LINKING FLUID-ROCK INTERACTION AND STRAIN ACCUMULATION IN A NATURALLY DEFORMED DIAMICTITE

Determining the rheology of earth materials is a first-order issue in tectonics, which requires an understanding of interrelations between deformation mechanisms, fluid-rock interaction, mineralogic reactions, and softening processes during progressive deformation. Detailed structural and geochemical analyses of a diamictite on Antelope Island, Utah, which was deformed during the Sevier orogeny, reveal gradients in strain and mineralogy, providing a natural laboratory to study processes of reaction softening and hydrolytic weakening. In the least deformed areas, the diamictite contains relatively equant clasts of granitic orthogneiss, quartzo-feldspathic paragneiss, and quartzite that sit within a micaceous matrix. With increasing strain, gneissic clasts display microcracking and alteration to quartz-muscovite-chlorite aggregates with clast axial ratios locally > 10:1. Quartz within deformed gneissic clasts underwent crystal-plastic deformation, and neocrystallized quartz grains formed during alteration. Quartzite clasts, however, remain relatively equant even in high strain areas with quartz remaining strong. Bulk geochemical analysis of gneissic clasts and matrix reveals that concentrations of Al, Ti, and Zr remain similar from low to high strain, suggesting only minor volume loss. Although bulk volume loss appears limited, Si, alkalies, and Mg were mobile at the grain scale. X-ray spectral mapping is underway to quantify mobility associated with alteration of feldspar and mafic minerals to muscovite and chlorite. Fourier transform infrared (FTIR) spectroscopy and synchrotron infrared radiation studies are also underway to quantify the amount and location of water within quartz grains. Preliminary results of detailed synchrotron mapping show gradients in water concentration across quartz grains, with the highest concentrations near some grain boundaries, suggesting that these boundaries formed pathways for fluid infiltration. Water concentrations also display an overall increase with increasing strain in gneissic clasts, suggesting that microcracks provided pathways for water to enter the quartz crystal structure. In detail, orthogneiss and paragneiss clasts display slightly different patterns of alteration and water content, which are the focus of ongoing studies.