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The AAPG/Datapages Combined Publications Database
Earth Science Bulletin (WGA)
Abstract
U-Th-Pb Systematics of Precambrian Rocks in the Laramie Mountains, Wyoming
Abstract
Uranium, thorium and lead concentrations and the isotopic composition of whole-rock samples of granite from the Laramie Mountains, Wyoming, suggest intrusion of the granite no later than 2530 ±80 m.y. ago.
The uranium in surface samples is present in amounts that are insufficient to account for the observed lead isotopic composition. However, some core samples of heavily fractured rock show an extreme isotopic disequilibrium between 238U and 206Pb. Their uranium concentrations are generally far in excess (up to 60%) of average amounts required to support the measured lead-206. Radioactive disequilibrium measurements indicate that large amounts of uranium were gained by these fractured rocks during the last 150,000 years.
Lead data on K-feldspar separated from the rocks analyzed suggest that lead has been assimilated by these minerals since time of crystallization.
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