Magmatic diversity in continental rifts : a case study on the Early Tonian, plutono-volcanic Salto da Divisa Complex, Araçuaí Orogen, Eastern Brazil.

Abstract

Exposures from plutonic roots to volcanic roofs in deeply eroded regions may disclose the architecture and igneous processes in ancient metamagmatic edifices, like it is the Early Tonian Salto da Divisa Complex (SADICO) of Eastern Brazil. Field, petrographic, lithochemical and isotopic (in-zircon U-Pb and Lu-Hf, and whole-rock Sm-Nd) studies were conducted on the SADICO, an anorogenic plutono-volcanic complex located in the northeast Araçuaí orogen. The SADICO magmatic record comprises (with U-Pb crystallization ages, and isotopic Hf and Nd data): i) pyroxenite (εNd(t): +2.2 to −5.3; Nd TDM: 1.2–1.7 Ga); ii) mafic-intermediate dykes (εNd(t):+2.1 to −5.9; Nd TDM: 1.2–1.8 Ga) and enclaves with OIB-like signature; ii) ferroan, A-type granitoid with mafic-felsic mingling-mixing features (885 ± 9 Ma; εHf(t): −5 to −7, Hf TDM: 2.0–2.1 Ga; εNd(t): −3.2 to −4.3, Nd TDM: 1.5–1.7 Ga); iii) fluorite-bearing, metaluminous to peraluminous, ferroan A-type granites, including biotite-amphibole granite (915 to 875 Ma; εNd(t): −2.8 to −5.8, Nd TDM: 1.6–1.9 Ga), biotite granite (894 ± 10, εHf(t): −4 to −11, Hf TDM: 1.9–2.3 Ga; εNd(t): −1.6 to −8.0, Nd TDM: 1.4–2.2 Ga), and amazonite-bearing two-mica granite (εNd(t): −4.0; Nd TDM: 1.7 Ga); iv) rhyolite (905 ± 24 Ma; εHf(t): −1 to −8.7, Hf TDM: 1.8–2.2 Ga; εNd(t): +3.1, Nd TDM: 1.1 Ga) and subvolcanic silica-undersaturated trachyte (912 ± 13 Ma, εHf(t): −14 to −18, Hf TDM: 2.5–2.7 Ga; εNd(t): +0.3, Nd TDM: 1.2 Ga), also with ferroan A-type signature. Our integrated petrogenetic model envisages mantle-derived magmas (ultramafic and mafic rocks) evolved by assimilation-fractional crystallization to felsic subvolcanic and volcanic rocks, interacting with granitic magmas produced by crustal anatexis (biotite-amphibole granite) and subsequently fractionated (biotite granite) and highly fractionated (two-mica granite) with the involvement of F-rich fluids up to subvolcanic levels.