Field, Petrologic, and Geochemical Relations of the Grouse Canyon Member of the Belted Range Tuff in the GTUF Rock Mechanics Test Area, U12g Tunnel, Nevada Test Site

摘要

This report summarizes the results of petrochemical studies of welded Grouse Canyon Member tuff in the G-Tunnel Underground Rock Mechanics Facility (GTUF), carried out in support of thermomechanical testing. The Grouse Canyon Member in GTUF is divided into three partial-cooling units (A, B, and C from the base upward), separated by tuffaceous breccias that mark partial cooling breaks. Unit B is most suitable for in situ experiments due to the presence of a thick, densely welded, relatively homogeneous central zone. Unit A is not favorable due to thinness, lithologic variability and extensive zeolitization. Unit B is not favorable due to variable degree of welding, and irregular zones of clay alteration. The Grouse Canyon Member is a marginally peralkaline comendite tuff; whole-rock agpaitic indices are close to 1.0. Phenocrysts present include sodic sanidine (An sub 4 Ab sub 55 Or sub 41 ), quartz, and minor clinopyroxene, fayalitic olivine, biotite, and Fe/Ti oxides. Devitrification yields axiolitic and spherulitic sodic sanidine and quartz, granophyric quartz, and minor sodic pyroxene and amphibole. These assemblages are characteristic of iron-rich peralkaline silicic tuffs. This differs slightly from the Topopah Spring Member at Yucca Mountain, in which there are no olivine or pyroxene phenocrysts and no ferromagnesian devitrification phases. Zeolitization in Unit A and the underlying Tunnel Bed 5 has added CaO from, and released Na sub 2 O to, groundwater under oxidizing conditions. 26 refs., 16 figs., 2 tabs. (ERA citation 10:051003)