Monogenetic volcanism as an indicator of mantle heterogeneities beneath Northern Patagonia

HALLER MJ; MASSAFERRO GI; DOSTAL J; MEISTER CM

Foz do Iguassu

Congreso; 2010 AGU Meeting of the Americas; 2010

American Geophysical Union

In Patagonia, a fringe of small-volume monogenetic basaltic lava fields extends about 350 - 450 km inland from the active subduction zone that runs parallel to the Andean range for 1,250 kilometers. Patagonian eruptions occurred mainly between 3.8-1.5 and 1 million years ago, with a few eruptions occurring in the Late Pleistocene (0.23 million years ago).  Plio-Pleistocene lava fields in Northern Patagonia are characterized by two distinct geochemical and petrographic groups of basalts that are interspersed spatially and temporally. The first group is dominated by alkali basalts, whereas the second group consists of subalkaline basalts with tholeiitic affinities. The alkaline group is characterized by low SiO2 content, with modal or normative nepheline, contains olivine phenocrysts, and these basalts classify as alkaline basalts, basanites and trachybasalts. The subalkaline basalts have higher SiO2 contents, contain plagioclase phenocrysts with disequilibrium features such as reaction rims and embayments, are quartz normative, and classify as basalts and basaltic andesites.  Both groups show positive correlations of highly to moderately incompatible elements plotted against highly incompatible element concentrations (e.g. Nb/Zr vs Nb; La/Sm vs La), and of REE/HFSE (rare earth elements/high field strength elements) plotted against REE concentrations (e.g. La/Nb vs La). Alkali basalts show greater LREE enrichment and steeper slopes than the tholeiitic basalts. In addition, alkaline basalts are depleted in K and Hf, whereas tholeiitic basalts are depleted in Ti.  Based on Fe8.0 – calculated magma segregation depths, the alkali basalts formed at deeper levels than the tholeiitic basalts. Similarly, partial melting calculations indicate that alkali basalts formed by smaller amounts of partial melting (3.5-4.6 %) than tholeiitic basalts (5.2-8.1 %).