NEW GEOCHEMICAL AND GEOCHRONOLOGIC CONSTRAINTS ON THE BALCONES IGNEOUS PROVINCE, TEXAS

New geochemical and geochronologic analyses have been conducted on phonolites, and nephelinites from the Balcones Igneous Province (BIP) of south-central Texas to investigate their petrologic and volcanic histories. A basalt from Brewster county was included to explore the idea that the BIP may extend west to the Trans Pecos region, Texas. The BIP is a late Cretaceous alkalic intraplate volcanic-plutonic complex stretching 400 km from near Austin to near Brackettville, Texas. It was emplaced along the buried Pennsylvanian Ouachita orogenic thrust front, a zone coinciding with the interior transition from Mesoproterozoic cratonic lithosphere to Jurassic oceanic lithosphere of the Gulf of Mexico basin. Geochemical analyses (n=14) have included major element, trace element, rare-earth element, radioisotope and electron microprobe techniques. Initial results suggest the nephelinites and phonolites formed from chemically primitive mantle peridotite sources by small degrees of partial melting in the garnet stability field. The phonolites are interpreted to have formed from a nephelinitic parental magma and may represent the final stages of magmatism in the province.  Zircon mineral separates (4 samples) were analyzed by the U/Pb SHRIMP-RG method. The nephelinite (1 sample) produced a ²³⁸U/²⁰⁶Pb age of 86 Ma, consistent with the oldest ages in the BIP. The zircons from the phonolites (2 samples) both produced ²³⁸U/²⁰⁶Pb ages of 77 Ma, over 14 Ma older than previously reported whole rock ⁴⁰K/⁴⁰Ar ages for those samples. The Brewster county basalt (1 sample) produced an age of 73 Ma, over 23 Ma older than any previously known volcanism in that region. Amphibole (3 samples), phlogopite (1 sample) and nepheline (4 samples) mineral separates were analyzed by ⁴⁰Ar/³⁹Ar step heating.  The results of the ⁴⁰Ar/³⁹Ar amphibole and phlogopite analyses indicate an age of 77 Ma for 3 of the phonolites, concordant with ²³⁸U/²⁰⁶Pb zircon ages. The ⁴⁰Ar/³⁹Ar nepheline ages (75 to 48 Ma) were consistently 10-50% younger than the zircon, amphibole and phlogopite ages, probably due to radiogenic ⁴⁰Ar loss in the nepheline. Magmatism occurred over a much shorter time period than previously reported, and recently targeted alkaline outcrops in Cohuila, Mexico may form part of the BIP, extending the province west into Brewster county, Texas.