The Mt. Stuart Batholith: A Test Case for the Application of Igneous Barometry to Paleomagnetic Studies of Granitic Batholiths
Ague, J J (1), Brandon, M T (1)
(1) Department of Geology and Geophysics, Yale University, P.O. Box 208109, New Haven, CT 06520-8109 United States
Abstract:
Granitic plutons provide an ideal target for paleomagnetic studies because they commonly contain high coercivity magnetic minerals, their homogeneity and coarse grain size makes it easy to identify areas that were deformed or altered after crystallization, and their slow cooling tends to reduce paleomagnetic "noise" relatedto secular variation. What is commonly missing, however, is a reliable estimate of paleohorizontal. The Cretaceous Mt. Stuart batholith (MSB), Washington Cascades, provides an ideal test case for application of igneous barometry to recovering paleohorizontal in granitic batholiths. Our studies were motivated by the discovery of Beck and coworkers in the 1970's and 80's of anomalously low paleomagnetic inclinations for the MSB, which were the first indication of young northward transport of Baja BC. Beck and coworkers considered both large-scale northward transport (3000 km or more) and regional tilting as causes for the anomalous inclinations, and ruled out large tilts on the basis of extant geological relations. Nonetheless, the tilt history of the MSB became a key question. Early arguments made to resolve this question were based on the dip of younger clastic strata, the distribution of cooling ages around the MSB, the predicted thermochronologic effects of tilting and differential exhumation, and the distribution of metamorphic pressures (P) in the surrounding country rocks.
We favored an approach using aluminum-in-hornblende-barometry (AH)because it allowed direct imaging of equilibration P within the MSB. Our preliminary results from the Peninsular Ranges batholith, California, and the MSB indicated systematic regional tilts and supported the soundness of this approach. This application, however, was not without problems: 1) the governing reactions for the barometer were unknown; 2) laboratory calibrations of the barometer differed and the effects of T, fluid composition, and magma composition were not well studied; 3) much of the MSB lacks the critical mineral assemblage needed for barometry; and 4) the NE lobe of the MSB was apparently altered by hydrothermal fluids. Problems 3) and 4) were addressed by careful field sampling and laboratory analysis. Our best-fit tilt results for the MSB were consistent with shallowly-dipping, planar isobaric surfaces; residuals to the fit were at the error level of the barometer and showed no systematic variation across the batholith. Furthermore, when restored, the characteristic paleomagnetic inclinations were not significantly different from other high quality paleomagnetic studies in Baja BC. Problems 1 and 2 were addressed by Ague (1997) which provided a thermodynamically-based AH reaction that can be used to directly estimate equilibration P. That work suggests that trends in regional P determined using the empirical AH calibrations are robust, but that the absolute values of the P estimates may need to be adjusted slightly. Our study demonstrates the potential for a happy marriage between paleomagnetics and igneous petrology.
