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Journal of Petrology | Volume 43 | Number 4 | Pages 663-703 | 2002
© Oxford University Press 2002
Petrogenesis and Implications of Calc-Alkaline Cryptic Hybrid Magmas from Washburn Volcano, Absaroka Volcanic Province, USA
1DEPARTMENT OF EARTH SCIENCES, MONTANA STATE UNIVERSITY, BOZEMAN, MT 59717, USA
2INSTITUTE OF MINERALOGY AND GEOCHEMISTRY, UNIVERSITY OF LAUSANNE, BFSH 2, 1015 LAUSANNE, SWITZERLAND
Received June 16, 2000; Revised typescript accepted October 18, 2001
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ABSTRACT |
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The petrogenesis of calc-alkaline magmatism in the Eocene Absaroka Volcanic Province (AVP) is investigated at Washburn volcano, a major eruptive center in the low-K western belt of the AVP. New 40Ar/39Ar age determinations indicate that magmatism at the volcano commenced as early as 55 Ma and continued until at least 52 Ma. Although mineral and whole-rock compositional data reflect near equilibrium crystallization of modal phenocrysts, petrogenetic modeling demonstrates that intermediate composition magmas are hybrids formed by mixing variably fractionated and contaminated mantle-derived melts and heterogeneous silicic crustal melts. Nd and Sr isotopic compositions along with trace element data indicate that silicic melts in the Washburn system are derived from deep-crustal rocks broadly similar in composition to granulite-facies xenoliths in the Wyoming Province. Our preferred explanation for these features is that mantle-derived basaltic magma intruded repeatedly in the deep continental crust leading to fractional crystallization, silicic melt production, and homogenization of magmas, followed by ascent to shallow reservoirs and crystallization of new plagioclase-rich mineral assemblages in equilibrium with the intermediate hybrid liquids. The implications of this process are that (1) some calc-alkaline magmas may only be recognized as hybrids on purely chemical grounds, particularly in systems where mixing precedes and is widely separated from crystallization in space and time, and (2) given the role ascribed to crustal processes at Washburn volcano, the variation between rocks that follow calc-alkaline trends in the western AVP and those that follow shoshonitic trends in the east cannot simply reflect higher pressures of fractionation to the east in Moho-level magma chambers in the absence of crustal interaction.
KEY WORDS: petrogenesis; magma mixing; calc-alkaline; Absaroka Volcanic Province; 40Ar/39Ar dates
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INTRODUCTION |
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The widespread, but poorly understood Challis–Absaroka volcanic episode affected large areas of the northwestern USA in the Eocene following Laramide crustal shortening (i.e. the ‘Challis arc’; Armstrong, 1978
). Volcanic rocks associated with this event are particularly voluminous in the Absaroka Volcanic Province (AVP) of northwestern Wyoming and southwestern Montana, USA, where >20 000 km3 of calc-alkaline to shoshonitic magmatic rocks are exposed in the Absaroka, Gallatin, and Beartooth Ranges (Fig. 1; Absaroka Volcanic Super group of Smedes & Prostka, 1972). Because the Absaroka volcanic rocks have long been presumed to have calc-alkaline compositional affinities and across-strike enrichments in K2O similar to magmas erupted in some continental volcanic arcs (e.g. Dickinson & Hatherton, 1967; Chadwick, 1970), early workers attributed their origin to shallow subduction of the Farallon plate beneath the North American plate during the Eocene (e.g. Lipman et al., 1972). However, this interpretation is now controversial for several reasons. First, spatial and temporal studies have been unable to decipher any logical time-transgression of magmatic activity throughout the northwestern USA during the Eocene, with the result that the ‘Challis arc’ was much wider than any modern arc and oriented perpendicular to the present plate margin (Fig. 1). On the basis of this distribution and geochemical arguments some workers now consider the Challis–Absaroka volcanic episode as resulting mainly from regional lithospheric extension and resultant decompression melting (Dudas, 1991; Hooper et al., 1995; Morris et al., 2000). Second, the origins of magmatic rocks in the AVP are poorly understood because there exist few detailed studies aimed at deciphering the petrologic evolution of magmas erupted from individual eruptive centers. This lack of detailed studies has led to uncertainty on a number of issues ranging from the role of crustal interaction during differentiation of the calc-alkaline magmas to the significance of the across-strike K2O enrichments in the AVP.
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To address some of these issues and provide a reference point for future studies we present results from our investigation of Washburn volcano, one of the principal eruptive centers for calc-alkaline magmas in the AVP and the type locality for the Washburn Group of the Absaroka Volcanic Supergroup of Smedes & Prostka (1972; Fig. 1). Overall, this work represents the first detailed study of a calc-alkaline (sensu stricto) eruptive center in the AVP. The goals of this paper are (1) to document the ages and ranges of major element, trace element, and isotopic (Sr and Nd) compositions of magmatic products at Washburn volcano, (2) to place constraints from phenocryst compositions on the crystallization history of Washburn rocks, (3) to evaluate the possible roles of different petrologic processes in producing the spectrum of compositions observed, and (4) to use the information in (1)–(3) to develop a generalized model for the evolution of calc-alkaline magmas at Washburn volcano and in the AVP.
