Mantle heterogeneities beneath Northern Patagonia

HALLER, M., MASSAFERRO, G., DOSTAL, J, Y MEISER, C

Foz de Iguazú

Congreso; AGU 2010 Meetings of the Americas; 2010

AGU

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 %). We interpret the different basalt characteristics to indicate mantle source heterogeneities from at least two different sources. The alkali basalts are interpreted to have been derived from an OIB-type garnet peridotite, phlogopite- and/or amphibole-bearing source. Tholeiitic basalts are interpreted to have evolved from a slab-derived fluid-enriched garnet source. The spatial and chemical relationship of the basalts indicates that the lithosphere beneath Patagonia is heterogeneous, composed by OIB-like and slab derived fluid-enriched domains. The results presented here, combined with the geophysical data support an extensional tectonic regimen for this volcanism.