TIME AS A SYMPTOM: GEOCHRONOLOGIC AND GEOCHEMICAL INSIGHTS INTO SILICIC INTRUSIVE MAGMATISM

Abstract

Geochronologic evidence from intrusive silicic rocks from many localities suggests most formed through incremental emplacement. However, there remain questions about the duration of melt residency, the extent and frequency of magma mixing, and what processes control emplacement rates. In this thesis I use high precision zircon geochronology and isotope geochemistry to explore these questions for two silicic intrusions. From the Tuolumne Intrusive Suite (TIC) I dated zircon included in K-feldspar megacrysts to understand the duration of megacryst growth and the spatial extent of magma mixing between two TIC units. In the younger of the two, megacryst-included zircon dates are less protracted and consistently younger than those from the older unit and the cores of the contact. This and the 1.5 Myr age difference between matrix zircon of the two units suggest that the spatial extent of magma mixing was restricted to the width of the gradational contact at any given point in time. The radiogenic Sr compositions of the megacrysts become increasingly less radiogenic from core to rim, suggesting the crustal component of the melt decreased over their growth. Additionally, these transects feature smaller scale oscillations that correlate with Ba concentration, indicating that the oscillatory Ba zoning observed in TIC megacrysts may be controlled by the same process affecting radiogenic Sr compositions. The stable Sr compositions of the megacrysts are overall lighter than their surrounding matrix. This may result from equilibrium or kinetic fractionation between the melt and K-feldspar and is evidence that such fractionation is resolvable even for heavier elements in high temperature systems. Finally, I measured zircon dates from the Eocene Duncan Hill pluton (DHP) in Washington. My results suggest it emplaced incrementally over 2.5 Myr and that some or all of the magmatic differentiation that produced the pluton’s compositionally distinct zones occurred prior to emplacement. This emplacement history differs from another nearby Eocene pluton for which there is geochronologic evidence of rapid magma chamber assembly. This difference may point to the importance of emplacement mechanisms, as the fast-assembling pluton is located directly on a transform fault and the DHP is only adjacent to a fault.