The origin and evolution of a continental volcano: Independence, Montana

Details The origin and evolution of a continental volcano: Independence, Montana The origin and evolution of a continental volcano: Independence, Montana

Dec 1985

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1985phdt........17m&link_type=abstract

Ph.D. Thesis Pennsylvania State Univ., University Park.

Computer Science

4

Geomorphology, Igneous Rocks, Lithosphere, Montana, Paleontology, Radioactive Isotopes, Volcanoes, Earth Core, Earth Planetary Structure, Geochronology, Magnesium Compounds, Neodymium Isotopes, Strontium 87, Strontium 89

Scientific paper

A volcano, Independence, in the Absaroka Mountains, Montana was studied. Its igneous record to elucidate the genesis of the volcanic rocks and the composition of the lithosphere in the Bearthooth region was studied. The magma parental to most of the extrusive rocks was a high magnesium andesite; the parent of the Independent stock was a basaltic andesite. Both these magmas evolved by precipitation of plagioclass-rich, olivine-free, magnetite-bearing assemblages of potassic compositions. Crustal contamination-fractional crystallization produced liquid lines of descent with only moderate potash enrichment. Minor crustal rock types can be important contaminants and can produce unexpectedly large deviations n the isotopic compositions of contaminated rocks. The mantle-derived rocks have low (Nd-143, Nd-144), and moderate (Sr-87/SR-86), which reflect the composition of the sub-Bearthooth mantle. The Pb-isotopic compositions of the high-magnesium andesites suggest that enrichment occurred 3.8 By ago. The Pb-isotopic compositions of the anatectites define a 3.3 By isochron, interpreted as the age of regional homogenization of the deep crust.

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