Age and thermal history of the Geysers plutonic complex (felsite unit), Geysers geothermal field, California: a 40Ar/39Ar and U-Pb study

Sixty-nine ion microprobe spot analyses of zircons from four granite samples from the plutonic complex that underlies the Geysers geothermal field yield Pb-207/Pb-206 VS. U-238/Pb-206 concordia ages ranging from 1.13 +/- 0.04 Ma to 1.25 +/- 0.04 (la) Ma. The weighted mean of the U/Pb model ages is 1.18 +/- 0.03 Ma. The U-Pb ages coincide closely with 40Ar/39Ar ape spectrum plateau and `terminal' ages from coexisting K-feldspars and with the eruption ages of overlying volcanic rocks. The data indicate that the granite crystallized at 1.18 Ma and had cooled below 350 degrees C by similar to 0.9-1.0 Ma. Interpretation of the feldspar 40Ar/39Ar age data using multi-diffusion domain theory indicates that post-emplacement rapid cooling was succeeded either by slower cooling from 350 degrees to 300 degrees C between 1.0 and 0.4 Ma or transitory reheating to 300-350 degrees C at about 0.4-0.6 Ma. Subsequent rapid cooling to below 260 degrees C between 0.4 and 0.2 Ma is in agreement with previous proposals that vapor-dominated conditions were initiated within the hydrothermal system at this time. Heat flow calculations constrained with K-feldspar thermal histories and the present elevated regional heat flow anomaly demonstrate that appreciable heat input from sources external to the known Geysers plutonic complex is required to maintain the geothermal system. This requirement is satisfied by either a large, underlying, convecting magma chamber (now solidified) emplaced at 1.2 Ma or episodic intrusion of smaller bodies from 1.2 to 0.6 Ma. (C) 1999 Elsevier Science B.V. All rights reserved.