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

摘要

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(SHRTMP-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.