A DETERMINATION OF THE ORIGIN OF GRANITIC ROCKS OF THE CAJON PASS SCIENTIFIC DRILL HOLE, SAN BERNARDINO MOUNTAINS, CA

The Cajon Pass Deep Scientific Drill Hole is located at the northern edge of the San Bernardino Mountains between the Los Angeles Basin and the Mojave Desert in Southern California. The well reached a depth of 3.5 km; this represents an unusual chance to study rocks formed at depth that have never been exposed to weathering, giving unique insight into the geology of the region. Coring recovered intact igneous and metamorphic rocks varying from granite to diorite, quartzite, and gneiss. Initial petrographic observations suggested a division of the granitic rocks into five units (Barth and Dorais, 2000). The goal of this study is to provide a new division of units based on whole rock geochemistry, mineral geochemistry, and geochronology. Unit I, at approximately 500-573m, and Unit II, at approximately 932-1082m and 1329-1470m, are composed of granodiorite. Units III, IV, and V are composed of compositionally expanded suites of diorite and quartz diorite, tonalite, granodiorite, and minor granite located from 1,470-3500m (Barth and Dorais, 2000). Zircons taken at depths of 1023.3m and 1351.5m in granodiorite, 1655.3m in gneiss, 2427.6m in granodiorite, and 3019.0m in fine grain tonalite were analyzed by ion microprobe to determine the ratios of 238U/206Pb and 207Pb/206Pb. The decay rate of each parent isotope is constant and these ratios are therefore proportional to the age of the mineral. The ratios were plotted on a Tera-Wasserburg graph, with the assumption that the system remained closed and all lead present is a result of the decay of Uranium. This analysis yielded two distinct age groups of 78.9 ± 1.3 to 81.3 ± 0.78 Ma in granitic granodiorite and tonalite, and 151.3 ± 1.3 Ma in gneiss. The tonalite (3019.0m) also contains two relic zircons at 1625.9 ± 12.5 and 1739.0 ± 10.0 Ma possibly incorporated into the melt from a source deeper than the core or a preexisting pluton. With the addition of this new geochronological data, evidence suggests the existence of at least two distinct plutonic suites, the first emplaced at 151.3 ± 1.3 Ma with a younger pluton of 78.9 ± 1.3 to 81.3 ± 0.8 Ma emplaced adjacent to and incorporating zircons from the older unit.