Textural and Thermal History of Partial Melting in Tonalitic Wallrock at the Margin of a Basalt Dike, Wallowa Mountains, Oregon

Columbia River Basalt Group dikes invade biotite–hornblende tonalite to granodiorite rocks of the Wallowa Mountains. Most dikes are strongly quenched against wallrock, but rare dike segments have preserved zones of partial melt in adjacent wallrock and provide an opportunity to examine shallow crustal melting. At Maxwell Lake, the 4 m thick wallrock partial melt zone contains as much as 47 vol. % melt (glass plus quench crystals) around mineral reaction sites and along quartz–feldspar boundaries. Bulk compositional data indicate that melting took place under closed conditions (excepting volatiles). With progressive melting, hornblende, biotite, and orthoclase were consumed but plagioclase, quartz, and magnetite persisted in the restite. Clinopyroxene, orthopyroxene, plagioclase, and Fe–Ti oxides were produced during dehydration-melting reactions involving hornblende and biotite. Reacting phases became more heterogeneous with progressive melting; crystallizing phases were relatively homogeneous. Progressive melting produced an early clear glass, followed by light (high-K) and dark (high-Ca) brown glass domains overprinted by devitrification. Melts were metaluminous and granitic to granodioritic. Thermal modeling of the partial melt zone suggests that melting took place over a period of about 4 years. Thus, rare dikes with melted margins represent long-lived portions of the Columbia River Basalt dike system and may have sustained large flows.