Mid-Tertiary magmatism in western Big Bend National Park, Texas, U.S.A.: Evolution of basaltic source regions and generation of peralkaline rhyolite

Abstract

Tertiary magmatism in the Big Bend region of southwestern Texas spanned 47 to 17Ma and included representatives of all three phases (Early, Main and Late) of the Trans-Pecos magmatic province. Early phase magmatism was manifested in the Alamo Creek Basalt, an alkalic lava series ranging from basalt to benmoreite, and silicic alkalic intrusions of the Christmas Mountains. Main phase magmatism in the late Eocene/early Oligocene produced Bee Mountain Basalt, a lava series ranging from hawaiite and potassic trachybasalt to latite, widespread trachytic lavas of Tule Mountain Trachyte and silicic rocks associated with the Pine Mountain Caldera in the Chisos Mountains. Late main phase magmatism produced trachyte lava and numerous dome complexes of peralkaline Burro Mesa Rhyolite (~29Ma) in western Big Bend National Park. Late stage basaltic magmatism is sparsely represented by a few lavas in the Big Bend Park area, the adjacent Black Gap area and, most notably, in the nearby Bofecillos Mountains, where alkalic basaltic rocks were emplaced as lava and dikes concurrent with active normal faulting.Trace element modeling, Nd isotope ratios and calculated depths of segregation for estimated ancestral basaltic magmas suggest that Alamo Creek basalts (εNdt ~6.15 to 2.33) were derived from depths (~120 to 90km) near the lithosphere/asthenosphere boundary at temperatures of ~1600 to1560°C, whereas primitive Bee Mountain basalts (εNdt ~0.285 to −1.20) may have been segregated at shallower depths (~80 to 50km) and lower temperatures (~1520 to 1430°C) within the continental lithosphere. Nb/La versus Ba/La plots suggest that all were derived from OIB-modified continental lithosphere. Late stage basaltic rocks from the Bofecillos Mountains may indicate a return to source depths and temperatures similar to those calculated for Alamo Creek Basalt primitive magmas. We suggest that a zone of melting ascended into the continental lithosphere during main-phase activity and then descended as magmatism died out.Variation within Burro Mesa Rhyolite is best explained by fractional crystallization of a mix of alkali feldspar, fayalite and Fe–Ti oxide. Comendite of the Burro Mesa Rhyolite evolved from trachyte as batches in relatively small independent magma systems, as suggested by widespread occurrence of trachytic magma enclaves within Burro Mesa lava and results of fractionation modeling. Trachyte may have been derived by fractional crystallization of intermediate magma similar to that erupted as part of Bee Mountain Basalt. εNdt values of trachyte lava (0.745) and two samples of Burro Mesa Rhyolite (−0.52 and 1.52) are consistent with the above models. In all, ~5wt.% comendite may be produced from 100 parts of parental trachybasalt.Negative Nb anomalies in some Bee Mountain, Tule Mountain Trachyte and Burro Mesa incompatible element plots may have been inherited from lithospheric mantle rather than from a descending plate associated with subduction. Late phase basalts lack such a Nb anomaly, as do all of our Alamo Creek analyses but one. Even if some slab fluids partially metasomatized lithospheric mantle, these igneous rocks are much more typical of continental rifts than continental arcs.We relate Big Bend magmatism to asthenospheric mantle upwelling accompanying foundering of the subducted Farallon slab as the convergence rate between the North American and the Farallon plates decreased beginning about 50Ma. Upwelling asthenosphere heated the base of the continental lithosphere, producing the Alamo Creek series; magmatism climaxed with main phase magmatism generated within middle continental lithosphere, and then, accompanying regional extension, gradually died out by 18Ma.