GEOCHEMISTRY OF THE BEE MOUNTAIN BASALT FORMATION, BIG BEND NATIONAL PARK, TEXAS

The Eocene Bee Mountains Basalt (BMB) Formation (Chisos Group) consists of a series of widespread lava flows that crop out extensively from western Big Bend National Park to southeastern Big Bend Ranch State Park in south Brewster and Presidio Counties, Texas. These lavas erupted between 34 and 33 Ma, during the post-orogenic phase of the Trans Pecos Magmatic Province (TPMP), an alkali/alkali-calcic igneous province active from 48 to 17 Ma. Geochemical evidence suggests that the BMB consists of two distinct suites of lavas ranging in composition from basalt to trachyandesite. The sodic suite consists of nepheline- or hypersthene-normative basalts to basaltic trachyandesites (48-53 wt% SiO₂) characterized by low TiO₂ (mostly <1.8 wt%) and P₂O₅ (mostly <0.45 wt%) with modest LREE enrichment relative to HREE (Ce_N/Yb_N = 3.4-5.3). The potassic suite consists of quartz- or hypersthene-normative basalts to trachyandesites (49-56 wt% SiO₂) characterized by higher TiO₂ (mostly >1.8 wt%), P₂O₅ (mostly >0.45 wt%), and Ce_N/Yb_N (5.6-10.1). The most primitive basalts in both suites are clearly evolved, as evidenced by low Mg# (0.57), Ni (<52 ppm), and Cr (<30 ppm).