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

Sixty-nine ion microprobe spot analyses of zircons from four granite sample s from the plutonic complex that underlies the Geysers geothermal field yie ld 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 Ar-40/Ar-39 ape spectr um plateau and `terminal' ages from coexisting K-feldspars and with the eru ption 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 Ar-40/Ar-39 age data using multi-d iffusion domain theory indicates that post-emplacement rapid cooling was su cceeded 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 M a is in agreement with previous proposals that vapor-dominated conditions w ere initiated within the hydrothermal system at this time. Heat flow calcul ations constrained with K-feldspar thermal histories and the present elevat ed regional heat flow anomaly demonstrate that appreciable heat input from sources external to the known Geysers plutonic complex is required to maint ain the geothermal system. This requirement is satisfied by either a large, underlying, convecting magma chamber (now solidified) emplaced at 1.2 Ma o r episodic intrusion of smaller bodies from 1.2 to 0.6 Ma. (C) 1999 Elsevie r Science B.V. All rights reserved.