COMPOSITION OF MODERN SAND FROM THE SIERRA NEVADA: CONSTRAINTS ON ACTUALISTIC PETROFACIES OF CONTINENTAL-MARGIN MAGMATIC ARCS

The Sierra Nevada provides a natural laboratory for study of actualistic sand(stone) petrofacies of continental-margin magmatic arcs. The northern Sierra Nevada consists of an active undissected magmatic arc, as personified by Mount Lassen and related volcanic centers. As the Mendocino triple junction has migrated north during the last 25 million years, active volcanism has ceased, and the Sierra Nevada has been uplifted along a major fault zone along its eastern margin. In the process, volcanic rocks have been eroded, and the underlying plutonic and metamorphic rocks have been exposed. A south-to-north transect of the Sierra Nevada is equivalent to a bottom-to-top profile of a magmatic arc. Modern sand reflects this evolutionary trend of dissection with time, so that the composition of sand derived from all parts of the Sierra Nevada provides base-level characterization of the dissection process. The data gathered from this study can be used to test and improve existing compositional models for the dissection of magmatic arcs. These models, in turn, are useful in reconstructing ancient magmatic arcs from the geological record. Previous work has involved study of modern sand from the Cascade volcanic arc of British Columbia, Washington, Oregon and northernmost California, and from the Transverse Ranges of southern California. The present study fills in the gap along the Pacific margin of the USA. As expected, northern Sierra Nevada sand is dominated by plagioclase and volcanic lithic fragments, and southern Sierra Nevada sand consists of subequal feldspar (more plagioclase than K-spar) and quartz, with minor lithic fragments. Quartz and metamorphic lithic fragments dominate where metamorphic terranes dominate. Actualistic sand(stone) petrofacies models may be refined using these data.