POTENTIAL FOR BADDELEYITE IN U-PB DATING OF MAFIC ALKALINE ROCKS FROM THE BALCONES IGNEOUS PROVINCE, TEXAS

Zircon (Zr₂SiO₄) is the most widely used mineral for U-Pb dating, yet it is uncommon in alkaline mafic rocks and its absence has hindered the dating of these rocks using the U-Pb decay scheme. Baddeleyite (ZrO₂) is a more common accessory mineral in alkaline mafic rocks and is considered to be a reliable geochronometer due to its high U and minimal initial Pb content. Although baddeleyite is a suitable substitute for U-Pb dating when zircon is not present, the development and popular use of baddeleyite in geochronology has been stunted due to a difficult extraction process and a study producing SHRIMP-II ion microprobe results that yielded statistically inaccurate variations in ²⁰⁶Pb/²³⁸U ratios relative to the orientation of the baddeleyite crystal with respect to the primary ion beam. The results of this study became known as the crystal orientation effect. In recent years, simplified methods for extracting baddeleyite have been demonstrated. A recent study produced SHRIMP-II U-Pb analyses of baddeleyite that showed good reproducibility, consistency with zircon ages, and an absence of the crystal orientation effect. These developments reinforce the potential for baddeleyite in geochronology. Much of the geochronologic data from the Balcones Igneous Province (BIP) of south Texas does not use modern geochronologic methods. The BIP is an alkaline igneous province 400 x 100 km in size closely approximating the boundary between Mesoproterozoic cratonic lithosphere of the Texas craton, and Jurassic age transitional lithosphere of the northern Gulf coast region. Felsic phonolitic samples (n=14) from the BIP have been successfully dated using U-Pb (zircon) and ⁴⁰Ar/³⁹Ar (amphibole, and mica) techniques. Of the 23 mafic samples dated (utilizing ⁴⁰Ar/⁴⁰K and ⁴⁰Ar/³⁹Ar techniques) from the BIP since 1971, most results appear problematic. Chemical analyses of mafic BIP samples (n=45) show a range of 150-722 ppm zirconium content, and although these rocks do contain accessory minerals capable of hosting zirconium, no attempts have been made to isolate baddeleyite for geochronologic study. U-Pb dating of baddeleyite from this region could provide more data with which we can further constrain the ages of mafic rocks from the BIP, as well as support a more wide-spread use of baddeleyite in geochronology.