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This is a progress report on some preliminary 40Ar/39Ar incremental heating experiments on feldspar separates from four samples of the felsite unit, a complex silicic batholith that intrudes the overlying Franciscan Complex (Late Jurassic to Late Cretaceous) and underlies the Geysers Geothermal Field, northern California (Schriener and Suemnicht, 1981; Thompson, 1989,1991). The felsite unit is only found in the subsurface but it appears to be an elongate body whose axis trends northwest-southeast and whose surface is shallowest in the southeast part of the field (Figure 1). It ranges in composition from granite to granodiorite (Schriener & Suemnicht, 1981; Thompson, 1991).

The apparent coincidence of the heat flow anomaly within the Geysers field (Walters and Combs, 1989) with the distribution of felsite within and below the zone of steam production suggests that the felsite unit may be the primary source of heat. Presently available K-Ar ages (0.9 Ma to 2.7 Ma) suggest, however, that the felsite unit may be too old to be the primary source of heat for the present thermal activity. Resolution of this apparent paradox should be of interest for the purposes of both exploration and field management. If the felsite unit is young (<1 Ma), for example, then it should be hot wherever it is found. If the felsite unit is old (>1 Ma), on the other hand, then it may be relatively cold outside of the region of present production. The felsite unit also may be a complex body emplaced over a significant interval of time

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