Felsic Columns Enclosed within Mafic Sills: An Interesting Enigma in Big Bend National Park, Trans-Pecos Texas

Big Bend National Park in Trans-Pecos Texas contains numerous Tertiary igneous intrusions. It is fairly common for some of these intrusions to be emplaced near the contact of the Ernst and San Vicente members of the Boquillas Formation. However, in the McKinney Springs Quadrangle located in the northeastern portion of the park, an enigmatic association of igneous rocks has been discovered at three locations within ±1 kilometer of Javelina Creek. Plagioclase-rich columns occur near the base of a mafic sill. The columns have a diameter of 15-25cm, and are spaced 1-3 meters apart in plan view. The long axes of the columns are oriented perpendicular to the base of the sill. Their overall distribution has yet to be positively determined, but observations suggest their height to be a minimum of one meter. Examination of hand samples suggests the columns are composed of ±90% plagioclase. However, due to a lack of thin sections at this time, their specific mineral composition has yet to be determined. There is no obvious contact aureole around the columns. This lack of metamorphism suggests that the minerals within the column crystallized contemporaneously with those within the main body of the sill. A more in-depth study, including thin sections and a detailed distribution both laterally and vertically, of this enigma is planned for the near future.