Geocronologia U-Pb LA-ICP-MS em zircão das rochas do Grupo Jacareacanga, Domínio Tapajós, Cráton Amazônico, Brasil

Abstract

The Jacareacanga Group is divided into the Sai Cinza and Cadiriri formations, which are composed of mica schists, chlorite schists, actnolite schists and talc schists, metacherts, quartzites with pelitic paragneisses and amphibolites. It comprises a volcanic-sedimentary sequence formed by siliciclastic and chemical sedimentation and mafic-ultramafic volcanism that correspond to a turbidite sedimentation with intercalated oceanic bottom basalts of a backarc basin or trench basin related to the earliest magmatic arc (Cuiú-Cuiú Arc) of the Tapajós Domain of the Tapajós-Parima Province from the Amazon Craton. The field data of mapping programs of the Geological Survey of Brazil – CPRM and the petrography and scanning eletron microscope analysis of this study supported the selection of samples for a new geochronological analysis of the Jacareacanga Group. The U-Pb LA-ICP-MS in zircon crystals dated a mica schist, an amphibolite and two pelitic paragneisses of this unit and gave 207Pb/206Pb ages close to previously dating obtained for this unit. However, these previous dating applied few appropriate geochronological methods (U-Pb TIMS in zircon and Pb-evaporation in zircon) and analyzed not enough amounts of zircon crystals to constrain the age of formation of metasedimentary rocks. The present study gets robust geochronological data obtained by an in situ dating method appropriated to analyze large amounts of zircon and identify the ages of detrital sources and constrained ages for the sedimentation of metasedimentary rocks of the Jacareacanga Group, distinguishing them from the ages of later events. A cutoff of ages < 2000 Ma was applied in order to distinguish the later events are related to the granitic intrusions that cut this unit. The pelitic paragneisses have given 207Pb/206Pb ages between 2008 and 2042 Ma and the mica schist ages of ca. 2056 Ma for the sedimentation age of low and high metamorphic grade metasedimentary rocks of the Jacareacanga Group. Furthermore, these rocks have detrital zircon populations of Rhyacian ages and subordinated of Archean ages that indicate detrital sources original from adjacent cratonic areas. The amphibolite did not furnish good geochronological data to constrain the age of basaltic volcanism associated to the sediments of the Jacareacanga basin. The geochronological data of this study show that the Jacarecanga basin was formed during the early Orosirian, and the oldest detrital sources support that was a basin of a continental margin magmatic arc of a Rhyacian to Archean crust continent.