FLUID-ROCK INTERACTION IN DEFORMED DIAMICTITE, WILLARD THRUST SYSTEM, UTAH

Diamictite deformed at greenschist facies within the Willard thrust system of the Sevier orogenic belt, northern Utah, displays varying relations between fluid-rock interaction, volume loss, and strain between the footwall (Antelope Island) and hanging wall (Fremont Island). Diamictite consists of pebble to boulder clasts comprised of red granitic orthogneiss, green paragneiss, and metaquartzite, which sit within a sandy to micaceous matrix. Most gneissic clasts were likely derived from basement of the Farmington Canyon Complex during Neoproterozoic glaciation; these basement rocks were subsequently locally exhumed during Sevier thrusting. By comparing geochemistry of diamictite with basement protoliths, characteristics of fluid-rock interaction across a major thrust can be evaluated.

On Antelope Island (footwall), red orthogneiss clasts and matrix have concentrations of Al₂O₃ (relatively immobile) that overlap with basement sources, indicating little to no volume change during deformation; SiO₂ concentrations also overlap. Green paragneiss clasts, however, display a slight increase in Al₂O₃ and decrease in SiO₂, suggestive of minor volume loss by dissolution of quartz. Strain and microtextural relations also indicate limited volume change. Na₂O decreases whereas MgO increases, reflecting alteration of sodic feldspar to phengitic muscovite, with precipitation of quartz in matrix, strain shadows, and microveins. Strain within clasts and matrix increases systematically toward the WNW, with X-Z ratios increasing from ~2:1 to ~10:1. Feldspar is almost completely altered to mica in high-strain samples, and precipitated quartz displays significant plastic deformation. On Fremont Island (hanging wall), significant increases in Al₂O₃ and decreases in SiO₂ of diamictite indicate ~20% volume loss, consistent with strain and microtextural relations. Compared to Antelope Island, Na₂O is higher and MgO is lower, reflecting alteration of biotite to Fe-rich chlorite and feldspar to muscovite plus paragonite, with net dissolution of quartz. X-Z strain ratios are overall lower, ranging from ~2:1 to 4:1. Differences in geochemical patterns are interpreted to record up-temperature flow in the footwall and down-temperature flow in the hanging wall, with focused fluid flow along the main fault zone.