EMPLACEMENT AND GROWTH OF RHYOLITIC SUBVOLCANIC INTRUSIONS OF THE BLACK HILLS IGNEOUS PROVINCE, SOUTH DAKOTA AND WYOMING

An integrated field and petrographic study of rhyolitic intrusive bodies in the 58 to 50 Ma northern Black Hills Igneous Province indicates the occurrence of multiple magma sheets within sills and laccoliths directly fed by dike systems. The rhyolitic intrusive bodies form domed laccoliths (with exposures of up to ~2 km in diameter and ~600 m thick) and a great number of sills and dikes along with diatremes and breccia pipes. Most of the bodies were emplaced in Paleozoic clastic rocks at depths of 1 to 3 km, although some vented to the surface apparently forming maar-diatremes. Rhyolites from this study can be divided into two families based on their petrography: A) aphanitic rhyolite and B) quartz bearing porphyritic rhyolite. Quartz phenocrysts have diameters up to 3 mm and are fractured and somewhat rounded with a partial resorption rim suggesting a probable surge in the temperature of the magma prior to final phase of emplacement.

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.