GEOLOGY AND K-AR DATING OF THE TUXTLA VOLCANIC FIELD, VERACRUZ, MEXICO

The Tuxtla Volcanic Field (TVF) is located on the coast of the Gulf of Mexico in the southern part of the state of Veracruz, Mexico. Volcanism began about 7 my ago, in the Late Miocene, and continued to recent times with historical eruptions in AD 1664 and 1793. The oldest rocks occur as highly eroded remnants of lava flows in the area surrounding the historically active cone of San Martin Tuxtla. Between about 3 and 1 my ago, four large composite volcanoes were built in the eastern part of the area. Rocks from these structures are hydrothermally altered and covered with lateritic soils, and their northern slopes show extensive erosional dissection that has widened preexisting craters to form erosional calderas. The eastern volcanoes are composed of alkali basalts, hawaiites, mugearites, and benmoreites, with less common calc-alkaline basaltic andesites and andesites. In the western part of the area, San Martin Tuxtla Volcano and its over 250 satellite cinder cones and maars produced about 120 km3 of lava over the last 0.8 my. A ridge of flank cinder cones blocked drainage to the north to form Laguna Catemaco. Lavas erupted from San Martin and its flank vents are restricted to compositions between basanite and alkali basalt. The alignment of major volcanoes and flank vents along a N55-degrees-W trend suggests an extensional stress field in the crust with a minimum compressional stress orientation of N35-degrees-E. In total, about 800 km3 of lava has been erupted in the TVF in the last 7 my. This gives a magma output rate of about 0. 1 km3/1000 year, a value smaller than most composite cones, but similar to cinder cone fields that occur in central Mexico. Individual eruptions over the last 5000 years had volumes on the order of 0.1km3, with average recurrence intervals of 600 years. The alkaline compositions of the TVF lavas contrast markedly with the calc-alkaline compositions erupted in the subduction-related Mexican Volcanic Belt to the west, leading previous workers to suggest that the TVF is not related to subduction. Trace-element signatures of TVF lavas indicate, however, that they are probably related to subduction. We suggest that the alkaline character of the TVF lavas is the result of low degrees of melting of a mantle source coupled with a stress regime that allows these small-volume melts to reach the surface in the TVF.