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Neogene volcanism at the front of the central Mexican volcanic belt: Basaltic andesites to dacites, with contemporaneous shoshonites and high-TiO₂ lava

¹ Department of Earth and Planetary Science, University of California, Berkeley, California 94720, USA
² Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA
³ Department of Geology and Geophysics, University of California, Berkeley, California 94720, USA, and Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA

As part of the continuing study of the young Mexican volcanic belt designed to document the ages and types of volcanism from the Gulf of California to the Valley of Mexico, a 50-km-wide segment of the central part of the belt has been mapped, and five types of lava have been found. Pliocene (3.77 Ma) shoshonite lava flows (K₂O >2 wt%; SiO₂ 52–58 wt%) form eroded plateaus more than 50 km behind the present volcanic front, but in the past 1 m.y. shoshonites have erupted closer to the volcanic front (300 km from the Middle America Trench). The shoshonite lava type is the most enriched in Ba, Sr, and Zr of the suite, and plagioclase phenocrysts are absent, presumably because of high contents of dissolved water (3–5 wt%).

Quaternary shield volcanoes and several cinder cones with small-volume lava flows are composed of high-TiO₂ lavas (>1.2 wt%), which have 51–57 wt% SiO₂, <6 wt% MgO, low Ni, Cr, Ba, and Sr, and high Nb (>9 ppm) and Zr (160–230 ppm). Pliocene high-TiO₂ lava is found within the eroded plateaus located 50 km behind the present volcanic front.

Quaternary basaltic andesite (52–57 wt% SiO₂) with up to 10 wt% MgO is found at the volcanic front, along with more siliceous andesite (57–63 wt% SiO₂). Representatives of the siliceous andesites (57–63 wt% SiO₂) are free of plagioclase phenocrysts, low in Al₂O₃ (15.7 wt%), but rich in MgO (5 wt%). Experiments reported elsewhere suggest that the magmas contained 3–7 wt% dissolved water. This andesite erupted along a normal fault between 0.3 and 0.005 Ma, and it is associated with dacite, similarly lacking plagioclase phenocrysts, but having comparatively abundant pyroxene. Other dacite in the Zitácuaro area is richly porphyritic, having plagioclase and hornblende; this dacite forms clusters of steep-sided domes or widespread pyroclastic deposits, and the latest eruptions, dated by radiocarbon and K-Ar, occurred between 0.03–0.05 Ma.

Estimates of the volume of magma erupted in the Zitácuaro–Valle de Bravo region in the past 1 m.y. (1.8 km³ · m.y.^–1 · km^–1) show that this area is somewhat less productive per 1 km of arc than those to the west (3.5 km³ · m.y.^–1 · km^–1) in the Michoacán-Guanajuato Volcanic Field (MGVF). A large volume of dacite has erupted in the Zitácuaro–Valle de Bravo region, which is rare in the MGVF. The cone density in the Zitácuaro–Valle de Bravo region (2.1/100 km²) is only slightly lower than the 2.6 cones/100 km² found in the MGVF.

Key Words: andesite • basaltic andesite • dacite • Mexican Volcanic Belt • shoshonite • eruption rate

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