EARLY OLIGOCENE IGNEOUS FLARE UP IN BIG BEND NATIONAL PARK, TEXAS: CONTEMPORANEOUS ACTIVITY AT DAGGER FLATS INTRUSIVE SILL COMPLEX AND PINE CANYON CALDERA

The igneous landscape dominating much of Big Bend National Park (BBNP) includes, but is not limited to, tectonically-controlled Tertiary dike and sill systems, shallow intrusive bodies, lava flows, and large ignimbrite-producing calderas. These igneous forms have been variably influenced or controlled by regional tectonism. Nearly all are associated in some degree with extensive fossil hydrothermal systems, which altered and mineralized surrounding rocks.

The terrain of Dagger Flats is represented by a series of low northwest-trending hills bounded by northwest-trending faults cored with thick sills that intruded into and are capped by the Cretaceous Boquillas Formation. Skarn deposits have formed in the cap rock directly above the sills. The sills are predominantly mafic with subordinate felsic compositions. The intrusion of magmatically driven hydrothermal solutions into marine carbonate sandstone and shale resulted in skarn mineralization characterized by quartz, wollastonite, garnet, zeolites, pyroxenes, and prehnite and preservation of delicate original features in the carbonate. High-resolution aeromagnetic mapping in BBNP shows a well defined sill as a northwest-trending structure with moderate amplitude positive anomalies and with similar magnetic characteristics as other nearby intrusive bodies. The size of Dagger Flats intrusion is ~15-22 km long by ~5 km wide and is comparable in size to or larger than, adjacent sills and intrusive bodies, including those in the McKinney Hills and the Rosillos Mountains. The Dagger Flats intrusive complex is somewhat smaller than the magnetic anomaly associated with the Pine Canyon caldera.

High-precision Ar/Ar radiometric dates of Dagger Flats sills indicate that these bodies were intruded over a 1.5-my interval at ~31-32 Ma, contemporaneous with peak intrusive and volcanic activity <10 km to the south at the Pine Canyon caldera. Intrusive activity at Dagger Flats occurred from 32.47+0.41 Ma to 30.95+0.61 Ma, while mostly rhyolitic volcanic activity at the Pine Canyon caldera ranged from 32.22+0.12 Ma to 30.86+0.41 Ma.