A New Workflow to Assess Emplacement Duration and Melt Residence Time of Compositionally Diverse Magmas Emplaced in a Sub-volcanic Reservoir

Abstract:
Construction durations of magma reservoirs are commonly inferred from U–Pb zircon geochronology using various statistical methods to interpret zircon U–Pb age spectra (e.g. weighted mean ages of concordant zircon populations). However, in compositionally different magmas, zircon saturation and crystallization are predicted to occur at different times relative to other mineral phases and the geological event of interest; for instance, magma emplacement. The timescales of these processes can be predicted by numerical modeling and measured using U–Pb zircon thermal ionization mass spectrometry (TIMS) geochronology, therefore creating an opportunity to quantify magma emplacement in space and time to constrain the size and longevity of magma reservoirs during pluton construction. The Jurassic tilted, bimodal (gabbroic and granitic) Guadalupe igneous complex (GIC) in the Sierra Nevada arc presents an exceptional opportunity to study the construction duration of a shallow (1–10 km) magma reservoir comprising multiple magma batches. We present a new workflow to constrain emplacement ages from zircon geochronology of compositionally different magma batches and evaluate melt-present timescales. High-precision U–Pb chemical ablation isotope dilution (CA-ID)-TIMS zircon ages are combined with MELTS modeling to calculate zircon saturation ages for each dated sample. Bayesian statistics are then used to compare calculated zircon saturation distributions with zircon age distributions from TIMS data to predict time, temperature, and melt fraction at zircon saturation and solidus. In addition, we use mineral thermometry and cooling rate calculations to relate zircon saturation ages to emplacement ages for felsic and mafic rocks, resulting in a best estimate for the total construction duration of 295 ± 110 kyr for the GIC. Rhyolites exposed at the top of the GIC are ∼2–3 Myr older and thus not part of the same magmatic system. The good agreement between Ti-in-zircon crystallization temperatures and calculated zircon saturation temperatures by MELTS implies that bulk-rock compositions of both mafic and felsic rocks are close to liquid compositions. Mafic and felsic magmas experienced extensive mingling at the emplacement level in a magma chamber (which, as defined here, has temperatures above the solidus of the respective rock composition) encompassing ∼60% of the exposed map area of the complex shortly after construction. Melt was present within the system for a total duration of ∼550 kyr as constrained by two-dimensional thermal finite-difference modeling using an incremental growth and sill emplacement model. The construction and melt-present timescales calculated in this study for the shallow GIC have implications for the potential of in situ differentiation, mixing and mingling timescales and eruption in shallow magmatic systems.

Suggested citation:
Ratschbacher, B.C. et al. (2018). A New Workflow to Assess Emplacement Duration and Melt Residence Time of Compositionally Diverse Magmas Emplaced in a Sub-volcanic Reservoir. Journal of Petrology 59 (9), 1787-1809.