Co-Existing Pyroxenes from the Ferrosyenites of Sivamaiai, Tamil Nadu, South India

The ferrosyenites of Sivamalai (Tamil Nadu, South India) contains ortho- and clinopyroxenes in addition to fayalitic olivine, quartz, palgioclase, orthoclase perthite and opaques. The orthopyroxene occurs as (a) submicroscopic rim around fayalite, (b) exsolved (001) and (100) lamellae within clinopyroxene and (c) discrete blebs close to clinopyroxene. The composition of orthopyroxene among the analysed samples varies from eulite to orthoferrosilite and that of clinopyroxene from ferroaugite to ferrohedenbergite. The ortho- and clinopyroxenes individually exhibit systematic intraspecimen and intralintergranular variations of Fe and Mg, with a restricted range of Ca. The Fe/(Fe+Mg) ratios in the pyroxenes (0.966-0.722) indicate their consistency iron-rich nature, and the orthopyroxenes, in particular. are among a few extremely iron-rich ones reported.

The occurrence of orthopyroxene lamellae within clinopyroxenes of ferrosyenites of Sivamalai is a diagnostic feature of slow cooling of a tholeiitic magma. The two compositional trends defined by the pyroxenes in the quadrilateral suggest strongly fractionated nature of the liquid. The mineral assemblage fayalite+quartz+ferrosilite-rich orthopyroxene suggests a minimum P of 6 kb. The Sivamalai pyroxenes are considered to have formed at -740°C and 7 kb. These pyroxenes are presumably crystalllized from a Fe-rich acidic liquid under low fO₂, moderate to high P and increasing SiO₂.