The Origin of the Meta-Anorthositic Gabbros and Garnetiferous Granulites of the Sittampundi Complex, Madras, India

The petrochemistry of the highly metamorphosed complex is discussed using new chemical analyses for major and trace elements of 21 meta-anorthositic hornblende gabbros, 9 mafic rocks, mostly garnetiferous granulites, and 15 country rocks and xenoliths. The igneous origin of the complex is confirmed, contrary to some earlier views that the anorthite-rich rocks were of metasedimentary calc-silicate origin. The granulites and meta-anorthositic rocks give congruent, and therefore consanguineous igneous trends of variation, suggesting that they crystallized from one magma being the mafic and felsic portions separated by variable crystal settling. The metamagmatic country rocks, especially the amphibolites, are not part of the same suite.

The importance of 'normalising' critical trace element contents (e.g, Cr, Ni, Sr) to constant mafic or felsic mineral percentage to allow for the widely differing abundances obtained from felsic-rich and mafic-rich rocks is demonstrated. The procedure allows igneous trends of variation to be much more critically appraised, and, in the present rocks, leads to the suggestion that some of the Cr in the meta-anorthositic rocks was originally present as chromite, a view in accordance with the occurrence of chromite-rich layers in the intrusion.

The hornblende in the meta-anorthositic rocks, which are pyroxene-free, is metamorphic in crystallization from the texture and the hornblende composition. Whether it is recrystallized primary igneous hornblende or was derived by metamorphism of pyroxene, olivine and plagioclase is uncertain. Some chemical criteria that might enable the crystallization of primary igneous hornblende, even when metamorphically recrystallized, to be recognised are suggested and although their application to the present intrusion does not lead to an unequivocal conclusion, they may be useful elsewhere.