The Geysers - Cobb Mountain magma system, California (Part 1): U-Pb zircon ages of volcanic rocks, conditions of zircon crystallization and magma residence times

Date

2003

Authors

Schmitt, Axel K
Grove, Marty
Harrison, Timothy
Lovera, Oscar
Hulen, Jeffrey
Walters, Mark

Journal Title

Journal ISSN

Volume Title

Publisher

Pergamon-Elsevier Ltd

Abstract

Combined U-Pb zircon and 40Ar/39Ar sanidine data from volcanic rocks within or adjacent to the Geysers geothermal reservoir constrain the timing of episodic eruption events and the pre-eruptive magma history. Zircon U-Pb concordia intercept model ages (corrected for initial 230Th disequilibrium) decrease as predicted from stratigraphic and regional relationships (1 σ analytical error): 2.47 ± 0.04 Ma (rhyolite of Pine Mountain), 1.38 ± 0.01 Ma (rhyolite of Alder Creek), 1.33 ± 0.04 Ma (rhyodacite of Cobb Mountain), 1.27 ± 0.03 Ma (dacite of Cobb Valley), and 0.94 ± 0.01 Ma (dacite of Tyler Valley). A significant (∼0.2-0.3 Ma) difference between these ages and sanidine 40Ar/39Ar ages measured for the same samples demonstrates that zircon crystallized well before eruption. Zircons U-Pb ages from the underlying main-phase Geysers Plutonic Complex (GPC) are indistinguishable from those of the Cobb Mountain volcanics. While this is in line with compositional evidence that the GPC fed the Cobb Mountain eruptions, the volcanic units conspicuously lack older (∼1.8 Ma) zircons from the shallowest part of the GPC. Discontinuous zircon age populations and compositional relationships in the volcanic and plutonic samples are incompatible with zircon residing in a single long-lived upper crustal magma chamber. Instead we favor a model in which zircons were recycled by remelting of just-solidified rocks during episodic injection of more mafic magmas. This is consistent with thermochronologic evidence that the GPC cooled below 350° C at the time the Cobb Mountain volcanics were erupted.

Description

Keywords

Keywords: crystallization; magma chamber; magmatism; residence time; uranium series dating; volcanic rock; zircon; United States

Citation

Source

Geochimica et Cosmochimica Acta

Type

Journal article

Book Title

Entity type

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DOI

10.1016/S0016-7037(03)00140-6

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