284 The Deformation-Metamorphism-Fluid Triplet Governing Plate Boundaries and Orogens

Investigating orogens requires establishing relationships between deformation, fluids, metamorphism, and timing from within and adjacent to faults and shear zones. We seek presentations that combine new and traditional structural, petrochronology, and other microanalytical techniques.

Wednesday, 22 October 2025
1:30 PM - 5:30 PM
HBGCC, 217B

Dowsing the shear zone: oxygen and hydrogen stable isotopes record progressive fluid-rock-deformation interactions across an evolving mylonitic front

Paper number: 284-2
Program: Technical Sessions
Time: 2:00 PM – 2:15 PM

Authors

Abstract

Aqueous fluids are essential “agents of change” in high-strain zones by promoting rheologic weakening, enhancing fracturing, and participating in metamorphic reactions. The isotopic composition of silicates in shear zones confirm the presence and provenance of these fluids, and can be an archive of topographic and climatic change during mountain building. In contrast, the absence of an aqueous fluid signature is utilized as evidence of deep, hot, and impermeable cratonic processes. However, evidence for or against aqueous fluids in the plastic crust is often elucidated across time and space from sparce evidence. Here we present new d’18O, and D’17O values of quartz, muscovite, biotite, and feldspar, paired with d2H values of muscovite and biotite from 23 localities within a continuous 80-km exposure of a well-constrained, mylonitic, quartzofeldspathic shear zone in the extensional Cordillera Blanca detachment, Peru. d’18O, d’17O, and D’17O values are internally consistent within single minerals, but vary from primary, magmatic ranges to values consistent with meteoric fluid interaction between phases within single samples. d’18O, d’17O, and D’17O values demonstrate decreasing heterogeneity along strike with the greatest variation in the southern segment of the shear zone and the least in the north. Oxygen isotopes also follow a general trend of increasing d’18O and d’17O values alongside decreasing D’17O values. This follows a decrease in the average and range of deformation and mineral-pair equilibrium temperatures across the structure, suggesting a spatial trend in the extent of water-rock interaction along strike. d2H values are consistent between mica species within single samples, but vary from -46 ‰ to -137 ‰ (muscovite) and -63 ‰ to -149 ‰ across the structure. We interpret these variations to reflect: 1) variable closure temperature of elemental and isotopic exchange for each phase, and 2) metamorphic phase growth and mechanical mixing during shear disconnecting potential equilibrium relationships between the phases. Collectively, these data reveal heterogeneous equilibration with infiltrating meteoric fluids during the competition between fluid percolation, crustal anatexis, and melt injection. Given the observed complexity of isotopic fluid tracers within a single structure, caution is urged when interpreting the presence, absence, relative quantity, or relative timing of fluid-rock interaction in high-strain terranes based on a spatially limited or single-tracer dataset.

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

doi: 10.1130/abs/2025AM-414561

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