PETROLOGIC STUDY OF ZONED DOLOMITE VEINS IN THE FRANCISCAN COMPLEX, SONOMA COUNTY, CA

A distinctively altered and veined, 1-2 m wide zone of sheared shale intermittently crops out for 1 km at Miwok, Coleman, and Arch Rock beaches along the Sonoma County coast. The zone trends N20W through outcrops of Franciscan graywacke (occasionally well bedded) and mélange, roughly paralleling the main trace of the San Andreas Fault (N35W). The sheared texture, weathering resistance, and abundant dolomitic veins distinguish the sheared zone from the foliated shale in the surrounding mélange.

The oldest dolomitic veins are amorphous patches, followed by deformed veinlets. They are composed of dolomite (to 12 mol% FeCO₃) often rimmed with Fe-rich magnesite. These veins are cut by up to three more sets of sequentially emplaced veins which become progressively less ferroan, straighter, more sparry, and vuggy. The final vein stage dolomite contains less than 2 mol% FeCO₃. Well-defined rhythmic Fe-zoning is present in many late stage veins. Calcite and silicates are absent in all veins.

We interpret the veins as having formed from hydrothermal fluids (with Mg > Ca) in a fault zone. The high Mg content of the fluid is attributed to abundant serpentinite in the Franciscan. The cause of the rhythmic Fe-zoning is unknown.