INTRA-CONTINENTAL RIFT VOLCANISM: PETROLOGICAL, PALEOMAGNETIC AND ROCK MAGNETIC DATA BEARING LAVA VENT PLUMBING AND ERUPTION DYNAMICS AT LEMPTéGY VOLCANO AND CERRO COLORADO

Monogenetic basaltic volcanic fields occur in tectonic settings ranging from extensional to convergent. Pleistocene to Holocene volcanic activity along both the Limagne Graben (France) and the Rio Grande rift (USA) occurred as a result of crustal extension, consequent lithospheric thinning, and mantle upwelling, resulting in abnormal heating of the lithosphere and antectic melting. Both Chaîne des Puys and Cerros del Rio Volcanic fields reflect very similar geological environments that shaped the cinder cones of Puy de Lemptégy and Cerro Colorado. Paleomagnetic data along with structural field mapping studies were analyzed in order to characterize and compare the timing of emplacement of effusive versus extrusive units and the formation and deformation of flow units during cone growth. Petrographic and rock magnetic analysis described principal mineral and magnetic phases that carry both the remanence and anisotropy. Strombolian acidity dominates the active period of the volcano and contributed the most to the growth of the cinder cone. Early eruptive stages are characterized by a gas-rich magma and more explosive activity characterized by microcrystalline, hypohyaline textures mostly composed of volcanic glass. Late effusive stages showed more complex petrographic textures, with dominant plagioclase and mafic minerals (olivine + augite). Paleomagnetic data from individual sites along the volcanoes flanks revealed statistically distinguishable mean from the expected Pleistocene magnetic field direction as well as between site locations. Rock magnetic experiments revealed SD to PSD grain behavior with Fe-Ti oxide phase, likely Med-Ti Titanomagnetite, as the principal magnetic phase carrying both the remanence and anisotropy. As observed in Lemptégy, a rather complex magma plumbing network evolved through the course of the eruption and this comparative study leads us to believe that the growth of a volcano occurs not on a simple central axis feeder but rather through a multi-feeder geometry system. This study shows that the dynamics governing different monogenetic volcanoes in similar geological settings are identical and thus a model can be extrapolated for monogenetic volcanism around the world.