INTRA-CONTINENTAL RIFT VOLCANISM: PETROLOGICAL, PALEOMAGNETIC AND ROCK MAGNETIC DATA BEARING ON 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 occurred along the Limagne Graben (France) and the Rio Grande rift (USA) as a result of crustal extension, consequent lithospheric thinning, and mantle upwelling, resulting in heating of the lithosphere and antectic melting. Both the Chaîne des Puys and Cerros del Rio Volcanic Field reflect very similar geological environments that generated the cinder cones of Puy de Lemptégy and Cerro Colorado. Paleomagnetic data along with structural field mapping studies are being conducted 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. A total of twenty Currie point estimates and Lowrie fuller tests were obtained to describe the principal magnetic phases that carry both the remanence and anisotropy. Petrographic analyses aim to describe transitional phases in lava flow composition and changes in vesicularity and hydrous phases as volcanic activity progressed. Preliminary observations indicate similar eruptive vent dynamics, with structural measurements pointing to a well-defined central vent surrounded by pyroclastic deposits (scoria, blocks and bombs), and flows. Changes in eruptive behavior are evident by successive textural transitions between effusive and explosive cycles. Rock and paleomagnetic analysis indicate SD to PSD grain size consistent with a Fe-Ti oxide phase, likely Medium-Titanium Titanomagnetite, as the principal magnetic phase carrying both the remanence and anisotropy. The presence of an iron sulfide (FeS) phase common to both volcanoes possibly reveals a low temperature magmatic source. After processing all data, subvolcanic deformation within the volcanoes’ plumbing systems and their external structure will be compared to general models of cinder cones evolution.