PETROLOGY AND GEOCHEMISTRY OF THE LAZUFRE VOLCANIC COMPLEX: EVIDENCE FOR DIVERSE PETROGENETIC PROCESSES AND SOURCES IN THE ANDEAN CENTRAL VOLCANIC ZONE

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 (CVZ). Here we present field relationships, mineral compositions and 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 ± 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 medium- to high-K calc-alkaline andesites to dacites. The typical phenocryst assemblage is plagioclase-orthopyroxene-clinopyroxene+/-amphibole. Magmatic inclusions and mafic glomerocrysts are present in most lava flow samples. Plagioclase and pyroxene phenocrysts in all rocks exhibit textures consistent with thermal disequilibrium including resorption and sieving in plagioclase and reaction rims around the pyroxenes. Major element trends of rocks from both centers include decreases in concentration for Mg, Fe, Ca and increases in concentration for K and Na with increasing SiO­₂ suggesting limited crystal fractionation. LIL/HFS elemental ratios are typical of CVZ arc-front magmas contaminated by small volumes of hybridized crust. Low ratios of ²⁰⁶Pb/²⁰⁴Pb, ⁸⁷Sr/⁸⁶Sr, and ¹⁴³Nd/¹⁴⁴Nd suggest partial melting of lower mafic crust as the dominant process in the generation of Lazufre extrusive rocks and relatively little involvement of felsic upper continental crust. We suggest the parental magma was modified after homogenization by small degrees of mixing and assimilation-fractional crystallization during ascent through the upper crust and the unique whole rock geochemical signatures are only obtained for each center during this final ascent. The results from this study suggest 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.