The origin of intermediate composition rocks at the volcano is best explained by mixing between variably fractionated and contaminated mafic magmas and heterogeneous silicic partial melts of Archean granulite-facies rocks in the deep crust. Despite chemical evidence for mixing, there is very little textural or mineral chemistry evidence in the hybrid magmas to support the model. A scenario where hybrid magmas produced in the deep crust ascend to shallow reservoirs and crystallize low-pressure mineral assemblages dominated by plagioclase is preferred to explain these relationships. These results have important implications for the interpretation of igneous textures and their bearing on rock-forming processes and the significance of across-strike geochemical variations in the AVP.
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TECTONIC AND GEOLOGIC SETTING |
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Existing geochronologic information indicates that the bulk of the magmas in the AVP were erupted between 54 and 38 Ma, following or possibly overlapping the latest phases of Laramide foreland thrusting, which ended at
55 Ma at this latitude (Love et al., 1975; Armstrong, 1978; Feeley et al., 1999; Hiza, 1999). Eruption of the Absaroka rocks also appears to temporally coincide with the onset of regional crustal extension in the northwestern USA. Evidence supporting this contention comes from abundant isotopic ages of metamorphic and igneous rocks exposed in the cores of extensional complexes in Idaho and Washington State that indicate tectonic elevation and denudation at 45–50 Ma (Burchfield et al., 1992), and interbedded tuffs in synextensional basin-fill deposits that range from 46 Ma to younger than 30 Ma (Janecke & Snee, 1993). Nevertheless, although temporally associated with regional crustal extension, there is little evidence within the AVP for the presence of major early Tertiary tectonic extensional faults. Several large-displacement extensional structures involving the volcanic rocks are present, but these are generally regarded as features related to east-directed gravitational sliding off the growing volcanic highland (Hauge, 1985).
At present, the AVP covers 23 000 km2 in northwestern Wyoming and southwestern Montana (Fig. 1). It has been tentatively correlated with other Eocene volcanic rocks in southwestern Montana, the Challis volcanic rocks of Idaho, and the Colville igneous complex of NE Washington, although these fields are not linked by outcrops (Fig. 1; Armstrong, 1978; Dudas, 1991). Undoubtedly, much of the original extent of the volcanic field is eroded or covered by Miocene and younger volcanic rocks of the Snake River Plain–Yellowstone Plateau fields, and continuity with volcanic rocks of similar age may be obscured. However, apparent regional geochemical trends within the AVP, such as increasing K2O contents eastward, suggest it may be separate.
According to correlations advanced by Smedes & Prostka (1972) for Eocene rocks exposed in Yellowstone National Park, three groups make up the Absaroka Volcanic Supergroup. In ascending stratigraphic order these are (1) the Washburn Group, (2) the Sunlight Group, and (3) the Thorofare Creek Group. The areal distribution of the three groups, illustrated in Fig. 1, in part reflects the prevalent thinking at the time of the study of Smedes & Prostka (1972) that volcanic activity migrated SE along two subparallel belts of intrusive–extrusive centers: the ‘K-poor’ (calc-alkaline) western belt and the ‘K-rich’ (shoshonitic) eastern belt (Chadwick, 1970). However, subsequent stratigraphic and geochronologic studies in eastern and southern areas of the volcanic sequence are incompatible with the nomenclature established in Yellowstone National Park (e.g. Brown, 1982; Eaton, 1982; Sundell & Eaton, 1982; Hague, 1985; Decker, 1990; Hiza, 1999). Even within the park itself, Smedes & Prostka (1972) encountered major difficulties in establishing regional stratigraphic subdivisions because many of their formational units consist of monotonous sequences of andesitic volcaniclastic rocks, which are difficult to distinguish at different localities. Furthermore, Hiza (1999) has recently argued that mafic magmas (i.e. <56 wt % SiO2) are characteristically potassic throughout the AVP and that if only these compositions are considered, little regular geographic variation in magma chemistry exists. In spite of these complications we illustrate the inferred location of the K2O dividing line of Chadwick (1970) and the stratigraphic subdivisions of Smedes & Prostka (1972) in Fig. 1 because this facilitates comparison with previous studies. Our work also demonstrates that mafic magmas erupted at Washburn volcano and the Electric Peak–Sepulcher Mountain eruptive center (Lindsay & Feeley, 1999, and this study) are not especially potassium rich. We recognize, however, that the stratigraphy and eruptive history of the AVP are undoubtedly more complex than originally envisioned (e.g. Hiza, 1999).
