SOURCE ROCK AGES AND DEPOSITIONAL AGE OF PROTEROZOIC METASEDIMENTARY ROCKS IN THE BURRO MOUNTAINS, MAZATZAL PROVINCE, SOUTHWEST NEW MEXICO: A COMPARISON OF LA-MC-ICPMS U/PB ZIRCON AGES AND ELECTRON MICROPROBE MONAZITE AGES

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, the age of deposition, and the age of metamorphism of the metasedimentary sequence was investigated using both LA-MC-ICPMS U/Pb isotope analysis of detrital zircons and electron microprobe analysis of monazite. Our previously reported microprobe monazite ages taken from monazite cores ranged from 2.3-1.7 Ga but low Pb concentrations resulted in 50-150 Ma uncertainties. Major element analyses and microprobe totals near 100% both suggest that alteration was not a factor in the low Pb concentrations of the cores, and common Pb would have to be >150 ppm to account for erroneous ages >1.68 Ga. The core ages older than 1.7 Ga were therefore interpreted to be from detrital grains.

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

The detrital zircon ages confirm that the monazite cores are probably detrital. The smaller beam size (10 microns) and in situ thin-section analysis makes EMP dating better for determining metamorphic ages and relating these ages to fabrics, provided Pb concentrations are >750 ppm. LA-MC-ICPMS analysis is rapid, precise, and is superior for constraining source and depositional ages. Source rocks for the Burro Mountains metasediments contain zircons that range in age from 1.65-1.89 Ga (and possibly Archean). 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 province and 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.