DIKE-WALLROCK INTERACTIONS IN GRANITOIDS OF THE WALLOWA MOUNTAINS, NE OREGON

Hundreds of Columbia River Basalt Group dikes have invaded granitoids of the Wallowa batholith and satellite plutons, providing a rare opportunity to examine the nature and variety of dike-wallrock interactions. Subparallel dikes strike roughly north-south, are steeply dipping, and commonly follow local joints in granitoid wallrock. Dike-wallrock interactions may be one or more of the following types: (1) basalt strongly quenched against wallrock; (2) dikes with zones of quenched partial melt developed in wallrock; (3) dikes containing wallrock screens and/or whole to disaggregated wallrock xenoliths; and (4) dikes that have eroded their margins. Type 1 dikes are the most common in the Wallowas and are a few cm to nearly 30 m thick with quench zones of 3 to 20 cm thick at the dike margins. Wallrock immediately adjacent to these dikes shows no textural evidence of reaction. Type 2 dikes are not common in the Wallowas. The partial melt zones in wallrock range from a few cm to nearly 4 m thick and may contain up to 50% quenched melt at the dike-wallrock margin. Xenoliths and screens in type 3 dikes range in size from a few cm to 6 m in diameter and occasionally have textural evidence of a few % partial melt. In type 4 dikes, coarse-grained basalt cross cuts the quench zone at dike margins. Zones of melting in wallrock, screens, and xenoliths are found at the contact with coarse basalt. The nature of the dike-wallrock contact (in addition to the textural nature of the dike itself) addresses the thermal history of the dike. For example, the presence of a partial melt zone correlates with dikes that were continuously or repeatedly active, as indicated by erosion of the quenched margin by coarser basalt. We believe that dikes with partially melted margins, rather than quenched margins, experienced high thermal flux and represent long-lived portions of the flood basalt feeder system.