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Significance of the Nova Brasilndia Metasedimentary Belt In View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Bern Open Repository and Information System (BORIS) TECTONICS, VOL. 23, TC6004, doi:10.1029/2003TC001563, 2004 Significance of the Nova Brasilaˆndia metasedimentary belt in western Brazil: Redefining the Mesoproterozoic boundary of the Amazon craton Eric Tohver,1,2 Ben van der Pluijm,1 Klaus Mezger,3 Eric Essene,1 Jaime Scandolara,4 and Gilmar Rizzotto4 Received 18 July 2003; revised 6 July 2004; accepted 16 September 2004; published 7 December 2004. [1] The Nova Brasilaˆndia metasedimentary belt belts in sedimentary environment, metamorphic grade, (NBMB) of western Brazil marks a fundamental and timing of deformation, signify that these two belts crustal boundary in the Amazon craton. The are not geologically continuous. The ‘‘Grenvillian’’ metasedimentary rocks of the NBMB (calc-silicates, deformation recorded by the NBMB belt marks the metapelites, quartzites, metabasites) contrast strongly final docking of the Amazon craton and Paragua with the older, polycyclic granitoid rocks of the craton within the Rodinia framework. The Aguapeı´ adjacent Amazon craton. Aeromagnetic anomalies belt, in contrast, seems to record only limited indicate that the belt is continuous for at least deformation internal to the Paragua craton. INDEX 1000 km in an E-W direction, although the TERMS: 8102 Tectonophysics: Continental contractional orogenic easternmost extent of the NBMB is covered by belts; 3660 Mineralogy and Petrology: Metamorphic petrology; the Cretaceous sediments of the Parecis Formation. 1035 Geochemistry: Geochronology; 8025 Structural Geology: Additional geologic evidence suggests that the belt Mesoscopic fabrics; 9360 Information Related to Geographic extends along an E-W trend for 2000 km. The Region: South America; KEYWORDS: Rodinia, Grenville mobile northern portion of the NBMB preserves vestiges of belt, Amazon craton, Paragua craton, geochronology, P-T path. Citation: Tohver, E., B. van der Pluijm, K. Mezger, E. Essene, an early high pressure-temperature (P-T) assemblage J. Scandolara, and G. Rizzotto (2004), Significance of the Nova (kyanite + staurolite) overprinted by sillimanite during Brasilaˆndia metasedimentary belt in western Brazil: Redefining prograde metamorphism. A higher metamorphic grade the Mesoproterozoic boundary of the Amazon craton, Tectonics, is observed in the southern portion of the belt, 23, TC6004, doi:10.1029/2003TC001563. with peak conditions calculated to be 800 MPa and 800°C for granulitic assemblages. The combined P-T 1. Introduction path demonstrates that the competing processes of imbrication (northern domain) and magma [2] Many recent plate reconstructions for the Mesoprote- generation (southern domain) are responsible for rozoic postulate that the Grenville orogeny represents world- regional metamorphism and crustal thickening. wide events leading to the assembly of the supercontinent Cooling from peak metamorphic conditions is Rodinia [Dalziel, 1991; Hoffman, 1991]. North America holds a keystone position in Rodinia based on the Neo- recorded by U-Pb monazite ages of 1090 Ma and proterozoic rift basins that circumscribe Laurentia [Bond et titanite ages of 1060 Ma. Integrated cooling rates al., 1984], with most reconstructions placing the southwest- of 2°–3°C/Myr from regional metamorphism are ern margin of Amazonia in proximity to some portion of calculated from these U/Pb ages combined with eastern Laurentia. The Grenvillian history of the eastern 40 39 Ar/ Ar ages of hornblende (970 Ma) and biotite Laurentian margin has been relatively well documented (910 Ma). The NBMB marks the Mesoproterozoic along the 3000 km length of this ancient mobile belt, limit of the SW Amazon craton. The discordance which stretches from the southernmost portion of cratonic of the NBMB to the NNW structural trend of the Laurentia typified by the Llano Uplift region of central Texas younger Aguapeı´ belt (200 km SE of NBMB), [Mosher, 1998] to the northeastern exposures of Labrador together with marked differences between the two [Krogh, 1994; Davidson, 1998]. In contrast, present knowl- edge of the Grenvillian history of the Amazon craton is less complete, making exposed portions of the belts all the more 1Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan, USA. interesting from the standpoint of Rodinia paleogeography. 