RIDGE SUBDUCTION VOLCANISM IN THE ZEBALLOS COMPLEX, SOUTHERN PATAGONIAN ANDES

The Zeballos Complex (47.0^oS, 71.4^oW) lies on the western side of Meseta del Lago Buenos Aires (MLBA) in the southern Andean backarc region, southeast of the Chile Triple Junction and east of ridge collision that occurred at 6 Ma. The Zeballos is an alkaline volcanic complex consisting of multiple lava flows, dikes, and volcanic necks that range from trachybasalts (47-50% SiO₂; 4-7% Na₂O+K₂O) to highly differentiated trachydacites (55-65% SiO₂; 6-10% Na₂O+K₂O) with an EM-1, OIB-like mantle source. Zeballos volcanics have ⁴⁰Ar/³⁹Ar ages ranging between (3.3 to 1.8 Ma), that fall within the upper range of other post-plateau basalts of the MLBA (3.4 to 0.125 Ma) and thus are linked with the ridge collision and development of upwelling asthenospheric mantle through a slab window between the Nazca and Antarctic plates (Ramos and Kay, 1992; Gorring et al, 1997). The trachyandesites and trachydacites in this suite are unique within the entire suite of Neogene southern Patagonian slab window magmas. Major element analyses of Zeballos Complex basalts are chemically similar to MLBA post-plateau lavas but have lower MgO (3-6%). Zeballos trachyandesites and trachydacites have chemical signatures indicative of significant crystal fractionation (MgO <1.5%, FeO <8%, CaO <5%) of olivine, pyroxenes, Ca-plagioclase, and Fe-Ti oxides from parental magmas that were similar to MLBA post-plateau basalts as well as similar LILE/LREE and LILE/HFSE ratios (Th/La<0.13; Ce/Pb>15). Trace element data indicate partial melting in the presence of garnet. Our preliminary hypothesis is that MLBA post-plateau basalts, Zeballos basalts, and Zeballos trachyandesites and trachydacites represent a comagmatic suite (± AFC). There is chemical evidence of contamination by subduction-related processes and/or crustal components as seen in (low Ce/Pb and Th/La) and many samples have high Pb. The highly alkaline character of the trachyandesites and trachydacites is unlike the distinctly more calc-alkaline signature of Cerro Pampa adakites, suggesting little influence of a slab melt component in the petrogenesis of these rocks.