ARC-RELATED RAPAKIVI GRANITES FROM THE RIBEIRA FOLD BELT, SE BRAZIL

Abstract

Arc-related late orogenic rapakivi granites from the Late Precambrian Ribeira Fold Belt (RFB) are presented and discussed. They occur intimately associated in space and time with Caledonian-type granites and appinites (mainly potassic diorites) and were emplaced during the Post-collisional Crustal Identation and Uplift stage of the RFB. The three rocks groups, together with shoshonitic and ultrapotassic ones, belong to the Pluriserial Ribeira Magmatic System 590 (PRMS-590), with acted between 620 and 570 Ma ago during the RFB back-arc development. Petrographic, geochemical and isotopic data define a high-K alkali-calcic nature and a lower crustal origin for the rapakivi granites. The crustal magmas may underwent or not further compositional changes either by mixing (up to 14 vol. %) with enriched mantle-derived appinitic magmas and/or by assimilation of fluid-rich upper crustal material. ε Nd(585) values for crust- and enriched mantle- derived rocks range between –12 and –16 & –7 and –10, respectively, and T DM ages range between 2.4 and 1.8 Ga. & between (fictive) 1.4 and 1.6 Ga., respectively. With increasing participation of the appinitic component during mixing, the rapakivi granites became progressively more alkaline and their ε Nd(585) and T DM values became less negative and lower, respectively. Geochemical and isotopic data show that the rapakivi magmas are derived from calc-alkaline biotite and hornblende- bearing tonalitic/ granodioritic sources by incongruent dehydration melting at low P t , high T, and f O2 typical for high-K rocks. The development of the rapakivi texture is controlled mainly by physical conditions during magma ascent and cooling than by magma composition. Rapakivi plutons were emplaced under decompressional conditions during rapid orogenic uplift as shown by expressive rising and unroofing of their partly only slightly other calc-alkaline host-rocks. Due to their compositional and isotopic similarity, same age and close spatial association, the Ribeira rapakivi and Caledonian granites are considered as products derived from a same source under slightly variable melting conditions.