title: Volcanology and Geochemistry of the South Flank of Mount Baker, Cascade Range, Washington
Creators: name: McKeever, Douglas
Publisher: name: Western Washington University
Year: 1977

description: <p>Major element chemical analysis of 97 rocks from the south side of Mount Baker was performed by energy dispersive x-ray fluorescence. Conclusions based on chemistry are:</p>
<p>1) Variation diagrams based on a large number of samples show that variation is not continuous nor, in general, smooth. Rocks from two major stratigraphic positions exhibit compositional coherency within each group and are interpreted as eruptive groups which were extruded over time intervals which were short compared to the lifetime of the volcanic center. The older of the two groups is called the Park Butte phase, and the younger is the Koma Kulshan phase (rocks of which form Mount Baker volcano).</p>
<p>2) Individual samples can be identified as belonging to one or the other of the two phases with good accuracy.</p>
<p>3) Koma Kulshan volcanic rocks are well clustered, are high in silica and alkalies, and are low in total iron, magnesium, and calcium compared to Park Butte volcanic rocks.</p>
<p>4) The Park Butte phase can be divided into three units in the study area based largely on silica and magnesium content. The Koma Kulshan phase consists of dozens if not hundreds of individual flows in the study area. Correlation of flow remnants based on comparison of major element chemistry does not appear possible at this point because variation is almost as great within single flows as it is among flows.</p>
<p>5) Black Buttes volcanic rocks cluster between the Koma Kulshan and Park Butte phases. The Black Buttes rocks analyzed closely resemble the high-silica Park Butte unit.</p>
<p>6) Samples from Table Mountain straddle the andesite-dacite line (Irvine and Baragar, 1971) and are unusually silicic considering their relatively old age.</p>
<p>7) The upper Sulphur Creek flow (less than about 10,300 years old) chemically resembles the Black Buttes phase which is much older. This is an argument against differentiation from a mafic magma.</p>
<p>8) Basalt is present in the Park Butte phase. 9) Baker volcanic rocks are transitional between a divergent and coherent suite (McBirney, 1968; Hopson, 1972).</p>
<p>Field observations show that the explosion index (percentage of fragmental material) is 5 to 10 for the Koma Kulshan and Park Butte phases on the south side of Mount Baker, and 70 or more for the Black Buttes phase. The difference in explosion index may be due in part to the rate of effusion: low for the Black Buttes eruptives, high for the Koma Kulshan and Park Butte eruptives.</p>
<p>Koma Kulshan and Park Butte lava flows are simple, block lava type, averaging 5 km long or less. Koma Kulshan flows are 5 to more than 50 meters thick, while Park Butte flows are more than 50 meters thick. Inclined platy jointing can be used to indicate the terminus of flows in the Koma Kulshan phase because it dips back toward the source at the flow terminus.</p>
<p>Intratelluric plagioclase phenocrysts and synneusis structure indicate much turbulence of the magma prior to eruption. Patchy and oscillatory zoning in plagioclase indicate turbulence in the magma chamber and a decrease in confining pressure during the rise of a water deficient magma. Relative scarcity of hydrous minerals and tephra in the Koma Kulshan and Park Butte phases may indicate low water pressure. Lack of crystal settling in the Park Butte phase is shown by uniform vertical distribution of olivine throughout the flows.</p>