GLEN R. HIMMELBERG Department of Geology, University of Missouri, Columbia, Missouri 65201 ROBERT A. LONEY U.S. Geological Survey, Menlo Park, California 94025 Petrology of the Vulcan Peak Alpine-Type Peridotite, Southwestern Oregon ABSTRACT general and therefore fundamental to the petrogenesis. This report is concerned essen- The alpine-type peridotite in the area of tially with the mineralogy and petrology of the Vulcan Peak, Oregon, is part of the larger Vulcan Peak peridotite; the structure and Josephine ultramafic complex in the Klamath petrofabrics are taken up in a separate report. Mountains geologic province. Partially serpen- tinized, foliated harzburgite with 15 to 30 per- GEOLOGIC SETTING cent orthopyroxene makes up approximately The Vulcan Peak peridotite is part of the 90 percent of the body. The remaining 10 per- larger Josephine peridotite (Wells and others, cent is dunite that occurs in the harzburgite as 1949) which lies within the Klamath Moun- concordant and discordant layers and as ir- tains geologic province of northwestern Cali- regular bodies. In general, the peridotite at Vulcan Peak is similar in structure, texture, mineralogy, and chemistry to the peridotite at Burro Mountain, California. Structures, tex- tures, and compositions of coexisting phases are consistent with high-temperature (1,000° to 1,200°C) deformation and recrystallization in the upper mantle, and tectonic emplacement into its present crustal position. Evidence to indicate whether the peridotite originated as a refractory residue during partial fusion processes that produced mafic melt or by crystallization from an ultramafic or picritic magma remains inconclusive; poikilitic clino- pyroxene enclosing olivine in some dunites, and certain chromitite textures, may represent relict igneous features suggesting a magmatic stage in the history of the peridotite. INTRODUCTION This report is part of a continuing study of selected alpine-type peridotite masses that began with the detailed structural and petro- logic investigation of the peridotite at Burro Mountain, California (Loney and others, 1971). The Burro Mountain peridotite and the perido- tite at Vulcan Peak, Oregon, here informally termed the "Vulcan Peak peridotite," are representative of the harzburgite subtype (Jackson and Thayer, 1972) of alpine perido- Figure 1. Index map showing location of the Vulcan tites. One objective of this study is to deter- Peak area relative to the Josephine ultramafic complex mine those features of the peridotites that are (black) and the Klamath Mountain geologic province characteristic of the harzburgite subtype in (after Hotz, 1971). Geological Society of America Bulletin, v. 84, p. 1585-1600, 7 figs., May 1973 1585 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/5/1585/3443254/i0016-7606-84-5-1585.pdf by guest on 28 September 2021 Figure 2. Geologic map of the Vulcan Peak area, 6 = 10-VP-68, ll-VP-68; 7 == 16-VP-68, 17-VP-68; x = sample location. 1 = 18-VP-68; 2 = 19-VP-68; 8 = l-VP-68,2-VP-68,3-VP-68; 9 = 4-VP-68, 6-VP-68, 3 = 15-VP-68; 4 = 14-VP-68; 5 = 12-VP-68,13-VP-68; 7-VP-68; 10 = 8-VP-68. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/5/1585/3443254/i0016-7606-84-5-1585.pdf by guest on 28 September 2021 VULCAN PEAK ALPINE-TYPE PERIDOTITE, OREGON 1587 fomia and southwestern Oregon (Fig. 1). The others (1971) in accordance with the classifica- peridotite is bounded on the east by a belt of tion established by Jackson (1968, p. 140). At intensely serpentinized peridotite, which sepa- Vulcan Peak, the ultramafic rocks consist of rates it from the rest of the Josephine complex harzburgite and dunite, and the general term (Fig. 2). During the late Mesozoic, the sedi- "peridotite" is used to include both. mentary and volcanic rocks of the Klamath As is common to most alpine-type perido- Mountains province were metamorphosed, de- tites, partial serpentinization is ubiquitous and formed, intruded by igneous plutons, and serpentine minerals generally make up 30 to 50 extensively thrust faulted (Irwin, 1964; Dott, percent of the peridotite. Locally, the amount 1965; Kays, 1968; Coleman, 1971a; Coleman of serpentine minerals may be as low as 10 per- and Lanphere, 1971; and Hotz, 1971). The cent and in other places the ultramafic rock is Vulcan Peak peridotite is, in its western edge, a serpentinite with no relict primary minerals in fault contact with the Dothan Formation of remaining. Except for the margins of the body Late Jurassic age (Fig. 2). In the vicinity of and certain zones in the interior, the serpen- Vulcan Peak, the Dothan Formation consists tinite is not sheared but blocky (see Coleman predominantly of graywacke with interlayered and Keith, 1971, p. 312-313 for definitions) shales and minor amounts of mafic volcanic and the primary rock type can be distin- rocks. Incipient metamorphism of the gray- guished. Serpentine from two specimens of wacke produced the association white mica- blocky serpentinite was identified as lizardite chlorite-albite-quartz and, in some samples, and