Seismic refraction studies indicate that the crust beneath the AVP at present is 45–50 km thick (Prodehl & Lipman, 1989). Exposed basement rocks include Archean crystalline rocks of the Wyoming Province (Fig. 1), which are mainly granitoid gneisses that intruded high-grade metasedimentary and metavolcanic rocks at 2·8 Ga, and shallow marine carbonate and clastic sedimentary rocks ranging in age from Cambrian to Cretaceous (Ruppel, 1972; Wooden & Mueller, 1988). Deep- to mid-crustal lithologies are represented by mafic to silicic Archean granulite-facies xenoliths carried to the surface by Eocene alkalic magmas in the Crazy, Bearpaw, and Highwood Mountains (Dudas et al., 1987; Collerson et al., 1989; Joswiak, 1992), and late Cenozoic basaltic magmas of the Snake River Plain (Leeman et al., 1985). Many of these xenoliths are similar to granulite-facies rocks worldwide in that they have relatively low contents of Rb, U, and heavy rare earth elements (HREE), although they are light rare earth element (LREE) enriched (Leeman et al., 1985; Joswiak, 1992).
Stratigraphy
Washburn volcano is a major calc-alkaline eruptive center in the AVP and is the largest Eocene volcanic center exposed in Yellowstone National Park. It is the primary source area for the Lamar River Formation, the eastern member of the Washburn Group of Smedes & Prostka (1972). The Lamar River Formation is particularly well known to visitors of Yellowstone National Park because in it are preserved the famous upright fossil forests (Dorf, 1964). Washburn volcano has been previously mapped by Schultz (1962; 1:30 000) and Prostka et al. (1975; 1:62 500; Fig. 2).
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The eroded northern flank of Washburn volcano in the vicinity of Mt. Washburn, and Hedges and Dunraven Peaks in the SW Washburn Range, consists of 1300 m of volcanic vent facies strata, mainly of the Lamar River Formation, that include dikes, lava flows, flow breccias, and debris flow deposits that dip up to 30° away from the primary vent region (Fig. 2; Prostka et al., 1975). The lava flows and dikes are largely pyroxene basaltic andesites and andesites, although numerous amphibole-bearing dacitic lava flows are present near the base of the sequence. With increasing distance from Mt. Washburn and Hedges and Dunraven Peaks the vent-facies rocks grade into alluvial-facies lithologies consisting of epiclastic volcanic conglomerate and breccia, volcanic sandstone and siltstone, and ashfall tuff deposits. The southern flank of Washburn volcano is truncated by the northern segment of the Yellowstone Caldera fault, exposing the interior of the volcano (Fig. 2). Here, fine-grained biotite tonalite of the Sulphur Creek stock intrudes stratigraphically low Lamar River Formation volcanic rocks. This stock is similar in composition and age (see below) to the Eocene volcanic rocks and represents a shallow intrusion related to Washburn volcano.
The samples examined in this study were chosen according to the stratigraphic units of Prostka et al. (1975; Fig 2) from well-exposed, vertical stratigraphic sections dominated by vent-facies rocks. Temporal and spatial variations in bulk chemistry for lava flows within the Washburn volcanic sequence are shown schematically in Fig. 3. In Fig. 3 (and subsequent figures) we designate with different symbols lava flows exposed on Mt. Washburn and those in the SW Washburn Range to the west of the Grand Loop Road because these have different compositional ranges. Lava flows and dikes in the SW Washburn Range consist of a crudely bimodal package of olivine + pyroxene basaltic andesites and amphibole-bearing dacites, whereas dikes, stratigraphically higher lava flows, and the Sulphur Creek stock to the east and NE on Mt. Washburn are predominantly olivine + pyroxene basaltic andesites and pyroxene ± amphibole andesites. Included in this latter sequence are the stratigraphically highest exposed lava flows on Mt. Washburn that Prostka et al. (1975) designated as part of the Langford Formation of the Thorofare Creek Group of Smedes & Prostka (1972; Figs 2 and 3). Although these flows were originally interpreted as younger (middle to upper Eocene) and erupted from vents much more distal than other units at the volcano, our work shows that they are comparable in age and composition with other flows on Mt. Washburn. We therefore consider all exposed units to be derived from the same or very similar magmatic systems.
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GEOCHRONOLOGY |
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To ascertain the timing of magmatism at Washburn volcano we determined 40Ar/39Ar ages from phenocryst and groundmass samples of the stratigraphically highest and lowest exposed lava flows together with a biotite separate from the Sulphur Creek stock (Table 1). The data are graphically presented in the form of 40Ar/39Ar age spectra and 39Ar/40Ar vs 36Ar/40Ar isochron diagrams in Fig. 4. All errors on ages and intercepts reported in Fig. 4 are 2
. The errors on individual steps, graphically represented by the width of rectangular boxes on the age spectrum diagrams, also represent a 2 level of confidence. The sample localities are shown in Fig. 2. Details of the analytical procedures are described in the Appendix and a summary of the results is presented in Table 1. The full dataset may be downloaded from the Journal of Petrology Web site at http://www.petrology.oupjournals.org.
