Sources of Neoproterozoic post-collisional granites from the Porto Alegre region, eastern Dom Feliciano Belt, Brazil: Sr-Nd-Pb isotopic constraints

Abstract

The Dom Feliciano Belt, southernmost Brazil and Uruguay, is a Neoproterozoic to Cambrian mobile belt that resulted from the convergence of the Kalahari, Congo, and Río de la Plata cratons. The present paper focuses on post-collisional granites located in the eastern sector of this belt, in the Porto Alegre region, RS, Brazil. The studied rocks yield crystallization ages of ca. 600 Ma and intrude subduction-related, calc-alkaline gneisses from the host basement. This work presents novel isotopic data (Sr-Nd-Pb) for representative samples of the Viamão, Porto Alegre, and Itapuã intrusive suites. For the Viamão Intrusive Suite, the results (87Sr/86Srt = 0.7089–0.7168; εNdt=−6.0 to −6.7; TDM = 1.4–1.5 Ga; 206Pb/204Pb = 18.20–19.71) suggest mixing of mantle (DM) and Paleoproterozoic crust-derived sources (Arroio dos Ratos Complex, a remnant of the basement). For the Porto Alegre Intrusive Suite, the results (87Sr/86Srt = 0.7090–0.7437; εNdt=−4.2 to −8.3; TDM = 1.3–1.8 Ga; 206Pb/204Pb = 18.89–22.19) suggest mixing of mantle (HIMU) and Tonian-Cryogenian crust-derived components (Chácara das Pedras Gneiss, a remnant of the basement). In turn, the results for the Itapuã Intrusive Suite (87Sr/86Srt = 0.7116–0.8733; εNdt=−1.6 to −6.0; TDM = 1.2–1.6 Ga; 206Pb/204Pb = 18.44–22.20) suggest mixing of the mantle (possibly HIMU) and a Nd-depleted, crust-derived component (in comparison to the source of the Porto Alegre Intrusive Suite). All the studied suites are interpreted as being formed in a Neoproterozoic post-collisional setting. The mantle-derived signatures (DM and HIMU) are interpreted as the result of the partial melting of remnants of former oceanic slabs related to previous subduction events. In turn, the crust-derived signatures are interpreted as the result of assimilation of basement rocks. We suggest that the post-collisional magmatism of this sector of the Dom Feliciano Belt is associated with a deep hot zone.