New Mexico Geological Society Annual Spring Meeting — Abstracts

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. We determined the age of monazite by analyzing three metapelitic rocks and one granite using an electron microprobe in order to evaluate the age of the protoliths and the timing of peak metamorphism. For each thin section, we analyzed >5 points on 4-7 monazite grains, for a total of 161 analyses. Most grains were zoned in concentrations of the light rare-earth elements, as well as 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 and high Th concentrations and younger ages. Other grains show minimal chemical zonation.

The ages have a histogram distribution that shows three distinct events. The oldest ages range from ~2.30 to ~2.125 Ga. The next youngest group of ages ranges from ~1.65 to ~1.375 Ga with peaks at 1.42 Ga and ~1.48 Ga. The youngest ages range from ~1.225 to ~1.100 Ga with a mean of ~1.175 Ga.

Typically, older ages have lower Pb, Th, and U concentrations resulting in larger analytical uncertainties. For example, core ages of ~2.20 Ga have 1 sigma errors of ~100 Ma whereas the younger ages of ~1.45 Ga have errors of ~8-10 Ma.

Although the oldest ages (> 1.7 Ga) are probably speculative given the large uncertainties and small number of analyses, perhaps they represent the ages of detrital monazite grains. The oldest protolith dated in the Burro Mountains is a diabase ~1.630 Ga (McLemore et al., in press). In the data presented, no major event is recorded at ~1.630 Ga, but a group of ages randomly cluster from ~1.600 -1.65 Ga which may be related to magmatism at this time. The age clusters at 1.42 Ga and 1.48 Ga are similar to the ages of granitic plutons common in the southwest U.S., such as the 1445 ±10 Ma Burro Mountain granite (Stacey and Hedlund, 1983). The older dates in the youngest group, ~1.225 -1.100 Ga, could represent metamorphism associated with granitic I intrusions in the Burro Mountains that yield Ar/Ar hornblende and biotite cooling ages of 1.22-1.20 Ga (McLemore et aI., 2000).