NORTHBRAE RHYOLITE IN THE BERKELEY HILLS, CA: A ROCK WELL-MISUNDERSTOOD

In 1914, Andrew Lawson described the Northbrae as a Pliocene volcanic flow. Since then, the Northbrae has been repeatedly reinterpreted and is currently mapped as that part of the Jurassic Coast Range Ophiolite known as the Leona Rhyolite. Though now mapped as one unit, the widespread Leona and the areally restricted Northbrae are distinguished from each other by their petrology, geochemistry, and morphology. Recent dating suggests that the Northbrae may be a Late Miocene flow. The geochemistry of the Leona Rhyolite shows a flat REE signature, similar to the keratophyres of the Coast Range Ophiolite. The Northbrae exhibits light REE enrichment and a negative Eu anomaly. In discrimination diagrams, the Northbrae plots as an anomalous ridge/within plate granite, whereas the Leona plots as a volcanic arc. The Northbrae formed as a glass flow or dome: it exhibits flowbanding, autobrecciated clasts, relict spherulites, and a microcrystalline matrix. The Leona contains no flowbanding or autobrecciation. Northbrae (and the related Cragmont Rock) exhibits silicification textures; Leona does not. In outcrop, Northbrae surfaces are frequently rounded, with some extremely polished areas. Leona outcrops, in contrast, are jagged, rough, and fractured. This research suggests that the Leona and the Northbrae Rhyolites have different petrogeneses and that they are distinct rock units. One hypothesis explaining these distinct lithologies is that the Northbrae may be an exotic block within the Franciscan Complex or the Coast Range Ophiolite. In Spring 2002, zircons collected from the Northbrae gave a tentative Late Miocene age for its formation. If correct, this date would preclude the Mesozoic exotic-block hypothesis. This date may indicate that the Northbrae is a newly recognized unit of the Late Cenozoic volcanics that erupted in the wake of the northward-migrating Mendocino Triple Junction. Northbrae’s rounded morphology, polished surfaces, and silicification could be explained by travel within the San Andreas fault system. The Northbrae may thus offer a new constraint on displacement within the San Andreas fault system.