Termocronologia por traços de fissão em apatita da Borba Sul da Bacia do Amazonas, na região de Itaituba (PA)

Abstract

The region of Itaituba (PA) located at the South portion of the sedimentary basin of the Amazon can be defined as a border zone between this basin and the Amazonian Craton. In this region outcrop sedimentary rocks, belonging to Pitinga, Maecuru, Ererê, Barreirinha, Curiri, Monte Alegre and Itaituba Formations, representatives of sineclise phase sedimentary basin of the Amazon; sedimentary rocks the stage rift gathered in Prosperança Formation; besides igneous rocks of Iriri Group, Parauari Intrusive Suite and Maloquinha Intrusive Suite, which compose the basement of the sedimentary basin. The fission track thermochronology of apatite was used in the investigation of thermotectonic evolution of the sedimentary basin of the Amazon region of Itaituba (PA), mainly from the dating of basement rocks of the basin. The method of fission tracks in apatite is a marker thermochronologic low temperature, recording temperatures of up to 120 º C. When subjected to conditions of temperatures above the traces of apatite fission are cleared, resetting the clock geochronological for this method, and registers, thus, a new event. Thus, this tool was used to investigate the age of the tectonic processes that affected the Amazon Basin, which were also recorded in the rocks of its basement. Given the scarcity of apatite in sedimentary rocks sampled units of the Amazon basin, only a sample of the Monte Alegre Formation sandstone has been dated by fission track method. However, in the basement could be dated 20 samples, representing various rocks including rhyolites, granites and olivine-diabase of Paleoproterozoic, belonging to the three units mentioned above. The sandstone of the Monte Alegre Formation, whose age of the lithostratigraphy and Mesocarbonífero, provided apparent ages in apatite fission track AFT) from 91 Ma. Statistically this sample revealed the existence of two populations of apatite grains: a population with a mean age of 105 Ma and another population with a mean age of 64 Ma, indicating that this sample was subjected to two events thermotectonic. This fact can be confirmed through the thermal modeling also showed that the presence of two events whose ages are 106 Ma and 58 Ma. Samples of the basement were divided into three groups according to ages TFA obtained and the results of mathematical models. In group 1 TFA ages ranged from 163.0 Ma and 258.7 Ma, for group 2 ages were established between 142.1 Ma and 147.4 Ma, while samples of group 3 were between 170.1 Ma and 180.9 Ma is noteworthy that the ages of crystallization of igneous bodies sampled are between 1.8 and 1.9 Ga. The mathematical modeling of samples of groups 1 and 2 record the same epsódios, a first cooling, followed by heating and finally an episode of cooling. This last cooling event occurred about 100 Ma for all samples of these groups, except the sample IT-16 passing through the final event of cooling for 62 Ma. The thermal histories of samples IT-22 and 23 show a period of accelerated cooling compared to previous samples, their thermal histories ending at 39 Ma and 35 Ma, respectively, suggesting the existence of neotectonic events in the region. The events of approximately 100 Ma recorded in the Monte Alegre Formation sandstone and models of the samples of the basement can be related to a compressional tectonics that occurred according to the opening of the eastern equatorial Atlantic and the convergent movement of Andean subduction zone, west of South American plate. In turn the events of Tertiary age (64-58 Ma) marked these rocks may show reflections of a process where dextral transform interacted plates South American, Caribean and Nazca, reactivating old fracture zones. The various ages obtained for samples of the basement (ranging from 142.1 Ma and 258.60 Ma) are markers of major tectonic events, but may be registering the different ages at which these samples passed by the isotherm of 120 ° C, indicating thus the occurrence of a tectonic with different behavior in different areas of this region.