New Mexico Geological Society Annual Spring Meeting — Abstracts

Field investigations of the Chain of Craters area of the Zuni-Bandera volcanic field suggest a strong temporal, spatial and chemical pattern of Quaternary basaltic volcanism. At least six episodes of volcanism can be defined in this area. The first episode consists of large volume tholeiitic flood-basalts. The second episode consists of alkalibasalts which have relatively low volumes of lava, but form high cinder cones. Peridotite xenoliths are associated with these cinder cones. The third episode consists of transitional basalts, between alkalic and tholeiitic basalts. This stage is represented by strongly aligned cinder cones, which become younger and more tholeiitic to the northeast, away form the second stage cinder cones. The fourth episode consists of tholeiitic basalts, forming lava shields and a few small cinder cones. This tholeiitic volcanism continues to the northeast of the earlier transitional episode volcanism. Other tholeiite flows occur throughout the area, being extruded at or in close proximity to second or third episode cinder cones. The fifth stage of volcanism consists of glomerocrystic basaltic-andesites. These flows occur near or actually extrude from fourth episode vents. The glomerocrysts occur mostly as radiating euhedral plagioclase crystals, with subhedral pyroxene and olivine crystals in them, and rounded clots composed of pyroxene and olivine crystals. The sixth episode is composed of both alkalic and tholeiitic basalts, with compositions in the same ranges as those of the second and fourth episodes respectively.