284-6 Petrochronology of Fluid-mediated Strain Localization in the Crust

Session: The Deformation-Metamorphism-Fluid Triplet Governing Plate Boundaries and Orogens

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Strain localization is a fundamental component of plate tectonics and crustal deformation. It is controlled by chemical and mechanical processes, yet exactly how specific chemical-mechanical feedbacks modulate strain localization requires further investigation. We leverage accessory phase petrochronology to track the timescale, reaction history, and kinematic evolution of mylonitic fabrics within a mid-crustal shear zone that experienced extensive fluid infiltration and syn-kinematic metamorphism. The Early Devonian Lincoln syenite in mid-coastal Maine, USA, is located within a broad zone of Late Devonian dextral shear. The pluton preserves a strain gradient from pristine igneous mineralogy and textures through an altered assemblage that hosts a mylonitic foliation at the pluton margin. Zircon, apatite and titanite petrochronology – combined with accessory mineral textures and microstructures – elucidate the timing and conditions of fluid-mediated processes modulating strain localization. Zircon ages from each sample domain record igneous crystallization at 417 Ma. Apatite from the altered and foliated domains preserves textures indicative of fluid-mediated dissolution and recrystallization by an interface coupled dissolution-reprecipitation (ICDR) process. Biotite pins apatite grain boundaries, kinematically linking dissolution-reprecipitation to deformation. Igneous domain apatite yield a mean U-Pb age of 373 Ma interpreted as a cooling (< 450 ºC) age consistent with regional ⁴⁰Ar/³⁹Ar hornblende ages. Altered and foliated domain apatite have a mean U-Pb age of 362 Ma interpreted as recording the timing of fluid-mediated recrystallization. Elevated Sr and depleted REE concentrations in altered and foliated domain apatite signal fluid-mediated feldspar dissolution and REE-partitioning with co-crystallizing titanite and allanite. Titanite occurs as rims on ilmenite in the altered domain, and as euhedral grains and grain aggregates in the foliated domain. Euhedral and aggregate titanite have BSE zoning consistent with recrystallization by ICDR. Electron backscattered diffraction analysis of titanite shows subgrain microstructures and grain boundary geometries suggestive of dynamic recrystallization. Titanite U-Pb ages document syn-kinematic growth and recrystallization between 375–355 Ma. Our results elucidate the timing of fluid infiltration at ~375 Ma and progressive strain localization until 355 Ma as the strong igneous framework evolved into a weak interconnected network of actinolite and biotite. The textural, timing and trace element signatures in both apatite and titanite positively reflect the chemical and mechanical processes responsible for strain localization in the crust.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8922

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