Geologic map of the Bannack-Grayling area, Beaverhead County, Montana

It should be possible to infer the thermal state of the source terrane for granitic bodies, provided we have independent means to establish the chemical nature of this terrane. The chemical nature of the granitic rocks, including their degree of hydration, implies the solidus temperature. The concentration of the heat-producing radioactive elements in the granite (K, U, and Th) probably provides an upper estimate of their concentration in the source rock, which is an important thermal parameter. The depth and ambient temperature of the country rock into which the granite magma intruded provide useful boundary conditions for the thermal regime at the crustal level of anatexis. These constraints in turn form the bases for estimating the subcrustal thermal flux as well as the effective thermal interface for enhanced heat flow from below that resulted in anatexis. These inferences, in combination with other field-based parameters such as uplift rates and permissible time lapses for the geological events, permit realistic thermal modelling for the formation of granitic batholiths. The procedure is applied to the Late Cretaceous Pioneer and Boulder batholiths in southwestern Montana, U.S.A. The modelling results suggest that mantle upwelling, not subduction or thrust loading, caused anatexis. The isotopic chemistry of the granitic rocks rules out direct mixing of mantle magma, and field relations rule out crustal thinning as causes for partial melting.

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Upper Cretaceous-Paleocene Synorogenic Conglomerates of Southwestern Montana

Wilson¹

1970

Bulletin

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Geologic map of the Bannack-Grayling area, Beaverhead County, Montana

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Anisotropy of fractal dimension of normal faults in northern Rocky Mountains: Implications for the kinematics of Cenozoic extension and Yellowstone hotspot's thermal expansion

Anisotropy of fractal dimension of normal faults in northern Rocky Mountains: Implications for the kinematics of Cenozoic extension and Yellowstone hotspot's thermal expansion

Using granite to image the thermal state of the source terrane

Upper Cretaceous-Paleocene Synorogenic Conglomerates of Southwestern Montana

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