The Late Devonian Elwell Formation is part of a late Paleozoic island-arc assemblage exposed in the northern Sierra Nevada, California. In the 10-km interval from Mount Elwell (Plumas County) to Packer Lake (Sierra County), it consists largely of numerous andesitic sills interleaved with phosphatic radiolarian chert and turbidity-current and debris-flow deposits. Peperite, formed by interaction of intrusive andesitic magma and still-moist, incompletely lithified “chert” not far beneath the sea floor, occurs sparingly. The whole assemblage was thoroughly recrystallized (greenschist-facies regional metamorphism) during the Late Jurassic Nevadan orogeny.

The phosphatic chert is composed of white, fluorapatite-rich lenses and nodules enclosed in a black matrix consisting of microgranular quartz and carbonaceous material (probably graphite). Radiolaria (family Entactiniidae) are so abundant that the chert is thought to have originated as radiolarian ooze. The phosphatic lenses and nodules probably formed by direct inorganic precipitation of fluorapatite from pore water within decaying radiolarians at the sediment-water interface.

The andesitic sills are generally aphyric and aphanitic; original textures varied from hyalopilitic in sill interiors to pilotaxitic at sill margins. The andesite was charged with plagioclase (now albite) microlites, and quench morphologies occur where sills intruded the wettest “chert.” Relatively thick sills commonly display columnar structure, whereas a few originally more viscous sills are flow laminated. The top of a sill emplaced essentially at the sediment-water interface is pillowed. Many sills bear xenoliths of phosphatic chert, and scarce quartz megacrysts (evidently derived from the underlying, dacitic Sierra Buttes Formation) appear nearly everywhere. Concentrations of major-element oxides and immobile trace elements indicate that the andesite originated as part of the tholeiitic rock association of island arcs; rare-earth–abundance patterns are particularly diagnostic of this paleotectonic setting.

Most peperite has poorly sorted (coarse ash to coarse block) andesitic fragments uniformly distributed through a voluminous black chert matrix. Where invaded “chert” retained the most pore water, quenched andesitic magma formed irregularly shaped lapilli and small pillows that initially exhibited ductile behavior. Rapid cooling below the softening point of glass resulted in brittle fracture of the lapilli during dispersal through the “chert” by steam explosions. Steam explosions also incorporated into the peperite rigid clasts of locally dewatered chert, and shattered already-formed phosphatic nodules. Where the invaded “chert” was better lithified, freezing sill margins simply granulated quietly in situ by thermal-contraction cracking of microlitic glass; fluid “chert” injected widely spaced contraction joints deep within the sills. In all cases, there was sufficient heated connate water to cause palagonitization and chloritization of andesitic fragments.

Greenschist-facies metamorphism produced two mineral assemblages in the interiors of andesitic sills, the second perhaps reflecting locally higher Pco2/Ph2o during metamorphism: (1) albite + quartz + epidote + actinolite + chlorite + Ti-rich mineral ± calcite ± stilpnomelane ± white mica, and (2) albite + quartz + chlorite + Ti-rich mineral + calcite ± stilpnomelane ± white mica. Epidote-rich specks occurring sporadically in the sills represent metasomatism on a scale of millimetres to centimetres; such specks are mineralogically zoned in andesitic fragments in intensely recrystallized peperite. Additional examples of local meta-somatic activity include syntectonic crystallization of calcite rhombohedra across original vesicle walls in a sill. Cleavage developed locally during regional metamorphism is parallel to axial planes of closely appressed folds that deformed andesitic fragments in peperite and flow laminae in sills.

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