Andesitic sills intruded a large region of the Rocky Mountain foreland of central western Montana during Late Cretaceous thrusting and folding. They provide an important time marker for deformation and thermal evolution in this part of the foreland basin. The sills predate emplacement of the Lewis-Eldorado-Hoadley (LEH) thrust plate over the Montana disturbed belt and inversion of the northeastern part of the Proterozoic Belt basin into the Purcell anticlinorium. The sills occur at high stratigraphic levels in the LEH plate on the south limb of the Purcell anticlinorium and occupy stacked thrust sheets in the LEH footwall on the noah limb of the Purcell anticlinorium. Biotite separated from a pyroxene-diorite sill in the hanging wall of the Lewis-Eldorado thrust south of the Purcell anticlinorium near Gold Creek, Montana, yielded a 39Ar/40Ar plateau age of 76±1.2 Ma (2s). The sill is part of a set of sills and dikes that intruded strata as young as the Campanian Golden Spike Formation. The sample site was never deeply buried and the date appears to record the crystallization age of the sill. North of the Purcell anticlinorium, a pyroxene-andesite sill intruded into Upper Cretaceous rocks near Gibson Reservoir yielded a biotite 39Ar/40Ar age of 58.8±1.5 Ma (2s). This site was tectonically buried beneath a stack of thrust sheets in the footwall of the LEH plate. We suggest that the age records cooling of the sill upon erosional denudation. Similar major and trace element geochemistry suggests that the sills may be genetically related. Thus, thrust loading of the foreland basin by the Lewis-Eldorado plate likely began after emplacement of the sills at 76 Ma.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah