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Origin and Emplacement of the Andesite of Burroughs Mountain, a Zoned, Large-Volume Lava £Ow at Mount Rainier, Washington, USA

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Origin and Emplacement of the Andesite of Burroughs Mountain, a Zoned, Large-Volume Lava £Ow at Mount Rainier, Washington, USA Journal of Volcanology and Geothermal Research 119 (2002) 275^296 www.elsevier.com/locate/jvolgeores Origin and emplacement of the andesite of Burroughs Mountain, a zoned, large-volume lava £ow at Mount Rainier, Washington, USA Karen R. Stockstill a;1, Thomas A. Vogel a, Thomas W. Sisson b;Ã a Department of Geological Sciences, Michigan State University, East Lansing, MI 48824-1115, USA b Volcano Hazards Program, US Geological Survey, Menlo Park, CA 94025, USA Received 2 February 2002; received in revised form 3 May 2002; accepted 3 May 2002 Abstract Burroughs Mountain, situated at the northeast foot of Mount Rainier, WA, exposes a large-volume (3.4 km3) andesitic lava flow, up to 350 m thick and extending 11 km in length. Two sampling traverses from flow base to eroded top, over vertical sections of 245 and 300 m, show that the flow consists of a felsic lower unit (100 m thick) overlain sharply by a more mafic upper unit. The mafic upper unit is chemically zoned, becoming slightly more evolved upward; the lower unit is heterogeneous and unzoned. The lower unit is also more phenocryst-rich and locally contains inclusions of quenched basaltic andesite magma that are absent from the upper unit. Widespread, vuggy, gabbronorite-to-diorite inclusions may be fragments of shallow cumulates, exhumed from the Mount Rainier magmatic system. Chemically heterogeneous block-and-ash-flow deposits that conformably underlie the lava flow were the earliest products of the eruptive episode. The felsic^mafic^felsic progression in lava composition resulted from partial evacuation of a vertically-zoned magma reservoir, in which either (1) average depth of withdrawal increased, then decreased, during eruption, perhaps due to variations in effusion rate, or (2) magmatic recharge stimulated ascent of a plume that brought less evolved magma to shallow levels at an intermediate stage of the eruption. Pre-eruptive zonation resulted from combined crystallization^differentiation and intrusion(s) of less evolved magma into the partly crystallized resident magma body. The zoned lava flow at Burroughs Mountain shows that, at times, Mount Rainier’s magmatic system has developed relatively large, shallow reservoirs that, despite complex recharge events, were capable of developing a felsic-upward compositional zonation similar to that inferred from large ash-flow sheets and other zoned lava flows. ß 2002 Elsevier Science B.V. All rights reserved. Keywords: Mount Rainier; Cascade Range; andesites; dacites; magmatic di¡erentiation; lava £ows 1. Introduction 1 Present address: Department of Geological Sciences, Systematic compositional zoning in a lava £ow University of Tennessee, Knoxville, TN 37996-1410, USA. * Corresponding author. Tel.: +1-650-329-5247; or £ow group provides information about the Fax: +1-650-329-5203. evolution and evacuation of the pre-eruptive mag- E-mail address: [email protected] (T.W. Sisson). ma body because it preserves a nearly instantane- 0377-0273 / 02 / $ ^ see front matter ß 2002 Elsevier Science B.V. All rights reserved. PII: S0377-0273(02)00358-X VOLGEO 2518 9-11-02 276 K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296 Fig. 1. Map of the lava £ow and block-and-ash-£ow deposits at Burroughs Mountain, Mount Rainier, WA, showing sample lo- cations (circles), vertical sampling traverses (brackets), and upper^lower lava boundaries (dashed lines). ous partial sampling of the magma reservoir sys- upward con¢guration of the e¡usive £ow. Unlike tem. This perspective is similar to that obtained large ash-£ow sheets, where silicic-¢rst, ma¢c-lat- from the study of chemically zoned or layered er eruption sequences are the rule, diverse zoning ash-£ow sheets (Smith, 1979; Hildreth, 1981). Un- styles in lava £ows suggest to us that competing like ash-£ow sheets, which commonly have a processes in£uence the succession of lava compo- ma¢c-over-silicic zonation (Smith, 1979), lava sitions. These processes are not well understood, £ows can be ma¢c at their base and become pro- in part due to a scarcity of well-studied zoned gressively more silicic upward (Carrigan and Ei- lava £ows. chelberger, 1990; Vogel et al., 1989), or can erupt The Burroughs Mountain lava £ow of Mount in a sequence from early silicic to progressively Rainier, WA (Fig. 1), was studied in detail be- more ma¢c compositions, preserved in successive cause it is an accessible and well-exposed repre- £ows or £ow lobes (Donnelly-Nolan et al., 1991; sentative of the large-volume andesitic lava £ows Kinzler et al., 2000; Coombs et al., 2000). that surround Mount Rainier. As such, its ¢eld, The ma¢c-upward zonation in ash-£ow sheets geochemical, and mineralogical features can help and the silicic-to-ma¢c progression of some lava to reveal the processes leading to sizeable erup- £ows have been attributed to eruption from tions of andesitic lava. The lava £ow is a pheno- chemically zoned and layered magma bodies. cryst-rich andesite that is chemically layered, with The shallow silicic portion erupts ¢rst, followed its upper layer being more ma¢c than its basal by evacuation of more ma¢c magma from pro- zone (Stockstill, 1999), similar to zoned ash-£ow gressively deeper in the chamber (Smith, 1979; sheets. However, the upper layer is itself zoned, Spera et al., 1986; Mills et al., 1997). Eichelberger becoming systematically more felsic upward. This et al. (2000) propose an alternate interpretation zoning re£ects temporal changes in erupted com- that deep silicic magma intrudes shallow ma¢c positions that provide insights into the magmatic magma, rises through it, and erupts ¢rst. The re- zoning of a crustal reservoir and the extraction of verse, silicic-upward zonation in some lava £ows magma during a relatively large andesite eruption. has been attributed to dynamic processes where two magmas of contrasting composition erupt to- gether. Lower viscosity ma¢c magma overtakes, 2. Geologic setting and description encapsulates, and outruns higher viscosity silicic magma during £ow in the conduit (Carrigan et The Burroughs Mountain lava £ow lies at the al., 1992; Carrigan, 1994) resulting in the silicic- northeast foot of Mount Rainier between 2350 VOLGEO 2518 9-11-02 K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296 277 and 1300 m elevation (Fig. 1). It is a large-volume nocrysts and as groundmass grains, but are oxi- (3.4 km3), up-to-350-m-thick and 11-km extend- dized with blebby and lamellar unmixing. Apatite ing andesitic lava £ow, which terminates where it forms inclusions in phenocrysts and less obvious abutted against glacier ice. The £ow’s thickness, needles in the groundmass. Olivine has been its situation high above the adjacent valley, and found in a few thin sections as rare strongly em- ice contact features show that the lava was im- bayed phenocrysts armored by granular orthopy- pounded by and entrenched into the margin of a roxene, and its former presence is inferred in oth- thick Pleistocene glacier (Lescinsky and Sisson, er samples from rare Fe^oxide^orthopyroxene 1998) that ¢lled the present-day White River Val- symplectites. Zircon, Fe^sul¢de, and baddelyite ley. Glacial erosion has cut cirques and cli¡s into are in very low abundance (0^3 grains per thin the north and south sides of the £ow that expose section) as 0.5^5 micron-diameter, rounded grains. nearly continuous sections from the £ow base to Baddelyite forms minute blebs on or within its eroded top. The gentle upper surface of the coarsely exsolved ilmenite, and may have precipi- £ow contrasts with the steep, glacially eroded tated from ilmenite during slow cooling and oxi- £anks, and the lava’s upper surface may be close dation. to the original £ow top, now stripped of its rubbly Quenched magmatic inclusions (Bacon, 1986) carapace. The lava erupted 496 ( þ 7) kyr ago, at are locally abundant in the lower portion of the the beginning of a period of vigorous volcanic lava £ow and in the block-and-ash-£ow deposits, activity spanning 500^420 kyr ago, the onset of but have not been found in the upper part of the which marked the beginning of modern Mount lava £ow. The quenched inclusions are ¢ne- Rainier (Sisson and Lanphere, 2000; Sisson et grained, have ellipsoidal shapes and open vesicles, al., 2001). Similar large-volume lava £ows at and lack phenocrysts, with the exception of traces Mount Rainier have been traced to radial-dike- of olivine and rare resorbed plagioclase grains fed £ank vents (Sisson, unpublished mapping), that were probably entrained from the host mag- and although no direct connection is preserved, ma during mingling. we consider it likely that the Burroughs Mountain Coarse-grained equigranular inclusions of lava £ow erupted through a radial dike system vuggy gabbronorite-to-diorite are widespread in exposed immediately to the southwest (Fig. 1). the lava £ow and reach sizes of 15 cm. Tertiary The lava £ow is porphyritic andesite-to-dacite rocks in the Mount Rainier region di¡er from with abundant medium-to-coarse-grained pheno- these coarse-grained inclusions in many respects, crysts of plagioclase (to 3 mm), and lesser pyrox- and we interpret the inclusions as products of the ene and hornblende (to 1.5 mm). Plagioclase phe- Quaternary Mount Rainier magmatic system. Pla- nocrysts are sharply de¢ned laths with relatively gioclase in nearby Tertiary rocks is characteristi- simple normal and oscillatory zoning parallel to cally clouded with alteration minerals, including phenocryst faces. Embayed zones interrupt oscil- sericite and carbonate, whereas plagioclase in the latory and normal zoning in many phenocrysts, coarse-grained inclusions is glass-clear and free of but the degree of embayment is small, has been alteration.
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