DETRITAL ZIRCON AGE PROVENANCE OF LATE PRECAMBRIAN TO EARLY CAMBRIAN SEDIMENTS FROM THE SAN BERNADINO MOUNTAINS, CALIFORNIA

In the San Bernadino Mountains of southern California, a discontinuous to semicontinuous succession of quartzites and phyllites records North America's transition from a rifted to a passive margin in the Eocambrian. Known in part as the Big Bear Group (Cameron, 1982), this succession can illustrate how provenance patterns change with the evolving margin. Using the SHRIMP-RG to measure the age distributions of detrital zircon populations, provenance patterns were determined for several units in the Big Bear and overlying Cambrian units, the Wood Canyon formation, and the Zabriske Quartzite. Age distributions displayed a trimodal distribution with peaks at ca. 1.0-1.2 Ga, 1.3 -1.5 Ga, and 1.7-1.8 Ga. These populations vary relative to one another within individual units, giving each unit a distinct pattern. Minor age populations appear as distinguishing features. The Zabriskie Quartzite for example possesses 2.6-3.4 Ga grains, a subpopulation entirely absent in the underlying Wood Canyon formation. The Wood Canyon is distinguished by an abundance of 1.0-1.2 Ga grains. The Zabriskie has little 1.0-1.2 Ga material relative to its much large 1.7-1.9 Ga grain population. The basal unit of this succession, the Wild Horse Quartzite differs from these upper units in having relatively equal proportions of 1.0 -1.2, 1.3-1.5 and 1.7-1.8 Ga material. Regardless of relative proportions, the age populations bear only some resemblance to nearby basement sources. Widespread cratonic or even exotic sources are necessary to create these provenance patterns. The nature of this source and how it fed sediment to the margin is an important factor in determining North America's role in Proterozoic continental arrangements.