Strengths and limitations of in situ U-Pb titanite petrochronology in polymetamorphic rocks: An example from western Maine, USA.

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Authors

Jesse B Walters, Alicia M Cruz-Uribe , Won Joon Song , Christopher Gerbi , Kimberley Biela

Abstract

Titanite is a potentially powerful U-Pb petrochronometer that may record metamorphism, metasomatism, and deformation. Titanite may also incorporate significant inherited Pb, the correction for which may introduce inaccuracies and result in geologically ambiguous U-Pb dates. Here we present laser ablation inductively coupled mass spectrometry (LA-ICP-MS)-derived titanite U-Pb dates and trace element concentrations for two banded calc-silicate gneisses from south-central Maine, USA (SSP18-1A & -1B). Single spot common Pb-corrected dates range from 400 to 280 Ma with 12–20 Ma propagated 2SE. Titanite in sample SSP18-1B exhibit regular core-to-rim variations in texture, composition, and date. We identify four titanite populations: 1) 399 ± 5 Ma (95 % CL) low Y + HREE cores and mottled grains, 2) 372 ± 7 Ma high Y + REE mantles and cores, 3) 342 ± 6 Ma cores with high Y + REE and no Eu anomaly, and 4) 295 ± 6 Ma LREE-depleted rims. We interpret the increase in titanite Y + HREE between ca. 400 and ca. 372 Ma to constrain the timing of diopside fracturing and recrystallization and amphibole breakdown. Apparent Zr-in-titanite temperatures (803 ± 36 °C at 0.5 ± 0.2 GPa) and increased XDi suggest a thermal maximum at ca. 372 Ma. Population 3 domains dated to ca. 342 Ma exhibit no Eu anomaly and are observed only in compositional bands dominated by diopside (> 80 vol %), suggesting limited equilibrium between titanite and plagioclase. Finally, low LREE and high U/Th in Population 4 titanite date the formation of hydrous phases, such as allanite, during high XH2O fluid infiltration at ca. 295 Ma. In contrast to the well-defined date-composition-texture relationships observed for titanite from SSP18-1B, titanite grains from sample SSP18-1A exhibit complex zoning patterns and little correlation between texture, composition, and date. We hypothesize that the incorporation of variable amounts of radiogenic Pb from dissolved titanite into recrystallized domains resulted in mixed ages spanning 380–330 Ma. Although titanite may reliably record multiple phases of metamorphism, these data highlight the importance of considering U-Pb data along with chemical and textural data to screen for inherited radiogenic Pb.

DOI

https://doi.org/10.31223/X5H32V

Subjects

Earth Sciences, Geochemistry, Geology, Other Earth Sciences, Physical Sciences and Mathematics, Tectonics and Structure

Keywords

trace elements, LA-ICP-MS, titanite petrochronology, calc-silicate rocks

Dates

Published: 2021-07-29 09:54

Last Updated: 2021-07-29 13:54

License

CC BY Attribution 4.0 International

Additional Metadata

Conflict of interest statement:
The authors declare no conflicts of interest.

Data Availability (Reason not available):
Data is available from the lead author upon request.

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