RAPID EMPLACEMENT AND GROWTH OF A RHYOLITIC INTRUSION: THE WOODVILLE HILL LACCOLITH, BLACK HILLS IGNEOUS PROVINCE, SOUTH DAKOTA

The Woodville Hill laccolith (WHL) is a well exposed (3.0 km x 1.7 km) rhyolite intrusion located in the 58 to 50 Ma northern Black Hills Igneous Province. Magma intruded into and deformed flat-lying shale units of the middle portion of the Cambrian Deadwood Formation. The rhyolite (~600 m thick) consists of three well defined and easily identifiable layers based on phenocryst content: 1) a lower white to cream garnet-bearing zone up to 300 m thick, 2) a middle cream to gray biotite bearing zone ~200 m thick, and 3) an upper white phenocryst-free zone ~100 m thick. Boundaries between layers are highly gradational and produce a mixed zone up to 80 m thick between the lower and middle zones. Garnet phenocrysts are of igneous origin and highly fractured, indicating a lower crustal generation. Presence of small miarolitic cavities indicate a shallow emplacement depth of ~1 km or less. Columnar joints found near wall rock exposures are well developed across zone boundaries.

The geometry of the intrusion is one of an elliptically shaped laccolith with its long axis trending ~N25W and parallels the trend of surrounding quartz-bearing dikes. The flat floor of the laccolith is exposed on the west side of the intrusion which also has a convex shape based on map patterns of igneous layers. On the east side, the intrusion punched upward through sedimentary units and the three internal layers here are subhorizontal, thus creating an overall trapdoor geometry. On the northeast side, the intrusion is in contact with an older quartz-bearing rhyolite dike system and shows vertical flow foliations against the dike.

The structural, field, and petrographic data indicate that the laccolith grew vertically by downward stacking of subhorizontal magma sheets of changing phenocryst composition which represent sill emplacements from a presumed unexposed dike system parallel to the long axis of the laccolith. Lateral emplacement of magma to the northeast was obstructed by older quartz-bearing dikes which caused the east side to punch vertically through country rocks instead of bending them like on the east side. The lack of sharp internal boundaries between layers and the presence of fractured igneous garnets indicate a rapid ascent from a lower crustal differentially zoned magma chamber and rapid emplacement of sill batches that inflated into a laccolith.