A TAFROGÊNESE ESTATERIANA NOS BLOCOS PALEOPROTEROZÓICOS DA AMÉRICA DO SUL E PROCESSOS SUSEQÜENTES.

Abstract

The Paleoproterozoic orogenic collages were very important for the origin of the older cratonic nuclei of the South American continent, with coalescence of various Archean blocks. Such fusion processes have been designated as the Transmazoninan Orogeny or Cycle, which is an oversimplification, because there were different (both in time and space) orogenetic events from 2.2 to 1.9 Ga (from the Rhyacian to the Orosirian period).

Subsequent global process of taphrogenesis took place on the newly formed Paleoproterozoic landmasses- from Amazonia to Uruguay – with a striking synchronization, in the 1.8-1.7 Ga time interval (Statherian Period). These widespread extensional regimes were responsible for the evolution of different types of tectonic setting (including both magmatism and basinal sedimentation), starting from the preliminary branching systems of rifts within the post-Orosirian landmasses.

Lithological assemblages for these periods/events, as well as they present records are variable within each occurrence.

Felsic to intermediate volcanics (rhyolites, rhyodacites, dacites) and anorogenic A-type granites (and rare gabbro-anorthosite bodies) are the most commom, together with minor mafic dike swarms and mafic-ultramafic layered igneous intrusions.

Some initial rift systems evolved to very large cratogenic basins, during an interval of hundreds of million years, with very typical quartzite-pelite-carbonate assemblages, of epicontinental seas and other continental environments. The age control for the upper sequences (Upper Mesoproterozoic) is still poor.

There are several knows systems, in different cratonic blocks, from northern South America (Amazonia) to Uruguay and Argentina (Rio de La Plata craton) which will be mentioned.

In all mentioned occurrences, there are a series of variables (geological and tectonophysical characteristics) still unknown. Relationships among time, heat flux and average factors of extension (β)  cannot yet be estimated due to the scarcity of geochronological data. The only possibility for estimating the magnitude of those processes is through comparison of the intensity of the preserved geological records, keeping in mind their different crustal levels at present.

Only small parts of these Proterozoic basins have remained as cratonic covers up to the present time. Some segments of them were first involved in collisional orogenic processes in Late Mesoproterozoic times (± 1.3 Ga), although geological records for this orogeny (so-called Uruaçuano) are still deficient.

Additionally, most of these basins were involves in the fission of the Rodinia supercontinent, during the initial steps of the Brasiliano/Panafricano Cycle (around 1000 Ma) and many of their structural trends have been reactivated during the Neoproterozoic orogenic collage resulting in the agglutination of Western Gondwana.

In such conditions, there are somes cases where the time interval is about 1.1 Ga from the basin-forming tectonics (Statherian Period) to the last period of orogenesis (c a 600 Ma, Neoproterozoic III).

Sometimes, it is possible to observe along the same structural trends, or even side-by-side, both fold belts (Brasiliano domains0 and weakly deformed covers (on the Brasiliano cratonic blocks), with coeval volcano-sedimentary piles.