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

Gardeweg P., Moyra C. (1991) The geology, petrology and geochemistry of the Tumisa volcanic complex, North Chile. (PhD thesis), Kingston Polytechnic, .


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.

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