A GEOPHYSICAL STUDY OF THE PRECAMBRIAN BASEMENT IN CENTRAL MONTANA (MAGNETICS, GRAVITY, PETROLOGY)

A high-sensitivity magnetic anomaly map produced from total-intensity aeromagnetic data collected as part of the National Uranium Resource Evaluation (NURE) program was used as the main data base in a study of the Precambrian basement in central Montana (lat 46(DEGREES)-49(DEGREES)N, long 106(DEGREES)-112(DEGREES)W). Qualitative and quantitative analyses of this map, together with complementary analyses of available gravity data, provided an insight into the gross lithologic makeup and structural framework of the crystalline basement in central Montana. Owing to the limited amount of direct geologic information on the Precambrian basement in this area, the study relied on the method of analogy to derive interpretations. Computer modeling of a well-defined, representative magnetic anomaly pattern over a small portion of the study area yielded a physical model of the crystalline basement in this particular area. Comparisons of the physical characteristics of this model with those of the exposed basement terranes in the Superior Province of the Canadian Shield suggest that the granitoid batholith-felsic orthogneiss terrane is the best geologic model for the crystalline basement in this particular portion of the study area. Similar, though more irregular, magnetic anomaly patterns over most of the remainder of the study area suggest that granitoid plutonic rocks and felsic gneiss country rocks may also be the primary constituents of the crystalline basement here. A structural model, consistent with the pattern of deformation produced in a rigid-plastic solid when indented by a rigid die, is proposed to explain the irregular magnetic anomaly patterns over much of the study area. A deformational event akin to mechanical indentation may thus have affected the crystalline basement in central Montana, possibly during the Hudsonian orogeny in late-Early Proterozoic time. This event may have come about during the consolidation of individual microcontinents to form a composite craton in North America, as evidenced by the possible crustal roots of an ancient volcanic arc and marginal depositional basin in north-central Montana, which may mark a continental suture zone. Several major Phanerozoic surface structures and igneous centers, such as the Little Rocky Mountains, the Bowdoin Dome, the Bearpaw Mountains, and the Central Montana Platform overlie integral parts of the pattern of interpreted basement deformation in central Montana and thus may have been controlled by the subjacent interpreted basement structures