GEOLOGIA DA REGIÃO DE PORTO NACIONAL, GO

Abstract

Geological mapping and lithostructural studies have been carried out at the Porto Nacional region, northern Goias State. Several units and features were recognized related to the long-term evolution during the Archean-Middle Proterozoic interval, and to the Brasiliano reactivation and Phamerozoic development. The oldest units are the Porto Nacional Complex (acid to basic granulites, kinsigites, anorthosites and leptinites) and the Matanca Suite (porphiroblastic gnaissic granitoid), representative of Archean highgrade and granite-gneiss terrains. These terrains are part of two crustal blocks, juxtaposed seemly during the Jequie Event (ca. 2.5-2,8 Ga through a large ductile low-angle shear zone trending N 30E. The Morro do Aquiles Formation is the psamo-pelitic sequence of the Santo Antonio Fold Belt supposed to have been developed during the Transamazonico Cycle. It present metamorphism of high greenschist facies and superposed deformations due to two or three phases. The granite and granophyre bodies (Lajeado Suite) may be interpreted as transamazonian post-tectonic intrusions (ca. 1.8 Ga)or cratonic manifestations aged 1.5-1.8Ga. The Natividade Group is a psamitic sequence with conglomeratic, pelitic and volcanic (?) intercalations, related to the Uruacuano Cycle. The low greenschist facies metamorphism and the Conceicao Anticlimorium developed during the first sin-metamorphism folding phase are prominent features of this unit. Gentle folding with N40W subvertical axial plane affected this sequence, as well as the Morro do Aquiles Formation and the Porto Nacional Complexo. The sedimentation of the imature and red beds of the Monte do Carmo Formation, the intrusion of dacite and andesite dikes (ca. 1.1 Ga), the general cataclasis and associated low-grade recrystalization, and the observed local crenulations, are related to Uruacuano lateorogenic intense faulting (Transbrasilian Lineament?). Diabase dikes frequently cut all the above Precambrian units. They are supposed to be related to fault reactivation and regional reheating of the Brasiliano Event (ca. 5OO Ma), and to the Mesozoic Wealdenian Reactivation, when most of the NE and NW faults were active. Silurian-Ordovician and Cenozoic sedimentary covers are present in the studied area, indicating more stable periods of its geolcgic history.