Large Grenville-age plutons are known to be present in the 1340-1370 Ma basement in Texas and eastern New Mexico, but their distribution and compositional ranges are poorly known. New U-Pb zircon dates (SHRIMP-RG, Stanford University) on subsurface samples have identified widespread presence of compositionally diverse Grenville-age (1070-1110 Ma) plutonic rocks. In the Texas Panhandle, core from a > 175 m thick, sill-like gabbroic intrusion yielded a date of 1081 ± 8.3 Ma. In situ differentiation of this tholeiitic magma led to a ∼ 7-fold increase in incompatible element concentrations as REE patterns remained essentially flat. Trace element data suggest an E-MORB-like source. To the south, alkali-feldspar granite with A-type affinities was recovered from drill core from the Abilene gravity minimum near Albany, Texas. It yielded a date of 1078 ± 23 Ma; similar to undeformed (post-orogenic) granites in the Llano uplift. Further west, an anorthosite xenolith from the Eocene Three Sisters intrusion in El Paso, Texas yielded a date of 1068 ± 30 Ma which is within error of the 1110 ± 19 Ma age determined for the main stage of the nearby A-type Red Bluff granitic suite. Xenoliths from Potrillo maar volcano in the center of the southern Rio Grande rift include monzonitic xenoliths with granulitic texture which yielded dates of ∼ 1072 Ma. In contrast, Potrillo maar xenoliths with igneous textures yielded U-Pb dates of ∼ 27 Ma. Development of the granulitic texture is interpreted to reflect granulite facies metamorphism during Tertiary time (26-27 Ma) on the basis of age zoning and U concentrations in the zircon crystals. Our results show that Grenville-age magmatism in the Texas and New Mexico subsurface was widespread and was coeval with syn- and post-deformation granites in the Llano uplift of central Texas. The compositions of dated samples suggest that mafic magmas were broadly tholeiitic in character and that granitic magmas were "A-type". These magmatic affinities are thought to be associated with crustal extension rather than with subduction. Moreover, the presence of anorthosite xenoliths in the El Paso-Kilbourne Hole region, the broadly tholeiitic character of mafic rocks, and the A-type nature of the granitic rocks are broadly similar to AMCG (Anorthosite-Mangerite-Charnockite-Granite) suites present in the Grenville province of eastern Laurentia.
- New Mexico
- Southern Laurentia