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6 hits

  • Thumbnail for SW1B

    This coarse-grained ophitic norite contains intergrown plagioclase and orthopyroxene. Chlorite alteration of pyroxene is pervasive.

  • Thumbnail for BB16

    This hypidiomorphic, equigranular, two-phase granite contains almost exclusively k-feldspar and quartz. A vein cross-cutting the sample is infilled with chalcedony.

  • Thumbnail for Trace Element Geochemistry and Petrology of Aden Crater and the Albuquerque Volcanoes
    Trace Element Geochemistry and Petrology of Aden Crater and the Albuquerque Volcanoes by Butler, Daniel Jay

    Aden Crater and the Albuquerque volcanoes are spatter and cinder cone systems with minor lava flows. They developed within the Rio Grande rift, a wedge shaped extensional feature in the southwestern United States, in the late Tertiary. Both systems are thus hypothesized to have a similar geochemistry that reflects upon comparable tectonic processes, settings, and time periods of eruption. The Aden volcanic field is a <80 Ka volcanic system in southern New Mexico, situated within the Rio Grande rift, approximately 25 miles north of the Mexican border. The eleven samples collected at the Aden system are from the spatter wall, lava lake feeder vent, and flow features. In thin section, plagioclase composition of An60 occurs alongside olivine crystals without reaction rims in a plagioclase, clinopyroxene, brown glass, and olivine groundmass. Major element geochemistry, determined by X-ray fluorescence (XRF), reveals an SiO2 content of 50-55% and an MgO content of 10-11%. The Aden basalts are classified as alkaline, within-rift basalts on IgPet classification diagrams. In-depth geochemical study, focusing on trace element geochemistry, reveals a close match on spider-type plots with Ocean Island Basalt and internal variation in heavy rare earth element (HREE) contents. Ytterbium divides the Aden basalts into three groups on rare earth element (REE) plots based on location, but there is not enough evidence in other elements or classification diagrams to support differentiation. There are no distinct differentiation or fractionation trends on SiO2 vs. V or Ni vs. Rb diagrams. The unusually high magnesium content and the pattern of spider diagrams suggest the presence of garnet in the source melt retaining these HREEs. Furthermore, the lack of distinguishable trends indicates a relatively short eruption from a single source without time for fractionation. The Albuquerque volcanoes lie eight kilometers to the southwest of Albuquerque, New Mexico, situated near the Albuquerque basin, within the Rio Grande rift. The volcanoes are situated in the center of the Rio Grande rift both from a north to south and east to west perspective. The eight samples collected at the Albuquerque volcanoes are include three from surrounding flows and five samples from the three main volcanoes: JA cone, Black Cone, and Vulcan. Phenocrysts of plagioclase (An69-65) and olivine occur in a grey groundmass. The basalts are classified as subalkaline, and their geochemical data plots most similarly to Enriched Mid Ocean Ridge Basalt (EMORB). Interpreting these data with relation to the geochemistry of the Albuquerque volcanoes, and other volcanic fields in the Rio Grande rift area, suggests that Aden Crater is not representative of central Rio Grande rift geochemistry. The geochemistry of the Albuquerque volcanoes is similar to the Servilleta Basalt and Taos Volcanic field, both within the Rio Grande rift, while the Aden basalts are most similar to the Ocate volcanic field and the Zuni-Bandera volcanic field, both outside the boundaries of the Rio Grande rift. This difference in classification reflects differences in levels of crustal trace elements like Zr, Nb, and K, yet Aden Crater and the Albuquerque volcanoes have a similar overlying crustal thickness. I believe that Aden Crater was sourced from a metasomatized garnet bearing mantle, such metasomatism caused by overlying remnants of the Farallon plate enriching Aden Crater in crustal trace elements not seen in other within-rift basalts.

  • Thumbnail for SW16

    Equant, subhedral plagioclase grains are the dominant phase in this thin section. Interspresed within, and crosscutting these plagioclase crystals are oblong, rounded zones of increased alteration. Each has a corona of a cloudy, isotripic phase, the outside border of which is often lined with clinozoisite crystals growing radially from the inclusion. The central part of the inclusion consists of zeolite prisms growing in each of three consistant orientations. Fractures crosscut the igneous fabric radially from these alteration zones.

  • Thumbnail for SW11

    Large, euhedral plagioclase crystals are more abundant in this norite than the pyroxenes. Exsolution lamellae are extensive in the pyroxenes. Minor clinozoisite is found growing in the cracks between pyroxenes.

  • Thumbnail for SW12

    Large, euhedral plagioclase crystals are of equal abundance as the pyroxenes in this norite. Exsolution lamellae are extensive in the pyroxenes. Opaque sulfide phases infill the interstices in portions of this thin section.