U-PB ZIRCON AGES OF SURFACE AND SUBSURFACE SAMPLES FROM TEXAS AND SOUTHERN NEW MEXICO: IMPLICATIONS FOR GRENVILLE-AGE AND TERTIARY MAGMATISM

The presence of 1340 to 1370 Ma basement in Texas and eastern New Mexico is well documented. Large, Grenville-age plutons are also present, but their distribution and compositional ranges are poorly known. New U-Pb zircon ages on subsurface samples (SHRIMP II, Stanford) have identified widespread presence of compositionally diverse Grenville-age (1070-1110 Ma) plutonic rocks. Alkali-feldspar granite core from the Abilene gravity minimum yielded an age of 1078±23 Ma; an age similar to undeformed (post-orogenic) granites in Llano uplift. In the Texas Panhandle, core from a >175 m thick gabbroic sill-like intrusion yielded an age of 1081±8.3 Ma. In situ differentiation of this tholeiitic magma led to an ~7-fold increase in incompatible element concentrations; REE patterns are essentially flat. Further west, an anorthosite xenolith from the Eocene Three Sisters intrusion in El Paso yielded an age of 1068±30 Ma, slightly younger than the main stage of the nearby Red Bluff granite (1110±19 Ma). Further west, in the center of the Rio Grande rift, crustal xenoliths from Potrillo Maar cluster into two ages groups: monzonitic granulites at ~1072 Ma and a suite of dioritic, monzonitic and monzogranitic xenoliths at 26-27 Ma. The fact that the Tertiary xenoliths contain ~1070 Ma inherited zircons, combined with the trace element geochemical data, suggest that they assimilated, or mixed with the partial melts of, local Grenville-age crust. This is consistent with their Nd model ages, which average 1.13 Ga (ε_Nd from –3.3 to –5.3). Moreover, zircons in the granulitic xenoliths yield discordant ²⁰⁶Pb/²³⁸U ages as young as 25 Ma, which could indicate granulite-facies metamorphism at ~26 Ma related rifting. Our results show that Grenville-age magmatism in the TX–NM subsurface was widespread and was coeval with syn- and post-deformation granites in the Llano Uplift of central Texas. However, the compositions of dated samples suggest “A-type” magmatic affinities rather than a subduction-related tectonic setting.