New Mexico Geological Society Annual Spring Meeting — Abstracts

The Precambrian rocks of the Big Burro Mountains, southwest New Mexico, include mafic to felsic plutonic rocks that intrude metasedimentary rocks of the Bullard Peak series and Ash Creek series (Hewitt, 1959). All units are cut by diabase dikes. It is unclear whether or not the metasedimentary rocks represent roof pendants, large xenoliths, or septa between plutons. At least one large intrusion, the granite of Burro Mountain, is deformed. The Ash Creek series includes calc-silicate schists, andalusite and cordierite bearing hornfels, serpentine-carbonate rocks, and phyllitic schists containing white mica and chlorite. Conditions of metamorphism are likely < ~4 kbar and < ~450°C, or upper greenschist facies, based on assemblages in the metapelitic rocks. The Bullard Peak series includes pelitic and quartzofeldspathic schists and gneisses, migmatites, amphibolites, and calc-silicate schists. Biotite, white mica sillimanite, and garnet are found within metapelitic units, suggesting conditions of peak metamorphism were < ~7 kbar and < ~650°C, or amphibolite facies.

New 1:24,000 scale mapping of metapelitic rocks in the Bullard Peak quadrangle reveals a consistent schistose to gneissic foliation that strikes generally NE and dips both NW and SE. The foliation is folded, and fold axes generally plunge to the NNW. Mineral stretching lineations defined by sillimanite and quartz rods ranges within 30° of N-S with shallow plunges. The granite of Burro Mountain is deformed with a fabric parallel to the foliation in adjacent metamorphic rocks. Other large intrusions apparently post-date deformation.

The timing of metamorphism is still unclear. Two fractions of zircon from a metapelitic gneiss in the Bullard Peak series yield a ~1550 Ma U-Pb date (Stacey and Hedlund, 1983), and a K-Ar biotite date from the same rocks is 1410±50 Ma (Hedlund, 1980). However, a number of ~1200 Ma ⁴⁰Ar/³⁹Ar biotite and hornblende dates were recently reported (McLemore et at, 2000). In order to better constrain the timing of peak metamorphism, we have initiated electron microprobe U-Pb dating of monazite (e.g., Williams et aI., 1999) from the metapelitic rocks.