EMPLACEMENT AND GROWTH OF RHYOLITIC SUBVOLCANIC INTRUSIONS OF THE BLACK HILLS IGNEOUS PROVINCE, SOUTH DAKOTA AND WYOMING
Many of the well exposed rhyolite sills examined for this study are directly connected to feeder dikes and show transitional relationships from dikes to sills. At a composite sill within the open-cut section of the Homestake mine, we identified at least 5 individually stacked magma sheets (sills) that connect separately to their own dikes. The magma sheets intruded horizontally into fine clastic units of the Deadwood Formation which are preserved as screens between sheets near the transition to dikes. Laterally away from the dikes the layers of sedimentary host rock are absent and the sill becomes a composite of the 5 sheets.
Within laccoliths, similar magma sheets can be seen on the outer edges of the plutons paralleling the adjacent host rocks. Inward, the laccoliths become more massive with little visible sheeting. At Bear Butte, the initial subhorizontal sheets were rotated to nearly vertical as new magma batches were added below. In this view, laccoliths are the result of incremental growth of downward-stacking magma batches, building sheeted plutons fed by multiple dikes. The earlier increments were emplaced into cooler wall rocks where they cooled below the solidus quickly and preserved sheeted relationships. Later increments intruded into progressively hotter environments and obscured the contacts between sheets giving it a massive appearance.