3 Microanalyses, Macro Implications: Using Microscale Analyses to Decipher System-to-Regional Scale Processes

Monday, 25 March 2019
8:00 AM - 12:00 PM
Kings Room

FORMATION OF TOPAZ-ENRICHED GNEISS IN THE EAST-CENTRAL COLORADO FRONT RANGE VIA CRYSTALLIZATION OF MESOPROTEROZOIC HALOGEN-RICH GRANITIC MAGMAS

Paper number: 3-8
Program: Theme Sessions
Time: 10:15 AM – 10:35 AM

Authors

Abstract

Metamorphic rocks enriched in economically valuable minerals occur in all mountain belts worldwide, yet their processes of formation are often incompletely understood. Topaz [Al2SiO4(F,OH)2] is a naturally occurring nesosilicate of great economic and industrial value, and is most commonly found in silica-rich igneous rocks. However, conspicuous 30-m-thick bands of topaz-, sillimanite-, rutile-, and quartz-rich (TSRQ) metamorphic gneiss occur in the Evergreen region of the east-central Front Range, Colorado, immediately adjacent to “normal” amphibolite and pelitic schist horizons. Bulk-rock geochemistry and isocon analysis shows that (e.g. Na, K, Mg, Ca) have been leached from these TSRQ rocks when compared to adjacent F-poor lithologies, indicating that the TSRQ gneiss must have interacted at some point with halogen-rich fluids. In addition, thermobarometry conducted via conventional and phase diagram-based techniques suggests pressure–temperature conditions of equilibration of ~700 °C and ~6.0 kbar, defining isotherms consistent with metamorphism in the middle continental crust during active mountain building. In-situ U–Pb dating of monazite within TSRQ units produced concordant ages mostly between representing the timing of F-rich fluid metasomatism. Together, these data are interpreted as recording metasomatic alteration in the middle crust by F-rich fluids released due to crystallization of nearby plutons, such as the Boulder Creek granodiorite at c. 1.65 Ga. As such, these topaz-rich horizons represent in-situ transformation of an older metamorphic assemblage, and not the metamorphosed products of weathered, pre-orogenic high-F tuffs or lava flows that were subsequently buried, as suggested by some previous studies. These results have far-reaching implications for constraining the behavior of halogens in the geological environment.

Geological Society of America Abstracts with Programs. Vol. 51, No. 2

doi: 10.1130/abs/2019SC-327202

© Copyright 2019 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.