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The geochemical evolution of Volcan Tatara-San Pedro, 36 degrees S, Southern Volcanic Zone, Chile

Andrew Howard Wulff, University of Massachusetts Amherst


This study involves an examination of geochemical data from the Tatara-San Pedro Volcanic Complex (TSPVC), an approximately 250 km2 Quaternary frontal volcanic arc center located at 36° S, 70.5° W in the Southern Volcanic Zone (SVZ) of the Chilean Andes. Every lava flow in almost thirty, mostly vertical, stratigraphic sections was sampled, and used in conjunction with field observations, K-Ar and 40Ar/39Ar ages, and paleomagnetic data, to construct generalized chemo-stratigraphic columns. These columns not only demonstrate changes in lava composition with time, but were used to reveal the sequence of petrogenetic processes that formed those lavas. Geochemical variation both within a single lava flow and amongst members of an eruptive package of lavas was established, using lava flows which had erupted at different times, and were characterized by different mineralogical and geochemical features. Paleomagnetic and geochemical data were also used to correlate lava flows and packages of flows erupted onto different geographic regions of the complex. Petrogenetic modelling suggests that magma supply rates may be the dominant parameter controlling lava compositions; high rates of supply produce compositionally monotonous sequences of mixed basalt and basaltic andesite, which largely comprise the main volcanic edifices. The final-erupted lavas in many of these sequences are the least contaminated and may represent the best proxies for recharge magma compositions. During periods of intermediate magma supply rates, increased cooling and crystallization produces greater compositional range between mixing endmembers, and mixed andesites are the result. Low supply rates are associated with either single, short-lived flank eruptions of mafic lava, or more evolved compositions derived independently from differentiation of magma within small volume chambers. The relationship between fractionated, erupted, and recharge volumes determines the nature of erupted sequences. The range of lava compositions at TSPVC, as the result of multiple sources, different crustal contaminants, and different petrogenetic processes, is almost as great as that exhibited by all volcanoes from the Southern Volcanic Zone. This suggests that the dominant control on SVZ compositions may not be crust:mantle ratio, but different petrogenetic processes and changing crustal contaminants with time.

Subject Area


Recommended Citation

Wulff, Andrew Howard, "The geochemical evolution of Volcan Tatara-San Pedro, 36 degrees S, Southern Volcanic Zone, Chile" (1998). Doctoral Dissertations Available from Proquest. AAI9929174.