UNRAVELING THE EMPLACEMENT HISTORY OF MAFIC TO FELSIC INTRUSIONS IN BIG BEND NATIONAL PARK, TEXAS USING HIGH-PRECISION 40Ar/39Ar AND U-PB ZIRCON GEOCHRONOLOGY

We present new data to unravel the complex intrusive history within Big Bend National Park (BBNP), with recent geologic field observations, age determinations, and geochemistry for a large number of identified intrusions. Though previous geochronology within the Park indicate that intrusive activity ranged from 80 to 17 Ma, our study indicates significant magmatic activity occurred in discrete episodes from 46.5 to 23 Ma. These shallow intrusions occur throughout BBNP and were emplaced as dikes, sills, plugs, and laccoliths that range in size from stringers a few centimeters thick, to laccoliths ten kilometers in diameter (Rosillos Mountains-RM). The intrusions vary in composition from gabbro to diorite to fayalite granite.

The age determinations presented in this study are 40Ar/39Ar and U-Pb zircon geochronology. The oldest intrusive rocks in the Park are gabbros dated at 46.5 Ma (U-Pb). The largest and most studied of these gabbros is located at Mariscal Mountain. A preliminary Ar-Ar age of 46.0 Ma was obtained from a pure separate of pyroxene. A hiatus in activity occurred from 46 to 43 Ma followed by several large laccoliths that were emplaced at 42.3 Ma (mafic rocks, using Ar-Ar) and 41.5 Ma (felsic rocks-Maverick Mountain, using U-Pb) in the northern part of the park. Another hiatus occurred from 40 to 34 Ma. Between 33.0 and 32.2 Ma a major magmatic pulse occurred (intrusive and extrusive) we collectively call the Pine Canyon event, which includes many units found throughout the Park. A localized magmatic period occurred between 30 and 27 Ma (Ar-Ar and U-Pb) with the emplacement of mafic and felsic intrusive rocks associated with the domal uplift of Sierra Quemada located on the SW flank of the High Chisos Mountains. Another intrusive event occurred from 29.4 to 28.8 Ma (Ar-Ar and U-Pb) with the emplacement of diorite dikes and intrusions at 29.0 Ma soon after the eruption of the Burro Mesa Rhyolite (29.4 Ma). A more mafic phase of activity occurred at 28.8 Ma (syenodiorite laccoliths at Pena and Rattlesnake Mountains) followed by the emplacement of a minor gabbro at 25.0 Ma (Iron Mountain). The final intrusive rocks emplaced within the Park were dated at 23 Ma (Ar-Ar) from a series of basaltic sills, cinder cones, and lava flows from the Black Gap Wildlife Management Area (E of the Park).