Earth and Planetary Sciences ETDs

Jane Pedrick

Publication Date

12-1-1995

Abstract

Proterozoic rocks in the Taos Range of northern New Mexico preserve components of a protracted tectonic history involving pre-1.7 Ga, 1.65 Ga, 1.4 Ga, and possible 1.1 Ga tectonism. The southern Taos Range is a 1.76 – 1.7 Ga volcano-plutonic complex with tectonically interlayered remnants of the Hondo Group. Similar rock types are exposed in the northern Taos Range where supracrustal rocks are sandwiched between ca. 1.65 Ga plutons. Supracrustal rocks preserve peak metamorphic conditions of T less than or equal to 650 degrees Celsius, 6-8 kb in the northern Taos Range, and T greater than or equal to 550 degrees Celsius, 6 kb in the southern Taos Range. In the northern Taos Range, relationships between field geometries, high-temperature microstructures, and peak metamorphic assemblages suggest that plutonism at ca. 1.65 Ga was synchronous with peak metamorphism and top-to-the-southeast thrusting. Lower peak metamorphic temperatures in the southern Taos Range are attributed to the lack of plutonism during this ca. 1.65 Ga contractional event. Nested clockwise P-T paths for the northern and southern Taos Range show heating with burial, followed by cooling and decompression to near triple-point conditions. This model of pluton-enhanced 1.65 Ga tectonism helps explain regional metamorphic zones in New Mexico and the Southwest. At ca. 1.4 Ga, new growth of radiogenic minerals was accompanied by crystallization and reactivation of older fabrics. The lack of any known 1.4 Ga plutons in the Taos Range suggests that the heat source was not local, and supports a model suggesting that regional 1.4 Ga plutonism was accompanied by widespread reheating to ~500 degrees Celsius and thermal softening of the middle crust, allowing the crust to flow and deform according to prevailing states of compressional stress. Continued or renewed post-1.4 Ga contractional deformation is indicated by low-temperature microstructures that also show top-to-southeast, thrust-sense deformation in the northern Taos Range. In contrast with previous models for the tectonic evolution of the Taos Range and northern New Mexico, no evidence is seen to support the existence of an extensional shear zone between an upper plate of medium-grade rocks and a lower plate of high-grade rocks. Current work suggests a protracted tectonic history, with peak contractional deformation and metamorphism at ca. 1.65 Ga correlative with the Mazatzal orogeny of Arizona, and renewed heating and contraction at ca. 1.4 Ga in association with a regional magmatic event and possible 1.1 Ga (Grenville) thermal and tectonic events. Chapter 2 includes an evaluation of the assumptions and input data used in one application of the Gibbs method. The Gibbs method was used to calculate how magnetite and ilmenite compositions correlate to fO2, assuming ideal mixing, and comparing results to established magnetite-ilmenite oxygen barometers based on experimental data. Oxygen fugacities calculated with the Gibbs method at temperatures of 700, 800, and 900 degrees Celcius, using reference conditions at the QFM buffer, were within one log unit fO2 of the published experimental data. Location of the reference conditions in T-fO2 space influences correlation between the calculated estimates and the barometers, but does not affect the relative ordering of "unknown" samples with respect to fO2. These results suggest that the Gibbs method can be used with confidence to investigate relative chemical gradients and monitor relative chemical potentials in real geologic samples without detrimental consequences due to reference choice or applicable simplifying assumptions.

Degree Name

Earth and Planetary Sciences

Level of Degree

Doctoral

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Brearley, Adrian

Second Committee Member

Papike, Jim

Third Committee Member

Williams, Mike

Language

English

Keywords

Tectonometamorphic, Proterozoic, Taos

Document Type

Dissertation

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