Petrology of the plio-quaternary volcanism of the South-Central and Meridional Andes

Abstract

Plio-quaternary volcanoes lay upon a continental crust which is thick and of Precambrian-Paleozoic age in the South-Central Andes (SCA, 18–28°S) whereas it is relatively thin and of Mesozoic-Cenozoic age in the Meridional Andes (MA, 32–42°S). Eleven calc-alkaline volcanic massifs (or groups of small volcanoes) and two shoshonitic volcanoes have been studied in the SCA. Eleven calc-alkaline volcanic massifs have been studied in the MA (one alkaline volcano has been approached in Argentinian Patagonia). Calc-alkaline lavas are mostly andesites and dacites in the SCA and high-Al basalts and basic andesites in the MA. Shoshonitic lavas are absarokites, shoshonites and banakites. No cognate xenoliths were found in the lavas. The texture of the lavas is microlitic with abundant (calc-alkaline lavas) or rare (shoshonitic lavas) phenocrysts. Distribution of phenocrysts opposes calc-alkaline lavas of the SCA (with hornblende and biotite) to those of the MA (with olivine). In calc-alkaline lavas, plagioclase is the most abundant mineral phase from high-Al basalts (An 90) to rhyolites (An 20); olivine is Fo 78−65 phenocrysts in basic lavas; clinopyroxene (Ca-Mg-rich augite) and orthopyroxene (bronzite) show little compositional variation with lava differentiation; amphibole is pargasitic or edenetic hornblende; crystal clots which result from resorption of hornblende are found; biotite is Ti-rich; phenocrysts of quartz are found in rhyolites and (rimmed by pyroxenes) in basic lavas of the SCA; microlites of magnetite are ubiquitous and phenocrysts occur only in the SCA, In the SCA, geothermometers and geobarometers give the following results: equilibration temperatures which decrease from 1000° C in andesites to 900° C in dacites; H 2O pressures vary between 1 and 4 kbar and oxygen fugacities are higher than the Ni-NiO buffer, basic lavas may have equilibrated at 50 km depth. In shoshonitic lavas, plagioclase and K-feldspar are present, Olivine is Fo 85, (phenocrysts) and Fo 65 (microlites); clinopyroxene is Cr-rich; orthopyroxene is bronzite in shoshonites (microlites) and banakites (microlites and phenocrysts); hornblende is pargasitic; biotite is T-rich in banakites. Conditions of differentiation of the lavas are high temperatures (850–1050°C), water pressures inferior to 2 kbar, oxygen fugacities above the Ni-NiO buffer and total pressures which correspond to depths of 80–120 km. Major- and trace-element distributions emphasize differences between calc-alkaline associations of the SCA and MA and between calc-alkaline and shoshonitic associations. In shoshonitic lavas, decreases in MgO and Ni, CaO and Cr, Al 2O 3 and Sr correspond to respective olivine, clinopyroxene and plagioclase fractionations; incompatible elements circum-Pacific characteristics (high Al 2O 3 constant Na 2O 3 increasing K 2O with differentiation), Andean characteristics (low CaO), and peculiar characteristics: MA lavas have lower MgO (olivine fractionation); in the SCA, lavas have lower TiO 2 and FeO (fractionations of magnetite, pyroxenes and hornblende). The origin of the Andean magmatism is not consistent with single-stage models (anatexis of continental crust; crystal fractionation of basaltic magma under amphibolite or eclogite facies; partial melting of peridotitic upper mantle material). A three-stage model is proposed. The first stage consists of partial melting of upper mantle material; loss in fluids of the subducted oceanic crust induces the melting; bubbles of primary magma are generated. The second stage consists of selective crustal contamination during the stay of the magma at the base of the continental crust. The third stage consists of low-pressure crystal fractionation which governs the differentiation of the lava series. The present model considers transversal and longitudinal zonation of the Andes. High degrees of melting, low rate of contamination and reduced crystal fractionation characterize the MA, whereas low degrees of melting, high rate of contamination and considerable crystal fractionation characterize the SCA. Garnet is involved in the genesis of shoshonitic lavas.