Evaluating the Usefulness of Geochronometers in the Packsaddle Domain for Refining the Timing of Metamorphism Via High-Resolution Petrography, Enchanted Rock State Natural Area, Llano Uplift, Texas

Author

Haylee Vike, Stephen F. Austin State UniversityFollow

Date of Award

12-14-2024

Degree Type

Thesis

Degree Name

Master of Science - Geology

Department

Geology

First Advisor

Liane M. Stevens

Second Advisor

R. LaRell Nielson

Third Advisor

Zachariah Fleming

Fourth Advisor

Jenny Rashall

Abstract

The Llano uplift, central Texas, exhumes Mesoproterozoic rocks with a complex history of metamorphism, deformation, and magmatism. There are three recognized metamorphic events related to the Grenville orogeny (~1.3-0.9 Ga). The timing of these metamorphic events is not well resolved due to past methods, technology, and lack of published geologic context for many of the current dates. These current time constraints on metamorphic timing could be further refined through modern geochronologic and petrochronologic methods.

A majority of U-Pb zircon dates from the Valley Spring (~1366-1232 Ma), Coal Creek (~1326-1275 Ma), and Packsaddle (~1257-1215 Ma) domains are interpreted as crystallization ages for igneous protoliths, or for syntectonic (~1119-1116 Ma) and post-tectonic (~1098-1071 Ma) plutons (i.e., Town Mountain Granite), and most of these are upper intercept dates for multigrain zircon fractions. Very few dates have been interpreted to represent the timing of metamorphism and these dates do not fully distinguish between M₁ (~1147-1128 Ma) and M₂ (~1134-1102 Ma) events. Additionally, the metamorphic dates that have been interpreted are only found in abstracts and lack geologic context.

The Packsaddle domain only has two imprecise geochronologic constraints on metamorphism. For this study, ten samples were collected from the Packsaddle domain at Enchanted Rock State Natural Area. Polished thin sections were prepared for petrographic analysis and high-resolution scanning in PPL and XPL. The petrographic analysis will consist of identifying geochronometers (e.g., zircon or monazite) and describing their relation to metamorphic and deformation textures (e.g., foliations, leucosomes, porphyroblasts). Identification of geochronometers and their geologic context allow for determination of the best dating techniques and interpretation of the subsequent dates in context of the rock’s history. This work is done as the basis for future research and analysis of the existing time constraints via newer in situ geochronologic and petrochronologic methods.

Repository Citation

Vike, Haylee, "Evaluating the Usefulness of Geochronometers in the Packsaddle Domain for Refining the Timing of Metamorphism Via High-Resolution Petrography, Enchanted Rock State Natural Area, Llano Uplift, Texas" (2024). Electronic Theses and Dissertations. 586.
https://scholarworks.sfasu.edu/etds/586

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