chrysotile. Serpentinization is a late-stage, pumpellyite. This association is equivalent to low-temperature process (Coleman and Keith, the pumpellyite zone as defined by Blake and 1971) that is probably still occurring today others (1967) and is probably a result of low- (Barnes and others, 1967; Barnes and O'Neill, grade regional metamorphism similar to that in 1969; Wenner and Taylor, 1971). Thus it is areas of the Franciscan Formation in the Cali- unrelated to the petrogenesis of the peridotite fornia Coast Ranges (M. C. Blake, Jr., unpub. and will not be considered further in this data). report. To the north, the Vulcan Peak peridotite is Harzburgite is the dominant rock type, in fault contact with a heterogeneous unit of making up 90 percent or more of the area amphibolite, clinopyroxenite, wehrlite, dunite, studied. The primary mineral assemblage is and intrusive mafic rocks. These rocks repre- olivine-orthopyroxene-clinopyroxene-chromian sent the southwestern end of the Chetco River spinel. The orthopyroxene generally makes up complex (Hotz, 1971). A potassium-argon age 15 to 30 percent; clinopyroxene and chromian of 147 m.y. (R. W. Tabor, 1971, written spinel are present only in accessory amounts, commun.) determined for muscovite from a less than 2 percent. pegmatite in the mafic igneous complex falls The harzburgite contains a prominent folia- within the 140 to 151 m.y. range of potassium- tion resulting from gradational or abrupt varia- argon ages from the northern part of the Chetco tions in the ratio of olivine to orthopyroxene; River complex (Hotz, 1971). As a metamorphic where the change is abrupt, the result is alter- event produced amphibolite-grade metamor- nating pyroxenitic and dunitic layers that phic rocks 150 m.y. ago in other parts of south- range in thickness from 1 to 15 cm (Fig. 3). In western Oregon (Coleman and Lanphere, some pyroxenitic layers, the orthopyroxene 1971), the 147-m.y. muscovite age above and content is as great as 65 percent (3-VP-68). the amphibolite in the Vulcan Peak area Texture oi: the harzburgite is xenoblastic possibly are a result of the same metamorphic granular, with the olivine and orthopyroxene event. The genetic relationships between the exhibiting interlocking grain boundaries. Or- clinopyroxene-rich peridotites, the mafic igne- thopyroxene grains are 3 to 4 mm long; they ous rocks, and the Vulcan Peak peridotite are contain clinopyroxene exsolution lamellae and uncertain. commonly exhibit kink banding. The average original olivine grain size is difficult to deter- STRUCTURAL RELATIONS mine because kink banding within grains and AND PETROGRAPHY OF grain boundary recrystallization have obscured THE PERIDOTITE the original grain boundaries. In general, Ultramafic rock nomenclature used in this though, it seems that the average olivine grain report is the same as adopted by Loney and size is approximately 2 to 3 mm in diameter Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/5/1585/3443254/i0016-7606-84-5-1585.pdf by guest on 28 September 2021 1588 HIMMEL3ERG AND LONEY £> i Figure 3. Pronounced foliation in harzburgite. harzburgite foliation at low angle. Note 6-in.-thick dunite below hammer crosscutung except in narrow shear zones where it is much Tabular bodies of dunite 10 to 50 cm thick, finer. Chromian spinel occurs in the harzburg- are either concordant o.: discordant relative to ite as disseminated, anhedral, reddish-brown the harzburgite foliation. Along strike, the grains approximately 1 mm in size. It is inter- tabular bodies of dunite commonly pinch out granular to the silicate minerals and is in all or intertongue with harzburgite. For simplic- aspects similar to spinel described from the ity, Loney and others (1971) referred to similar Burro Mountain harzburgite (Loney and occurrences at Burro Mountain, California, as others, 1971). sills and dikes. Unfortunately these terms carry Dunite is ubiquitous in the Vulcan Peak unwarranted genetic implications and will not area, but volumetrically makes up less than 10 be used here. percent of the peridotite. It occurs both as The primary mineral assemblage of all the irregular masses and tabular bodies. In maxi- dunites consists of olivine, accessory chromian mum dimension, the irregular bodies range spinel, and a trace of ori:hopyroxene and (or) from 1 m to approximately 50 m. Although clinopyroxene. The texture is xenoblastic contacts are commonly discordant in detail, the granular with the olivine grain size generally in bodies generally parallel foliation in ths the range of 3 to 5 mm. Kink bands are com- harzburgite. In many bodies, dunite and mon to olivine. harzburgite are intricately intertongued, prob- Chromian spinel occurs in the dunite either ably as a result of folding. Where the modal as 0.1 to 0.2 mm subhedral to euhedral grains content of chromian spinel is great enough, it that are intergranular to, and enclosed in, may form discontinuous planar concentrations olivine or as anhedral grains exclusively inter- defining a foliation in the dunite.