Post-orogenic and non-orogenic alkaline granite associations: the Saibro intrusive suite, southern Brazil — A case study

Abstract

Post-orogenic alkaline granites of the Late Proterozoic to Cambrian Saibro intrusive suite in the Sul-rio-grandense shield (southern Brazil) were emplaced in a cratonic area during late strike-slip movements accompanied by shear episodes at the end of the Brasiliano orogeny. The suite is made up of rare syenites and abundant hypersolvus and subsolvus granites. Both metaluminous amphibole + biotite-bearing and peralkaline arfvedsonite + aegirine-bearing rock types are present. together with associated volcanic formations. Field relations, textural, mineralogical and geochemical data are consistent with a derivation of residual granitic liquids from a parental syenitic magma through crystal-liquid fractionation of alkali feldspars, plagioclase, amphibole, Fe-Ti oxides and apatite. Sr isotope data favour an ultimate origin in the mantle but do not exclude derivation from a Rbdepleted and F-enriched granulitic lower crust. Crystallization occurred in a water-enriched and rather oxidizing environment, as a result of which the entire suite has a transitional character between that of an orogenic and an anorogenic setting. The alkaline granites are characterized by relatively high agpaiitic indices ( AI= Na 2 O+K 2 O Al 2 O 3 ) and moderate to high contents of large ion lithophile elements (LILE) and high field strength elements (HFSE). Associated F-rich fluids are responsible for a complete low-temperature reequilibration of the major rock-forming minerals (alkali feldspars converted into a microcline + albite assemblage, biotite or aegirine replacing amphibole). As a consequence, the mafic silicate minerals are dominantly sodic (amphibole, aegirine) and potassic (biotite). Though regarded as related to A-type, within-plate granites, these alkaline granites are here ascribed to post-orogenic geodynamic setting at the time of their emplacement.