The Devonian Achala batholith of the Sierras Pampeanas: F-rich, aluminous A-type granites

RAPELA, C.W.; BALDO, E.G; PANKHURST, R.J; FANNING, C.M

Bariloche

Simposio; VI South American Symposium on Isotope Geology; 2008

INGEIS - CIG

The Devonian-Carboniferous granites of the Sierras Pampeanas have received scant attention compared to the Cambrian (Pampean) and especially the widespread Ordovician (Famatinian) granites. The largest of these bodies, the Achala batholith, is considered typical of this magmatism, and therefore its age, geochemistry and isotopic signature have been revisited in order to better characterize the Achalian episode. U-Pb SHRIMP crystallization ages of the main monzogranitic facies (379 ± 4 Ma) and younger intrusive tonalites (369 ± 3 Ma) indicate that the major, central sector of the batholith, was emplaced during a relatively short time interval, in the Upper Devonian. Dominated by 70–74% SiO2 monzogranites, the batholith has however a wider, 60–76 % SiO2 range, within the fields of the calc-alkalic to alkali-calcic series. Achala granites are termed aluminous A-type as they are all peraluminous, and typically show enrichments in F, Ga, Nb, high Ga/Al ratios and low K/Rb ratios. åNdt values from -1.1 to -4.0 coupled with inherited zircons, suggest that the Achala granites represent variable proportions of a juvenile mantle component and crustal melts formed by dehydration melting of biotite-bearing restites. If this origin is correct, a very significant heat source would have been required to generate the F-rich, hybrid Achalian magmas