Geochemistry of cherts and associated rocks in the Franciscan and Shimanto Terranes

Analyses of 122 Jurassic-Cretaceous rock samples from the Franciscan Terrane, California, and 24 Cretaceous samples from the Shimanto Terrane, southwest Japan, show that the cherts and associated rocks accompanied by pillow basalts are enriched in Fe and Mn which may have been derived from hydrothermal activities. The results of the fraction analyses for terrigenous, basaltic, hydrothermal, and biogenic end-members in the Franciscan rocks show that hydrothermal emanations caused iron and manganese enrichment in the rock samples and played an important role in the formation of siliceous rocks. Vertical variations of the hydrothermal fraction and TiO2-normalized values for hydrothermal elements such as Fe, Mn, Ni, Zn, Y and Pb show a possibility that the bedded cherts of the Franciscan Terrane were deposited in a hydrothermal field on or near some active oceanic ridge, for more than 30 Ma and were a remnant slice of oceanic crust added to the North American continent. On the other hand, from the field occurrences and vertical variations of TiO2-normalized values of hydrothermal elements, the Shimanto rocks are inferred to have been deposited in a hemipelagic environment such as marginal and inter-arc basins.