METAMORPHIC XENOLITHS AND FRACTURE PATTERNS IN GLENN SPRING SILL, BIG BEND N.P.- NEW DATA FROM THE NORTHERNMOST COAHUILA BLOCK

Subangular to subrounded xenoliths of quartzite, quartz graphite schist, meta-quartz monzonite (crude compositional lineation), rare marble and metachert occur throughout the exposed Glenn Spring porphyritic microgranite to rhyolite porphyry sill in SE Big Bend National Park. Lengths of inclusions range from 0.5 micron to 1 m and, in four areas surveyed, concentrations vary between 9 or 10/sq m (0.5-1.5 cm size population) and 3 or 4/sq m (>3 cm population). Quartzite and schist are pervasively sheared and isoclinally folded.

Compositions, fabrics, metamorphic grades and deformational styles for schist, quartzite, and marble xenoliths replicate those in nearby outcrops of Lower Permian (277±2 Ma) metasedimentary strata in Sierra del Carmen, Coahuila. Felsic metaigneous rocks in the vicinity (none known from Sierra del Carmen) include Silurian to Devonian granitic gneiss boulders within Marathon Basin flysch and Paleozoic granite gneiss in the Pemex #1 La Perla well, Coahuila.

Glenn Springs arroyo slices 10-20 meters into the sill from its northern termination south about 1.5 km to where it plunges beneath the overlying Cretaceous cover. Although the fracture trends remain constant (primary set: N10-20 W; secondary set: N40-50 E), the liesegang banding changes both vertically into the sill and down dip from north to south. The upper 0.5 m for the entire length of exposure is so highly altered that no identifiable bands can be recognized. From the north edge of the sill for 200 m south, all fractures have seven liesegang bands. This pattern terminates laterally against massive blocks (2-4 m) with only one or two bands. This change coincides with a west-trending topographic break across the sill and a point where the stream drops into a narrow canyon. The southern 200 m has fractures spaced 0.5-2 m apart with 13 bands. Beneath this top 10-15 m are widely spaced fractures with 5-6 bands or locally deeper with 2 bands. Our mapping indicates that the sill covers an area at least 35% greater than previously reported and that the N10-20 W fracture set is ubiquitous.