PETROLOGICAL AND GEOCHEMICAL DISTINCTIONS BETWEEN THE NORTHBRAE AND LEONA RHYOLITES OF THE BERKELEY HILLS, CA

Scattered throughout the Berkeley Hills are silica-rich rocks, which have been mapped as Tertiary (Dibblee, 1980; Knox, 1973; Radbruch, 1969; Robinson,1956), but which are now considered to be Upper Jurassic (Jones and Curtis, 1991). The geologic map of the San Francisco-San Jose Quadrangle (California Division of Mines and Geology, 1991) identified these rocks as "rhyolite of uncertain age" and grouped them as part of the Coast Range Ophiolite. Graymer et al. (1996) characterized them as volcanic rocks (keratophyre and quartz keratophyre) overlying the Coast Range Ophiolite (USGS Open File Reports 96-252 and 94-662). Though now mapped as one unit, the widespread Leona and the areally restricted Northbrae may be distinguished form each other by their petrology, their geochemistry, and their geomorphology. Petrology indicates that the Northbrae formed as a glass flow or dome: it contains flow-banding, autobrecciated clasts, relict spherulites, and a microcrystalline matrix. The Leona exhibits no flow-banding or autobrecciation. It contains glomerocrysts of plagioclase in a matrix of plagioclase laths. Northbrae exhibits resilicification textures, and Leona does not. In outcrop, Northbrae surfaces are frequently rounded, with some extremely polished areas (not slickensides). Leona outcrops, in contrast, are jagged, rough, and fractured.

The geochemistry of the Leona Rhyolite shows a flat REE signature, similar to the keratophyres of the Coast Range Ophiolite (Blake and Jones, 1981). By contrast, the Northbrae samples all show light REE enrichment and a negative Eu anomaly. Northbrae plots as an anomalous ridge on the Nb v. Y diagram (Pearce,1984). Northbrae plots within the Iceland zone on the Zr/Nb v. Zr diagram (Shervais,1987).

This research suggests that the Leona and the Northbrae Rhyolites do not have the same petrogenesis and that they are distinct rock units. Some of our data suggests affinities between the Northbrae and the Franciscan Complex. The foregoing suggests that the accretionary history, as well as the mapping, of the San Francisco Bay area are more complex than previously accepted.