HR: 0800h
AN: T51A-0433    [Abstracts]
TI: Growth of Syntectonic Fibrous Veins in Shale, Ouachitas Orogen, Arkansas
AU: * Cervantes, P
EM: pcervantes@tamu.edu
AF: Department of Geology and Geophysics, Center for Tectonophysics, Texas A&M University, College Station, TX 77845 United States
AU: Wiltschko, D V
EM: d.wiltschko@tamu.edu
AF: Department of Geology and Geophysics, Center for Tectonophysics, Texas A&M University, College Station, TX 77845 United States
AB: Tectonic fibrous quartz and quartz-calcite veins in the Lower Ordovician Mazarn Formation display veinlets (5 to 25 $\mu$m wide) separated by 15 $\mu$m wide host segments parallel to the vein edge throughout the vein length, their number increase in fine host laminae and disappear in coarse host laminae. At the vein tip, fibers: a) are only quartz, b) range in width between 20 and 120 $\mu$m, c) discontinuously show host bands parallel to the vein edge. At the tip, the vein edge is irregular. At the center of the vein the vein all is smooth and, a) fibers are composed of both calcite and quartz, b) quartz fiber widths range between 30 and 300 $\mu$m, c) wide fibers ($>$100 $\mu$m) show inclusion trails parallel to the fiber long axis and, d) 10% of the fibers span the width of the vein. Vein fibers widen with distance from the vein tip. Cathodoluminescence (CL) in quartz fibers show growth bands parallel to vein-host interface. Width of growth bands is similar to width of veinlets. Growth bands disappear towards the center of fibers. Calcite in the host is found in distinct coarse-grain laminae and is absent in fine-grain laminae. Fluid inclusion data shows two thermal episodes of growth, one at around 210 $\deg$C and a second one at 130 $\deg$C. These observations suggest that the mechanisms of growth at the tips and central part of the vein are different. Specifically, at the tips vein growth takes place by cracking followed by competitive crystal growth (higher temperature diffusion-controlled process). Fiber width at this stage is determined by the initial crack aperture. Veins widen by addition of material at vein-host interface. At center of vein fibers widen by a combination continued precipitation on fiber walls and replacement of calcite by quartz.
DE: 8010 Fractures and faults
DE: 8025 Mesoscopic fabrics
DE: 8030 Microstructures
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting