Petrology and geochemistry of the Lazufre volcanic complex : evidence for diverse petrogenetic processes and sources in the Andean Central Volcanic Zone

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Montana State University - Bozeman, College of Letters & Science


The Lazufre volcanic complex is an area of active surface uplift (~25°14'S) situated between two potentially active Quaternary volcanic centers, Lastarria and Cordon del Azufre, in the Andean Central Volcanic Zone. Studies incorporated geologic field relationships, mineral compositions, textures, and whole rock geochemical and isotopic data to develop a petrogenetic model to identify the source area and petrogenetic processes for the Lazufre magmatic system. Whole rock K-Ar dates of lavas from Cordon del Azufre place the most recent eruptions at 0.6-0.3 Ma + or = 0.3 Ma. The most recent eruptive activity at Lastarria has been dated at ~0.5-0.1 Ma. Volcanic rocks erupted from Lazufre are andesites to dacites and conform to a medium- to high-K calc-alkaline suite. Typical phenocryst assemblage is plagioclase-orthopyroxene-clinopyroxene-amphibole. Magmatic inclusions and mafic glomerocryst are present in most lava flow samples. Plagioclase and pyroxene phenocrysts in all rocks exhibit textures consistent with thermal disequilibrium. Important geochemical characteristics of these rocks include negative correlations for Mg, Fe, Ca and increased K and Na with increasing SiO 2 suggesting limited crystal fractionation. High Cr and Ni in some of the more mafic samples indicate mingling of a more mafic magma with a large volume of more silicic magma. Large ion lithophile elements are elevated at higher SiO 2 content, suggesting assimilation of more felsic rocks. A low range in 206 Pb/204 Pb, 87 Sr/86 Sr, and 143 Nd/144 Nd suggest partial melting of lower mafic crust as the dominant process in the generation of Lazufre extrusive rocks and indicate that there was relatively little involvement of ancient or felsic continental crust in magmagenesis of the area. The original magma was modified by homogenization and small degrees of mixing and assimilation and fractional crystallization during differentiation through ascent of the mid and upper crust. The results from this study are significant in that a multitude of differentiation processes and magma sources, specifically, a considerable mafic lower crustal component were involved in the generation of Lazufre Volcanic Complex magmas in the Andean Central Volcanic Zone.




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