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AMS-07 Crustal evolution of the cratonic nuclei of South America

I- and S-type granitoid rocks: Nd-Pb isotopic constraints on the Paleoproterozoic geodynamic evolution of the central region of the Guyana Shield

Marcelo Esteves Almeida, CPRM - Geological Survey of Brazil (Brazil)
Moacir José Buenano Macambira, UFPA - Federal University of Pará (Brazil)
Franck Poitrasson, Université Paul Sabatier - LMTG (France)
Candido Augusto Veloso Moura, UFPA - Federal University of Pará (Brazil)
Elma Costa Oliveira, UFPA - Federal University of Pará (Brazil)
 

In Uatumã-Anauá domain (southeastern Roraima, Brazil), central part of the Guyana Shield, previous geochemistry and zircon geochronology show granitoid rocks associated to different magmatic series, yielding also distinct interval ages. New Nd-Pb isotopic data yield evidence of crustal origin, suggesting dominant Rhyancian and local Neoarchean sources for these granitoids. In the Northern Uatumã-Anauá domain, the 1.97 Ga Martins Pereira Granite show Nd T_DM model ages with 2.67-2.33 Ga range, including negative εNd_1.97 (-0.92 to -4.74) and high ²⁰⁷Pb/²⁰⁴Pb (15.58-15.70) ratios. This renders the Martins Pereira Granite a good example of I-type high-K calc-alkaline magmatism generated by reworking and recycling of a pre-existing, older crust (e.g. Siderian-Archean sources), related probably to the final stages of Anauá Arc evolution (collisional event). The S-type Serra Dourada Granite is related also to this collisional stage (1962 Ma) and their Nd T_DM model ages (2.53-2.39 Ga) and εNd_1.96 (-1.08 to -4.34) imply in older crustal-derived sources, such as metasedimentary rocks of Cauarane Group. This group shows 2.46-2.27 Ga (local 2.64 Ga) Nd T_DM model ages and εNd_1.97 +1.23 to -4.16. The Martins Pereira and Serra Dourada granitoid rocks were generated probably during the docking and amalgamation of the Anauá Arc to the Transamazonian (Rhyacian) and Central Amazonian (Siderian-Archean) provinces. In the southern Uatumã-Anauá domain, the I-type high-k calc-alkaline Caroebe Suite (1.90-1.89 Ga) presents very homogeneous ²⁰⁷Pb/²⁰⁴Pb ratios (ca. 15.54). The εNd_1.89 (+0.46 to -2.05, locally -2.60 and -4.27) and Nd T_DM model ages (2.29-2.16 Ga, locally 2.47-2.37 Ga) are similar to the 1.89 Ga Igarapé Azul Granite (εNd_1.89 -0.02 to -1.61 and Nd T_DM 2.28-2.14 Ga), however this last shows more scattered ²⁰⁷Pb/²⁰⁴Pb ratios (15.42-15.65). The field observations (no remnants of ocean crust and ophiolites, acretionary prism sucession, arc-related basins, etc.), geochemical evidence (absence of low- to medium-K calc-alkaline rocks) and the isotopic data (no conspicuous juvenile pattern) show that mantle-derived origin is not a viable model for the Caroebe-Igarapé Azul granitoids. Their origin involves probably partial melting of Rhyacian TTG metaigneous rocks (juvenile crust?) and Orosirian metavolcano-sedimentary sources, with minor older sialic crustal sources contributrion (Siderian-Archean), in the extensional setting driven by underplating mechanism or soft collision.