Ion microprobe dating of zircons provides insight into the history of granitoid generation beneath Medicine Lake volcano (MLV), northern California. Part of the MLV geothermal system is hosted by a 319 ± 8 ka granodiorite intrusion that may be over 6 km in diameter. A sample of this intrusion was hydrothermally altered by 171 ka, indicating that the geothermal system was active by that time. Post-100-ka magmatism resulted in crustal melting, evidence for which is found in granitoid xenoliths erupted with numerous lava flows. Zircons within many of these xenoliths, analyzed by the ion microprobe (230Th techniques) have ages less than 20,000 years, and as young as the age of eruption in the very latest Holocene. Sustained crustal input of basaltic magma generates the heat to melt crust, and acts as a continuing heat source to the shallow geothermal system.

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