Age and thermal history of The Geysers plutonic complex (felsiteunit), Geysers geothermal field, California; a (super 40) Ar/ (super39) Ar and U-Pb study

Abstract

Sixty-nine ion microprobe spot analyses of zircons from four granitesamples from the plutonic complex that underlies the Geysersgeothermal field yield (super 207) Pb/ (super 206) Pb vs. (super 238)U/ (super 206) Pb concordia ages ranging from 1.13+ or -0.04 Ma to1.25+ or -0.04 (1sigma ) Ma. The weighted mean of the U/Pb model agesis 1.18+ or -0.03 Ma. The U-Pb ages coincide closely with (super 40)Ar/ (super 39) Ar age spectrum plateau and ""terminal"" ages fromcoexisting K-feldspars and with the eruption ages of overlyingvolcanic rocks. The data indicate that the granite crystallized at1.18 Ma and had cooled below 350 degrees C by approximately 0.9-1.0Ma. Interpretation of the feldspar (super 40) Ar/ (super 39) Ar agedata using multi-diffusion domain theory indicates thatpost-emplacement rapid cooling was succeeded either by slower coolingfrom 350 degrees to 300 degrees C between 1.0 and 0.4 Ma or transitoryreheating to 300-350 degrees C at about 0.4-0.6 Ma. Subsequent rapidcooling to below 260 degrees C between 0.4 and 0.2 Ma is in agreementwith previous proposals that vapor-dominated conditions were initiatedwithin the hydrothermal system at this time. Heat flow calculationsconstrained with K-feldspar thermal histories and the present elevatedregional heat flow anomaly demonstrate that appreciable heat inputfrom sources external to the known Geysers plutonic complex isrequired to maintain the geothermal system. This requirement issatisfied by either a large, underlying, convecting magma chamber (nowsolidified) emplaced at 1.2 Ma or episodic intrusion of smaller bodiesfrom 1.2 to 0.6 Ma.

10 samples from this reference