Controls on parental melt diversity generated from sub-orogenic lithospheric mantle (SOLM) during post-orogenic magmatism in the Araçuaí belt, SE Brazil

Most post-orogenic magmatic suites are dominated by granitoid rocks and “crustal” isotopic signatures, commonly interpreted as intra-crustal melting or interaction of mantle-derived melts with crustal materials. This interpretation is challenged in the Cambrian Araçuaí belt (SE Brazil), whose post-orogenic suite has a northern part similarly dominated by granites and charnockites, but a southern part exposing lower crustal composite plutons with gabbros, diorites, occasional syenites and minor granites. This study characterizes one of these plutons, Mimoso do Sul (dated here to 494–501 Ma), which exposes (i) mafic rocks that range from MORB-like tholeiitic norites to enriched potassic monzogabbros and (ii) a highly alkaline, potassic suite, including glimmerites, monzogabbros, monzodiorites, syenites and granites. The most primitive gabbros (X Mg = 0.75–0.60) show age-corrected 87 Sr/ 86 Sr and εNd values of ∼0.7078 and − 10.5, different from any regional country rocks - ruling out a derivation by crustal contamination. Instead, the arc-like trace element signature, including negative HFSE anomalies and elevated LILE, was likely imparted by subduction-related fluids or melts infiltrating the lithospheric mantle during the formation of the Rio Doce Arc (∼600 Ma). Yet, the highly unradiogenic Nd is at odds with typical subduction-zone signatures and requires an additional ancient metasomatim of the lithosphere. The diverse mafic rock types have indistinguishable isotopic compositions, but quite distinct trace element characteristics, interpreted to reflect variably intense metasomatism in the sub-orogenic mantle lithosphere and varying degrees of melting thereof. Further, for the alkaline differentiation suite, major and trace elements and Sr Nd isotopes support a nearly closed-system fractionation with some (unsystematic) Sr isotope variability due to minor crustal assimilation. Extended to post-orogenic magmatism in general, our results imply that the “crustal” signature and the variable magmatic lineages arise from a heterogeneously metasomatized sub-orogenic lithospheric mantle (SOLM).