253-5 U-Pb Zircon Dating of Granitic Pegmatites in the Crystal Mountain District in the Northern Colorado Front Range

Abstract:

The Crystal Mountain pegmatite district, located around the Big Thompson and Buckhorn Canyons in the Proterozoic rocks of the northern Colorado Front Range, typically contains quartz, feldspar, muscovite, tourmaline, garnet, beryl, and rare-metal trace accessory minerals. The pegmatites are classified as lithium-cesium-tantalum pegmatites, which are thought to be formed from the last fractionated melts of peraluminous granitic plutons. The nearby c.1.7 Ga Big Thompson Canyon tonalite, the c.1.7 Ga Northern Front Range granodiorite, and the c.1.4 Ga Silver Plume granite all have this composition, but no definitive evidence links the pegmatites to any of these plutonic suites or other magma sources.

Four samples of zircons from pegmatites across the district were U-Pb dated via laser ablation-inductively coupled plasma-mass spectroscopy. Two samples from and near the Storm Mountain pegmatite produced highly discordant data with a range of dates. From one sample, the oldest cluster of dates produce a Pb-207/Pb-206 weighted mean of 1733+/-16 Ma, whereas a chord fitted through the youngest dates produce an upper intercept of 1452+/-24 Ma. A second sample collected directly from the Storm Mountain pegmatite contains an old cluster of dates with a Pb-207/Pb-206 weighted mean of 1771+/-26 Ma, and a young cluster of dates with a Pb-207/Pb-206 weighted mean of 1346+/-22 Ma. A sample from the Bull Elk pegmatite also produced mostly discordant data, but a cluster of 11 concordant analyses yielded a concordia age of 1419+/-6 Ma. A sample from the Black Tourmaline pegmatite likely contains common lead that produces c.4.4 Ga dates. However, a handful of analyses appear to be free of this issue and results in an old cluster of dates with a Pb-207/Pb-206 weighted mean of 1733+/-29 Ma, and a young cluster of dates with a Pb-207/Pb-206 weighted mean of 1440+/-25 Ma.

The Bull Elk pegmatite is thought to have formed at c.1419 Ma, whereas the other pegmatites are thought to have formed at c.1730 Ma, but were affected by lead-loss at c.1350-1450 Ma. The c.1771 Ma age of the Storm Mountain pegmatite is problematic as it is older than the country rock protolith. Regardless, the ages demonstrate that the district was formed by multiple processes that spanned the Proterozoic history of the area.

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