| Late Cretaceous alkaline volcanism in the Balcones Igneous Province (BIP) of Texas provides a means of investigating the petrogenesis, timing, and duration of activity in an intraplate tectonic setting. Magmatism in the BIP is strongly correlated with the discontinuity separating Grenville age cratonic lithosphere from Jurassic age transitional lithosphere, a zone coinciding with the southern margin of Laurentia during the Late Cretaceous. The BIP is a segment of a larger Late Cretaceous magmatic belt stretching from the Trans Pecos of west Texas, to west central Mississippi, collectively forming a Northern Gulf Margin Magmatic Zone (NGMMZ). All volcanism in the NGMMZ is associated with same structural discontinuity. New geochemical (mineral chemistry, major and trace element), and radiogenic isotope (Sr, Nd, and Pb) analyses are used to model the petrogenesis of the BIP, and test various hypotheses that commonly explain the formation of similar type systems. New modeling suggests that the BIP melts were derived by small degrees of melting of depleted asthenosphere with FOZO and FOZO-HIMU affinities, facilitated by high volatile (exp. CO 2) budgets in the mantle. Stress regimes in place over the region were permissive of small-scale extension normal to the trace of the structural discontinuity, allowing melts to quickly ascend to the surface. Modeling further suggests that exhaustion of volatile budgets in the mantle provided an effective end to the igneous activity, as stress regimes remained constant long after the cessation of activity in the BIP. The results allow commonly invoked models (rifting, lithospheric delamination, edge convection, and mantle plumes) that explain similar occurrences to be rejected, as each hypothesis fails by test. A new extensional focusing model is developed, relating the coincidence of structural discontinuities, volatile budgets in the mantle, and imposed stress regimes to explain the petrogenesis of the BIP. New geochronological analyses, and reinterpretation of previously reported data from the BIP suggest magmatism occurred in two discrete phases, one mafic and one felsic. Mafic magmatism is restricted to 81.5 to 84.1 Ma, and felsic magmatism is restricted to 76.2 to 78.8 Ma. There is no evidence for comagmatic activity between the two phases. |
