New Mexico Geological Society Annual Spring Meeting — Abstracts

The Proterozoic deformational history in the San Andres Mountains is being determined through field mapping and ongoing geochronology. The oldest rock type is a deformed granitic gneiss that has been dated at 1.63 Ga (Roths 1991). Amphibolite dikes are both older and younger than underformed granitic plutons dates at 1.46 Ga (Roths, 1991). Amphibolite is boudinaged suggesting extensional deformation. Based upon field data extension was in an E-W direction, emplacing the amphibolite, and then in a N-S direction to form the boudins. Within the boudins a foliation exists that occurred concurrently with boudin formation. Smaller younger granites crosscut the boudins and will be dated to constrain emplacement and deformation. Metasedimentary rocks include quartzite and phyllite. The orthogneiss has been subjected to deformation producing the gneissic banding and a second event that folded this earlier fabric. The metasedimentary rocks are deformed and appear to have been affected by the same events that produced the gneiss. Preliminary results from U-Pb of detrital zircons from the quartzite suggest that the sediments were locally derived from the 1.6 Ga plutons. The younger undeformed plutonic rocks provide constraints on the timing of deformation.