Grenville-age A-type and related magmatism in southern Laurentia, Texas and New Mexico, U.S.A.

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.