Proterozoic accretionary belts of the Amazonian Craton Umberto

Proterozoic accretionary belts of the Amazonian Craton Umberto G. Cordani Institute of Geosciences University of Sao Paulo, Brazil

TECTONIC EVOLUTION OF THE AMAZONIAN CRATON IN THE PROTEROZOIC -- The SW part of the Amazonian craton was formed by continuous amalgamation of magmatic arcs, largely juvenile, the roots of which are now exhumed. - Accretionary belts, driven by subduction, formed a very large domain, at least 2700 km long and about 1300 km wide -- Duration of the process is about 700 m. y. , starting at 2. 0 Ga. An ocean basin was closed, to the SW of a large continent formed by the Guyana Shield plus the West African craton. -- The units decrease in age from NE to SW, and lithologies are practically only granitic (s. l. ). Basement was not detected.

EARTH THERMAL EVOLUTION Layered mantle convection Subduction slabs restricted to upper mantle Whole-mantle convection Oceanic lithosphere down to lower mantle SUPERPLUMES AVALANCHES SUPERCONTINENT CYCLE

PALEOPROTEROZIC Woopmay Orogen ARCHEAN Slave Craton

INTRA-OCEANIC MAGMATIC ARCS WITHIN ACCRETIONARY BELTS Absence of continental basement rocks Stacking, “Soft-collision” and accretion processes Positive or slightly negative Nd on granitoids Sm-Nd model ages similar or slightly older than radiometric ages of rocks (U-Pb on Zircon)

Províncias tectônicas da parte NW do Craton Amazônico

Províncias tectônicas da parte SW do Craton Amazônico

VENTUARI-TAPAJÓS and RIO NEGRO-JURUENA MAGMATIC ARCS Predominantly granitoid material, deformed and undeformed Archean basement not detected Radiometric ages roughly between 2. 0 and 1. 5 Ga, decreasing from NE to SW Post-orogenic volcanic-sedimentary basins with similar or slightly younger ages. Sm – Nd model ages always younger than 2. 1 Ga Nd (T) from slightly negative to about (+3)

Reconstrução da posição relativa de Laurentia, Baltica, Amazonia e Australia há 1. 65 Ga. Y-M=Yavapai-Mazatzal Lb=Labradorian G=Gothian R-J=Rio Negro-Juruena (a) - Pesonen et al (2001) (b) - Modelo alternativo (c) - Ahäll & Larson (2000) e Geraldes et al. (2001)

RONDONIAN-SAN IGNACIO PROVINCE and SUNSAS BELT

RONDONIAN-SAN IGNACIO PROVINCE Presence of Paleoproterozoic basement Large metamorphic belts of Mesoproterozoic age Volcanic-sedimentary units of Meso-Neoproterozoic age Radiometric ages between 1. 3 and 1. 5 Ga Sm – Nd model ages between 1. 7 and 2. 2 Ga Nd (T) between (-3) and (+2) Juvenile magmatic arcs (Pensamiento complex and related rocks) U-Pb radiometric ages between 1. 4 and 1. 5 Ga Sm – Nd model ages between 1. 5 and 1. 7 Ga Nd (T) about (+4)

MAIN CONCLUSIONS -- The SW part of the Amazonian craton was formed by continuous amalgamation of magmatic arcs, largely juvenile, the roots of which are now exhumed. - Accretionary belts, driven by subduction, formed a very large domain, at least 2700 km long and about 1300 km wide -- Duration of the process is about 700 m. y. , starting at 2. 0 Ga. An ocean basin was closed, to the SW of a large continent formed by the Guyana Shield plus the West African craton. -- The units decrease in age from NE to SW, and lithologies are practically only granitic (s. l. ). Basement was not detected.