DIKE EMPLACEMENT AND GEOCHEMISTRY OF A HIGH-ALUMINA OLIVINE THOLEIITE, BLIZZARD GAP, OREGON

One of the most striking features of the NW Great Basin region is the widespread distribution of late Miocene-to-Recent high-alumina olivine tholeiite basalt (HAOT). Few feeder dikes, however, have been described for this significant group of lavas. Here we describe the field characteristics of an HAOT dike at Blizzard Gap, Oregon and unravel its petrogenetic history through the examination of the major- and trace-element geochemistry. The dike strikes N64°W, which is parallel to the overall strike of most normal faults in the region. However, the dike is also offset along a NE-trending normal fault suggesting a change in the orientation of maximum tensional stress after dike emplacement. The dike outcrop is ~ 0.5 km long, well-exposed, with large outcrops varying from 20 m in width and 30 m in height to rubble on the earth's surface. Truncated at the base of Blizzard Gap summit, the dike crosses HWY 140 as it winds its way SE to the summit. Here, it feeds the uppermost HAOT lava on the adjacent plateau surface. Two main joint patterns delineated, with each differing in joint direction and spacing. Cooling joints are spaced ~19 cm apart, perpendicular to the dike margins, whereas platy joints are spaced ~3 cm apart, parallel to the dike margins due to flow-alignment and subsequent shear. The behavior of incompatible elements demonstrates that the dike magma had been modified by fractional crystallization, and the specific behavior of CaO, Sr and Al2O3 indicates that plagioclase was a crystallizing phase. Crystal fractionation, however, was not the sole petrologic process responsible for modifying the basalt chemistry. Crustal contamination was also a controlling factor, as indicated by the relative variability of incompatible element pairs like Ba and Zr.