Earth and Planetary Sciences ETDs

Publication Date

6-2-1967

Abstract

Vertical compositional zoning occurs within the basal simple cooling units of two welded ash-flow tuff sheets, the Kneeling Nun and Caballo Blanco formations, in the Cobre Mountains of southwestern New Mexico. The sheets are on the southern edge of the Datil-Mogollon volcanic province. The Kneeling Nun (33.4 m.y.) varies abruptly but systematically upward from a modal quartz latite to latite, and the Caballo Blanco (29.8 m.y.) from a modal rhyolite to capping quartz latite. Both are crystal-rich, and in each the amount of ferromagnesian and plagioclase phenocrysts and phenocryst percent increases upward, while the quartz and alkali feldspar/plagioclase ratio decreases. The range and nature of zoning is similar to ash-flow deposits elsewhere, but is stronger, more complex, and takes place within a single eruptive episode. The cooling units, certain detailed petrochemical characteristics of the vertical zoning, and features of the regional biotite trace element variations are discussed. A rapid and quantitative direct-reading emission spectrometric technique was developed and is briefly described for the determination of Fe, Mg, and trace elements in biotites, utilizing a high-current d.c. arc, Cd and Lu as internal standards, and inert Ar/O atmospheres. The petrographic data and spectrochemical analyses of biotite and enclosing rock (minus heavy minerals) for K, Na, Ca, Fe, Mg, and 12 trace elements are presented for 22 representative samples from two sections of the zoned units. Consideration of these and the areal parameters indicates that (1) each simple cooling unit resulted from the successive eruption and continuous deposition of increasingly lower levels of a strongly compositionally zoned magma, (2) turbulence during deposition was limited, and (3) lateral sorting was marked. Within limits, each deposit is an inverted replica of the magma within the chamber from which it erupted. The modal and biotite trace element variations are consistent with the upward gradation and differentiation to progressively rhyolitic melts within the chambers. The entire Caballo Blanco and the basal Kneeling Nun are considered to represent equivalent magma levels. Initial compositional zoning is believed to have resulted from probable crystal settling in a homogeneous magma concurrent with upward volatile and alkali transfer. Compositional variations indicate crystallization in the upper levels of the magma chambers occurred in previously zoned melts, and under the influence of a variation in PH2O, T, and PO2. Late stage crystallization was simultaneous through the chambers, but at differing rates. The upper two thirds of the Kneeling Nun cooling unit represents lower magma that was in part a crystal accumulate, and had in part crystallized more completely. The distribution of water through the magmas was a fundamental control of the vertical modal and chemical variations. Biotites in the upper levels of the chambers were used to detect gradients in intensive variables. An abrupt change in PO2 is suggested by a change in biotite Fe/Mg bulk composition. Preliminary indications suggest crystallization of late biotite at around 800°C, and at upper limits of 1 kilobar PH2O, 4% H2O, and 2 miles in depth. These are considered to approximate the conditions at eruption. Evidence indicates that the Kneeling Nun and Caballo Blanco ash-flows erupted from broad and shallow magma reservoirs, and it is suggested that the key to ash-flow type eruption is depth rather than extreme pressure. An analysis of variance on 11 trace elements in 48 biotite samples selected at random in a 5 cubic mile portion of the Kneeling Nun ash-flow sheet in the Cobre Mountains indicates that Co, Ni, Sr, and to a lesser extent La do not vary significantly. The range of variation of 8 trace elements in 54 Kneeling Nun and 12 Caballo Blanco biotites from the simple cooling units in the Cobre Mountains, and 57 random biotite samples from southern Datil-Mogollon ash-flow and related deposits indicates (1) the compositional range of one simple cooling unit (Caballo Blanco) exceeds or is comparable to that of a group of similar ash-flow deposits that differ in time and space, and (2) ash-flow magma zoning processes are efficient concentrators of rare elements in roof regions, and the connection with certain ore deposits is implied.

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

Second Committee Member

Edgar F. Cruft

Third Committee Member

Abraham Rosenzweig

Fourth Committee Member

Albert Masakiyo Kudo

Fifth Committee Member

J. Paul Fitzsimmons

Language

English

Document Type

Dissertation

Download

Included in

Geology Commons

Share

COinS