The Cambrian post-orogenic A-type granitoids from SE Brazil: metasomatized lithospheric mantle sources, crystal fractionation and minor crustal assimilation

Abstract

The role of mantle vs. crustal contributions to A-type granitoids are heavily debated, in line with the wide compositional diversity among these magma types. The Cambrian post-orogenic Venda Nova and Várzea Alegre Plutons of the Araçuaí belt, SE Brazil, turn out to represent an endmember case where the clear “crustal” isotopic signature is already present in the primitive mantle melts and hence mantle source. The mildly alkaline Venda Nova Pluton comprises wehrlites, gabbronorites, diorites, quartz syenites and titanite granites with mineral (clinopyroxene, amphibole, apatite) and bulk rock age-corrected εNd (500 Ma) values of -11.8 to -9.5 and 87 Sr/ 86 Sr (500 Ma) values of 0.7069–0.7080. The more calc-alkaline Várzea Alegre Pluton comprises gabbronorites, monzodiorites, charnockites, and monzo- and syeno-granites with mineral and bulk rock εNd (500 Ma) of -10.1 to -7.6 and 87 Sr/ 86 Sr (500 Ma) of 0.7067–0.7079 in the gabbronorites, but more variable values (0.7075–0.7097) in the charnockites and granites. Mixing models between the Venda Nova gabbronorites and potential crustal assimilants, combined with a mineral fractionation model, show that granite genesis primarily resulted from crystal fractionation from gabbronoritic melts with little crustal assimilation. For Várzea Alegre, the greater scatter in εNd points to a more heterogeneous mantle source, while the larger range of 87 Sr/ 86 Sr points to some localized assimilation of country rock metapelites or S-type granitoids. In both cases, the least differentiated gabbroic rocks already show geochemically enriched isotopic signatures which lack corresponding crustal compositions in the Araçuaí belt. We attribute this crustal signature to a lithospheric mantle source that was metasomatized both in ancient times to generate strongly negative Nd isotopes and more recently by fluids/melts related to subduction along the ~ 600 Ma Rio Doce arc, now incorporated into the Araçuaí belt. This contribution highlights that apparently “crustal” isotopic signatures in A-type granitoids can be derived from metasomatic enrichment of the lithospheric mantle rather than crustal assimilation.

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