct (Ca-poor) from Pl I and such plagioclase composition only occurs in association with Am. The coronae assemblages show very systematic occurrences, Grt-Qtz always develop at the Cpx-Pl I contact that lacks any orthopyroxene (Opx) exsolution at Cpx rim and Am-Pl II develops only at Cpx-Pl I interface, where the Cpx rim essentially contains Opx exsolution. The relation suggests Cpx+Pl I→Grt+Qtz and Cpx+Opx+Pl I→Am+Pl II as probable reactions in the separate corona domain. Nowhere in this rock, Grt and Am show any replacement relation between them. Results from pseudosection and conventional geothermobarometry demonstrate that both the assemblages could be stable at the similar P-T condition (~675ºC and ~5.5-6 kbar). Considering the chemistry of coronal assemblages and their reactants, a number of chemical potential diagrams are used to understand if the difference in local domain chemistry could explain the genesis of separate corona composition. μMgO–μCaO diagram indicates that relative variation in CaO/MgO in reaction domains could explain the reason for formation of Am in Cpx (+Opx)-Pl I margin and Grt in Cpx (-Opx)-Pl I contact. These diagrams also support the interpretation from other methods that both the coronal reactions likely to have occurred at the same P-T condition. Meticulous observations from petrography along with the results from thermodynamic analyses suggest that the distinct coronae assemblages developed at the interface of Cpx-Pl I in the dyke rocks are guided by micro-domainal variation in pre-reaction assemblage rather than broad change in physico-chemical condition during metamorphism.