Petrology and Chemistry of Volcanic Rocks of the Southern Andes
Pliocene to Quaternary volcanic rocks of andesitic to dacitic composition predominate in the northern (33–37°S; Province I), and southern (48–56°S; Province III) sectors of the SVZ, whereas rocks of basaltic composition predominate in the central sector (37–46°S; Province II). Many stratovolcanoes in the latter sector have precaldera lavas of basaltic composition and postcaldera lavas of dacitic composition (e.g. Quetrupillán, Lanín and Puyehue volcanoes). Hornblende is a common phenocryst phase in lavas of Provinces I and III (e.g. Marmolejo-San José volcanic group and Mt. Burney volcano), but are uncommon in Recent lavas from Province II, particularly between 37–42°S (e.g. Antuco, Callaqui, Lonquimay, Villarrica, Puyehue, Osorno, Puntiagudo and Calbuco volcanoes). Nevados de Chillán (36°50′S) located near the boundary of Provinces I and II is transitional, with lavas of andesitic composition lacking hornblende. Lavas from Province II generally have relatively flat chondrite-normalized REE patterns with low La/Sm ratios and HREE contents greater than 8 × chondrites; lavas from Province I have variable La/Yb ratios with HREE ranging from about 15 × chondrites to less than 5 × chondrites. Most samples from stratovolcanoes such as Puyehue and Nevados de Chillán, have very subparallel REE patterns; other volcanic suites have REE patterns of varying slope with poor correlations of La/Sm, La/Yb and total REE with major elements, especially SiO2 content. Miocene volcanism in the SVZ is represented by subvolcanic rocks of basaltic to dacitic composition distributed along the Longitudinal Depression (33–42°S) and High Andes Cordillera (33–38°S). Manifestations of Miocene igneous activity associated with porphyry copper deposits are also observed between 28–33°S where Quaternary volcanism is absent.
KeywordsVolcanic Rock Volcanic Belt Garnet Peridotite Volcanic Group Quaternary Volcanic Rock
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