TIMING OF PROTEROZOIC MAGMATISM IN THE RANCHO LA LAMINA REGION, SONORA, MEXICO: IMPLICATIONS FOR THE MOJAVE-SONORA MEGASHEAR HYPOTHESIS

New U-Pb ages were obtained from zircons from 3 intrusive rocks from the La Lamina Ranch area of northern Sonora, Mexico, north of Cucurpe. These data were collected at the Stanford/USGS facility and the University of Arizona to address the presence and location of a major crustal fault known as the Mojave-Sonora megashear. This structure was postulated to explain the pattern of Proterozoic basement rock ages in Sonora where 1.8-1.7 Ga rocks occupy the Caborca block SW of the fault and 1.7-1.6 Ga rocks occupy the North American block NE of the fault. Our samples are located ~30 km northeast of the main trace of the megashear. An undeformed granite was analyzed using multicollector LA-ICPMS. 20 grains are concordant and yield a concordia intercept age of 1772 ± 9 Ma (MSWD=1.2). Zircons from a strongly foliated granite were imaged using CL and analyzed with the SHRIMP. Grains are both euhedral and anhedral with most having concentric oscillatory zonation. 10 grains were analyzed. U concentrations range from 180-550 ppm and U/Th ratios range from 1-3. All ages are concordant and the same within 2s error with a mean 207Pb/206Pb age of 1737 ± 13 Ma (MSWD=2.4). 23 zircons from a hornblende gabbro were analyzed using the SHRIMP. Many of the grains have thin, high-U rims. One population at 1738 ± 9 Ma has a large MSWD (3.7) and significant scatter about the mean. Another has a mean age of 1404 ± 4 Ma (MSWD=1.1). Most of the older grains have U/Th < 5, whereas most of the younger population has U/Th > 9. We interpret the younger, high U/Th group as reflecting metamorphism at 1404 Ma. The older population may record its intrusive age but zircon abundance and U concentrations from 200-2700 ppm seem unlikely in gabbro and may reflect inheritance from granitic basement rocks as the ages and U/Th are similar to the other sample analyzed from the same area. The ages of 1.77-1.74 Ga are much older than the other ages from the North America block NE of the megashear. The data support one of several interpretations: (1) the major fault lies north of La Lamina; (2) the La Lamina basement was thrust northward from the Caborca block; (3) the distribution of basement ages in northern Sonora is not yet adequately established to locate major crustal structures. Our mapping suggests the La Lamina basement block overlies a south-vergent reverse fault and therefore was not derived from Caborca.