Geology and Tertiary Igneous Activity of the Hen Egg Mountain and Christmas Mountains Quadrangles, Big Bend Region, Trans-Pecos Texas

Cretaceous sedimentary rocks and Tertiary intrusive, extrusive, and volcaniclastic rocks dominate the geology of the Hen Egg Mountain and Christmas Mountains quadrangles in the Big Bend region of Texas. The Cretaceous rocks consist of limestones, shales, mudstones, and sandstones that are host rocks for the Tertiary intrusions.This report focuses on the Tertiary intrusive and extrusive rocks. The intrusions were emplaced in several distinct intervals between 47 and 20 Ma ago (Eocene to Miocene) but are dominantly 44 to 40 Ma old. All intrusions are small (less than a few kilometers in diameter) and were shallowly emplaced as laccoliths, sills, and dikes. The Eocene intrusions fall into two petrographically and chemically distinct suites: (1) a silica-undersaturated series from nepheline-normative hawaiite to syenite and (2) a silica-oversaturated series from trachyandesite to high-silica, peralkaline rhyolite. The silica-undersaturated rocks dominantly form sills or dikes and, more rarely, laccoliths having little structural relief. The silicic rocks form laccoliths having considerable structural relief, including trap-door laccoliths, and, less commonly, sills or dikes. The undersaturated rocks occur primarily in the central part of the Hen Egg Mountain quadrangle; the oversaturated intrusions occur throughout both quadrangles.A single nepheline-normative hawaiite sill was emplaced 28 Ma ago (late Oligocene) in the southeastern corner of the Christmas Mountains quadrangle. Abundant north-northwest-trending dikes of strongly nepheline-normative hawaiites and basalts were emplaced at 24 to 20 Ma (early Miocene), contemporaneous with initial Basin and Range faulting. These latter rocks commonly host mantle xenoliths.Silicic volcanism accompanying late Eocene intrusion occurred in two areas. A caldera complex in the Christmas Mountains represents an unusual eruptive laccolith, a caldera type not previously recognized. These calderas developed on the Christmas Mountains dome, a large (8 x 5 km), elliptical laccolithic uplift that resulted from emplacement of the caldera magma body. The complex produced an extensive series of rhyolitic to quartz trachytic airfall and ash-flow tuffs, silicic lavas and domes, and coarse debris-avalanche deposits; these rocks are divided into five stratigraphic sequences. At least four caldera collapses accompanied eruption, and the fill in the two youngest calderas dips steeply inward, indicating post-eruption subsidence. More minor volcanism consisted of massive air-fall or ash-flow tuff associated with high-silica, peralkaline rhyolite domes in the northwest corner of the Hen Egg Mountain quadrangle.The major structural features of the quadrangles are domes produced by intrusive emplacement. Laramide folding either did not occur or was a minor influence. Three north-northwest-trending Basin and Range fault zones transect the quadrangles. Displacement is mostly down to the west and less than 1 km.The only significant mineral production in the area was of fluorite formed at the contacts between alkalic, silicic intrusions and Cretaceous carbonate host rocks. The presence of numerous prospects, the unusual trace-element enrichment in several of the fluorite deposits, and the overall geologic setting and types of igneous rocks suggest significant potential for deposits of rare metals.