Mineralogy of a scapolite-bearing rock from Rajasthan, Northwest Peninsular India

Abstract

Electron-microprobe analyses of coexisting phases from a scapolite-garnet-epidote-calcite-plagioclase-hornblende-pyroxene (sphere-hematite-magnetite) rock of the Aravalli Group (Early Precambrian) at Karera, district Bhilwara, Rajasthan, reveal that chlorine is an important constituent of both scapolite (Me_71.3 and hornblende, making the latter a dashkesanite variety. The amphibole also contains an unusually high K₂O (3.7 wt.%), and is a chlor-potassium hastingsite. The epidote contains 41% pistacite and shows complete substitution of Al by Fe³⁺ in the Al-site and to some extent also in AIO and/or Al(OH) sites. The garnet is also rich in ferric iron and has a mol. composition Pyr₂₃Alm_8.5Gro₁₃And_54.5 Sp_1.0. The pyroxene is dominantly a hedenbergite. The phase relations and textural as well as geological criteria exclude metasomatic processes and favour equilibrium recrystallization of the scapolite-bearing assemblage; the chlorine presumably derived from an evaporite component of the Aravalli metasediments. The geothermometry based on the fractionation of Na and Ca between scapolite (EqAn 43.3) and plagioclase (An_85-90 yields metamorphic temperatures of around 700°C for the scapolite-bearing rock which are in agreement with those obtained by other mineral equilibria in the associated pelitic assemblages.