USING THE TI-IN-BIOTITE GEOTHERMOMETER TO INVESTIGATE THE THERMAL STRUCTURE OF THE CRUST DURING THE SEVIER OROGENY

We applied the Ti-in-biotite thermometer (Henry et al., 2005, Am Min) to garnet-zone and lower grade rocks of the Pequop Mts., northeast Nevada. Our goals were to better understand the temporal and causal relationship between metamorphism and ductile deformation during the Sevier orogeny and to test the utility of this geothermometer in medium to low-grade metamorphic rocks. The field area is a southeast dipping homocline of metamophosed Cambrian rocks. Metamorphic grade increases down-section from biotite-zone rocks of the Dunderberg Shale to garnet-zone rocks of the lower Killian Springs Formation and upper Prospect Mt. Quartzite. Garnet and biotite porphyroblasts are syn- to pre-kinematic to a dominant layer-parallel foliation, which we interpret to have formed during crustal extension.

Five samples from among three garnet-zone schists in the upper Prospect Mt. Quartzite and lower Killian Springs Formation yield a mean peak T of 607 +/-21ºC (95% confidence). Scatter of results is less within a schist than among different schists, suggesting unaccounted-for factors affecting the geothermometer related to rock composition. Seven other samples are spread out across 1.6 km within a single Dunderberg Shale schist, 600 m higher in the section. These yield a mean peak T of 568+/-5ºC, but to account for variability due to rock composition we assign a higher uncertainty of +/-21ºC, the same as for the garnet-zone rocks. The difference in temperatures between the biotite- and garnet-zone rocks is 39+/-30ºC. This would yield a geothermal gradient during metamorphism of 65+/-50ºC/km if there were no deformation following peak metamorphism. Whereas the lowest part of this confidence interval is permissible for orogenic metamorphism, the rest is not. Therefore, there was likely substantial thinning perpendicular to compositional layering following peak metamorphism, consistent with the dominant foliation forming by crustal extension following peak metamorphism and possibly triggered by it.

While our geothermal gradient result is reasonable, refinement of the Ti-in-biotite thermometer is needed to fix the dependence on rock composition. Moreover, our temperatures from the garnet zone are too high: more than high enough to grow staurolite and 55ºC higher than results obtained from garnet-biotite thermometry.