Rocky Mountain (53rd) and South-Central (35th) Sections, GSA, Joint Annual Meeting (April 29–May 2, 2001)

Paper No. 0
Presentation Time: 2:00 PM

SYNCHRONOUS SHEARING AND MAGMATISM AT 1430 MA AND SUBSEQUENT MAGMATISM AT 1410 MA IN THE COLORADO PROVINCE


STEIN, Holly J., AIRIE Program, Department of Earth Rscs, Colorado State University, Fort Collins, CO 80523-1482 and SIMS, Paul K., US Geol Survey, MS 905, PO Box 25046, Denver, CO 80225-0046, hstein@cnr.colostate.edu

We employ Re-Os dating of molybdenite in field context to discern the timing of Mesoproterozoic magmatism and its relation to intracratonic NE-trending shear zones in the Colorado Province. The data in this study demonstrate that the Re-Os chronometer is robust, enduring subsequent Cretaceous-Tertiary magmatism. Molybdenite samples hosted in granites, pegmatites, felsic dikes, and shear zones were analyzed from the Climax and Empire areas. An isochron age of 1430.3 ± 2.3 Ma agrees with the weighted average 1433.5 ± 2.9 Ma model age for the six molybdenites. The isochronous behavior of the different molybdenite occurrences supports syndeformational magmatism. At Empire, a subsequent episode of magmatism is recorded by molybdenite deposition at 1341 ± 5 Ma. To the south in the Transition Zone (TZ), which divides two major terranes (Yavapai and Mazatzal), the weighted mean model age for two molybdenites is 1409.8 ± 3.4 Ma. This documents a younger episode of Mesoproterozoic magmatism in the TZ. These two magmatic events (~1430 and ~1410 Ma) determined by Re-Os molybdenite dating are supported by two age populations (~1435-1430 and ~1410 Ma) for felsic xenoliths in the Colorado Plateau determined by U-Pb zircon dating.