U-Pb geochronology of Proterozoic igneous and metasedimentary rocks in southern New Mexico: Post-collisional S-type granite magmatism

Abstract

A bstract — U-Pb zircon ages, major element geochemistry, and Nd isotopic data are presented from several localities in the southern part of the Proterozoic Mazatzal province of southern New Mexico, excluding the Burro and San Andres Mountains. These data indicate that the bulk of granitic magmatism in the study area occurred at 1655 Ma, and that the granites are largely undeformed or only locally deformed. This group of igneous rocks is statistically younger than ~1675 Ma orthogneisses, indicating that a regional deformational event occurred between 1675–1655 Ma. The 1655 Ma granites are leucocratic with high SiO 2 and Al 2 O 3 and low MgO, have both biotite and muscovite, and thus are S-type granites. Nd isotopic compositions of three samples ranges from ε Nd = 1.1 to –2.0, but no useful mantle model ages were obtained owing to Sm/Nd modification after crystallization. We present a model for their generation as crustal melts following a collision. Our data also show that 1460 Ma plutons are present in the Fra Cristobal Range, Organ Mts., Antelope Hill in the southern San Andres Mountains, and likely at San Diego Mountain. This even was accompanied by metamorphism as indicated by younger zircon growth in granites at Cookes Peak and by metamorphic titanite growth at Mud Mountain. These new ages provide additional information regarding the development of the southern Mazatzal province.