The Nova Brasil¿ndia metasedimentary belt (NBMB) of western Brazil marks a fundamental crustal boundary in the Amazon craton. The metasedimentary rocks of the NBMB (calc-silicates, metapelites, quartzites, metabasites) contrast strongly with the older, polycyclic granitoid rocks of the adjacent Amazon craton. Aeromagnetic anomalies indicate that the belt is continuous for at least 1000 km in an E-W direction, although the easternmost extent of the NBMB is covered by the Cretaceous sediments of the Parecis Formation. Additional geologic evidence suggests that the belt extends along an E-W trend for ~2000 km. The northern portion of the NBMB preserves vestiges of an early high pressure-temperature (P-T) assemblage (kyanite + staurolite) overprinted by sillimanite during prograde metamorphism. A higher metamorphic grade is observed in the southern portion of the belt, with peak conditions calculated to be 800 MPa and 800¿C for granulitic assemblages. The combined P-T path demonstrates that the competing processes of imbrication (northern domain) and magma generation (southern domain) are responsible for regional metamorphism and crustal thickening. Cooling from peak metamorphic conditions is recorded by U-Pb monazite ages of 1090 Ma and titanite ages of ~1060 Ma. Integrated cooling rates of 2¿--3¿C/Myr from regional metamorphism are calculated from these U/Pb ages combined with 40Ar/39Ar ages of hornblende (~970 Ma) and biotite (~910 Ma). The NBMB marks the Mesoproterozoic limit of the SW Amazon craton. The discordance of the NBMB to the NNW structural trend of the younger Aguape¿ belt (200 km SE of NBMB), together with marked differences between the two belts in sedimentary environment, metamorphic grade, and timing of deformation, signify that these two belts are not geologically continuous. The Grenvillian deformation recorded by the NBMB belt marks the final docking of the Amazon craton and Paragua craton within the Rodinia framework. The Aguape¿ belt, in contrast, seems to record only limited deformation internal to the Paragua craton.