CONSTRAINTS ON THE EVOLUTION OF THE CORDILLERAN MARGIN FROM DETRITAL ZIRCON ANALYSES IN THE MIDDLE MEMBER WOOD CANYON FORMATION, MARBLE MOUNTAINS, SOUTHEASTERN CALIFORNIA

Detrital zircon data from the middle member Wood Canyon Formation (Lower Cambrian) exposed in the Marble Mountains in eastern California were collected to examine the provenance ages of this tectono-stratigraphically important unit. Despite numerous studies, considerable controversy remains as to where in the Neoproterozoic--Cambrian stratigraphy that deposits associated with the break-up of Rodinia give way to sediments clearly deposited along the Laurentian passive margin. Subsidence models predict that rifting may be as young as Early Cambrian, with the coarse-grained, arkosic nature of middle Wood Canyon strata a possible signature of such tectonic activity. However, from a sedimentologic and stratigraphic perspective, the Wood Canyon Formation possesses characteristics of having been deposited on a tectonically stable platform, with evidence for rifting being rooted in the Neoproterozoic Kingston Peak Formation. In order to address this problem from a different perspective, we have done single-crystal U-Pb geochronology on more than one hundred thirty detrital zircons from different levels in the middle member of the Wood Canyon Formation. In the Marble Mountains, middle member rocks rest nonconformably on 1.4 and 1.7 Ga granitic basement characteristic of the region. Consequently, local bedrock sources should have supplied abundant zircons if they had been tectonically exposed during rifting. A sample collected directly above the unconformity is dominated by grains of ~1670 Ma, which is consistent with local derivation. Zircons collected from everywhere else in the unit are almost entirely ~1100 Ma in age. A local source for the sample resting on bedrock most likely represents reworking of the regolith upon which the unit lies. The abundance of zircon ages at ~1100 Ma indicates that the great bulk of middle Wood Canyon Formation detritus was delivered from a distant Grenville terrane source, not from local crystalline bedrocks. These data suggest that by the time of deposition of the middle member Wood Canyon Formation, any relic rift shoulder had been deeply dissected such that the continental margin was receiving far-traveled sediments from the cratonic interior. This scenario is more consistent with a passive margin setting rather than a rift origin for the Wood Canyon Formation.