A retrogressive sapphirine-cordierite-talc paragenesis in a spinel-orthopyroxenite from southern Karnataka, India

Abstract

Within amphibolite facies Peninsular gneisses in the south of the Dharwar craton, units of Sargur supracrustal rocks contain ultrabasic enclaves. One of these enclaves is an orthopyroxenite which comprises bronzite, spinel and minor phlogopite preserving coarse-grained, relic textures of probable igneous origin. After incorporation into the gneisses the enclave evolved through several distinct stages, elucidation of which allow an assessment of its metamorphic history.Firstly, deformation during closed system, anhydrous recrystallisation caused the coarse-grained textures to be partially overprinted by similar mineral assemblages but with a granoblastic texture. Secondly, open system hydration caused retrogression of the bronzite to alumino-gedrite at the margins of the enclave. Subsequently, the penetration of these fluids along grain boundaries caused reactions between spinel and bronzite to produce reaction pockets carrying assemblages of peraluminous sapphirine associated with cordierite and talc. The differences in the mineral assemblages in each pocket coupled with slight variations in their chemistry, suggest that equilibrium did not develop over the outcrop. Because sapphirine + magnesite is present in some pockets, it is evident that CO2 was also a component of the fluid.Phase relations from the MASH portion of the FMASH system, to which the chemistry of the reaction pockets approximates, suggest that the hydrous metamorphism causing the changes depended upon the assemblage enstatite + spinel + vapour which exists at PT conditions above the position of I16, --760 ~ at 3 kbar and below I21 at --765 ~ at 5.6 kbar (Seifert, 1974(Seifert, , 1975, where sapphirine is replaced by kornerupine. The suggested path of reaction occurred between I18 and I21. Subsequent reactions related to I20 cause the formation of cordierite. Talc formation has to be modelled in a different reaction grid.The metamorphism recorded by these reactions is thus at a maximum of amphibolite facies and is interpreted to have formed during the uplift and cooling history of the gneiss complex when hydrous fluids were free to migrate. Given the complex high-grade metamorphic history of this part of the Dharwar craton this event is likely to be late Archaean or Palaeoproterozoic in age.

An occurrence of igneous orthorhombic amphibole, Eriksberg gabbro, southern Sweden

“…This value is substantially higher than temperatures of formation estimated for metamorphic gedrite, which range from 750 to 800 ∞C at similar pressures (e.g., Friend et al 1993;Dasgupta et al 1999). The geothermometry on Ca-amphobile ranges from 878 to 927 ∞C, which implies that the gedrite crystallized between 900 and 925 ∞C.…”

“…Anderson (1980) pointed out the reaction relation between liquid and olivine in the stabilization of igneous hornblende in arc-related magmas. Data is from Friend et al (1993;diamond), Hisada and Miyano (1996;plus sign), Ouzegane et al (1996;asterisk), Deer et al (1997;circle) and Dasgupta et al (1999;triangle gedrite-bearing assemblage. However, textural relations suggest that plagioclase was not reacting when the gedrite crystallized (Fig.…”