New Mexico Geological Society Annual Spring Meeting — Abstracts

Basaltic volcanism within the 2500 km² Zuni-Bandera volcanic field has been episodic, beginning at about 700 ka with the eruption of voluminous tholeiitic lavas in the southern and western parts of the field. These lavas include the Fence Lake flow and the basalts of the North Plains. These flows were of low viscosity and individual flows traveled up to 100 km from probable sources beneath younger cinder cones and flows of the Chain of Craters. Activity along the Chain of Craters was more complex with the eruption of both alkalic and tholeiitic lavas and an apparent younging toward the northeast along the chain. Three flows with vents near the northeast end of the chain have conventional K-Ar apparent ages of about 150 ka. Dates on three other tholeiitic vents from northeast of the Chain of Craters; EI Calderon, Twin Craters, and EI Tintero range from 92 to about 70 ka The two youngest flows within EI Malpais National Monument are the alkalic Bandera flow and the tholeiitic McCartys flow, which erupted about 11 and 3 ka, respectively. Both of these very young flows are voluminous, covering large areas within the national monument.

The results of about 100 chemical analyses of basalts from the Zuni-Bandera volcanic field have been compiled and statistically analyzed. In general, the tholeiitic basalts are relatively homogeneous in composition despite their age differences. Silica contents range from 47.5 to 53%, MgO contents from 6.2 to 10.5%Al, K₂O from 0.18 to 0.96%, and Mg numbers from 58 to 63. With the exception of EI Calderon, the tholeiitic vents appear to be monogenetic. Results from EI Calderon will be reported by Cascadden et al. (this abstract volume). The relative homogeneity in composition for the tholeiites probably reflects the larger degrees of partial melting (~10%) required to generate these basalts.

Compositional variability is much greater within the alkali basalts and it is possible to "fingerprint" many of these flows. Within the alkali basalts, SiO₂ ranges from 44 to 54%, MgO from 5 to 11 %, K₂O from 1.1 to 2.4%, and Mg numbers from 49 to 65. Again most of the vents appear to be monogenetic. Our data suggest that only the Paxton Springs volcano was polygenetic.