COEVAL SILICA-UNDERSATURATED AND -OVERSATURATED SYENITE IN THE BOFECILLOS VENT RING DIKE, BIG BEND RANCH STATE PARK, WEST TEXAS

The ~27 Ma Leyva Canyon volcano in Big Bend Ranch State Park represents a magmatic episode transitional between Laramide subduction and Basin and Range extension and comprises lava flows of mostly quartz trachyte that erupted from the Bofecillos vent. The vent is defined by two topographically-high arcuate ridges that form a ring dike around a central caldera. The southern ridge comprises ne-normative coarse-grained syenite with a cumulate texture at the base that grades to a phenocryst-poor microsyenite at the top. The northern ridge is coarse-grained syenite that contains ne-normative, hy-normative, and qtz-normative compositions. Syenite contains large crystals of anorthoclase and titanaugite (w/ aegirine-augite rims), with lesser Fe-rich olivine (mg#=0.23-0.52) + plagioclase + oxides + apatite ± nepheline ± pyrochlore ± baddeleyite ± analcime ± zirconolite. Minor amphibole (ferro-pargasite, edenite, richterite, magnesio-katophorite) and biotite are late phases. Syenite from the southern ring dike follows a trend of increasing K₂O (2.51-5.17 wt.%) over a narrow SiO₂ range (49.39-53.15 wt.%), whereas that from the northern ring dike shows a similar range in K₂O contents but a wider range in SiO₂ (54.90-62.66 wt.%). Distinctions between the southern, silica-undersaturated rocks and the northern, silica-saturated to -oversaturated rocks are also observed in the compositional trends of TiO₂, Al₂O₃, MnO, CaO, Sr, Nb, and Zr. We envision a sub-volcanic magma chamber that experienced open-system behavior, in which magma in the upper part of the chamber was contaminated (hyp- and qtz-normative), whereas that in the lower part remained uncontaminated (ne-normative). Asymmetrical collapse of the caldera floor led to compression of the magma chamber and intrusion of the silica-saturated magma along the northern ring fault. Continued caldera collapse further compressed the chamber, tapping the lower, uncontaminated magma, which was emplaced along the southern ring fault. Quartz trachyte lava compositions plot at the terminus of the silica-oversaturated syenite trend, confirming results of a previous study (White & Urbanczyk, 2001, JVGR) that suggested they were the products of open-system behavior.