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Plagioclase and amphibole phenocrysts and a sample of fine-grained groundmass were separated from a dacitic lava flow at the base of Hedges Peak (MW971), the stratigraphically lowest eruptive unit, to date the initiation of volcanic activity at Washburn volcano. Theoretically, all three samples should yield identical ages because all were at high temperatures immediately before extrusion and cooled rapidly once emplaced. The amphibole sample yielded an 40Ar/39Ar spectrum with evidence of excess argon in the initial heating steps, and most of the gas released defined ages of 60 Ma (Fig. 4a). The integrated total fusion age (roughly equivalent to a K–Ar age) is 59·3 Ma; however, no age plateau was obtained. The corresponding K/Ca ratios determined from the step heating data indicate a relatively homogeneous sample, possibly with some K-rich phyllosilicate contamination consistent with the high K/Ca ratio in the first step. Data from the amphibole sample failed to plot on a statistically meaningful isochron, precluding an independent evaluation of the isotopic ratio of trapped argon and the age of the sample. An internally discordant 40Ar/39Ar age spectrum was obtained from the plagioclase, which has an integrated fusion age of 54·7 Ma (Fig. 4b). Intermediate heating steps defined ages of 50 Ma, but the last 30% of gas released defined ages in excess of 60 Ma. The plagioclase data also failed to plot on a statistically meaningful isochron. The groundmass sample also has an internally discordant 40Ar/39Ar age spectrum, with a integrated fusion age of 54·06 Ma (Fig. 4c). Apart from the initial heating step, the K/Ca ratios are consistent with argon degassing from a relatively homogeneous sample. Isotopic data obtained from intermediate heating steps, representing 50% of the total argon released, plot on an isochron defining a sample age of 55 Ma (Fig. 4d). Because the isochron age is calculated making no assumptions about the isotopic composition of the trapped argon, we consider this age as the most reliable estimate for dating the emplacement of lava flow MW97-01. Moreover, the isochron age is similar to the integrated fusion ages of both the groundmass and the plagioclase. This interpretation suggests that the amphibole age of 59 Ma is unreliable, and it is consistent with no published reports of such old eruptive ages being described from the AVP. We therefore consider the amphibole data as anomalously old, probably because of extraneous argon. A similar result was obtained in the biotite data of sample MW9746 described below.
The 40Ar/39Ar age spectrum obtained for the groundmass of sample MW9743 is internally discordant with an integrated fusion age of 44·4 Ma (Fig. 4e). The highly variable K/Ca ratios are consistent with argon degassing from reservoirs of strongly varying retentivity and composition. When plotted on an isochron diagram, however, the data from heating steps making up 97% of the argon released define a relatively precise (MSWD = 3·9) age of 51·9 Ma for the effusion of this lava (Fig. 4f). Taken together, the isochron ages from the groundmass samples of the stratigraphically lowest and highest samples indicate that the >1 km thick accumulated Washburn volcanic pile was constructed in 3 my.
Biotite from the Sulphur Creek stock sample MW9746 yielded an 40Ar/39Ar age spectrum with evidence of extraneous argon in the first few heating steps with an integrated fusion age of 61·6 Ma and a well-defined age plateau of 53·5 ± 0·4 Ma (Fig. 4g). However, an isochron with these data (MSWD = 3·2) indicates a slightly younger age of 52·6 ± 0·2 Ma (Fig. 4h). The isochron age of 52·6 Ma is our preferred date for the sample, and the 40Ar/36Ar ratio of the trapped argon component (323) is significantly greater than present-day atmosphere (295·5) and is consistent with ‘excess’ argon as previously inferred for the amphibole in sample MW971.
On the basis of the 40Ar/39Ar data presented here we interpret magmatism at Washburn volcano to have commenced as early as 55 Ma and possibly continued until at least 52 Ma. These ages bracket the 53·4 ± 0·3 age reported by Hiza (1999) for a dacitic block and ash flow at the base of Sepulcher Mountain, located 50 km to the NW of Mt. Washburn (Fig. 1). The sequence of rocks exposed on Sepulcher Mountain represents the type section for the Sepulcher Formation, the western member of the Washburn Group of Smedes & Prostka (1972). Because these rocks are compositionally and petrographically identical to rocks exposed at Washburn volcano (Lindsay & Feeley, 1999), we concur with the opinion of Smedes & Prostka (1972) that rocks within the Sepulcher and Lamar River Formations were erupted from similar and nearly contemporaneously active volcanic centers. In addition, our results are also consistent with the suggestion of Hiza (1999) that the oldest rocks in the AVP are calc-alkaline lavas at present exposed in the northwestern part of the field.
