Clay fractions for K Ar dating, separated from Franciscan-Assemblage (late Mesozoic) metagraywacke and argillite from The Greysers Coring Project corehole SB-15-D, and a "background" outcrop consist of illite. mixed-layer illite/smectite. mixed-layer chlorite/smectite, and chlorite with minor calcite and quartz. Except for the chlorite/smectite, these phases were formed during both subduction-related Franciscan regional metamorphism and late Cenozoic (post-1.1 Ma) hydrothermal alteration and mineralization related to The Geysers hydrothermal system. The chlorite/smectite is exclusively hydrothermal in origin. In spite of careful efforts to physically separate the metamorphic and layer silicates: prior to analysis. all samples contained variable contributions from both sources. The proportion of illite plus illite/smectite clays in the < 5 m fractions of the samples ranges widely from about 41 to 97 wt.%. K2O contents of the dated clay fractions (less than or equal to 3 mum) also ranged widely, from 1.60 to 8.0 wt.%. Despite having been heated to at least 300 C. matrix illite and illite;smectite do not show depth- and temperature-dependent trends of downward-increasing illite interlayers as is the case in many old sedimentary basins (ef Texas Gulf Coast and North Sea oil fields) and other geothermal systems (e.g. Valles caldera and Salton Sea). Fourteen mixed-layer illite. smectites of different size fractions (0.1-0.35. 0.35 1 and 1-3 mum) from the SB-15-D meta-graywackes and argillites land a nearby "background" sample of the same rock type) yielded K-Ar dates ranging widely from 105.5 Ma (the outcrop sample) to 1.5 Ma. Although there is: a general decrease in apparent ape with depth, most of the clays yielded middle Tertiary dates (35.4-19.4 Ma). The outcrop age is consistent with published data for Franciscan regional metamorphism. The youngest of the core dates, 2.3 and 1.5 Ma, are slightly older than the earliest Geysers felsite intrusions (1.1 Ma), and substantially older than vein aduluria from the corehole (0.57 Ma). Thus, even these youngest illite/smectite dates for the SB-15-D cores likely reflect minor contamination from metamorphic phases. These K-Ar dates do, however, indicate that pre-existing detrital and metamorphic clays have not been totally degassed even though the host rocks have resided in a high-temperature geothermal system for more than a million years. We conclude that the partially reset metamorphic-hydrothermal illite/smectite dates probably reflect the effects of low host-rock permeabilities. of sluggish reaction kinetics of the metamorphic layer silicates, and of "fines migration" of Franciscan Complex clays, along fractures, induced by hydrothermal fluid circulation. Published by Elsevier Science Ltd on behalf of CNR.