Petrografia de metabasitos da mina de diamantes de Romaria, Romaria-MG

Abstract

The diamond mine of Romaria - MG is inserted in the urban perimeter of the homonymous city and is explored since 1867 in conglomerate breccia. The lithologies associated with the mine correspond to micaschist interspersed with metabasites of the Araxá Group, superimposed to this are sandstones of the Botucatu Formation being covered by vulcanoclastic rocks of the Uberaba Formation. Due to the peculiar characteristics of the deposit, several scientific researches were carried out with the intention of better understanding the genesis of the diamonds. However, the studies did not focus on the lithologies that enclosing the deposit, especially those related to the metamaphic bodies present in the region of the mine. Thus, the present work consisted of petrographic analyzes of schist and metamaphic bodie of the mine basement. The observed lithological results consist of sedimentary and igneous protolites. The rocks derived from sedimentary protolites are represented by the Grt-Bt-Mus Qtz schist, where it was possible to distinguish a regional metamorphic phase M1 described by Seer (1999), whose metamorphic apex is delimited by the pair Grt + Bt between the amphibolite facies in the garnet zone and the green schist facies in the biotite zone (~500°C), in addition, a retrometamorphic phase (RM2) characterized by the growth of muscovite and quartz on remains of skeletal garnet . The igneous protolites comprise basic rocks, which gave rise to the amphibolite, which shows a granulometric variation between the thicker edge, with crystals of hornblenda and polycylitic texture, and the center of the body is expressed with finer granulation of well-preserved hornblende crystals, both portions contain a thin matrix composed of Qtz + Plg and oxides. Amphibolites may represent a metamorphic apex in amphibolite facies, in the garnet zone, bounded by the pair Hbl + Plg, in addition to the retrometamorphic paragenesis defined by the occurrence of Ep + Act overgrown at the expense of the hornblenda. The variation in the observed granulation is attributed to the acting difference of the metamorphism on the body, with a greater action on the edges that receives a greater quantity of water, allowing for the growth of the crystals, while the center of the body, more rigid, and relatively more anhydrous, generates smaller crystals. The lack of orientation of this amphibolytic body points to a metamorphism in the absence of deformation, indicating that the rock experienced only thermal effects of the tectonic event, which would possibly place the body in more distant portions of the main deformational epicenter.