Isotopic and trace element characteristics of rhyolites from the Valles Caldera, New Mexico. Final technical report

Abstract

This report is a summary of work supported by DOE grant No. DE-FGO5-87ER13795 that was completed or is still in progress. The stated purpose of this grant was to collect geochemical information (trace element, radiogenic isotope and stable oxygen and hydrogen isotope) on samples from core holes VC-I and VC-2a in the Valles caldera in order to establish a consistent detailed intracaldera stratigraphy and relate this to extracaldera volcanic rock units of the Jemez Mountains. Careful stratigraphic control of the intracaldera units is necessary to evaluate models of caldera formation, ignimbrite deposition, and resurgence. Combined stable and radiogenic isotope and trace element data will also provide major insights to petrogenesis of the Bandelier magma system. The composition of non-hydrothermally altered samples from outflow units of the Bandelier Tuff and related volcanics must be known to assess isotopic variations of intracaldera ignimbrite samples. On detailed examination of the VC-2a core samples, it became apparent that hydrothermal alteration is so extensive that no geochemical information useful for stratigraphic fingerprinting or petrogenesis could be obtained, and that correlation with other intracaldera units and extracaldera units must be made on the basis of stratigraphic position and gross lithologic characteristics. Accordingly, we emphasize geochemical data from the extracaldera Bandelier Tuffs and related units which will be useful for comparison with proposed drill hole VC-4 and for any future studies of the region. The stable isotope, radiogenic isotope and trace element data obtained from this project, combined with existing major and trace element data for volcanic rocks from this area, provide an extensive data base essential to future Continental Scientific Drilling Program projects in the Jemez Mountains of New Mexico.