Geochronology of Bimodal Alkaline Volcanism in the Balcones Igneous Province, Texas: Implications for Cretaceous Intraplate Magmatism in the Northern Gulf of Mexico Magmatic Zone

Small-volume Late Cretaceous monogenetic alkaline volcanism along the southern margin of North America resulted in a broad igneous belt more than 1200 km long from the Trans Pecos region of west Texas to central Mississippi, collectively forming a northern Gulf of Mexico magmatic zone (NGMMZ). The locus of igneous activity is associated with the discontinuity separating Mesoproterozoic cratonic lithosphere and Jurassic transitional lithosphere, a zone approximating the southern margin of Laurentia, the subsurface trend of the Pennsylvanian Ouachita orogenic belt, and the trace of the Miocene Balcones fault zone in Texas. Although previous studies have attempted to determine the ages of igneous activity in the region, few well-constrained geochronologic data using modern high-resolution techniques are available. We determined the age of eruption for the Balcones igneous province (BIP), a 400-km-long subsegment of the NGMMZ, using modern 40Ar/39Ar and U-Pb geochronology methods. Our results suggest that previously reported K-40/40Ar and 40Ar/39Ar ages underestimate the age of igneous activity by as much as 17%. New ages from this study, along with reevaluation of previous results, suggest that BIP igneous activity occurred in two discrete phases, each lasting only 2.6 m.yr. and separated by 2.7 m.yr.: older mafic volcanism occurred between 81.5 and 84.1 Ma and younger felsic volcanism between 76.2 and 78.8 Ma. The total interval of 8 m.yr. for BIP igneous activity is much shorter than had previously been inferred. The best geochronologic results are obtained from U-Pb dating of zircon mineral (phonolites) separates and 40Ar/39Ar dating of phlogopite (nephelinites and basanites) and amphibole (phonolites) separates.