Earth and Planetary Sciences ETDs

Publication Date

4-5-1977

Abstract

This report is the result of detailed mapping of a part of the northern Mount Taylor volcanic field of west-central New Mexico. More than 100 volcanic vents were mapped, including 8 maars, 5 pit craters, and 15 distinct trachyte flow domes. Three successive periods of volcanism were identified: (1) Tapia stage (pre-Mount Taylor, 3.26± .20 m.y.): an alkali basalt-trachyte suite, identified here for the first time on the North American continent; includes basanitoid, hawaiite, mugearite, benmorite and trachyte. Of these, the trachyte is the most voluminous (about 3 km3), the basanitoid and hawaiite (''early basalt") the least abundant. The late history of this phase involved the building of a low, irregular shield of relatively mobile, voluminous trachyte flows probably underlain by a complex of basalt centers; viscous trachyte flow domes were erupted onto this at the center and along a 10-km diameter ring centered on the central complex; explosive eruptions of trachyte pumice, pumice flows and spatter occurred on this ring and from the summit of flow domes, followed by the extrusion of black, glassy trachytes from one of the craters. (2) Cebolleta stage (late and post-Mount Taylor, 2.68± - .36 m.y.): early andesine porphyritic alkali basalt erupted from distinct fissures which had been controlled by northeasterly trending normal faults. Augite-olivine porphyritic alkali basalt with augite phenocrysts up to 2 cm were erupted from similar fissures, as were the several varieties of hawaiite which concluded this stage. Several of the fissures were re-activated as faults with movements up to 30 m transecting cinder cones. (3) Alejandro stage: augite-olivine-andesine porphyritic alkali basalt initiated this stage. Crystal-rich in the early flows, phenocrysts decreased in both size and abundance in later basalts. The youngest flows are olivine microporphyritic alkali basalt transitional to hawaiite. Fissures are well-developed in this stage and are associated with prominent rows of cinder-and-spatter cones, maars and pit craters. Lherzolite inclusions and megacrysts, particularly of augite and minor olivine and andesine, characterize the early phase of each stage. In the Cebolleta and Alejandro stages the phenocrysts decrease in abundance. Ab content of the groundmass increases in all of the stages in vertical stratigraphic succession. Trace element data suggests that fractional crystallization was responsible for the range of composition in the Tapia stage and likely for the Cebolleta stage as well. The decrease in phenocryst content of the lavas of the Cebolleta and Alejandro stages is thought to be the result of progressively greater degrees of assimilation or resorption of crystal cumulates (left over from the Tapia stage) from basalt through hawaiite compositions. Strontitun isotopic data indicate that there is low radiogenic strontitm in the source (averge value, 87Sr/86 = .7045) and no significant contamination from crustal material. The largest of the eight identified maars, the Grande maar, is more than 1. 5 km in diameter. Maars are the result of phreatomagmatic explosions, excluding three late Tapia stage trachyte maars which may have resulted from volatile enrichment of late liquids in a crystallizing trachyte stock. The basaltic maars are indicative of a wet environment more than 2 m.y. ago. The numerous pit craters formed when high-level basaltic magma bodies were drained by adjacent vents on the same fissure. Two of the pit craters are more than 1 km in diameter. Structures in siliceous flows are analogous to features in glaciers including a previously unreported type of structure defined here as "cylindrical foliations"; these lay on the margins of flows and are essentially opposite in dip to those in the main body of the flow.

Degree Name

Earth and Planetary Sciences

Level of Degree

Masters

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Wolfgang Eugene Elston

Second Committee Member

Albert Masakiyo Kudo

Third Committee Member

Klaus Keil

Document Type

Thesis

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