The U-Pb and 40Ar/39Ar ages presented above can be divided into different groups which define the tectonic history of different blocks related to the collision and amalgamation of the Neoproterozoic around the southern limit of the São Francisco Craton. These final stages occur at different times and overprint differently the Brasília and Ribeira belts (Figure 7). The older U-Pb monazite ages between 642±1 and 620±1 Ma (H359) from migmatites and high-grade paragneisses of the Guaxupé Domain in the southern Brasília belt, are considered to date the main metamorphic phase associated with granitic plutonism, collision and a tangential tectonic regime (Figure 7). This is supported by U-Pb data and geologic constraints from the Socorro Domain indicating pre- to syn-collisional processes between 650 and 628 Ma (Hackspacher et al., 2003; Piuzana et al., 2002; Pimentel et al., 2004).
The U-Pb monazite ages from the Guaxupé and Socorro Domains between 613±1 and 607±3 Ma represent late stage metamorphism of the Brasília belt associated with eastward thrusting and nappe stacking. Pegmatites intruding upper crustal level have concordant monazite ages of around 598 Ma (Fig. 5I). They were associated with the final magmatic evolution, popossibly the same event reported by Janasi et al. (2001) in the Guaxupé Domain, between ca. 610 and 580 Ma. This late magmatism is interpreted to represent the post-orogenic uplift of the Brasília belt (Wernick, 1998) and marks the final closure of the Goianides Ocean (Fischel et al., 1998; Pimentel et al., 2000; Piuzana et al., 2002). Magmatic ages between 595 and 560 Ma record the main period of tectonic activity in the Ribeira belt (Machado et al., 1996) suggesting that magmatism in the easternmost part of the Brasília belt could be a response to tectonic and possibly magmatic activity in the Central Ribeira belt. Biotite 40Ar/39Ar cooling ages of 599±1 and 587±1 Ma from the paragneisses of the Guaxupé Domain indicate that the nappe system cooled from the 700 oC closure temperature of monazite to 350 oC over 10-20 myr, possibly as a result of exhumation of the southern Brasília belt.
Figure 7. Biotite and muscovite age-histogram
U-Pb monazite and 40Ar/39Ar biotite and muscovite age-histogram of the Southern Brasília and Central Ribeira belts. See text for details.
The 40Ar/39Ar ages in the range between 580 – 520 Ma, defining groups 2 and 3, include rocks south of the Jacutinga shear zone (Socorro Domain) and reflect a complex history of exhumation of the Nappe System. Transpressional structures in the Brasília belt characterize deformation during this time and are interpreted to be related to the tectonic evolution of the Central Ribeira belt (Figure 7) as described in the regional geology. At that time the main tectonic activity in the Ribeira belt changed from a compressive to an extensional regime where ductile shear zones evolved to brittle systems.
In the Socorro Domain, the 40Ar/39Ar ages from 571±1 and 560±1 Ma down to 537±1 and 521±1 Ma, are significantly younger than those north of the shear zone. This delayed cooling reflects either different times of exhumation, or partial resetting due to reheating during transpressional tectonics related to the Ribeira belt. Supported by a component of vertical displacement along the strike-slip shear zones, and contrasting metamorphic conditions on either side of Jacutinga shear zone (greenschist and amphibolite facies), we favour a later exhumation for this domain, but further interpret the variability of apparent ages of single samples as representing overprinting by tectonic activity related to the Ribeira belt. The apparent cooling ages around 520 Ma from samples located along the NE-SW trending sub-vertical strike-slip Jacutinga and Três Corações shear zones between the Guaxupé and Socorro Domains (Fig. 3) define both the latest time and the westernmost limit of the effects of the Ribeira belt tectonic on the Brasília belt. The youngest group of 40Ar/39Ar ages, between 505 and 490 Ma, is limited to the Metasedimentary Succession Domain (see Fig. 3A-B), south of the Jacutinga shear zone, are identical to typical cooling ages in the Ribeira belt along the Atlantic coast further to the east. Thus, we interpret these ages to reflect final uplift.
The 40Ar/39Ar cooling ages of samples ANDT, DIT, FDA, ITM TR and TI45 (Group 4, Table 4) are the youngest and suggest a trend ageing to the NW within the Metasedimentary Succession Domain. Although more data is required to establish this trend, the data suggest progressive partial reheating related to exhumation of the Central Ribeira belt.
In summary, integration of U-Pb and 40Ar/39Ar ages, supported by geologic documentation, indicates that the southern Brasília belt was reactivated by the younger Central Ribeira belt. Such interaction took place from the end of the Neoproterozoic to early Phanerozoic, during the assembly of West Gondwana. The southern margins of the Brasília belt reached the main collision phase with the São Francisco Craton at 620 Ma (Table 1) and peak metamorphism at 610-590 Ma as indicated by monazite ages. This was followed by exhumation and cooling before a late overprinting as a result of collision with the Central Ribeira belt and subsequent transpression. This late event caused the resetting of cooling ages to between 580 and 520 Ma. Late cooling ages recorded by samples from the Jacutinga shear zone marks the northwestern limit of a progressive regional cooling, associated to the Ribeira belt transpressional tectonics.