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

摘要

Mineralogy, geochemistry and isotopic data for Ediacaran, Ordovician to Devonian, and Upper Triassic cordieritebearing 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 x 10(-3) SI), negative epsilon Nd (-7.3 to -5.0) and high delta(18)Ozircon values ( 9 parts per thousand), suggest a major aluminous metasedimentary source. Data for the Chilean pluton (small volume, associated with coeval metaluminous granites, low delta(18)Ozircon values (6.6-7.4 parts per thousand), slightly negative epsilon Nd (-4), and high MS values (1.4-2.5 x 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 degrees C; and 3.6-4.0 kbar and at 670-700 degrees C for their cordierite. The Nanuque magma formed P 6 kbar, T = 750-800 degrees C, and magma emplaced at 3 kbar. Their cordierite formed at P = 5.0-5.2 kbar and T = 720- 740 degrees C. P-T conditions of the Los Tilos magma emplacement are P similar to 3 kbar and T similar to 670 degrees C. Cordierite-paramorph fl-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.