The geology, petrology and geochemistry of the Tumisa volcanic complex, North Chile

Tumisa (5,658 m) is a Pleistocene composite volcano located in the western margin of the Upper Cenozoic volcanic chain of the CVZ in northern Chile. It consists of a ~ 25 km[sup]3 non-welded deposit of block-and-ash flow and small-volume ignimbrites, small flank domes and a double peak formed by two cones, the younger a composite of lava flows and domes. The lava flows, domes and blocks of the pyroclastic flows are coarse-grained, crystal-rich dacite (host lava) with dominant plagioclase (An[sub]30-50) and magnesio-hornblendes with different proportions of orthopyroxene (En[sub]62-68), biotite and quartz phenocryts. Accesory phases include Fe-Ti oxides and apatite. These mineral assemblage coexist in disequilibrium with Mg-olivine and Mg-orthopyroxene. In addition to disequilibrium textures and mineral assemblages, there are widespread fine-grained, dark mafic inclusions. The inclusions are interpreted as blobs of hot (> 1100°C) basic magma containing < 5% crystals (Mg-olivine, Mg-orthopyroxene, Cr-rich spinel), which quenched on intrusion into a cool (~ 770°C), wet dacitic magma in a shallow level chamber (4-14 km). Repetitive supplies of the basic magma from depth triggered eruptions in a slowly cooling magma chamber. Mingling and partial hybridization of compositionally distinct multiple end-members was the dominant evolutionary process, combined with limited fractional crystallization, mainly in the basic magma. Post-mixing crystallization produced strongly contrasting mineral compositions due to temperature and compositional gradients. Calcic plagioclase (An[sub]50-74) and low-SiO[sub]2/high-TiO[sub]2 hornblende crystallized as prismatic or acicular aggregates in the inclusions (hyalodoleritic textures), as thin reversely zoned rims on resorbed phenocrysts and as groundmass grains. Clinopyroxene formed as acicular crystals in the inclusions, groundmass grains in the dacites and as reaction coronas around quartz. Mechanical transfer of phenocrysts between the two magmas and partial hybridization shifted whole-rock compositions (58.9-66.2% SiO[sub]2 for the host lavas; 52.7-58.4% SiO[sub]2 for the inclusions). The compositions are typical of normal calc-alkaline volcanoes from the western margin in the Central Andes. Isotopic ratios ([sup]87 Sr/[sup]86 Sr: 0.7055-0.70683; [sup]143 Nd/ [sup]144 Nd: 0.51239 to 0.51255, [epsilon][sup]Nd: -2.1 to -4.8) are within the normal range for parental magmas in this region and reflect minimal interaction with crustal material

Geochemistry of Nevado de Longaví Volcano (36.2°S): a compositionally atypical arc volcano in the Southern Volcanic Zone of the Andes

The Quaternary Nevado de Longaví volcano of the Andean Southern Volcanic Zone (SVZ) has erupted magmas that range in composition from basalt to low-silica dacite, although andesites are the dominant erupted magma type. Amphibole is a common phenocryst phase in andesites throughout the volcano, and it is the dominant mafic phase in Holocene dacites and their included mafic enclaves. Compositions of magmas erupted at Longaví volcano define arrays that diverge from trends delineated by neighboring frontal-arc volcanoes. Although mafic compositions at Longaví are broadly similar to basalts at other SVZ centers, Longaví intermediate and evolved magmas have systematically lower abundances of incompatible major (K2O, P2O5) and trace elements (Rb, Zr, Nb, REE, Th, etc), as well as high Ba/Th, Sr/Y, and La/Yb ratios. Longaví volcano magmas define two differentiation series with regard to enrichments of Rb (and other incompatible elements) with increasing silica. A high-Rb series that includes the oldest units of the volcano comprises basalts to andesites dominated by anhydrous mineral assemblages with chemical compositions similar to other SVZ magmatic series. The series with low Rb, on the other hand, includes the Holocene units that evolved from basaltic andesites to dacites by means of fractional crystallization wherein amphibole and calcic plagioclase dominate the mineral assemblage. Magmas parental to low-Rb series are interpreted to be high-degree mantle melts, highly hydrous and oxidized, formed as a response to high fluid inputs into the subarc mantle. Enhanced water transport to the subarc mantle is a plausible effect of the subduction of the oceanic Mocha Fracture Zone that projects beneath Nevado de Longaví. Volcanoes located over oceanic fracture zones further south along the SVZ have erupted hornblende-bearing magmas that share some chemical similarities with Longaví volcano magmas