PETROLOGY OF AN UNMETAMORPHOSED QUARTZ KERATOPHYRE BODY, FRANCISCAN COMPLEX, SONOMA COUNTY, CA

Quartz keratophyres are felsic flows or tuffs mineralogically characterized by quartz and albite phenocrysts and geochemically distinguished by a high soda/ potash ratio (5.4 in this body). They are commonly associated with high soda basalts (spilites). Quartz keratophyres occur most commonly in California in the Coast Range Ophiolite, but have also been described as independent bodies, as at Little Black Mountain (Erickson, 1995 and Bailey et al, 1964).

The quartz keratophyre body studied is located near Occidental, California (UTM 10 S 0505611E, 4251853N NAD27) and was mapped in recon by Christensen (1973). The study area has 2 exposures; a 40m roadcut of keratophyre breccia intruding brecciated metabasalt and a ~50m thick sequence of keratophyre tuff, perhaps caldera fill, exposed in a quarry ~50m south of the roadcut. Metabasaltic xenoliths in keratophyre exhibit greenschist facies mineralogy of chlorite +/- epidote +/- quartz and are variably reacted. It is not known if these xenoliths are spilitic. The tuff contains primocrysts of plagioclase, now pure albite, and quartz in a matrix of minute fragments and local layers rich in ~1mm spherical bodies (Pele's tears); no relict shards or pumice are present. A Pearce (1984) granitoid spidergram shows a strong arc signature. The absence of pumice and probable burial of the keratophyre by Tithonian shale (Christensen, 1973 and Gradstein et al, 1994) indicates a submarine eruption and a minimum age of eruption of 147 +/- 3Ma. Contrary to prevailing theory in keratophyre formation (Dickinson, 1962), this unit has not undergone posteruptive metamorphism. Diagenetic alteration has occurred as shown by abundant leaf-green celadonite in the tuff, plagioclase alteration to pure albite in tuff and lava, and SiO2=84% in the tuff.