WIDESPREAD GRENVILLE-AGE MAGMATISM IN THE BASEMENT OF WEST TEXAS AND ADJACENT NEW MEXICO

Much of the crystalline basement of central and western Texas and eastern New Mexico consists of various granitic and rhyolitic rocks. These are commonly associated with mafic intrusions that range in dimensions from m-scale to giant layered intrusions. Previous dating has shown an abundance of 1340 to 1380 Ma quartz monzonite, quartz syenite, and rhyolite: the southern parts of the granite-rhyolite province. Grenville-age felsic magmatism was thought to be limited to the Red Bluff granite/Thunderbird Rhyolite in the Franklin Mountains and the granites of the Llano province (~1120 to 1080 Ma). In contrast, and in spite of 1560 to 1440 Ma depleted mantle model ages (Nd), dated mafic rocks (Pecos mafic intrusive complex) are broadly Grenvillian (1165 Ma). Recent U-Pb (zircon) dating has identified additional, widespread, Grenville-age (1070-1110 Ma) plutonic rocks in the Texas/New Mexico basement. A core of alkali-feldspar granite from the Abilene gravity minimum yielded an age of 1078±23 Ma; similar to undeformed (post-orogenic) granites in Llano uplift. Xenoliths of monzonitic granulite from Potrillo maar (west of El Paso) yielded ages of ~1072 Ma. A similar 1068±30 Ma age was determined for an anorthosite xenolith in the Eocene Three Sisters intrusion in El Paso. These rocks, along with the 1110±19 Ma A-type Red Bluff granite, are indicative of a large Grenville-age anorogenic magmatic event in the El Paso region. In the Texas Panhandle, a >175 m thick gabbroic sill-like body intrudes rhyolitic flows that are part of the granite-rhyolite province. Zircons from a differentiated zone in this sill yielded an age of 1081±8.3 Ma. This is similar to a recent ~1100 Ma 40Ar/39Ar age reported from a gabbroic sill in the basement of eastern New Mexico. These data suggest that Grenville-age magmatism in southern Laurentia was long-lived (1165 to 1068 Ma) and much more widespread than previously thought. It was apparently bimodal, and locally required huge volumes of mafic magmas to form intrusive units such as the Pecos mafic complex and Red Bluff granite.