Geocronologia U-Pb e tipologia de zircão detrítico aplicadas a estudos de proveniência sedimentar das formações Piriá e Igarapé de areia, Cráton São Luís/Cinturão Gurupi (NE-Pará/NW-Maranhão)

Abstract

Modern analytical equipment such as the Multicollector-Inductively Coupled Plasma Mass Spectrometer, connected to a Laser Ablation microprobe (LA-MC-ICP-MS), made U-Pb geochronological analysis of zircon more accessible and allowed the collection of large amount of data. As a result, the dating of detrital zircon grains has become an important tool in sedimentary provenance studies, since it may provide direct information about the ages of the igneous and/or metamorphic rocks of the source areas that contributed to the deposits of a sedimentary basin. This tool, together with the morphological analyzes of the dated detrital zircons, was applied to provenance studies on the sedimentary rocks of the Igarapé de Areia and Piriá formations, that crop out in the northeast of Pará and northwest of Maranhão states. These sedimentary successions occur in the midst of rocks of the São Luís Craton (SLC), and Gurupi Belt (GB) geotectonic domains. The former is of Paleoproterozoic age and the GSB incorporates significant portions of the SLC strongly reworked in the Neoproterozoic. The Igarapé de Areia Formation is composed of medium grain arcosian sandstones with trough cross stratification, marked by levels of opaque minerals, indicating deposition in medium to highenergy fluvial systems. The maximum depositional age of this unit is 1940 ± 55 Ma, and the sedimentary contribution is restricted to sources from the Rhyacian period (2052 - 2269 Ma), which is consistent with the age of crustal growth of Saouth American Plataform. The Piriá Formation occurs in both geotectonic domains. For the succession that lies over the GB, here called the West Segment, detrital zircons from two sandstones of the arkose and greywacke interbedded with pelitic layers lithofacies, and from one conglomerate of the conglomeratic lithofacies were analyzed. The geochronological dates from the sandstones present subordinate peaks of Neoproterozoic (509 - 951 Ma) and Mesoproterozoic ages (1243 - 1630 Ma), and a main peak of Paleoproterozoic age, mostly Rhyacian (2050 - 2266 Ma). The conglomerate facies is made up of ortho- and paraconglomerates whose constituents indicate igneous and metamorphic sources. The ages of the detrital zircon grains are basically between 1950 and 2298 Ma, which suggest, from the geochronological point of view, that the sediments may have come from a Paleoproterozoic source area, unlike the sandstones of this segment. On the other hand, the Piriá Formation rocks that occur on the SLCF, East Segment, correspond to arkose with plane-parallel stratification interlayered with siltites, and were probably deposited in portions of lower energy of fluvial channels. The histogram of cumulative ages of the detrital zircon from an arkose shows the predominance of younger source areas (525 - 824 Ma). The maximum depositional age for the Piriá Formation is estimated at 509 ± 17 Ma. The discrepancy in the distribution of zircon ages between the arkoses of the East and West segments of the Piriá Formation indicates the contribution of different source areas, suggesting that these segments could represent sub-basins or sedimentary successions deposited in different periods of time.