21 Advancements in the Proterozoic tectonics of western North America

This session highlights recent developments in understanding of the Proterozoic tectonic record of western Laurentia, particularly its relation to critical mineral resources. Insights derived from all disciplines and approaches are welcome.

Tuesday, 19 May 2026
9:00 AM - 12:00 PM
Alvarado F

Tectonic Setting and Preliminary U-Pb Garnet Dating of Lithium-Cesium-Tantalum Felsic Pegmatites in the Northern Colorado Front Range

Paper number: 21-1
Program: Technical Sessions
Time: 9:05 AM – 9:25 AM

Authors

Abstract

The area around the Hyatt Mine, located about 5 km north of the Big Thompson Canyon in the Proterozoic rocks of the northern Colorado Front Range, contains many examples of beryl- and tourmaline-bearing felsic pegmatites. The Hyatt Mine was mined for its beryl, as a source of Be, mainly in the 1930’s through 1940’s. Main mineralogy of these pegmatites include quartz, alkali feldspar (albite to K-feldspar), and muscovite. In addition to the beryl and tourmaline, garnet too is a common accessory mineral, but trace quantities of various Li, Nb, and Ta-bearing minerals can be found. These pegmatites are typically classified as lithium-cesium-tantalum pegmatites, which are thought to originate from the last, highly fractionated melts of peraluminous granitoids. Three such granitoids are found in the area, including the Big Thompson Canyon Tonalite Suite, with an intrusive age of c. 1736 Ma, the Northern Front Range Granodiorite, with an intrusive age of c. 1731 Ma, and various granitic intrusions part of the c. 1450-1350 Ma Silver Plume Granite. Based on field relationships, Marget Fuller Boos, who mapped the pegmatites in the 1950’s, believed the pegmatites were related to the Big Thompson Canyon Tonalite suite. The pegmatites are hosted by sillimanite-grade schists and quartzite, which were deposited in an extensional back-arc basin at c. 1760 Ma. The pegmatites appear to be affected by the first deformation phase (c. 1725 Ma), but it is less equivocal that the pegmatites are affected by later folding of unknown age. To better constrain the timing of pegmatite emplacement and their possible tectonic setting and origins, garnets from the pegmatites were U-Pb dated via laser ablation-inductively coupled plasma-mass spectrometry. Data is highly scattered due to high common Pb content, but the three samples with reasonably well-defined chords on Tera-Wasserburg plots have lower-intercepts of 1909 +/- 353 Ma, 1643 +/- 143 Ma, and 1566 +/- 41 Ma, which are interpreted to be estimates of magmatic age. Given the scatter, it is impossible to attribute their origin to any of the three peraluminous granitic suites with confidence, but the Big Thompson Canyon Tonalite Suite and Northern Front Range Granodiorite, both subduction related plutonic suites, currently appear to be the most likely source based on this preliminary garnet U-Pb data.

Geological Society of America Abstracts with Programs. Vol. 58, No. 4

doi: 10.1130/abs/2026RM-410556

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