Cordierite-bearing granitic rocks in South America: Contrasting sources and conditions of formation

Abstract

Mineralogy, geochemistry and isotopic data for Ediacaran, Ordovician to Devonian, and Upper Triassic cordierite-bearing granitoids respectively from Brazil, Argentina and Chile are discussed here. Syn-collisional Lower to Middle Ordovician, and post-collisional Upper Devonian garnet-free granitoids from Argentina intruded greenschist-to amphibolite-facies metasedimentary rocks. Garnet-bearing and garnet-free monzogranites of the Nanuque Suite, Brazil, intruded high-grade gneisses and migmatites, and in the High Andes Belt, the Los Tilos garnet-free granite was emplaced into an early Permian batholith during an extensional event. Mineralogical, geochemical and isotopic characteristics of the granites from Argentina and Brazil, such as presence of peritectic and (±) restictic cordierite, low-magnetic susceptibility (MS) values (0.03–0.2 × 10 −3 SI), negative εNd (−7.3 to −5.0) and high δ 18 Ozircon values (>9‰), suggest a major aluminous metasedimentary source. Data for the Chilean pluton (small volume, associated with coeval metaluminous granites, low δ 18 Ozircon values (6.6–7.4‰), slightly negative εNd (−4), and high MS values (1.4–2.5 × 10 −3 SI) are compatible with a less-evolved source, possibly a metaluminous granitic rock. P-T magma crystallization conditions of the cordierite-bearing granites from Argentina are 2–4 kbar and 630–720 °C; and 3.6–4.0 kbar and at 670−700 °C for their cordierite. The Nanuque magma formed P > 6 kbar; T = 750–800 °C, and magma emplaced at ∼3 kbar. Their cordierite formed at P = 5.0–5.2 kbar and T = 720−740 °C. P-T conditions of the Los Tilos magma emplacement are P ∼ 3 kbar and T ∼670 °C. Cordierite-paramorph β-quartz nodules in this pluton, surrounded by leucocratic rims, have been formed by biotite dehydration reactions. This study confirms that cordierite in granites can be produced from different source rocks by a number of processes, at different P-T conditions.