Migmatitos gerados por water-fluxed melting no Sudeste de Roraima

Abstract

This research studied migmatites, gneisses, and metagranitoids of the Martins Pereira Suite, located in the eponymous region in southern Roraima state. Geologically, this suite is part of the Uatumã-Anauá Domain within the Ventuari-Tapajós Province of the Amazonian Craton. The Martins Pereira Suite, dated at approximately 1.97 Ga, is predominantly composed of high-K, calcium-alkaline bluish-gray metagranitoids, with portions exhibiting varying degrees of partial melting. The recorded migmatites display a wide range of morphologies, from "patches" (leucosome pockets estimated to be 1.9 Ga), formed under low-melting and deformation conditions, to stromatic and folded types, especially near shear zones. These zones acted as channels for aqueous fluid migration, facilitating rock melting at temperatures around 700°C, in the upper amphibolite facies. Temperature was estimated from specific microtextures, including quartz with chessboard patterns and mantle-core structures in feldspars, supplemented by Ti-biotite thermometry and comparisons with literature on the melting of granitic protoliths with added water. It was noted that leucosomes lack anhydrous peritectic minerals and exhibit diffuse contacts with the surrounding rock. The melanosome consists of biotite (Bt), plagioclase (Pl), titanite (Tnt), and epidote (Epi). These characteristics, along with low melting temperatures, suggest that migmatization occurred through the Water-Fluxed Melting mechanism, possibly via the reactions KF-1 + Qtz-1 + Pl-1 (An43-27) + H₂O = Melt and Pl-1 + Qtz-1 + Bt-1 + H₂O = Melt + Tnt-2. The melting resulted in the paragenesis KF-2 + Pl-2 (An28) + Qtz-2. A second migmatization event was also identified, similarly associated with Water-Fluxed Melting, characterized by the reaction KF-2 + Qtz-2 + Pl-2 + H₂O = Melt [KF-3 + Pl-3 (An24-21) + Qtz-2]. However, it was not possible to determine whether these migmatization events represent a single continuous tectonometamorphic episode, dated to 1.9 Ga, or if they constitute distinct events in time (Itã Event?). This study highlights the significance of migmatization in the Martins Pereira Suite, emphasizing the role of shear zones in the melting of protoliths under specific conditions in this portion of the Amazonian Craton.