Variation of illite/muscovite super(40)Ar/ super(39)Ar age spectra during progressive low-grade metamorphism: an example from the US Cordillera

• Published in: Contributions to mineralogy and petrology Vol. 164; no. 3; pp. 521 - 536
• Main Authors: Verdel, Charles, Pluijm, Ben A, Niemi, Nathan
• Format: Journal Article
• Language: English
• Published: 01.09.2012
• Subjects: USA, California; Germany, Berlin; USA, Nevada
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-012-0751-7

super(40)Ar/ super(39)Ar step-heating data were collected from micron to submicron grain-sizes of correlative illite- and muscovite-rich Cambrian pelitic rocks from the western United States that range in metamorphic grade from the shallow diagenetic zone (zeolite facies) to the epizone (greenschist facies). With increasing metamorphic grade, maximum ages from super(40)Ar/ super(39)Ar release spectra decrease, as do total gas ages and retention ages. Previous studies have explained similar results as arising dominantly or entirely from the dissolution of detrital muscovite and precipitation/recrystallization of neo-formed illite. While recognizing the importance of these processes in evaluating our results, we suggest that the inverse correlation between apparent age and metamorphic grade is controlled, primarily, by thermally activated volume diffusion, analogous to the decrease in apparent ages with depth observed for many thermochronometers in borehole experiments. Our results suggest that complete resetting of the illite/muscovite Ar thermochronometer occurs between the high anchizone and epizone, or at roughly 300 degree C. This empirical result is in agreement with previous calculations based on muscovite diffusion parameters, which indicate that muscovite grains with radii of 0.05-2 mu m should have closure temperatures between 250 and 350 degree C. At high anchizone conditions, we observe a reversal in the age/grain-size relationship (the finest grain-size produces the oldest apparent age), which may mark the stage in prograde subgreenschist facies metamorphism of pelitic rocks at which neo-formed illite/muscovite crystallites typically surpass the size of detrital muscovite grains. It is also approximately the stage at which neo-formed illite/muscovite crystallites develop sufficient Ar retentivity to produce geologically meaningful super(40)Ar/ super(39)Ar ages. Results from our sampling transect of Cambrian strata establish a framework for interpreting illite/muscovite super(40)Ar/ super(39)Ar age spectra at different stages of low-grade metamorphism and also illuminate the transformation of illite to muscovite. At Frenchman Mtn., NV, where the Cambrian Bright Angel Formation is at zeolite facies conditions, illite/muscovite super(40)Ar/ super(39)Ar data suggest a detrital muscovite component with an apparent age greater than or equal to 967 Ma. The correlative Carrara Fm. is at anchizone conditions in the Panamint and Resting Spring Ranges of eastern California, and in these locations, illite/muscovite super(40)Ar/ super(39)Ar data suggest an early Permian episode of subgreenschist facies metamorphism. The same type of data from equivalent strata at epizone conditions (greenschist facies) in the footwall of the Bullfrog/Fluorspar Canyon detachment in southern Nevada reveals a period of slow-to-moderate Late Cretaceous cooling.