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PETROGRAPHY AND SILICATE MINERAL CHEMISTRY |
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 TOPABSTRACTINTRODUCTIONTECTONIC AND GEOLOGIC SETTINGGEOCHRONOLOGY PETROGRAPHY AND SILICATE MINERAL... CHEMICAL COMPOSITIONSPETROGENESIS OF WASHBURN VOLCANO...DISCUSSIONCONCLUSIONSAPPENDIX: ANALYTICAL METHODSREFERENCES |
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Washburn igneous rocks investigated in this study are generally non- to slightly vesicular (<3 vol. % vesicles), porphyritic to partially glomeroporphyritic lavas and dikes. In thin section the rocks are hypocrystalline with intersertal to pilotaxitic groundmass textures. Phenocryst contents range from 44 to 5% (by volume) with the total decreasing with increasing SiO2 contents until
63 wt % SiO2 (Fig. 5; Table 2). Dacitic rocks have widely varying phenocryst contents. Changes in mode with bulk composition are regular throughout the suite. Basaltic–andesitic rocks contain plag 
cpx > ol ± opx; andesitic rocks contain plag > cpx > opx ± amph ± ol; and dacitic rocks contain amph + plag ± cpx ± opx ± bio (Fig. 5). All rocks also contain Fe–Ti oxide microphenocrysts. Glomerocrysts are common of cpx ± Fe–Ti oxides, ol + cpx ± Fe–Ti oxides ± plagioclase ± opx, and plag + amph ± Fe–Ti oxides. Mineral inclusion patterns and the occurrence of minerals in the glomerocrysts, together with textural features and compositions of the phenocrysts described below, suggest the following generalized crystallization sequences. Basaltic andesitic magmas crystallized olivine followed by Fe–Ti oxides, clinopyroxene + plagioclase, and orthopyroxene. Andesitic magmas crystallized plagioclase followed by clinopyroxene + orthopyroxene, Fe–Ti oxides and then amphibole in some cases. Dacitic magmas, in which pyroxenes are rare or absent, precipitated plagioclase followed by Fe–Ti oxides, amphibole, and then biotite when present. Groundmass assemblages include glass (partially to pervasively devitrified), plagioclase, clinopyroxene, and orthopyroxene. Zircon and apatite are common accessory phases in the intermediate and silicic lavas. Xenocrysts, identified by non-equilibrium compositions (see below) or magmatic reaction textures, are present but rare. Furthermore, we identified no clear petrographic evidence for mixing or mingling between compositionally disparate magmas, such as the presence of undercooled blobs of mafic magma that are frequently found in many andesitic to dacitic rocks (Bacon, 1986; Wilcox, 1999).
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The Sulphur Creek stock is tonalitic to quartz dioritic with plag > qtz > bio > cpx = opx > Fe–Ti oxide > amph. Texturally, the stock is fine grained, phaneritic, and subophitic in that late-crystallizing anhedral quartz partially encloses elongate plagioclase and biotite grains.
Olivine
Olivine generally occurs as equant, euhedral to subhedral phenocrysts in basaltic andesitic lavas. In the majority of these samples the phenocrysts range in size from about 4·0 to 0·5 mm, although populations in individual samples typically have a much narrower range. In a few samples the size variation is continuous from a maximum of 1·0 mm to a minimum of 0·1 mm. Additionally, a few andesitic and dacitic rocks contain small amounts of olivine (Fig. 5). Although we did not analyze any of these grains, they are probably xenocrysts or relicts because grain boundaries are ragged, variably resorbed, and have corona of clinopyroxene or orthopyroxene plus plagioclase. In nearly all rocks olivine is partially altered to bowlingite, particularly along crystal faces and fractures.
The total analyzed variation in the compositions of olivine in the basaltic andesitic rocks ranges from Fo85 to Fo72; within single phenocrysts, rims are normally zoned 
7 mol % Fo content relative to core compositions (Table 3). Fe2+/Mg ratios of olivine phenocryst cores are plotted versus those of three whole rocks in Fig. 6a. In this figure the whole-rock Fe2O3/FeO weight ratio is assumed to be 0·24 based on the Kilinc et al. (1983) expression for Fe speciation, an f(O2) of nickel–nickel oxide (NNO; Huebner & Sato, 1970), and a temperature of 1000°C (see below). In general, minimum olivine core Fe2+/Mg ratios increase with increasing rock Fe2+/Mg ratios and appear to indicate equilibrium between the olivine and the bulk rock based on a KD (=[Fe/Mg]ol/[Fe/Mg]rock) = 0·3 ± 0·03 (Roeder & Emslie, 1970; Wagner et al., 1995).
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Pyroxene
Clinopyroxene occurs in Washburn volcanic rocks as isolated, euhedral to subhedral phenocrysts up to 6 mm across, as compact, often rounded glomerocrystic aggregates up to 1 cm in diameter, as a microphenocryst phase in the groundmass, and as subhedral to euhedral rims surrounding orthopyroxene cores in basaltic andesitic rocks. In dacitic rocks clinopyroxene phenocrysts may be surrounded by corona of amphibole. Orthopyroxene occurs as: (1) individual subhedral to euhedral lath-shaped phenocrysts 0·5–1·0 mm across; (2) as crystals in monomineralogic clots and the glomerocrysts described above; (3) as smaller crystals together with plagioclase and Fe–Ti oxides in reaction coronas surrounding amphibole phenocrysts of some dacitic rocks; (4) as microphenocrysts in the groundmass.
