36 Rates, Dates, and Plates: Petrochronological Approaches to Unraveling Tectonometamorphic Histories

This session welcomes contributions  applying modern approaches in metamorphic petrology, geochronology, and structural geology to unravel the dynamics of convergent margin processes from the Appalachians and beyond. We encourage submission from Early Career participants.

Tuesday, 24 March 2026
8:00 AM - 12:00 PM
CCC, Room 22/23

Monazite Date Resetting in the Athabasca Granulite Terrane: An Investigation of Fluid-Based Coupled Dissolution Reprecipitation Processes in the Lower Crust

Paper number: 36-9
Program: Technical Sessions
Time: 10:40 AM – 11:00 AM

Authors

Abstract

Monazite has been widely used to constrain the timing of tectonic events. One pitfall of monazite is that it can be susceptible to coupled dissolution reprecipitation. This fluid-mediated process can produce new compositional domains within monazite crystals. The redistribution or partial removal of U, Th, and Pb, can lead to spurious dates and misinterpretations of tectonic histories. When Pb is entirely removed from altered regions of a crystal, the reset dates can help establish the timing of fluid-alteration processes which can occur significantly below the closure temperature for solid-state diffusion (as low as 300ºC). Monazite from the Upper Deck domain of the Athabasca Granulite Terrane (AGT) in northern Saskatchewan exhibits textures and compositional signatures that can be attributed to dissolution reprecipitation processes. To investigate this phenomenon, monazite crystals were mapped and dated using high resolution compositional X-ray mapping and in situ U-Th-Pb dating techniques on a Cameca SX-100 and the Ultrachron electron microprobes. Key features observed in complex crystals include: (1) embayments or finger-like channels from the rim to the core, (2) overgrowths with high Th concentrations outboard of the embayments and cores, (3) younger compositional domains cross-cutting core domains along narrow crystallographic-controlled planes, (4) irregular domains with dates younger than the domains surrounding them, and (5) domains with significant µm-scale compositional heterogeneity. High-confidence monazite dates (<10 Ma 2-σ error) in the AGT are associated with at least two periods of metamorphism from ~2.60 to ~2.55 Ga and ~1.95 to ~1.85 Ga. Dates between ~2.50 Ga and ~2.00 Ga, many with significant uncertainty, are difficult to assign to a specific tectonic event. If date resetting occurred, it could have significant implications for the interpretations of tectonic events in the AGT. For example, some ca. 2.35 Ga dates might be interpreted to reflect the Arrowsmith orogeny, an event of uncertain extent proposed to the west. Textural evidence of dissolution reprecipitation is typically recognizable, and care must be taken to evaluate if altered domains reflect fluid infiltration events or spurious dates due to partial Pb loss. The goal of our ongoing work is to examine the geochemistry of monazite crystals from the Upper Deck domain to gain insights into the dissolution reprecipitation process and then apply these insights to the current interpretations of the regional tectonic history. 

Geological Society of America Abstracts with Programs. Vol. 58, No. 2

doi: 10.1130/abs/2026NE-411449

© Copyright 2026 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.