THE PRECAMBRIAN EVOLUTION OF THE BURRO MOUNTAINS, SOUTHWEST NEW MEXICO, AS DETERMINED BY ELECTRON MICROPROBE U-PB DATING OF MONAZITE IN METAPELITES

The Burro Mountains of southwestern New Mexico consist of Precambrian metasedimentary rocks intruded by Proterozoic diabase and granitic rocks. The metasedimentary rocks are pervasively deformed. These rocks contain a well-defined gneissic foliation that strikes NW to NE and with dips >65°. A N-S mineral stretching lineation defined by sillimanite is also present. Fold axes trend N-S and axial planes are steeply dipping. The age and tectonic setting of this deformation has not previously been investigated. We determined the age of monazite in metasedimentary rocks using an electron microprobe in order to evaluate the timing of peak metamorphism. For each thin section, we created a chemical map and then analyzed >5 points on 4-7 grains. Most grains were zoned in concentrations of the light rare-earth elements, Th, U, and Pb. The cores of some grains contain high Y and low Th and U. These cores give the oldest ages. Rims of some of these grains have lower Y concentrations and younger ages. Other grains show minimal chemical zonation. Geochronology revealed five episodes of monazite growth: 2.3-2.2 Ga, 1.75 Ga, 1.48 Ga, 1.44 Ga, and 1.27-1.25 Ga. The 2.3-2.2 Ga ages are from three analyses of the core of one grain. Low Pb concentrations result in uncertainties of ± ~150 Ma (all reported errors are 1 sigma). The 1.75 Ga ages are from three analyses on two grains with uncertainties of ± ~50 Ma. The 1.48 Ga ages are based on fifteen analyses on seven grains, with individual uncertainties of ± ~10 Ma. The 1.44 Ga ages are from seven analyses on five grains with uncertainties of ± ~10 Ma. The youngest ages (1.27-1.25 Ga) are based on only two analyses with uncertainties of ± ~20 Ma. Although the oldest ages are probably speculative given the large uncertainties and small number of analyses, perhaps they could represent the ages of detrital monazite. 1.48 Ga and 1.44 Ga ages are similar to the ages of granitic plutons common in the southwest U.S., such as the 1445 ± ~10 Ma (U-Pb zircon; Stacey and Hedlund, 1983) Burro Mountain granite. ~1.25 Ga ages are probably related to granitic intrusions in the Burro Mountains that yield Ar/Ar hornblende and biotite cooling ages of 1.22-1.20 Ga (McLemore et al., 2000). Thus, the main episodes of monazite growth can be linked to the extensive 1.4-1.5 Ga magmatic event.