### Petrographic and chemical reconnaissance study of some granitic and gneissic rocks near the San Andreas fault from Bodega Head to Cajon Pass, California

By: Donald C. Ross

                            https://doi.org/10.3133/pp698

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#### Abstract

This petrographic and chemical study is based on reconnaissance sampling of granitic and related gneissic rock in the California Coast and Transverse Ranges. In the Coast Ranges, granitic rocks are restricted to an elongate belt, the Salinian block, between the San Andreas and Sur-Nacimiento fault zones. These rocks have a considerable compositional range, but are dominantly quartz monzonite and granodiorite. Moist of the Salinian block seems to be a structurally coherent basement block of chemically related granitic rocks. However, on both the east and the west sides of the block, gneiss crops out in abundance; these rocks may be structurally separate from the main part of the Salinian block. In the Transverse Ranges, the granitic and related rocks are dominantly of granodiorite composition, and in many areas granitic and gneissic rocks are intimately intermixed.

Chemically the rocks of the California Coast and Transverse Ranges are somewhat intermediate in character between those of the east-central part of the Sierra Nevada batholith and those of the western part of the Sierra Nevada batholith and the southern California batholith. Probably the closest similarity is to the east-central Sierra Nevada rocks, but the rocks of the Coast and Transverse Ranges are somewhat higher in Al2O3 and lower in K2O than Sierran rocks of the comparable SiO2 content.

Granitic basement rocks of the Salinian block are now anomalously sandwiched between Franciscan terranes. The petrographic and chemical data are compatible with the concept that the Salinian rocks were originally part of the great batholithic belt along the west coast, which is exemplified by the Sierra Nevada hatholith. It also seems most likely that the Salinian block was transported from somewhere south of the Sierra Nevada batholith by large-scale right-lateral movement along the San Andreas fault zone.

#### Study Area

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