2Now at the Instituto de Geocieˆncias (GMG), Universidade de Sa˜o [3] Three positions have been proposed for the Meso- Paulo, Cidade Universita´ria, Sa˜o Paulo, SP Brazil. proterozoic paleogeography of the Amazonia-Laurentia 3 Institut fu¨r Mineralogie, Universita¨t Mu¨nster, Mu¨nster, Germany. connection within Rodinia, based on different lines of 4Companhia de Pesquisas de Recursos Minerais (CPRM), Porto Velho, RO Brazil. evidence. Hoffman [1991] and Sadowski and Bettencourt [1996] used broad geological similarities, such as igneous Copyright 2004 by the American Geophysical Union. age provinces or metamorphic ages, to pair Amazonia with 0278-7407/04/2003TC001563 the central Grenville Province of Ontario and New York at a TC6004 1of20 TC6004 TOHVER ET AL.: MESOPROTEROZOIC MARGIN OF AMAZON CRATON TC6004 later interval, circa 1.0 Ga. Dalziel [1991] chose a more Sunsa´s and Aguapeı´ belts exposed in eastern Bolivia and northerly position for Amazonia alongside Newfoundland along the Bolivia-Brazil border, respectively; and the Nova and Labrador, based largely on matching the Iapetan rifting Brasilaˆndia belt exposed in southern Rondoˆnia (Figure 1). histories of Laurentia with the Precordilleran margin of The former two belts have been interpreted as delimiting the South America [Bond et al., 1984], which is now recog- Paragua craton, a polycyclic block of at least Paleoprotero- nized to constitute a separate terrane, exotic to South zoic age and of subcontinental dimensions exposed almost America [Thomas and Astini, 1996]. New paleomagnetic exclusively in eastern Bolivia [Litherland et al., 1986; Saes, data for the early ‘‘Grenvillian’’ interval (circa 1.2 Ga) of 1999; Geraldes et al., 2001]. The interior of this block is the Amazon craton supports a collision with the southern- characterized by the preservation of K-Ar ages and older most portion of Laurentia, i.e., the Llano region of central structural trends considered to mark the end of cratonization Texas [Tohver et al., 2002]. Observations of large-scale at circa 1.3 Ga [Litherland et al., 1989]. The Sunsa´s- strike-slip deformation of the Amazon craton basement Aguapeı´ belts comprise quartzitic and conglomeratic units rocks, together with observations from Laurentia, suggest deformed during late Grenvillian (circa 1.0 Ga) times. large-scale displacement between Amazon and Laurentia, Stratigraphic correlations between the Sunsa´s and Vibosi possibly reconciling the Hoffman [1991] and Tohver et al. groups of Bolivia and the Aguapeı´ Group of Brazil have [2002] paleographic models. According to this transpressive been advanced on the basis of lithology [Litherland et al., model of Grenville orogenesis, the evolution of Rodinia was 1989; Saes, 1999], with recent SHRIMP dates from the marked by both thrust faulting and strike-slip tectonics bottom of the Aguapeı´ sequence exposed in Mato Grosso, along the greater Grenville collisional zone, comprised of Brazil, suggesting a maximum depositional age of 1231 ± both its North and South American equivalents. 14 Ma [Santos et al., 2001]. Deformation of the Sunsa´s belt [4] One of the principal tasks in placing Amazonia in a is manifested in a network of curvilinear, strike-slip shear Rodinian reference frame is identifying the craton’s principal zones, with mylonitic belts observed in an en echelon trend structures, particularly those that are relevant to its Meso/ that generally strikes WNW. Extensive granitic intrusions Neoproterozoic history. The classical work of Almeida and accompany the deformation of the Sunsa´s belt along these Hasui [1984] defines the Amazon craton at its southern and mylonitic belts, followed by many postkinematic granites eastern margins by the presence of Brasiliano/Panafrican [Litherland et al., 1986, 1989]. In contrast, magmatism is belts (double black line, Figure 1, bottom inset). However, almost entirely absent from the Aguapeı´ belt, which is the emphasis on structures related to Gondwanan history marked by greenschist facies mylonites generated during distracts from the somewhat older signs of a previous thrusting and strike-slip deformation along its NNW trend supercontinent, thus obscuring the role of Amazonia in the [Barros et al., 1978] with minor S-type granite magmatism constructive phase of Rodinian history. We contend that the [Geraldes et al., 2001]. Compared to the Sunsa´s belt, only Meso/Neoproterozoic cratonic margin of Amazonia is such a localized deformation affected the Aguapeı´ belt (Figure 2a), structure overlooked by the traditional model. In this con- with the external margins of the belt (Serrania Huanchaca in tribution, new geochronological, petrological, and structural the west and Rio Branco in the east) still displaying data from the Nova Brasilaˆndia Metasedimentary belt horizontal or gently folded bedding of preorogenic strata (NBMB) are presented as evidence of high-grade deforma- [Litherland and Power, 1989; Saes, 1999; Fernandes, tion under transpressive conditions during late Mesoprotero- 1999]. Assuming that the lithostratigraphic correlation zoic times. Previously published aeromagnetic data from this
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