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

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.