Mapeamento geológico da Porção Sul de Uruará, Oeste do Pará: evolução do conhecimento geológico do Extremo Noroeste do Domínio Bacajá, Província Transamazonas

Abstract

Based on the integration of field and petrographic data, the interpretation of shaded relief images and airborne geophysics (gamma spectrometry and magnetometry), and a review of previous work, this paper proposes an integrated geological map for the study area, located in the southern portion of Uruará-PA, in the extreme northwestern of the Bacajá Domain, south-central Transamazonas Province. The results allow us to reflect on the evolution of geological knowledge in the area, as well as to compare the different lithostratigraphic units and discuss the tectonic evolution of these rocks. The proposed geological map includes five lithostratigraphic units: (i) São José Complex (2.50 Ga); (ii) Uruará Orthogneiss (2.50 Ga); (iii) Canaã Granite (2.10 Ga); (iv) Sant’Ana Granite (2.12–1.99 Ga); and (v) Uirapuru Granite (undated), in addition to a body from the Arapari Intrusive Suite. These rocks are overlain to the west by the volcanic rocks of the Iriri Domain and to the north by the sedimentary rocks of the Amazon Basin. Structurally, the bodies of the different units occur elongated in a NW-SE to E-W direction and show little deformed aspects in low-strain to strongly deformed domains near shear zones, which are subparallel to the NW-SE mylonitic foliation, with dips > 60°/NE. The foliation trajectory transposes the different units and suggests a continuity of the structural pattern following the regional trend (NW-SE), indicating that these rocks were simultaneously deformed under main NE-SW forces. Three families of structural lineaments occur in the area: (i) the NW-SE to WNW-ESE family, defined by magnetic lineaments, mainly affects the São José Complex and is interpreted as sinistral transpressive shear zones, dominated by pure shear, as indicated by field data; (ii) the NE-SW family, essentially defined by a large magnetic lineament and truncating the regional NW-SE trending lineaments. This structure was interpreted as an inferred shear zone, since no points (outcrops) have been mapped on this lineament and appears to displace the regional shear zones counterclockwise. The origin of this structure may be associated with different events, from the final stage of the Transamazonian Thrust itself (2.2–1.9 Ga) to younger events, such as the breakup of the Pangea supercontinent (~201 Ma); and (iii) the NNE-SSW family, defined by positive relief lineaments, is associated with small mining operations and occurs under cataclastic breccias, suggesting that they are fault zones that affected the crust in a brittle regime after the Trans-Amazonian Event. In terms of tectonic setting, the São José Complex and the Uruará Orthogneiss, with protoliths dated at 2.50–2.44 Ga, were formed in an Archean-Siderian island arc environment and subsequently metamorphosed during the collisional peak of the Trans-Amazonian Event (~2.1 Ga). On the other hand, the Canaã, Sant’Ana, and Uirapuru granites were emplaced contemporaneously with the regional collisional peak (~2.1 Ga) and can be interpreted as syntectonic granites formed in a syn- to late-collisional environment, with emplacement controlled by preexisting shear zones. A second pulse from the Sant’Ana Granite, dated at 1.99 Ga, is associated with crustal relaxation in a post-orogenic setting in relation to the Trans-Amazonian Event.