Datação pelo método de traços de fissão em apatita da região da estrutura dômica de Monte Alegre, Bacia do Amazonas (PA)

Abstract

In the northern portion of the Amazonas Sedimentary Basin, south of the Monte Alegre city (PA), a semi-elliptical structure defined by a group of small hills occurs, which is known as the Monte Alegre Dome. In the central portion of the dome, Devonian rocks of the Ererê Formation crop out, while toward the edge of the structure the Neodevonian to Mesocarboniferous successions, represented by the Barreirinha, Curiri, Oriximiná, Faro and Monte Alegre formations, are exposed. Diabase dykes and sills of about 200 Ma (Penatecaua Magmatism) cut this set of rocks. The formation of this dome structure has been related to the emplacement of dykes and sills of diabase. However, the contribution of Tertiary tectonism, present throughout the Amazon Basin, cannot be ruled out for the formation of the dome, since in the eastern and southern portions of this structure the Cretaceous to Tertiary rocks of the Alter do Chão formation are in faulting contact with rocks of the Ererê formation. In this study we applied the apatite fission track thermochronology in the diabase of the Penatecaua Magmatism in order to determine the age of formation of the dome structure, and to investigate the role of the Tertiary tectonics in the development of this structure. The apatite fission track method (AFT) is applied to study the thermotectonic evolution of a region, since the quantification of the fission tracks present in the mineral permits to estimate the age at which the rock passed through a given temperature (blocking temperature), which may be related to a tectonic event (uplift / subsidence) that occurred in the area. The mafic rocks studied in the region of Monte Alegre provided ATF apparent average ages between 53.2 and 43.6 Ma. These ages are interpreted as representing major tectonic events that occurred in the Tertiary (Early to Middle Eocene) that uplifted to shallower crustal levels and the diabases and the country rocks. The thermal histories obtained after modeling the AFT data allowed the recognition of four events of cooling, which occurred at 140-110 Ma, 110-100 Ma, 55-45 Ma and 30 Ma-recent. Just one sample (TFMA-12), located outside of the dome structure, did not record the Eocene tectonic event. The ATF data modeling also permitted to estimate cooling rates ranging from 0.4ºC / Ma for slower events, up to 2.82°C / Ma for faster events. Based on these cooling rates it is suggested that 3 to 3.35 km of rocks section was removed due to the reactivation of faults at different moments during the geologic evolution of the region, mainly along the Cenozoic. The apparent ATF ages between 43.6 and 53.2 Ma, as well as the significant cooling thermal event recorded in the Eocene by the data modeling revealed the important role that the Tertiary (Eocene) tectonic in the formation of dome structure of Monte Alegre. It is believed that the apparent ages of fission tracks (43.6 and 53.2 Ma) would place limits on the age of this Tertiary tectonics.