Fractures containing juvenile magmatic pyroclasts were encountered during drilling into a 600-year-old feeder dike beneath the Inyo Domes chain, California. The Inyo Domes consist of a north-south trending, 10-km-long chain of domes, rhyolitic tuff rings, and phreatic craters. Boreholes were cored through the 51-m-diameter conduit of Obsidian Dome, the largest of the Inyo Domes, and through an unvented portion of the intrusion (dike) 1 km to the south. Pyroclast-bearing fractures were intersected in both holes: (1) 7- to 40-cm-thick fractures in welded basaltic scoria and quartz monzonite country rock are adjacent to the conduit at depths of 400--411 m and 492--533 m; they contain gray, clastic deposits, which show truncated cross bedding and convolute bedding; (2) adjacent to the dike, massive fracture fillings occur at depths of 289--302 m (129 m east of the dike) and 366--384 m (95--87 m east of the dike).

The fracture fillings consist of mineral clasts derived from the quartz monzonite, quartz monozonitic and basaltic lithic clasts, and juvenile glass pyroclasts. Angular mineral components are present in the same ratio as in the surrounding quartz monzonite country rock. Juvenile glassy and hyalocrystalline pyroclasts make up from less than 1% up to 22% of the deposits. They consist of blocky obsidian clasts, equant, blocky glass pyroclasts with vesicularities of 0--30%, and small pumices with vesicularities of 30--40%. Intrusive pyroclasts differ from erupted pyroclasts in their generally lower vesicularity, higher crystallinity, and the presence of solution pits and clay coatings indicative of prolonged contact with water. The presence, orientation, and texture of fracture fillings strongly resemble those of propped man-made hydrofractures. We interpret these fractures as naturally occurring hydrofractures. The apparently horizontal fracture orientations may have been controlled by perturbations of maximum principal stress by the dikes or by preexisting sheet fractures in the quartz monzonite country rock. Assumption of elastic moduli and fracturing properties for the Sierran basement rock allows calculation of fluid over-pressures 5 to 9 MPa in excess of overburden stress. These overepressures are consistent with either vapor exsolution from decompressed magma or rapid heating of groundwater. However, the textural and chemical similarity of the pyroclasts to phreatomagmatic tephra that appears late in the explosive eruption sequece suggests that heating of groundwater by the dike/conduit caused the fracturing. Such fracturing around volcanic conduits may play an important role in the development of hydrothermal circulation. ¿ American Geophysical Union 1988