Vol. 31 No.4 December 2008

Journal of International Geoscience

Published by the International Union of Geological Sciences

by Girardi, V.A.V., Teixeira, W., Bettencourt, J.S., Andrade, S., Navarro, M.S., and Sato, K.

Trace element geochemistry and Sr-Nd characteristics of Mesoproterozoic mafic intrusive rocks from Rond?nia, Brazil, SW Amazonian Craton: Petrogenetic and tectonic inferences

Abstract

          The Amazonian Craton comprises an Archean domain surrounded by four successively younger Proterozoic tectonic provinces. Within the Rio-Negro-Juruena province the Serra da Providência Intrusive Suite (1.60 and 1.53 Ga) consists of A-type rapakivi granites, charnockites and mangerites genetically associated with diabase dikes, gabbros and amphibolites. The original mafic melts were derived from a depleted mantle source (eNd(T) +2.5 to +2.8; eSr(T) –12.1). Underplated mafic magma induced melting of a short-lived felsic crust, thus originating coeval felsic-mafic magmatism in a continental intraplate setting. The Colorado Complex, assigned to the Rondonian-San Ignacio province, comprises 1.35–1.36 Ga intrusive bimodal magmatism represented by monzonite gneisses associated with amphibolite, gabbro and metadiabase dikes intercalated with metasediments with detrital zircon that yield U-Pb ages of 1.35 to 1.42 Ga. Mafic samples display juvenile signatures (eNd(T) 0.0 to +5.2; eSr(T) –5.0 to –30.7) and are less contaminated than the Serra da Previdência and Nova Brasilandia ones. The generation of the basaltic magma is related to the subduction of an oceanic slab below the peridotite wedge (intraoceanic arc setting). Fluids and/or small melts from the slab impregnated the mantle. The Nova Brasilandia Sequence (Sunsas-Aguapeí province) comprises a metasedimentary sequence intruded by 1.10–1.02 Ga metadiabases, gabbros, meta-gabbros, and amphibolites associated with granitic plutons (bimodal magmatism). The original tholeiitic magmas, derived from a depleted source (eNd(T) = +3.1 to +5.0), in a proto-oceanic setting, underwent subsequent contamination by the host rocks, as indicated by the isotopic and trace element data.

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