Earth and Planetary Sciences ETDs

Publication Date

4-23-1979

Abstract

Similarities in the petrography, whole rock and mineral chemistries, trace element abundances, and initial Sr ratios of 1-5 MY andesitic through rhyolitic rocks erupted in the central-northern Rio Grande rift, New Mexico allow for their study as a single calc­alkalic volcanic suite. Ti-rich ferroan pargasites and pargasitic hornblendes found in some of these lavas and their inclusions indicate that hornblende is a stable phase from lower crustal/upper mantle to near-surface pressure conditions and may be 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 Plateau volcanic field possess textural, mineralogical, and bulk chemical features that indicate a progressive crystallization sequence of silicates in the magmas: olivine, plagioclase, augite, hypersthene, hornblende, biotite, sanidine and quartz. Fe-Ti oxides may crystallize at different intervals in this sequence in the different magmas. Hornblende appears to be a liquidus or near-liquidus phase in some lavas from Cerro Tome and Cerro de Los Lunas as a result of increased alkali and water contents in those magmas. Increased 87Sr/86Sr with SiO2 and 1/SrO and the results of Wright-Doherty mixing calculations indicate that crustal contamination along with crystal fractionation involving combinations of the above phases results in the differentiation of the rift calc-alkalic suite. FeO/MgO ratios (0.9-1.9 wt. %) of fractionating anhydrous assemblages (olivines, pyroxenes, and oxides) and of hornblende phenocrysts are appropriate for producing the calc-alkalic trend on AFM plots. Andesitic magmas are believed to be generated beneath the Rio Grande rift by partial melting of a hornblende-bearing upper mantle source rock based on the initial Sr ratios, fractionated REE patterns, and the hydrous nature of the lavas. Spatially and temporally related alkali basalts are also believed to be products of hydrous upper mantle partial melting.

Degree Name

Earth and Planetary Sciences

Level of Degree

Masters

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Albert Masakiyo Kudo

Second Committee Member

Klaus Keil

Third Committee Member

Douglas Gridley Brookins

Project Sponsors

The New Mexico Geological Society, Sigma Xi, the Sun Oil Company, N.M.E.R.B. and U.S.G.S. Geothermal Grants (numbers ERB76-264 and 14-08-001-G255, respectively), Dr. Albert Masakiyo Kudo, N.A.S.A grants NGL 32-004-063 and 32-004-064, Dr. Klaus Keil

Language

English

Document Type

Thesis

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