CRETACEOUS EXTENSIONAL FOCUSING OF FOZO AND FOZO-HIMU DERIVED MELTS IN THE BALCONES IGNEOUS PROVINCE, TEXAS

Late Cretaceous alkaline mafic to intermediate volcanism along the northern rim of the Gulf of Mexico basin is strongly correlated with the transition from Grenville age cratonic lithosphere to Jurassic age transitional lithosphere. Igneous centers in western Mississippi, central to southwest Arkansas, northeast Louisiana, south central, and Trans Pecos Texas form a larger northern Gulf margin igneous province, as all centers are associated with or near the same lithospheric transition. Igneous activity was relatively short-lived within individual domains, occurred from 108 (Prairie Creek) to 65 (Jackson Dome) Ma, produced small volumes, and resulted in a range of compositions including lamproites, lamprophyres, ijolites, nephelinites, basanites, alkali basalts, phonolites, and syenites in various portions of the region. U/Pb and ⁴⁰Ar/³⁹Ar dates from the Balcones Igneous Province (BIP) in Texas show that activity occurred between 77 and 88 Ma. New isotopic (Sr, Nd, Pb) evidence from the BIP suggests the melts were derived from FOZO and FOZO-HIMU asthenospheric sources by low degree melting of primitive mantle. Modeling suggests melting was facilitated by high volatile (esp. CO₂) contents in the upper mantle. Maximum principal compressional stresses in the BIP were oriented near parallel to the Greenville-Jurassic transition during the Late Cretaceous, allowing small-scale extension perpendicular to the discontinuity, initiating decompression melting of volatile enriched mantle. Although FOZO and HIMU are commonly thought to represent deep mantle sources (plumes), there is no evidence for mantle plume activity in the BIP. BIP sources are from the relatively shallow upper mantle (< 150 km). Our study provides evidence of significant spatially related geochemical heterogeneities in the upper mantle below south central Texas during the Late Cretaceous. Although a number of mechanisms are capable of producing intraplate volcanism with characteristics similar to the BIP (i.e. mantle plumes, edge convection, lithospheric delamination, and rifting), each can be eliminated based on the lack of predicted evidence. Our extensional focusing model provides the best explanation for magmatism in the BIP, and may be applicable to other parts of the northern Gulf margin igneous province.