Earth and Planetary Sciences ETDs

Publication Date

11-20-1995

Abstract

The mid-Tertiary Boot Heel volcanic field is located in the Basin and Range Province of southwestern New Mexico and southeastern Arizona. The field is defined on the basis of nine regional ash-flow tuffs, which have been correlated throughout the area, and for which probable source cauldrons have been identified. These tuffs are: tuff of Woodhaul Canyon (35.3 Ma); Bluff Creek Tuff (35.1 Ma); tuff of Steins (34.4 Ma); Oak Creek Tuff (33.5 Ma), tuff of Black Bill Canyon (stratigraphically constrained to 32.7-33.5 Ma); Gillespie Tuff (32.7 Ma); tuff of Horseshoe Canyon (27.6 Ma); Park Tuff (27.4 Ma); and Rhyolite Canyon Tuff (26.9 Ma). The first six caldera-forming eruptions occurred from 35.3 to 32.7 Ma. After a 5 million year hiatus, the final three tuffs erupted in quick succession, from 27.6 to 26.9 Ma. The age groups exhibit distinctive petrologic and chemical differences, and to a lesser degree, distinct eruptive styles.

The 35.3-32.7 Ma activity occurred as subduction was ceasing along this section of the western coast of North America. Volatile flux through the upper mantle and crust was high due to dehydration reactions in the subducted plate; amphibole is a common phenocryst phase in the older tuffs, and was probably an important cumulate phase in the more mafic source magmas. Chemical and isotopic similarities indicate that the 35.3-32.7 Ma Boot Heel volcanic rocks may be cogenetic with lower-crustal meta-diorite granulites found in basalt flows and tephras of the Plio-Pleistocene Geronimo volcanic field. The meta-diorite protoliths may have formed during mid-Tertiary time, by mixing of underplated basaltic magmas with partial melts of Precambrian lower crust.

The 27.6-26.9 Ma eruptive pulse contained no subduction component; the source magmas were relatively H2O-poor. Amphibole was no longer part of the fractionating assemblage, leading to comparative enrichment in those elements which are strongly partitioned into hornblende (e.g., the HFS elements). The young magmas underwent greater degrees of crystal fractionation before volatile concentrations reached levels able to support effusive pyroclastic eruptions􀀜 al o, many rhyolitic magmas erupted passively. as lavas. Hence, the young tuffs tend to be highly evolved – rhyolitic or high-silica rhyolitic in composition, whereas the older tuffs are dacitic to rhyodacitic. Trace element concentrations reflect these differences; young tuffs have steep enrichment and depletion patterns, while older units have relatively flat trends.

Neodymium, Sr, and Pb isotopic compositions of the volcanic rocks indicate that source magmas were mixtures of mantle-derived basaltic magmas and Precambrian crust, and suggest that the compositional differences between the two age groups are primarily due to changes in the fractionating crystal assemblage rather than changes in the composition or relative proportion of mantle and crustal melt components. Although Nd isotopes indicate that the younger magmas contained a slightly larger mantle component (they are about 1.5 εNd units more positive than the older rocks), the much higher Nd concentrations in the young rocks (50-100%) magnify the sensitivity of the isotopic system, and preclude any major change in the relative proportion of components. Strontium and Pb isotopes corroborate this, showing no variation between the two age groups.

Initial 87Sr/86Sr values within the tuffs vary inversely with silica content. Some examples can be attributed to post-emplacement metasomatism; however, for most units, contamination of the low-Sr rhyolitic upper parts of the magma chamber with a high-Sr, low-87Sr/86Sr component is probable. Possible contaminants might be limestone, abundant in the thick (several km) Paleozoic section in the this area, or andesitic magma from deeper in the magma chamber, which occurs as magma clots most of the tuffs of the Boot Heel field.

Degree Name

Earth and Planetary Sciences

Level of Degree

Doctoral

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Wolfgang Eugene Elston (Co-Chair)

Second Committee Member

Albert Masakiyo Kudo (Co-Chair)

Third Committee Member

William C. McIntosh

Fourth Committee Member

Bruce M. Thomson

Project Sponsors

Funding for this project was provided by a number of sources. Early work was funded by the National Science Foundation and supported by teaching assistantships from the Department of Earth and Planetary Sciences of the University of New Mexico. Further support in the form of research assistantships was received from Sandia National Laboratories. The Department of Earth and Planetary Sciences and the Student Research Allocation Committee (SRAC) provided small scholarships for travel and research expenses. Other funding. in the form of free or reduced cost analytical work, was also provided. INAA analyses were done at no cost by Los Alamos National Laboratories and by the Oregon State Reactor Sharing Program. Rb-Sr isotopic analyses were carried out in the UNM Geochronology Laboratory under the auspices of Douglas Brookins, with the able assistance of Dave Ward. Sm-Nd and Pb isotopic analyses were performed at the University of Kansas Geochronology Laboratory, with guidance and help of Douglas Walker and Jon Lynn. The Southwestern Research Station of the American Museum of Natural History, in the Chiricahua Mountains, provided excellent field accommodations at low cost. The Nature Conservancy and the Gray Ranch Corporation granted access to the Animas Mountains and adjacent areas.

Language

English

Document Type

Dissertation

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Included in

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