EPISODIC LATE CRETACEOUS VOLCANISM IN THE BALCONES IGNEOUS PROVINCE, TEXAS

The Balcones Igneous Province (BIP) of Central Texas erupted along the transition between the Texas Mesoproterozoic craton and buried transitional crust thought to be marked by the Ouachita fold belt and Jurassic passive margin.   Our geochronologic study of BIP nephelinites and phonolites suggest that volcanism was active from 76.2 to 83.5 Ma, a span of only ~7 Ma. Primitive mafic nephelinite and basanite samples yield phlogopite ⁴⁰Ar/³⁹Ar ages between 81.5 and 83.5 Ma, representing the earliest stages of volcanism in the BIP. More evolved felsic phonolites yield amphibole ⁴⁰Ar/³⁹Ar and zircon U/Pb ages ranging from 76.2 to 78.4 Ma, representing a second stage of BIP volcanism.  This is a much tighter range than previous efforts, which resulted in an age range of 63-86 Ma.  Xenocrystic zircons from BIP phonolites range in age from 558 to 982 Ma; no Mesoproterozoic zircon ages occur, suggesting that the Texas craton does not lie beneath the BIP. Neoproterozoic age zircons derived from Gondwana are documented in metasediments from the Ouachita thrust and fold belt, which we suspect underlies the BIP. The BIP is the south central Texas segment of the Northern Gulf Margin Magmatic Zone (NGMMZ) that collectively includes volcanic centers in Trans Pecos Texas (72.6 Ma),  to as far east as Prairie Creek, Arkansas (108 Ma), and Jackson Dome, Mississippi (65 Ma). All Late Cretaceous volcanic centers of the NGMMZ occur near the discontinuity separating Mesoproterozoic cratonic lithosphere to the north and west, and Jurassic extended lithosphere to the south and east.  Mafic and felsic BIP episodes were not contemporaneous, but were separated in time by ~3 Ma. Previous work has suggested that volcanism in the BIP can be related to regional stress regimes in place during the Late Cretaceous that caused small scale extension along the craton-transitional boundary.  Previous geochemical results suggest that the primitive nephelinites and basanites from the BIP were derived by small scale melting (< 1%) of HIMU and FOZO mantle sources in the upper mantle.