Pyroxene compositions are typical of calc-alkaline rock series. Clinopyroxenes (Wo34–46En43–52Fs5–16) are mainly augite, although a few phenocrysts have endodiopsidic cores, particularly in more magnesian-rich rocks, and orthopyroxenes are bronzite–hypersthene (Wo2–5En67–81Fs16–30; Table 4). All Washburn clinopyroxenes have low Al (Al/6 oxygens < 0·18) and Ti (Ti/6 oxygens < 0·02), suggesting crystallization under low-pressure, shallow-crustal conditions (Fig. 7a). Pyroxene zoning patterns are generally normal. Relative to cores, most rims are enriched in ferrosilite content by an amount <9%, and they have lower contents of Cr, Al, and less commonly Ti. In addition, a few analyzed pyroxene grains are reversely zoned. The majority of these are clinopyroxene phenocrysts with rimward enrichments in Mg (7% En) and less typically Ca (4% Wo) and Ti. Reverse zoning is less common in orthopyroxene and typically at the limit of analytical resolution (1% En). The origin of the Mg-rich clinopyroxene rims may be related to recharge and mixing with less evolved magmas or, as suggested by the lack of reverse zonation in the Wo component for many of these grains, late precipitation of Fe–Ti oxides in the groundmass and lowering of FeO in the liquid.
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Compositions of pyroxene phenocryst cores are compared with whole-rock compositions in Fig. 6b and c. With the exception of an obviously xenocrystic grain in a low-Mg dacite (MW974; the only cpx grain observed in this particular thin section), the relationships illustrated are consistent with an equilibrium KD (=[Fe/Mg]pyx/[Fe/Mg]rock) between 0·2 and 0·3 for clinopyroxene, which is similar to values (0·20–0·25) found experimentally in 1 atm, silica-saturated and undersaturated arc lavas (Grove & Baker, 1984; Kennedy et al., 1990). Equilibrium KD values for orthopyroxene in Mg-rich basaltic andesitic rocks appear higher than for clinopyroxene, but probably reflect early precipitation of clinopyroxene followed by orthopyroxene in mafic magmas. Magmatic temperatures calculated for rims of touching clinopyroxene–orthopyroxene pairs in three lavas using the methods of Wood & Banno (1973) and Wells (1977) are in the range of 1012–1017°C for a basaltic andesite (MW9730), 981–999°C for an andesite (MW9721), and 967–968°C for a high-silica andesite (MW9743; Table 4).
Plagioclase
Plagioclase is the most abundant mineral in Washburn rocks, with the exception of a few dacitic and basaltic andesitic samples in which amphibole and clinopyroxene, respectively, are slightly more abundant. In all bulk-rock compositions plagioclase occurs as large (0·3–4·0 mm), euhedral to subhedral phenocrysts in isolation or in glomerocrysts, and as equant microphenocrysts and lath-shaped microlites in the groundmass. Phenocryst habits are generally lath-shaped, although grains become increasingly equant in more silica-rich rocks. There appears to be no compositional distinction between plagioclase grains in isolation or in glomerocrysts. Compositions of plagioclase phenocryst cores for the Washburn suite span from An83 to An40 (Table 5), although greater than two-thirds of phenocryst cores are between An65 and An50 (Fig. 8).
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Although oscillatory zoning of small magnitude is common, overall core to rim zoning patterns of plagioclase phenocrysts are relatively uncomplicated; over half of all crystals analyzed are normally zoned <15 mol % An and <20% display reverse zoning of any magnitude. The maximum extent of reverse zoning observed is <12 mol % An and with very few exceptions it is <5 mol %. Groundmass crystals in all rock types have compositional ranges that are similar to the ranges of rim compositions and thus are either identical to, or slightly more Na rich than core compositions (Fig. 8).
The relationships between plagioclase phenocryst cores and whole-rock Ca/Na ratios are illustrated in Fig. 6d. With the exception of a few Ca-rich phenocrysts in the dacitic lavas, the relationships illustrated are consistent with an equilibrium KD (=[Ca/Na]plag/[Ca/Na]rock) between one and three for the andesitic and dacitic lavas. These values suggest early crystallization of most plagioclase cores under low to moderate magmatic water contents, based on the data of Sisson & Grove (1993), who showed that KD varies with melt water content, from 1·0 at anhydrous conditions to 5·5 for melts with 6% water at 2 kbar. The more Ca-rich cores may represent grains crystallized under higher water contents, unequilibrated crystals from higher temperature or pressure stages of evolution, xenocrysts derived from mixing with more mafic magmas, or low-density refractory solids retained from zones of deep-crustal melting and silicic magma production (see Feeley & Dungan, 1996). Because dacitic and andesitic rocks have relatively high proportions of these Ca-rich cores, whereas they are less abundant in more mafic rocks (e.g. Fig. 6d), we favor the latter hypothesis (see below). The relationships in Fig. 6d for the basaltic andesitic magmas suggest equilibrium KD values around one for most plagioclase cores. These values suggest crystallization from magmas with lower water contents relative to more evolved magmas or precipitation of plagioclase concurrently or following saturation of high-Ca clinopyroxene. Also shown in Fig. 6d are equilibrium relationships between plagioclase phenocryst rim and liquid (i.e. groundmass) Ca/Na ratios determined for five rocks by electron microprobe analyses. The plagioclase phenocryst rims in these rocks straddle or plot slightly above the 3·0 KD reference line. These relationships suggest equilibrium growth of plagioclase rims under higher water contents than for cores, perhaps because of residual water build-up during crystallization of predominantly anhydrous mineral assemblages.
