T96 Approaches for Extracting Shear Zone History from the Ductile Rock Record: Probing Their Initiation, Evolution, and Reactivation II

Tuesday, 27 October 2020
5:30 PM - 8:00 PM
Meeting Rooms

PETROCHRONOLOGY AND MICROSTRUCTURE OF TITANITE ACROSS A SHEARED PEGMATITE DIKE FROM THE ROAN WINDOW, NORWAY

Paper number: T96-4
Program: Technical Sessions
Time: 5:55 PM – 6:05 PM

Authors

Abstract

Titanite (CaTiSiO5) is a promising ductile deformation petrochronometer. Common in felsic and mafic lithologies, titanite incorporates U during (re)crystallization sufficient for U-Pb isotopic dating, and also incorporates all REEs and many trace elements, including Zr, whose concentration is temperature- and pressure-sensitive. Titanite can also recrystallize during deformation, suggesting it may record the timing, P-T information, and co-crystallizing phases of deformation if it can be shown that chemical and isotopic trends are related to the formation of microstructures, and not volume diffusion or secondary growth. Importantly, P-T-t interpretations can only be considered if Pb and Zr remain coupled in response to cooling and deformation.

To evaluate titanite’s ability to record conditions and timing of ductile deformation, we collected U-Pb isotopic and geochemical data from a suite of titanite grains across a sheared pegmatite dike from the Roan Window of Norway’s Western Gneiss Region. Deformation is localized in the center of the dike, parallel to the dike margins, resulting in a strain gradient from undeformed pegmatite to ultramylonite over ~4 cm. Zircon from a neighboring dike yielded a weighted mean 206Pb/238U age of 396.0±1.5 Ma (2σ, MSWD=1.0), which we infer to be the crystallization age of the adjacent deformed dike. Titanite analyses from the deformed dike yield a lower intercept 206Pb/238U date of 387.4±7.7 Ma (2σ, MSWD=2.2). The titanite date is ~ 9 My younger than zircon, indicating that it does not reflect dike crystallization. Additionally, although overdispersion is common for titanite U-Pb data, we explore the causes of elevated MSWDs in this study and evaluate the extent to which it is associated with deformation. We supplement petrochronology results with EBSD data from titanite and major phases to evaluate the correlation between microstructure, U-Pb systematics, and geochemistry. Results so far indicate that a significant portion of the geochemical variability (Zr, ∑REE, Eu/Eu*) is related to sector zoning. However, 207Pb/206Pb is elevated in proximity to deformation twins, suggesting that deformation may modify the proportion of nonradiogenic Pb in titanite, and therefore promote decoupling of isotopic data and P-T sensitive geochemical information.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-359220

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