GEOCHRONOLOGY OF IGNEOUS ROCKS IN THE CHOCOLATE MOUNTAINS REGION AS A MEANS TO INTERPRET TECTONIC EVOLUTION OF SOUTHEASTERN CALIFORNIA

The lower Colorado River trough, part of the Colorado River extensional corridor, includes overlapping tectonic regimes of extension and strike-slip (Sherrod and Tosdal, 1991). Existing whole rock and mineral K-Ar ages of volcanic rocks are inconsistent with stratigraphy, which causes problems when evaluating timing of faulting and magmatism. We have begun a geochronologic study to better understand the timing of extension and strike slip faulting, and the role of magmatism in the area. A total of 78 reliable single grain U/Pb zircon ages, 54 from tuffs and 24 from granites, were obtained on the USGS SHRIMP-RG ion microprobe. Rock ages were calculated from weighted means of 206Pb*/238U ages; pre-magmatic grains are very few and not included in average ages. A sample from the Quechan volcanic rocks in the southern Palo Verde Mountains has an age of 23.4±0.4 Ma. Three samples from the overlying silicic tuff composite unit yielded ages of 24.1±0.3 Ma (Felipe Pass tuff), 23.4±0.2 Ma (Black Hills tuff), and 23.2± 0.2 Ma (ignimbrite of Ferguson Wash). The granite of Mount Barrow yielded an age of 24.2±0.2 Ma, and the granite of Peter Kane Mountain has an age of 24.3±0.9 Ma. These data indicate that plutonism and explosive volcanism were synchronous in this region in latest Oligocene to early Miocene time. Volcanic rocks in the central part of the area dip 30-70 degrees toward the E-NE and are cut by moderately NW-dipping normal faults. Along the southwest margin of this faulted magmatic terrain, 40Ar/39Ar ages record rapid cooling of lower plate rocks in the Orocopia Mountains between 22 and 26 Ma and in the Gavilan Hills between 24 and 30 Ma. This indicates that extension on a regional northeast-directed normal fault system began prior to, but overlapped the eruption of the tuffs and intrusion of the granites. These observations indicate that silicic magmatism and initiation of extension were nearly synchronous on a regional scale, but extension continued after eruption of the tuffs. Volcanic units are locally capped by gently-dipping basalts; ages are locally stratigraphically inconsistent, but indicate a tentative upper age limit for extension between 13.4 and 9.6 Ma. Continuing work includes clarifying timing of extension to strike-slip and investigating characteristics of that transition with respect to magmatism.