Geodynamical evolution of the Transamazonian belt of the southeastern Guiana Shield - north of Amazonian Craton: Paleoproterozoic crustal growth vs. reworking of Archean crust

Category Other
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Travassos da Rosa-Costa, Lْcia۱; Lafon, Jean Michel۲
Holding Date 17 September 2008

The eastern portion of Guiana Shield, northern portion of Amazonian Craton, exposes an exceptionally wide Paleoproterozoic orogenic belt, which tectonic evolution is related to the Transamazonian orogenic cycle (2.26-1.95 Ga). Over the last decade, geophysical and mapping programs, and extensive geochronological studies in southeastern Guyana Shield (i.e. Brazil -Amapá and northwestern Pará states; French Guiana and Suriname) improved significantly the geological knowledge of this area. The Transamazonian orogenic evolution involves multi-stage crustal growth, mainly during Rhyacian time, with both juvenile Paleoproterozoic crustal accretion and subsequent crustal reworking, inclusive of Archean crust. The occurrence or lack of Archean remnants in this belt, allowed the individualization of distinct tectonic domains. In Suriname and French Guiana, Nd isotopic signature of magmatic rocks (-0.37<εNd<+2.3) and TDM values (2.40-2.19 Ga) discard any significant contribution of Archean crust and define a Paleoproterozoic juvenile domain.
Conversely, in Amapá and northwestern Pará states, an expressive continental landmass, named Amapá Block, has been recognized and reworking of Archean crust played a major role during the Transamazonian event. This block is constituted of a basement assemblage, with Archean precursors, dated mainly at 2.85-2.79 Ga and at 2.66-2.58 Ga, but Paleoarchean remnants as old as 3.32 Ga have been also identified. During the Transamazonian orogeny, this Archean basement was deformed, cross-cut by several granitic plutons and experienced high-grade metamorphism. Close to the limits of the Amapá Block, εNd values (-6.61 to +1.74), and TDM ages (2.83-2.24 Ga), reveal an origin for the Paleoproterozoic units by mixing of juvenile Paleoproterozoic magmas with Archean components. The Transamazonian orogenic evolution in the southeastern Guyana Shield began with the formation of juvenile oceanic crust at 2.26-2.20 Ga, dated on rocks derived from oceanic tholeiitic magmas. Widespread calc-alkaline granitoids dated at 2.19-2.13 Ga and coeval metavolcanosedimentary sequences registered the main period of crustal accretion, related to subduction settings (island arc or magmatic arc). Granitic magmatism and associated migmatization produced by reworking of Paleoproterozoic crust along transcurrent shear zones are well constrained at 2.11 to 2.08 Ga. In the southwestern Amapá Block, it is the time of granulite-facies metamorphism over Archean basement rocks, contemporaneous to the development of a thrusting system that marks the collisional stage of the Transamazonian orogen. Transcurrent tectonic dominated between 2.07 and 2.03 Ga, producing deformation, migmatization and granitic magmatism along strike-slip corridors. Late granitic magmatism at 1.99 Ga marks the end of the Transamazonian event in the region. No significant post-Transamazonian magmatism or metamorphic event has been recognized in southeastern Guyana Shield.