The Eruptive History of the Talpa-Mascota-San Sebastian Volcanic Field in Western Mexico.

The eruptive history of the Talpa-Mascota-San Sebastian (TMSS) volcanic field in the Jalisco Block (JB) of western Mexico is presented. The JB is bounded by the Tepic-Zacoalco and Colima grabens, as well as the Middle America Trench where the Rivera plate subducts beneath North America. The TMSS volcanic field spans ∼2030 km² and contains ∼123 small cones and flows of minette, absarokite, basic hornblende lamprophyre, basaltic andesite, and andesite. The petrology of these lavas is described in Lange and Carmichael (1990, 1991) and Carmichael et al. (1996). Of the ∼123 distinguishable eruptive units within this volcanic field, 26 samples have been dated by the ⁴⁰Ar/³⁹Ar method, and are combined with 10 dates from a previous abstract and nine dates from the literature (for a total of 45). The oldest lavas (2.35 to 0.5 Ma) are found in the Talpa region, whereas the youngest lavas (predominantly < 0.5 Ma) are found in the Mascota and San Sebastain regions to the north. There is thus a clear trend of volcanism becoming younger to the north, away from the trench. On the basis of these ages, field mapping, and the use of ortho airphotos and DEMs, it is estimated that a combined volume of < 12 km³ erupted in the last 1 Myr. The dominant lava type is basaltic andesite ( ∼44 %), followed by minette ( ∼20 %), basic, hornblende lamprophyre ( ∼17 %), andesite ( ∼13 %), and absarokite ( ∼6 %). Thus more than half of the eruptive material (57 %) is andesite and basaltic andesite, which erupted in close spatial and temporal association with the highly potassic lavas. There is no time progression to the type of magma erupted. The volumes of the potassic lava types are dwarfed by the amount of intermediate, calc-alkaline magma ( ∼360 km³) that has erupted over the same time period (< 1 Ma) within the Tepic-Zacoalco graben in western Mexico. These age results confirm that the potassic lavas of Mascota (not unlike those erupted 3-4 Myr ago in the Sierra Nevada batholith; Farmer et al., 2002) are part of line of similar volcanism that youngs to the N-NW. The Los Volcanes field was active from 3.4-1.5 Myr ago (Wallace and Carmichael, 1992), whereas two dates from the Ayutla volcanic field give ages of 4.4 and 4.5 Ma (Righter and Rosas-Elguera, 2001). It thus appears that this line of potassic volcanism within the interior of the Jalisco block is not a coeval "volcanic front", which raises the possibility that it may not be directly related to active subduction of the Rivera plate.