CORREÇÃO QUANTITATIVA DOS EFEITOS DE METASSOMATISMO EM ROCHAS VULCÂNICAS BÁSICAS. EXEMPLO DOS METABASITOS DO ESPINHAÇO MERIDIONAL (MG-BRASIL)

Resumo

A new correction procedure is described, by which analytical data for basic metavolcanic rocks may be tested to detect and correct for qualitative and quantitative variations, due to metassomatic modifications. The method involves plotting the analytical data in terms of oxide molecular proportion ratios, in form log X/Z vs. log Y/Z just as described in Beswick and Soucie's paper from 1978. The quantitative correction follow the results obtained by this method, and is based on the assumption that the K₂O grades plotted on graphs evaluate regularly, because the primary mineralogy of the analysed rocks was all qualitatively identical. The metabasic suite of lavas from the southern part of the Espinhaço (MG-Brazil) is analysed and corrected, testing the proposed method. The differences observed between the results obtained with corrected and non corrected original analyses are commented, and the results are dimentioned and compared. It has shown that alkalies, lime, magnesia, iron and silica have moved appreciably and differently in parts of the studied mafic bodies. Caution is suggested in the characterization of ancient volcanics by simple comparison of their non corrected compositions with those of young volcanics.