COMPOSITE PLUTONS, COMPOSITE ZIRCONS: GEOCHRONOLOGICAL AND GEOCHEMICAL PERSPECTIVES ON ZIRCON STRATIGRAPHY AND MAGMA EVOLUTION FROM U-PB TIMS-TEA

Geochronology is important for understanding magmatic processes due to the temporal dependence of advection and thermal diffusion on melt fraction, mobilization and differentiation of magmas. U-Pb geochronology of granitoids from every tectonic setting and emplacement depth routinely yields zircon dates spanning 10⁵-10⁶ years at the handsample scale. This observation raises a critical question: how should high-precision zircon age spectra be interpreted in the context of magma genesis and pluton assembly? Moreover, this question underscores the need to integrate textural, petrologic and field data in order to accurately interpret geochronologic data.

Our efforts to bridge the gap between zircon dates and magmatic processes involves linking (1) high spatial resolution LA-ICPMS REE+Hf zircon geochemistry to (2) high temporal precision U-Pb ID-TIMS zircon geochronology + solution ICPMS trace element geochemistry, or TIMS-TEA. We apply this to the Bergell Intrusion, a ~30 Ma composite pluton preserving a 12 km crustal transect. Samples of tonalite, K-spar megacryst-bearing granodiorite, and hybridized intermediate granitoid each record 500-700 ka of zircon growth, with zircon geochemistry measured by LA-ICPMS and TIMS-TEA evolving over this time interval. TEA data exhibit compositional trends on (1) the pluton-scale (e.g. Zr/Hf) over ca. 2. Ma, consistent with the evolution of a lower-crustal parental melt sourcing all Bergell rocks, (2) the lithology-scale (e.g. Lu/Hf) over ca. 1 Ma, suggestive of two intermediate reservoirs corresponding to the tonalitic and granodioritic lithologies, and (3) the handsample-scale (e.g. Th/U) over ca. 500 ka, representing in situ differentiation and melt evolution at the level of emplacement. Dating of multiple fragments from individual zircons shows that single crystals preserve the entire crystallization sequence apparent from dating multiple zircons, providing insight into zircon growth rates and helping to distinguish core-rim mixing from continuous zircon growth. Cumulatively these results demonstrate the ability of zircon to discern multiple geochemical processes operating at different crustal levels and ultimately place time constraints on crustal differentiation.