Orthopyroxenes from Sukinda ultramafites and the nature of the parental magma

Chromite-bearing ultramafites occur as synkinematic intrusions during the Eastern Ghat orogenic movement (2800—3000 m. y.), in Eastern India. The ultramafic instrusives are composed mainly of dunite, bronzitites, websterite and their metamorphic derivatives. These were emplaced by successive 'pulsation' intrusions and show some layered features. Pure (=-99,9%) orthopyroxenes separated from bronzitite, peridotite. websterite etc. were analysed by Philips automatic PW 1212 X-ray fluorescence spectrometer. The analyses revealed very low A1 and Ca content compared to most other widely known localities of layered types. The En values (80—83%) are also lower than those of many alpine and stratiform complexes. The crystallochemical considerations of the orthopyroxenes lead the author to assume a peridotite primary magma rather than a basic magma, occurring below the substratum in the Eastern Ghat region of E. India. To account for the crystallization of monomineralic rocks such as dunite, enstatitite, chromitites etc., liquids of these rock compositions are assumed to have been locally produced by partial fusion of peridotites. Absence of any literature in this direction leads the author to presume that Sukinda magma was compositionally unlike any of the presently known uppermantle magma genetic zones. The intrusive magma, however, might be contaminated during orogeny to change the path of crystallization