Petrogenesis of the Fifes Peak volcanics, south-central Cascades Range, Washington

Conference · Thu Apr 01 04:00:00 EST 1993 · Geological Society of America, Abstracts with Programs; (United States)

OSTI ID:5292067

Babcock, R S; Thompson, J M.S. ^[1]

---

1. Western Washington Univ., Bellingham, WA (United States). Geology Dept.

During the late Oligocene (ca. 27--23 Ma.), a voluminous section of subduction-related basalt to rhyolite flows and interbedded pyroclastics, comprising the Fifes Peak Formation, were extruded in the south-central Cascades Range of Washington State. Microprobe analyses reveal that basalts and basaltic andesites contain megacrysts of aluminous clinopyroxene and spinel (hercynite) that may be xenocrysts derived from an upper mantle source zone or early, high pressure magmatic phases. Mass balance calculations indicate that the compositional variation of the Fifes Peak basalt and andesite was controlled mainly by high-pressure clinopyroxene-dominated fractionation, coupled with lower pressure fractionation of plagioclase + orthopyroxene + magnetite/ilmenite. Trace element data and mass balances also show that the more silicic lavas must contain a significant crustal component (such as a high Ba/Nb pelagic sediment). The dacites and rhyolites were probably formed mainly by crustal melting, rather than contamination of mantle-derived magmas. The Fifes Peak volcanics have a typical arc-type trace-element distribution with distinct depletions in Ta, Nb, and Ti and enrichment in Th and large-ion-lithophile elements. All of the analyzed Fifes Peak flows are light-rare-earth-element enrichment, with a mean La/Yb ratio of 7.6. There is no indication of an OIB mantle source array, nor the low fluid-flux subduction conditions that characterize magmas generated in the southwest Cascades. Apparently the anomalous nature of the subduction zone in that region of the Cascades arc does not extend northward to the south-central Cascades.

• OSTI ID: 5292067
• Report Number(s): CONF-9305259--
• Conference Information: Journal Name: Geological Society of America, Abstracts with Programs; (United States) Journal Volume: 25:5
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
CASCADE MOUNTAINS
CENOZOIC ERA
CHEMICAL COMPOSITION
CHEMISTRY
DEVELOPED COUNTRIES
GEOCHEMISTRY
GEOLOGIC AGES
GEOLOGIC FORMATIONS
GEOLOGY
IGNEOUS ROCKS
MICROANALYSIS
MOUNTAINS
NORTH AMERICA
PETROCHEMISTRY
PETROGENESIS
PETROLOGY
ROCKS
SUBDUCTION ZONES
TERTIARY PERIOD
TRACE AMOUNTS
USA
VOLCANIC ROCKS
WASHINGTON