Petrology and Geochemistry of the Duzel (Ordovician) and Gazelle (Silurian) Formations, Northern California

Abstract

ABSTRACT Co, REE and to a lesser degree Mn, Ti, and Zr, reflect the same lithologic provenances for sandstones in the Duzel and Gazelle Formations as do rock and mineral fragment distributions in these sandstones. Most other elements show significant depletions or less commonly enrichments relative to the inferred source area composition. Mineralogical and geochemical data indicate quite different source area compositions and tectonic settings for the Duzel and Gazelle Formations. The clastic sediments of the Duzel appear to have been derived primarily from plutonic-metamorphic terranes and to have been deposited on a broad, semi-stable shelf or slope. Gazelle clastic sediments reflect eastward-lying contemporary calc-alkaline volcanic sources mixed in varying amounts with plutonic-metamorphic and sedimentary sources. The Gazelle Formation was probably deposited in one or more small eugeosynclinal basins lying west of an active volcanic chain built on or near the edge of the continent. It is not possible with existing data to determine if the Duzel Formation was deposited near its present geographic location (the in-situ model), or if it represents a fragment of an extinct or more highly evolved Ordovician arc system that was later swept in from the Pacific basin by sea-floor spreading and welded onto the continent (the Swept-in model).