Na-Poor Ferro-Augites and Ferro-Hedenbergites from Ferrosilite-Bearing Ferrosyenites of the Prakasam District in Andhra Pradesh, India: Further Evidences for a Tholeiitic Parentage

The clinopyroxenes from the ferrosyenites in the Prakasam district of Andhra Pradesh are characterized by their subcalcic nature, enrichment in Fe with concomitant impoverishment in Mg, and low amounts of (calculated) Fe³⁺, Na, Al and Ti - features which specifically typify their subalkaline (tholeiitic) parentage. The acmite component is insignificantly low in the analyzed clino-pyroxenes (Na_3.60-5.74 Mg_10.01-27.09 Fe_67.17-85.16) and bears no relationship whatsoever either with the hedenbergite component or with the calculated Fe³⁺ content. The results of the Fe³⁺ calculations were suspect, especially when the mineral analyses show low SiO₂, low totals, Si+Al<2, and total cations more than 4 per formula unit of 6 oxygens. These strongly coloured green clinopyroxenes, commonly mistaken as sodium-rich acmitic pyroxenes, are indeed sodium-poorferro-augites and ferro-hedenbergites. The high Fe/Mg ratios in the clinopyroxenes (2.4-8.2; Ca_43.3-45.6Mg_5.8-16.0Fe_39.6-50.5), coupled with the extremely high Fe/Mg ratios in the coexisting olivines (35.5-61.8; Fa_97.3-98.5) and orthopyroxenes (3.6-11.8; FS_78.2-91.3) are an indication of the very highly fractionated nature of the magma. Clinopyroxenes of comparable chemical characteristics are reported from ferrosyenitic rocks elsewhere in the world, which in contrast to these of the Prakasam district, are not cospatial with (feldspathoidal) alkaline rocks. The occurrence of ferrosyenites, containing Fe-rich orthopyroxene (eulite-ferrosilite) and inverted pigeonite, in the vicinity of alkaline rocks in the Prakasam district, is interpreted as purely fortuitous and merely coincidental, not having any genetic or causative relationship.