Cordilleran Section - 97th Annual Meeting, and Pacific Section, American Association of Petroleum Geologists (April 9-11, 2001)

Paper No. 0
Presentation Time: 8:10 AM


GRASSE, Scott W.1, BARTLEY, John1 and GLAZNER, Allen F.2, (1)Department of Geology and Geophysics, Univ of Utah, 135 S. 1460 E., Rm 719, Salt Lake City, UT 84112, (2)Department of Geological Sciences, Univ of North Carolina at Chapel Hill, CB# 3315, Mitchell Hall, Chapel Hill, NC 27599-3315,

The North Mountain pluton (NMP) has characteristics widely thought to typify plutons emplaced as hot Stokes diapirs. In the hot Stokes diapir model, space in the crust is made for buoyantly ascending granitic magma by ductile flow of thermally softened (by the diapir) wall rock down the margins of the intruding pluton. Field observations expected for a pluton emplaced as hot Stokes diapir include a domal shape, a flow channel consisting of highly strained rocks surrounding the pluton, and concentric foliation in the margin of the pluton. The NMP is equant in plan view, and is surrounded by a thin (~50 m) screen of schist and gneiss that separates the NMP from the overlying Lookout Peak granodiorite. All contacts and fabrics are broadly concordant. Field mapping and structural analyses have yielded the following. 1) Screen rocks contain lit-par-lit injections of both NMP and the Lookout Peak granodiorite. LP dikes form boudins within a fabric crosscut by NMP dikes. Dikes of NMP also contain xenoliths of the screen and Lookout Peak. 2) The Lookout Peak granodiorite nearest the screen contact contains a strong magmatic foliation. A high enclave abundance in this lower zone suggests it represents the floor of the host that we interpret to have been domed during NMP emplacement. The presence of a submagmatic to solid-state fabric within the outer 100 m of the NMP, and ductile shear bands within and near the screen, accommodated minor amounts of late-stage doming. The observations lead to the interpretation that a preexisting schist guided lit-par-lit injections of both the NMP and its host, and that the NMP was intruded initially as a sill that later domed the overlying host rock. The NMP lacks any evidence of a flow channel, and has a domed, rather than intrusive contact, suggesting a laccolith-type emplacement mechanism.