OLIGOCENE MAGMATISM AND XENOLITHS OF ACCRETED LITHOSPHERE - GLENN SPRING AND ADJACENT INTRUSIONS, BIG BEND NATIONAL PARK

Three NNE-aligned felsic intrusions dominate the bedrock geology of Glenn Spring quadrangle, Big Bend National Park, west Texas: Chilicotal Mt. (~6.8 sq km), Glenn Spring (~3.3 sq km), and Talley Mt. (~1.9 sq km) sills. They were emplaced in Upper Cretaceous to Paleocene siliciclastics, and the outer few meters of all are strikingly liesegang-banded. The age of the Glenn Spring porphyritic microgranite is 30.5 Ma (Ar-40/Ar-39, K-feldspars; D. Miggins, USGS), falling within the regional Eocene-Oligocene magmatic episode; Chilicotal Mt. alkali syenite and Talley Mt. porphyritic microgranite remain undated. All are Ti-rich (titanaugite, titanite, Ti-biotite); sodic amphibole is abundant and olivine is present in the Chilicotal sill, but both are absent from the other two. The Glenn Spring and Talley Mt. masses may be comagmatic (analyses in progress). The massive syenite that caps Chilicotal Mt. is intruded by a distinctive basalt porphyry sill, and dikes of that composition cut both the Chilicotal and Tally Mt. bodies.

Magmas feeding the intrusions ascended through lithosphere that was accreted to Laurentia during the late Paleozoic collision with Gondwana. Subangular to subrounded microxenoliths to boulders (>1 m) of quartzite, schist, and marble throughout the Chilicotal Mt., Glenn Spring and Talley Mt. sills attest to tectonism at the ancient margin. In inventoried areas of Glenn Spring sill, abundances ranged from 3 or 4 to 9 or 10 xenoliths per sq m, increasing toward the center of the intrusion. Compositions, metamorphic grade, and deformational history are essentially identical to those of Ouachita metamorphic rocks (~277 Ma) exposed in the Sierra del Carmen, Coahuila, ~35 km farther east. In addition, the xenolith suite includes meta-quartz monzonite, which is not yet documented from Coahuila outcrops, and which requires geochemical/geochronological assessment of this possible evidence of collisional anatectic melting.