Amphibole
Amphibole is present in more silica-rich Washburn rocks as phenocrysts ranging in size up to 4 mm. In many of these rocks amphibole is rimmed by opacite or less commonly by a fine-grained mass of plagioclase + Fe–Ti oxides ± pyroxenes, presumably as a result of breakdown during decompression (Foden & Green, 1992). In addition, amphibole phenocrysts in many samples are partially altered along cleavage surfaces and cracks to a fine-grained mass, perhaps bowlingite, and a few samples have crystals with cavernous interiors, presumably as a result of incomplete crystal growth. In these latter crystals alteration can be substantial. All Washburn amphiboles have (Ca + Na)M4 > 1·34 and (Na)M4 < 0·67 and thus are calcic amphiboles according to the classification scheme of Leake (1978) and Hawthorne (1981). Amphibole phenocrysts in andesitic rocks range from pargasite to pargasitic hornblende, whereas those in dacitic rocks are pargasitic hornblende to edenite (Leake, 1978; Hawthorne, 1981; Table 6).
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Figure 7b plots AlIV vs (Na + K) cations in the A site of amphibole phenocrysts in Washburn rocks. Experimental studies demonstrate that AlIV exhibits correlated increases with (Na + K)A with rising crystallization temperature. The higher AlIV and (Na + K)A in amphiboles from the andesitic rocks relative to most grains in the dacitic rocks are consistent with their less evolved nature and inferred higher liquidus temperatures. A few amphibole crystals from the dacitic rocks have cores with high AlIV and (Na + K)A that plot at the high-temperature end of the experimental data arrays. These cores probably represent unequilibrated regions from higher-temperature (or pressure) stages of evolution because they are compositionally similar to phenocrysts in the andesitic rocks and are surrounded by normally zoned rims similar to most other amphibole crystals in the dacitic lavas.
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CHEMICAL COMPOSITIONS |
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We analyzed 46 rocks from Washburn volcano for major and trace element abundances by X-ray fluorescence (XRF) spectrometry. Sample locations are shown in Fig. 2. On the basis of the XRF analyses, a set of 17 rocks was selected for additional determination of rare earth element (REE) and other trace element contents by instrumental neutron activation analysis (INAA) and a further subset of these (n = 9) for determination of Sr and Nd isotopic ratios by thermal ionization mass spectrometry (TIMS). All data are reported in Table 2 and details of the analytical procedures used are described in the Appendix.
Major elements
Variations in major element compositions of Washburn rocks are illustrated with respect to SiO2 in Fig. 9. According to the classification schemes of Peacock (1931), Peccerillo & Taylor (1976), and Le Maitre (1989), Washburn igneous rocks form a medium- to high-K, calc-alkaline suite (alkali–lime index 60; not shown) ranging in composition from basaltic andesitic through dacitic. In these respects the compositions of Washburn rocks share broad overall similarities with other Eocene calc-alkaline igneous rocks in the Absaroka Volcanic Province and elsewhere in the ‘Challis arc’ (e.g. Norman & Mertzman, 1991; Hooper et al., 1995; Dostal et al., 1998; Lindsay & Feeley, 1999; Morris et al., 2000). As is typical of calc-alkaline suites, MgO, Fe2O3, CaO, TiO2, and MnO decrease with increasing SiO2, whereas Na2O and K2O increase. Trends for Al2O3 and P2O5 are diffuse and show no obvious correlation with SiO2.
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With respect to identification of magmatic sources, differentiation processes, and compositional evolution of the Washburn magmatic system, the important major element features are: (1) samples from the SW Washburn Range consist mainly of a bimodal package of basaltic andesitic and dacitic rocks, whereas stratigraphically higher units on Mt. Washburn range continuously from basaltic andesitic through dacitic rocks, but are dominated by andesitic compositions; (2) trends with respect to SiO2 for many elements are linear, except for MgO, Al2O3, and P2O5, which have more diffuse distributions of data; (3) at a given SiO2 content, rocks from Mt. Washburn tend to have lower MgO and higher Al2O3, respectively, than rocks from the SW Washburn Range.
