Origin of Paleozoic volcanics, northern Sierra Nevada, California: trace element and isotopic evidence

Conference · Tue Jan 01 04:00:00 EST 1985 · Geol. Soc. Am., Abstr. Programs; (United States)

OSTI ID:6596562

Hannah, J L; Crock, J G; Goldberg, S A

Oceanic arc settings for Devonian and Permian volcanic sequences in the northern Sierra Nevada are suggested by: 1) abundant andesites and dacites; 2) the overwhelming predominance of submarine pyroclastic and epiclastic rocks; 3) localized vent facies; 4) absence of phenocrystic K-feldspar, hornblende, or biotite. Abundances of relatively immobile rare earth elements (REE), Ti, Y, Zr, and Nb, are typical of island arc tholeiites. Whole rock delta/sup 18/O values of 9.2 to 13.1 per thousand reflect low-temperature alteration. Relict quartz and augite phenocrysts, however, have retained original igneous isotopic signatures, yielding average delta/sup 18/O values of 8.2 and 6.0 per thousand, respectively. These low values preclude significant crustal contamination during magma ascent. Initial /sup 87/Sr//sup 86/Sr ratios for unaltered relict augite from Devonian andesite average 0.7082; initial ratios for augite from the Permian volcanics average 0.7045, suggesting a more primitive (back-arc.) magma source. Relatively high initial ratios from the Devonian volcanics require contamination of the magma by older, high Rb/Sr material. The contaminant is most likely a slab-derived component, as crustal assimilation or alteration processes would also increase oxygen isotope ratios. Whole rock initial /sup 87/Sr//sup 86/Sr and La/Yb ratios increase systematically through the Devonian sequence. These variations, which are not readily attributed to alteration, may reflect increasing contribution of subducted sedimentary material during arc maturation and accretionary prism growth.

• Research Organization: Univ. Vermont, Burlington (USA)
• OSTI ID: 6596562
• Report Number(s): CONF-8510489-
• Conference Information: Journal Name: Geol. Soc. Am., Abstr. Programs; (United States) Journal Volume: 17
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580300* -- Mineralogy
Petrology
& Rock Mechanics-- (-1989)
580400 -- Geochemistry-- (-1989)
CALIFORNIA
CHEMICAL COMPOSITION
CHEMISTRY
DEVONIAN PERIOD
FEDERAL REGION IX
GEOCHEMISTRY
GEOLOGIC AGES
IGNEOUS ROCKS
ISOTOPE RATIO
MINERALOGY
NORTH AMERICA
PALEOZOIC ERA
PERMIAN PERIOD
PETROGENESIS
ROCKS
TRACE AMOUNTS
USA
VOLCANIC ROCKS