ADVANCES IN UNDERSTANDING THE TIMESCALES OF MAGMATIC PROCESSES FROM HIGH-PRECISION U-PB GEOCHRONOLOGY

Despite conceptual and analytical advances in applying geochronology to igneous rocks over the past 10 years, gaps in linking mineral dates to specific magmatic processes persist and limit a full understanding of crustal evolution. Although zircon is the most exploited geochronometer, uncertainty in relating its growth, resorption, and transport to melt chemistry and phase assemblage compromises the accuracy of age interpretations.

Our recent work addresses how zircon geochemistry and growth respond to AFC processes. We use in situ geochemical and textural data to measure zircon stratigraphy and link these data with sub-grain ID-TIMS U-Pb geochronology and solution trace element analysis (TIMS-TEA). The goals are to a) build geochemical time-series for magmatic systems that can be related to, e.g., fractional crystallization of specific phases and/or magma mixing, b) integrate this information with field observation, geologic mapping, petrography, and whole-rock geochemistry to model parameters such as melt fraction and magma viscosity with time, and c) input these data into robust models of magmatic systems.

This talk highlights examples from two young magmatic systems. In one zoned intrusive suite (ca. 30 Ma Bergell Intrusion, N. Italy; 15-25 km paleodepth), some zircon trace element trends record evolution from tonalitic to granodioritic compositions in the deep crust over ~2 Ma (e.g. Zr/Hf), while other proxies document ~0.4 Ma of magma differentiation at the emplacement level (Th/U). In an upper-crustal granitic pluton (ca. 7 Ma Mt. Cappane pluton; 5 km paleodepth), petrographic characterization and in situ microsampling of zircon prior to TIMS-TEA constrains the timescales of magma mixing and megacryst growth during pulsed, top-down emplacement in <100 ka.

A hinderance to our efforts is a lack of quantification of the zircon saturation state of magmas over a range of spatiotemporal scales. As a possible way forward, we have integrated phase equilibria modeling (MELTS) with trace element modeling and existing experimental data for zircon saturation over a wide range of parameter space. These models predict zircon saturation and trace element compositions as a function of time, which can be compared to real U-Pb TIMS-TEA data as a means of testing hypotheses for the evolution of these systems.