EMPLACEMENT MECHANISMS AND PETROLOGIC CHARACTERISTICS OF BASALT FLOWS OF THE BLACK GAP SYNCLINE, WEST TEXAS

The Black Gap (BG) syncline is an open, NNW-trending fold on the eastern margin of the Trans-Pecos magmatic province of west Texas. The fold is defined by more than 14 Early Miocene basalt flows that range from 2–6 m thick and exhibit pahoehoe textures on both flow tops and bases. All flows include: (1) a distinct upper vesicular zone (UVZ), which ranges from 40 to 70% of total flow thickness; (2) a middle dense zone with few or no vesicles with a thickness that varies proportional to total flow thickness; and (3) a lower vesicular zone at the base of flows, which is ~0.5 m thick regardless of flow thickness. Based on these characteristics, the presence of vesicle cylinders, and distinct vesiculation patterns, inflation was the dominant emplacement mechanism for all BG flows. Since inflation is limited to slopes of <2°, nearly all rotation associated with syncline formation occurred after lava emplacement. Samples from the dense zones were collected on both the east and west limbs of the syncline for geochemical and petrographic analysis. The lavas have subophitic to intergranular textures, with sparse olivine phenocrysts typically altered to iddingsite, and a groundmass consisting of plagioclase, olivine, clinopyroxene and minor opaques. BG lavas have limited silica contents (~50 to 52 wt %) and are calcalkaline, in contrast to dominantly alkaline, high-K, mafic to intermediate older (~30 to 47 Ma) flow units exposed in Big Bend National Park (BBNP) to the west. Trace element modeling of assimilation fractional crystallization processes precludes petrogenetic linkages between BG and BBNP units. Trace element tectonic discrimination diagrams suggest that BG basalts were derived from a mantle source distinct from BBNP basalts.