New Mexico Geological Society Annual Spring Meeting — Abstracts

The Burro Mountains in SW New Mexico expose Proterozoic metasedimentary rocks cut by several plutons ranging in age from 1.6-1.2 Ga. The age of the source rocks and the age of deposition was investigated using LA-MC-ICPMS U/Pb isotope analysis of individual detrital zircons. Our previously reported electron microprobe ages of detrital monazite ranged from 2.3-1.7 Ga but low Pb concentrations resulted in 50-150 Ma uncertainties.

Zircons from the one of these metasedimentary rocks were analyzed in order to better constrain the age of the source rocks and the depositional age. A 35-micrometer laser beam was used to analyze zircon cores. We analyzed 49 different zircons which have a range of ²⁰⁷Pb/²⁰⁶Pb ages from 1655 ± 37 to 1892 ± 62 Ma with a mean of 1725 Ma. The 1ð uncertainties range from 18-62 Ma with a mean of 25 Ma. Histograms indicate a main peak of ages at 1725 Ma with smaller peaks at 1680 Ma, 1792 Ma, and 1841 Ma. One grain yielded a Late Archean age of 2878 ± 26 Ma.

Source rocks for the Burro Mountains metasedimentary rocks therefore contain zircons that range in age from 1.65-2.88 Ga. Most Mazatzal province source rocks are thought to be <1.7 Ga, whereas Yavapai basement rocks are as old as 1.85 Ga and may be providing some of the source material for these samples, though Mojave or Wyoming province or other sources cannot be ruled out. The metasedimentary rocks were deposited between 1655 Ma (the youngest detrital zircon) and 1633 Ma (the age of a cross-cutting diabase), which is consistent with the ages of other Mazatzal province volcano-sedimentary sequences. Comparison of these data with those from detrital zircon studies in adjacent areas in eastern Arizona indicates that maximum age of deposition, 1655 ± 37 Ma, is the same as for sedimentary rocks in the Little Dragoon Mountains (1655 ± 4 Ma). The detrital zircon ages in the Burro Mountains are significantly older than those from the Cochise Block (1697-1729 Ma) and Pinal Block (1678-1731 Ma, with one grain at 2448 Ma) in SE Arizona. This probably reflects a greater contribution from Yavapai province igneous rocks as well as from older continental rocks either in the continental interior or possibly from a source other than North America.