Dating Ductile Deformation and Metasomatism using Apatite Petrochronology

Margo Odlum, UNLV
Drew Levy, drewlevy@nevada.unr.edu
Daniel Stockli, stockli@jsg.utexas.edu

Abstract

Chronologic constraints paired with microstructural and geochemical information is critical for understanding the P-T-t evolution of ductile deformation and metasomatism. Apatite dynamically recrystallizes during deformation, dissolves and reprecipitates during fluid flow, and chemically tracks metamorphic and metasomatic reactions making it a powerful tool for dating and geochemically characterizing deformation and metasomatism.

We integrate microstructural, U-Pb, and geochemical analysis of apatite grains from an exhumed mylonitic shear zone in the St. Barthélémy Massif, French Pyrenees, to understand how deformation and metasomatism are recorded by U-Pb dates and geochemical patterns. Granitic samples from the hanging wall preserve magmatic apatite U-Pb crystallization ages that overlap with zircon and monazite U-Pb ages of ∼300 Ma, granitic trace and rare earth element (TREE) trends, and primary magmatic growth zoning. Granulite and amphibolite grade gneisses in the footwall have Early Cretaceous dates (~100-120 Ma) and are dynamically recrystallized. Electron backscatter diffraction (EBSD) analysis documents crystal plastic deformation characterized by low-angle boundaries (<5°) associated with dislocation creep and evidence of multiple slip systems. U-Pb date maps of single grains indicate dates in deformed grains reflect, and are governed by, low-angle dislocation boundaries. Apatite TREE geochemical signatures are enriched in light rare earth elements. Single grain elemental maps indicate that TREE and U-Pb behavior is decoupled in the high-grade gneiss samples suggesting REEs record higher temperature processes than U-Pb isotopic systems. Apatite from (ultra)-mylonitic portions of the shear zone, known as the Main Mylonitic Band, show evidence of metasomatism with mixed U-Pb dates (300-100 Ma), mixed TREE trends, and patchy microtextures. The youngest U-Pb dates constrain the timing of metasomatism. Collectively, results demonstrate that crystal-plastic microstructures and fluid-interactions can markedly change apatite isotopic signatures, making apatite petrochronology a powerful tool for dating and characterizing the latest major deformation and/or fluid events.

Session

Session 6: Advances in Geology, Geochronology, Geophysics