SIGNIFICANCE OF LATE MIOCENE BASALT DIKE INTRUSION AT YUCCA MOUNTAIN

One of the more contentious - and still unresolved- debates over geologic hazard at Yucca Mountain involves basaltic intrusion and volcanism at or near the mountain. Arguments center mainly on recurrence intervals and whether basaltic magmatism in the subjacent mantle is growing or declining, as well as physical effects of intrusion within a repository. A large part of uncertainty hinges on the small population of late Miocene to Pleistocene basalt intrusions near Yucca Mountain; only one basalt dike swarm is known at Yucca Mountain. The dikes comprise a swarm about 500 m wide and a kilometer long that strikes N15°-20°W across the northern end of Jet Ridge, the hanging wall block of the Solitario Canyon fault. The dikes are exposed over a vertical section of almost 150 meters. Individual dikes are about 30 cm wide. The dikes have a vesicular core zone up to 25 cm thick. Generally smooth, sharp contacts and sparse xenoliths indicate non-violent intrusion exerting sufficient stress to propagate and expand within a local fracture network. At least one of the dikes vented resulting in a small accumulation of well-preserved local spatter. The dikes are hyperthene-normative trachybasalt, compositionally similar to other local late to post-Miocene basalt. The Ar/Ar age of the dikes ranges from 11.32 to 11.55Ma. The dikes are probably related to the poorly preserved basalt intruded in the nearby Solitario Canyon fault and which has a K/Ar date of around 10Ma. The physical features of the dikes indicate mild single-event intrusion and venting, in marked contrast to the explosive devolatization predicted in published Yucca Mountain intrusion models. This basalt study was not included in the Yucca Mountain Final Report of the Igneous Consequences Peer Review Panel of 2003. The dike data can help resolve some uncertainties raised in the Igneous Consequences report, and the ages provide a baseline for erosion rate at Yucca Mountain.