O metamorfismo da serra Tepequém (Estado de Roraima)

Abstract

The Serra Tepequém, located in the center north portion of the Roraima State, is considered as a residual relief of the Paleoproterozoic sedimentary rock of the Guiana Shield, northern Brazil. This region is recognized by the most prominent gold and mainly diamonds occurrences, stratigraphically positioned on the base of the Paleoproterozoic Roraima Supergroup. This unit were interpreted as a cratonic covers accumulated in the final phase of the last important regional metamorphic event, the Transamazonian Cycle (2.25 -2.00 Ga). The Serra Tepequém constitutes an opened mega synclinal with interflank distances of 6 km in length, with subhorizontal axis trending N70ºE and subvertical axial plane. The rock unit constitutes a metavolcanosedimentary sequence with about 700 meters of thickness. Basal metadacites and metalitharenites are covered by a level of metabasalt succeeded upsection by intercalations of 300 meters thick of metasiltstone (slate) and fine metarenite. Conglomerates hosting gold and diamonds are intercalated with metarenites suggesting fining upward cycles. Slate cleavage parallel to the axial plane is defined by preferential orientation of the small flakes of white micas. Elongated pebbles and elliptical spots define the lineations that occur according to the axial folds. Kaolinite and quartz were not observed in the pelites in contrast with abundant pyrophyllite and white micas (muscovite/illite) that suggest the neoformation through the reaction kaolinite + quartz = pyrophyllite + water. This reaction defines the beginning of the metamorphism in these pelitic rocks. A high temperature zone of prehnita-pumpellyite facies inside in the very low grade metamorphism is defined in the metabasalts due the presence of actinolite and pumpellyite. The slates and the metabasalt have respectively P-T conditions at 2,0 Kb and 345±20 ºC (actnolite appearance) and 3,9 Kb and 430±15 ºC (pyirophyllite disappearance). The metavolcanosedimentary rocks show a metamorphic gradual increase transition to a supracrustal succession displaying a lateral NW-SE trending in the innermost portion of the Central Guyana Belt. Slates of the Serra Tepequém grade to phyllittes, quartzits and mafic schists, outcrooping in the Vila Brasil, mica schists and amphibolites in the Serra Cauarane, and finally to kinzigitic paragneisses and migmatites in the Serra Murupu. The presence of metamorphic paragenesis and fabrics, generated in the compressive tectonic setting, indicate very low grade orogenic metamorphism in portions of the Paleoproterozoic cover (Roraima Supergroup) of the Guiana Shield. It implies in the existence of a regional metamorphic event, younger than the Transamazonian Cycle.