The construction of a composite magma intrusion underneath “The Geysers” geothermal reservoir (California) based on zircon ages, trace elements, and isotopic compositions

One the world’s largest geothermal reservoirs, “The Geysers” in the California Coast Ranges, is underlain by a composite granitic pluton at shallow depth (~1–3 km, based on geothermal well penetration). Published U-Pb zircon geochronology indicates that this Geysers Plutonic Complex (GPC) intruded between c. 1.8 and 1.1 Ma in three major pulses: the oldest formed a cap of orthopyroxene-biotite microgranite porphyry, followed by orthopyroxene-biotite granite and hornblende-biotite-orthopyroxene granodiorite dominating at deeper levels. Lavas and minor pyroclastic deposits of the overlying Cobb Mountain Volcanic Center erupted between c. 1.2–1.0 Ma. The Geysers-Cobb Mountain plutonic-volcanic association shares common magmatic origins rooted in asthenospheric upwelling into a migrating slab window, where lower-crustal hybridization of mantle-derived magmas was followed by upper-crustal intrusion and differentiation. When and how shallow intrusions or eruptions were fed from this common source, however, remains unclear. This can be reconstructed from crystal-scale analysis of trace elements, oxygen and hafnium isotopes in zircon that can uniquely track magmatic processes in an evolving, long-lived magma system.

GPC microgranite zircons display strongly negative Eu anomalies, high levels of incompatible trace elements, and near-solidus Ti-in-zircon temperatures (~670 °C for aTiO₂ = 0.55 and aSiO₂ = 1). This is distinct from zircons from GPC granite and granodiorite that have moderately negative Eu anomalies, inconspicuous trace element enrichments, and variable Ti-in-zircon temperatures (~850–700 °C). Unlike trace elements, O and Hf isotopes in zircon are indistinguishable between GPC microgranite porphyry and the main population of granite-granodiorite zircons (δ¹⁸O = +4.76 to +9.18; εHf = +1.4 to +10.7). There is, however, a subgroup of zircon in GPC granite and granodiorite with elevated δ¹⁸O (~8.05) and lower εHf (~4.4) indicating that some late-stage melts experienced higher degrees of assimilation compared to the other magma types. Zircons from Cobb Mountain lavas are similar to those from the GPC granite and granodiorite, but distinct from the granophyre.

We set up a thermal model for zircon crystallization to satisfy the following observations: (1) evolved magma from which zircon crystallized was continuously present between c. 2.1 and 1.1 Ma, and (2) crystal recycling from the GPC microporphyry stage in subsequent intrusive or eruptive pulses was negligible. A magma reservoir at ~7 km depth which incrementally grew in three stages matches requirements imposed by zircon ages and compositions: (1) initial magma accumulation at low recharge fluxes starting at 2.1 Ma (0.1 km³/ka), (2) a brief flare-up at 1.6 Ma (4 km³/ka for 50 ka), (3) a return to low recharge fluxes (0.1 km³/ka) between 1.3 and 1.1 Ma. The total injected magma volume amounts to ~300 km³, three times the volume of the GPC as constrained by geothermal wells. According to this model, magma accumulation was long-lived, thus capable of sustaining protracted geothermal activity, but the main igneous growth occurred almost instantaneously. One implication is that accumulation of large volumes of magma can be rapid, and may require special circumstances that are only realized ephemerally despite overall long-lived magmatic activity.