PETROGRAFIA DE DUAS OCORRÊNCIAS DE PROVÁVEL ECLOGITO EM BODOCÓ E FLORESTA NO ESTADO DE PERNAMBUCO, BRASIL

Abstract

Petrographic investigations on two Ti mineralizations at Bodocò and Floresta, respectively 600 and 400 km westwards from Recife, in the State of Pernambuco, Brazil, revealed the presence of probable eclogites among the ore hosting mafics. At Bodocò the mafics are conformably enclosed by gneisses and migmatites of the Pajeú-Paraíba Foldbelt, 1.5 km southwards, from the tectonic contact with the Middle to Young Proterozoic Pianco-Alto B rígida Foldbelt. The gneissic sequence is supposed by some authors to be the northernmost exposure of the precursor of the São Francisco Craton in its Lower Proterozoic configuration. An early eclogitic assemblage of garnet + omphacite + rutile + zoisite is preserved in the mafics as relics, successively replaced by four retrograde parageneses, almost identical to those found in C type eclogites in the Italian Alps and Norway: 1. replacement of omphacite by clinopyroxene + placioclase and, later, ferro-glaucophane + plagioclase symplectites and of rutile by tjjtanite and/or ilmenite; 2. iron enrichment and recrystallization of the amphibole to crossite or magnesio-riebeckite; 3. transformation of the mafics into, biotite, chlorite and epidote; 4. poiquiloblastic growth of plagioclase, forming banded amphibole-chlorite gneisses. The optical identification of the pyroxenes and amphiboles was confirmed by X-ray difractometry and is coherent with the textural relationship if isochemical reactions are assumed. At Floresta the metamafics are enclosed by Lower Proterozoic to Archean gneisses and migmatites or, as magaxenoliths, in 0.89 Ga old granites, a few kilometers northwards from the Pernambuco Lineament. This lineament marks the northern limit of Pernambuco-Alagoas Massif, a supposed fragment of the precursor of the Sao Francisco Craton, with the Pajetf-Parafba Foldbelt. Of the supposed eclogitic assemblage only garnet and rutile are preserved in this case. Garnet crossitites identical to those found at Bodocò, and plagioclase + clinopyroxene symplectites in garnet amphibolites without individualized plagioclase grains, are supposed to have been formed by retrogressive metamorphism of eclogites. The symplectites recrystallize to coarse aggregates of normal hornblende and plagioclase, evolving gradually to banded garnet amphibolites. PETROGRAPHY OF TWO OCCURRENCES OF PROBABLE ECLOGITES AT