Nature of the Boulder Batholith of Montana
The Boulder batholith formed as a thin and shallow mass that spread over a floor of premagmatic rocks and that was covered for the most part only by its own volcanic ejecta. The original roof consists of volcanic rocks of the same age and composition as the intrusive rocks beneath it. Eastward, the volcanic overflow from the batholith lies upon prebatholithic sedimentary rocks and may have been pushed eastward over them. The westernmost part of the batholith, by contrast, is roofed by prebatholithic sedimentary rocks, which are remnants of an eastward-thrust mass. All premagmatic rocks dip beneath both the north and south ends of the batholith, which lies atop the downtilted stratigraphic piles. The ends of the batholith are scalloped by wide cusps, which mark lobate advances of bathtub-shaped plutons. The plunges of the axes of the narrow, tightly squeezed anticlinal horns between the cusps flatten into the batholith, and the floors of the cusps presumably flatten correspondingly. The straight east margin of the batholith may be controlled by a single, flanking fold of the floor rocks, rolled down in front of the spreading plutonic mass. The batholith was emplaced during a small part of the period of regional eastward thrust faulting of miogeoclinal and foreland-basin strata over the cratonic crystalline basement. The spreading batholith produced westward, reversed overturning in structures to the west, and to the east and south, it caused outward spreading in an arcuate bulge broadly concentric to itself. The bottom of the Boulder batholith is not exposed. Analogy with complexes of greater age and deeper erosion elsewhere suggests that a migmatite terrain formed where feeder magma rose through the crust and was in part enveloped by older rocks flowing back under the spreading batholith. Away from feeder zones, the outer part of the batholith presumably is underlain by sedimentary rocks whose metamorphism decreases downward beneath the granite.