New Mexico Geological Society Annual Spring Meeting — Abstracts

Major and trace element characteristics of whole rocks from the Doña Ana Mountains, south-central New Mexico, range from trachyandesite to rhyolite. The isotopic character of the Doña Ana caldera-fill rhyolite is different than trachyte and trachyandesite in the ~43-41 Ma Palm Park formation, the local country rock in the area. The Doña Ana caldera-fill rhyolite was previously assumed to be the volcanic equivalent of an exposed monzonite pluton, but isotope ratios of feldspar crystals indicate that the caldera-fill rhyolite originates from a distinct source. Haga (1994) suggested that the Doña Ana caldera-fill rhyolite was magmatically related to the Squaw Mountain tuff, a 36.1 Ma tuff associated with the nearby Organ Mountains Caldera. The chemical composition and isotope character of the 36.1 Ma Doña Ana caldera-fill rhyolite, however, is significantly different than the Squaw Mountain rhyolite tuff. Sr, Nd, and Pb isotopes define distinct fields for Doña Ana Mountain volcanic rocks. Overall, rocks of the Doña Ana Mountains are chemically distinct and separate from volcanic rocks of the Organ Mountains caldera system, consistent with two unrelated caldera systems existing near each other at ~36 Ma.