GEOCHEMISTRY, STRATIGRAPHY, AND CORRELATION OF VOLCANIC ASHES IN A LATE CENOZOIC EXTENSIONAL BASIN, GUANAJUATO, MEXICO

The San Miguel de Allende graben, Guanajuato, Mexico, contains numerous important fossil collecting localities, and has yielded significant discoveries of vertebrate fossils. The fossils represent a major terrestrial biotic interchange following the formation of the Panamanian land bridge. A composite stratigraphic section totaling 46 meters spans Pliocene (~ 4.8 Ma) to Early Pleistocene times and is composed of sediments from floodplain, pedogenic, and volcanic environments.

Electron microprobe analyses of rhyolitic volcanic glass from 14 samples reveals that 6 separate eruptions of volcanic ash were deposited during the Pliocene in the basin. They are defined by differences in major element geochemistry. Two magma series (medium K and high K) are represented in the ashy deposits. The high K series is marginally peralkaline and is probably associated with rifting behind the magmatic arc and are distinct from normal calcalkaline volcanism along the front of the Trans Mexican Volcanic Belt (TMVB). The medium K series is more similar to typical arc rhyolites.

The lower and middle Pliocene sediments have been correlated between eight measured sections throughout the basin using four distinct volcanic ash beds. Three of the ashes have radiometric ages: 3.36 (±0.04 at 1ó) Ma using 40Ar/39Ar, 3.9 (±0.3) Ma using fission-track, and 4.4 and 4.8 (±0.3) Ma using fission-track.

This preliminary tephrachronologic framework from the San Miguel de Allende graben may now be used for precise correlations between this and other TMVB basins, and for interpreting the Pliocene tectonic and volcanic evolution of central Mexico in the context of an important vertebrate fossil record.