We have developed a new quantitative method to estimate paleohorizontal in granitic plutons using the aluminum-in-hornblende (AH) barometer. The method is used to correct previously published paleomagnetic data from the 93–96 Ma Mount Stuart batholith of the Cascades Mountains, Washington State, for the effects of postemplacement tilting. AH barometry was done on 46 samples from the batholith using the compositions of hornblende rims coexisting with the full mineral assemblage required for pressure estimation. High-contrast back-scattered electron imaging was used to ensure that the analyzed hornblendes were not significantly affected by subsolidus alteration.
The success of the AH barometry is indicated by two observations. First, increases in the Al content of the hornblendes are governed almost entirely by a pressure-sensitive tschermak-type substitution. Second, amphibole-plagioclase thermometry indicates that assemblage equilibration occurred at or very near magmatic conditions (≈650°C) and that temperature has a negligible effect on our pressure estimates. AH barometry results indicate that the depth of crystallization across the batholith decreases systematically from ≈0.3 GPa in the northwest to ≈0.15 GPa in the southeast, consistent with independent barometry for the contact aureole of the batholith and regional structural and stratigraphic relations. Using a best-fit planar-tilt model and bootstrap analysis of uncertainties, we estimate that the paleohorizontal plane has a strike of 43° ± 30.4° and dip of 7° ± 2.0° southeast (±95% confidence).
Our estimated paleohorizontal allows us to restore the paleomagnetic data of Beck et al. (1981) and to estimate the original paleolatitude of the Mount Stuart batholith. Beck et al. found that the southern part of the batholith yielded a number of sites with a well-defined high-coercivity remanence. The carrier of this remanence was not resolved, but the following four lines of evidence strongly suggest that the published directions were acquired shortly after emplacement of the batholith. (1) The “stable” sites all came from the shallowest and most rapidly cooled portions of the batholith as indicated by our AH results and concordant K/Ar ages for hornblende/biotite pairs. (2) The high coercivity component was always normal in polarity, which is consistent with emplacement of the Mount Stuart batholith at the end of the Cretaceous long normal. (3) Sites from the batholith and the contact aureole gave similar directions. (4) The directions show no indication of tilt-related smearing. After restoration, the paleomagnetic data indicate 42° ± 11° clockwise rotation and 3100 ± 600 km of northward offset (±95% confidence). This result verifies Beck et al.'s original interpretation that the Mount Stuart batholith originated at the paleolatitude of northern Mexico.