Decompression Melting of FOZO and FOZO-HIMU Sources by Extensional Focusing, Balcones Igneous Province, Texas

The NW margin of the Gulf of Mexico basin experienced widespread, small volume, mafic to intermediate alkaline magmatism during Late Cretaceous time. Centers ranging from Trans Pecos to central Texas, SW Arkansas, NE Louisiana, and west central Mississippi collectively comprise a Northern Gulf Margin Magmatic Zone (NGMMZ). All centers are spatially correlated with the discontinuity separating Grenville age (~1.3 Ga) cratonic lithosphere and Early Mesozoic age (~160-210 Ma) transitional lithosphere. Magmatism occurred between 65 Ma (Jackson Dome, MS) and 108 Ma (Prairie Creek, AR). New U/Pb and ⁴⁰Ar/³⁹Ar results from the Balcones Igneous Province (BIP) of central Texas, a smaller segment of the NGMMZ, demonstrate magmatism occurred in two phases: nephelinites and basanites erupted between 81.5-84.1 Ma, and phonolites erupted between 76.2-78.8 Ma. All BIP nephelinites are primitive (molar Mg# > 65), indicating little mantle fractionation; steep REE patterns lacking Eu anomalies indicate formation by low degree melting of a garnet peridotite source (< 150 km depth).  Melting was probably facilitated by high volatile (esp. CO₂) budgets.  New Sr, Nd, and Pb isotopic results from the BIP suggest derivation from FOZO and HIMU-FOZO. FOZO and HIMU sources are commonly associated with deep mantle plume sources, however, there is little evidence for mantle plumes in the BIP based on observed time-space-composition relations. Several mechanisms capable of producing BIP magmatism (e.g. rifting, lithospheric delamination, edge convection, and mantle plumes) have been considered and rejected. Our preferred interpretation is that BIP magmatism reflected lithospheric stresses: maximum compressional stresses orientated parallel to the discontinuity between cratonic and transitional lithospheres resulted in minor extension perpendicular to the discontinuity, initiating decompression melting of volatile charged asthenospheric mantle, magma ascent, and surface eruption. Magmatism in the BIP is best explained by our extensional focusing model, and may also explain magmatism elsewhere in the NGMMZ.