Transcript Mid-Proterozoic accretionary belts in the Amazonian Craton

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
G
E
O
C
I
Ê
N
C
I
A
S
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
G
E
O
C
I
Ê
N
C
I
A
S
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
G
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C
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C
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A
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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.