Intrusive emplacement and thermal history of the Geysers Plutonic Complex, northern California: New insights from in-situ U-Pb zircon dating
Abstract
The Geysers Plutonic Complex (GPC) is a unique example of a Quaternary intrusion that is exposed at shallow subsurface levels (>0.7 km depth). It spatially overlaps with a major surface heat-flow anomaly that is associated with one of the world's largest geothermal fields, known as The Geysers. Based on drill hole penetration, the GPC appears as an elongated northwest-trending keel shaped body which has an areal extent of ~50 km2. It has been petrographically subdivided into microgranite porphyry, granite and granodiorite and compositional similarities of these subunits linked them to the extrusive rhyolites and dacites from the Cobb Mountain volcanic field (CMVF) that overly the GPC at its eastern margin, but little is known about the relative timing of the intrusive and related volcanic activities. Age determination of the GPC requires in-situ techniques due to xenocrystic contamination of drill cuttings that are generally the only available materials from the GPC. Meaningful ages reported in the literature are limited to four samples from the GPC granite unit that range from 1.13 to 1.25 Ma (238U/206Pb ages uncorrected for initial 230Th deficit). Here we present U-Pb zircon ages for an extended sample set that covers all subunits of the GPC and the CMVF. These ages provide new constraints on the onset and duration of intrusive emplacement of and eruptive tapping from a shallow magma body. Apparent 238U/206Pb ages (1.52 - 1.74 Ma, 1σ uncertainty typically <5 % relative) for five samples of microgranite porphyry including a microgranitic dike rock exceed the ages determined for the granite and granodiorite units (1.13 - 1.25 Ma, 14 samples total). One well at the eastern margin of the GPC penetrated 0.8 km of previously unidentified biotite-orthopyroxene-hornblende granite that yielded slightly younger 238U/206Pb ages between 1.05 and 1.08 Ma (three samples). U-Pb zircon ages from the CMVF (Alder Creek rhyolite: 1.24 +/- 0.04 Ma; Cobb Mountain rhyodacite: 1.24 +/- 0.04 Ma; Cobb Valley rhyodacite: 1.20 +/- 0.05 Ma) closely overlap with the age range of the granite and the granodiorite samples. Initial 230Th deficit in zircon results in radiogenic 206Pb contents that are on average by 5-10 % too low, and therefore an average +0.10 m.y. age correction is required. After applying this correction, we obtain an average zircon crystallization age for the CMVF that is about 0.20 m.y. older than the eruption ages implied by Ar-Ar sanidine ages (1.15 +/- 0.01 to 1.01 +/- 0.06 Ma). The eruption ages, however, are similar to the corrected zircon crystallization ages of the youngest GPC unit, the biotite-orthopyroxene-hornblende granite. The data indicate a complex intrusive history of the GPC and a time span of at least 0.8 m.y. for its formation. Volcanic activity is temporally and spatially linked to the last intrusive phase but the erupted magmas appear to consist of remobilized older materials of previous intrusions. Intrusive activity within the main body of the GPC ceased at ~1.0 Ma which suggests that the GPC at its presently known extent is unlikely to be the heat source for the present-day heat-flow anomaly at the Geysers steam field.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2001
- Bibcode:
- 2001AGUFM.V32E1025S
- Keywords:
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- 1035 Geochronology;
- 3640 Igneous petrology;
- 8035 Pluton emplacement;
- 8130 Heat generation and transport;
- 8135 Hydrothermal systems (8424)