Famatinian cordierite- bearing magmatism on the proto-Andean margin of Gondwana

ALASINO, P., DAHLQUIST, J., RAPELA, C, GALINDO, C., CASQUET, C., PANKHURST, R., SAAVEDRA, J., BALDO E.

Santiago, CHILE

Congreso; International Congress on the Geology and Geophysics of the Southern Hemisphere; 2007

     Petrological and geochemical data from Cerro Toro unit and Mazán granite, in Sierra de Famatina and Sierra de Mazán, respectively, as well as the Tuaní Granite in the Sierra de Chepes (Dahlquist et al. 2005), indicate that the Famatinian peraluminous magmatism in the Sierras Pampeanas, define trends  from intermediate (60.7-69.9%) to felsic (71.3-73.6%)  compositions, with mineral  associations formed by Pl, Mc, Qtz, Bt, Ms, Crd, ± Gtr and Zrn, Mon, fluor-Ap, Opq (mainly ilmenite and/or titanomagnetite). The granites show distinctive high Aluminium Saturation Index (1.39-2.07), that increases towards the most mafic varieties defined by cordierite- bearing granitoids with a high FeOt+MgO+TiO2 (5.7-11.3%) and intermediate Na2O + K2O (5.2-6.7 %) contents. All granitic suites  define similar magmatic trends in the high peraluminous field of the A-B multicationic diagram, indicating they crystallized in medium- to low- P conditions (A=60-180 and B=100-190), similar to those observed in other cordierite bearing S-type granites (e.g., Cooma granodiorite in the Lachlan Fold Belt, SW Australia). Moreover, discriminatory source diagrams show the trends of the cordierite-bearing suites are projected in the greywacke field, with Rb/Ba = 0.16-0.56, Rb/Sr = 0.70-2.83 and CaO/Na2O = 0.34-0.69 ratios. Euhedral crystals of magmatic cordierite [Mg/(Mg+Fe+Mn) = 0.59 ± 0.04; Na+K = 0.05 ± 0.01, n = 23], devoid of mineral inclusions, are typical of all these granitoids, accompanied by plagioclase (An = 29.7 ± 4.8, n = 23), alkaline feldspar (Or = 84.9 ± 7.1, n = 6), biotite [Fe/(Fe+Mg) = 0.53 ± 0.06, n = 31] and garnet (Alm = 72.2 ± 1.3, Prp = 12.2 ± 1.6 and Spss = 12.2 ± 2.7, n = 10). This peraluminous magmatism is the result of a regional anatexis of a plagioclase-rich, immature continental platform sedimentary source, during a time-restricted episode (484-479 Ma, U/Pb SHRIMP, Pankhurst et al. 2000), with a high geothermal gradient.