Geochemistry of Andesites and Related Rocks, Rio Grande Rift, New Mexico

  1. Robert E. Riecker
  1. Charles Zimmerman and
  2. A.M. Kudo

Published Online: 21 MAR 2013

DOI: 10.1029/SP014p0355

Rio Grande Rift: Tectonics and Magmatism

Rio Grande Rift: Tectonics and Magmatism

How to Cite

Zimmerman, C. and Kudo, A.M. (1979) Geochemistry of Andesites and Related Rocks, Rio Grande Rift, New Mexico, in Rio Grande Rift: Tectonics and Magmatism (ed R. E. Riecker), American Geophysical Union, Washington, D. C.. doi: 10.1029/SP014p0355

Author Information

  1. Department of Geology, The University of New Mexico, Albuquerque, New Mexico 87131

Publication History

  1. Published Online: 21 MAR 2013
  2. Published Print: 1 JAN 1979

ISBN Information

Print ISBN: 9780875902142

Online ISBN: 9781118664988

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Keywords:

  • Andesites rocks;
  • Mineral chemistry;
  • Petrography;
  • Rock chemistry;
  • Strontium and rubidium concentration

Summary

The petrography, whole rock and mineral chemistry, trace element abundances, and initial Sr ratios of 1—4 M.Y. old Rio Grande rift calc-alkalic volcanic rocks and their inclusions indicate that pargasitic hornblende may be a stable phase from lower crustal/upper mantle to near-surface pressure conditions and is important in controlling the generation and fractionation of this andesitic suite. Andesitic rocks from Cerro Tome, Cerro de Los Lunas, Cerros del Rio, and the Taos volcanic field contain similar mineral assemblages, bulk chemistry, LREE-enriched patterns, and initial Sr ratios of 0.7036 to 0.7075. Increased initial 87Sr/86Sr with SiO2, petrographic evidence, and the results of Wright-Doherty mixing calculations suggest crustal contamination of andesitic magmas of up to about 20 percent by volume in some lavas to be coincident with crystal fractionation. Initial fractionation of low-silica andesites by plagioclase + olivine ± magnetite ± pyroxene ± crustal contamination is followed by hornblende-plagioclase fractionation in the high-silica andesites and more silicic rocks. FeO/MgO ratios (0.9—1.9 wt. %) of the fractionating anhydrous assemblage and in the hornblende is appropriate for producing the calc-alkalic trend on AMF plots. Pargasitic compositions from phenocrysts, xenocrysts, and grains found in metamorphic and igneous inclusions are strikingly similar: SiO2 = 41—46% by wt.; TiO2 = 1—4; Al203 = 10—12; total Fe as FeO = 10—17; CaO = 10—12; MgO = 9—14; Na2O = 1.5—3; K20 = 0.5—1.5; F = 0.1—0.7. The consistency of compositions suggests a wide pressure range of stability for these Fe-Ti-bearing pargasites at the magmatic temperatures (950°C to 1180°C) estimated by the two-pyroxene and plagioclase-melt geothermometers and at oxygen fugacities approximated by the Ni-NiO buffer. Andesite genesis in the Rio Grande rift is believed to be the result of partial melting of a hornblende-bearing upper mantle source rock because of the high fractionated REE patterns and the hydrous nature of the andesitic magmas. Spatially and temporally related alkali basalts are also believed to be products of hydrous upper mantle partial melting.