Meen (1985) emphasized plots of K2O vs K2O/MgO and MgO for discriminating rocks at Independence volcano (Fig. 2), believed to be related exclusively by high-pressure fractional crystallization of mantle-derived parents in the absence of crustal contamination from those that interacted with crustal materials. In theory, fractional crystallization of magnesian minerals plus plagioclase leads to production of evolved liquids that fall within narrow coronal bands characterized by large increases in K2O compared with smaller increases in K2O/MgO and decreases in MgO (Fig. 10a). In contrast, contamination of magmas with low-K2O/MgO crustal melts produces trajectories with low slopes on such plots. Figure 10a illustrates a plot of K2O vs K2O/MgO for Washburn volcano rocks. Many samples from the SW Washburn Range define a narrow coronal band nearly identical to those formed by suites of rocks at Independence volcano inferred by Meen (1985) to be uncontaminated. The SW Washburn rocks that define this band cannot represent a single uncontaminated liquid line of descent, however. As discussed below, variations in trace element contents and isotopic ratios are inconsistent with this proposition. Furthermore, nearly all Washburn rocks, including the most mafic samples, form a shallow array in Fig. 10b that cuts Independence ‘uncontaminated’ trends at high angles. We interpret the Washburn array as a mixing line between high-MgO basaltic magmas and low-MgO crustal melts. This implies that the compositions of all Washburn rocks were affected by crustal interaction and none can be assumed to be direct uncontaminated differentiates of primary mantle-derived magmas.
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Trace elements
Variations in trace element abundances of Washburn volcanic rocks display an interesting set of relationships that are similar to, but somewhat more complex than for major elements. The important features are as follows.
Many trace elements (e.g. Ba, Rb, Y, Zr, Sc) display overall linear, but diffuse variations with respect to SiO2, although highly compatible trace elements such as Cr and Ni scatter widely so that some intermediate composition rocks have relatively high concentrations of these elements (Fig. 11). These features do not reflect xenocrystic olivine as there is no correlation between modal percent olivine and Ni and Cr contents in the andesitic and dacitic rocks (Table 2). They are also not due to contamination during sample preparation because the samples were crushed in tungsten-carbide grinding bowls, which does not cause analytically significant contamination for Ni and Cr (or MgO; Johnson et al., 1999). It is noteworthy that, analogous to variations in MgO, rocks from Mt. Washburn generally have lower Ni and Cr contents than most equivalent bulk composition rocks from the SW Washburn Range. Moreover, the samples that form a narrow coronal band in Fig. 10a do not define a single liquid line of descent resulting from fractionation of magnesian minerals plus plagioclase in diagrams illustrating variation of Ni and Cr with SiO2. Accordingly, diagrams such as Fig. 10a may not uniquely identify magmas strictly related by simple closed-system fractional crystallization (see Meen, 1985; Meen & Eggler, 1987).
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Both Sr and Zr increase slightly with increasing SiO2 contents, indicating that fractionation of plagioclase and zircon (or crustal melting with these phases residual) were not major processes in the evolution of the Washburn suite as a whole. In contrast, Y decreases with increasing SiO2, indicating that fractionation of large amounts of clinopyroxene or amphibole, or contamination by crustal melts with low Y contents was important. On the basis of location and Sr, Zr, Y, and Rb contents, three groups of dacitic rocks can be distinguished at Washburn volcano; two from the SW Washburn Range, designated types 1 and 2, and one from Mt. Washburn, designated type 3. On a comparative basis, type 1 dacites (including one high-silica andesitic lava) have higher Rb, Zr, and Y, and lower Sr contents than type 2 dacites (Fig 10). Type 3 dacites have similarities to and differences from types 1 and 2 in that they have Y contents similar to those of type 2 dacites, Sr contents similar to or greater than those of type 1 dacites, and Zr and Rb contents intermediate between those of both types.
All Washburn volcanic rocks have LREE-enriched, chondrite-normalized REE patterns (Fig. 12). For the suite as a whole andesitic and dacitic rocks tend to have lower concentrations of HREE, higher concentrations of LREE, and flatter middle (MREE) to HREE patterns (e.g. Tb–Lu) than basaltic andesitic rocks. As a result of the first two features chondrite-normalized REE plots display fanning (cross-over) patterns with fulcrums centered between Nd and Sm. All Washburn rocks have either no Eu anomaly or small positive anomalies that increase from 1·00–1·03 for basaltic andesitic rocks to 1·01–1·15 for andesitic and dacitic rocks. These relationships are consistent with the increasing Sr concentrations with SiO2, in that both indicate that fractionation of plagioclase was not a major process during evolution of the Washburn suite. Furthermore, the decreasing HREE contents with increasing SiO2 wt % virtually require that differentiation at Washburn volcano involved mixing with, or assimilation of, crustal melts with low HREE contents. They cannot reflect fractionation of substantial amounts of amphibole, the only major phenocryst phase in Washburn rocks with mineral–melt Kd values >1 for the MREE and HREE. Fractionation of substantial amphibole results in depletion of MREE relative to HREE and the production of concave-upward REE patterns.
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Mid-ocean ridge basalt (MORB)-normalized trace element plots for Washburn basaltic andesitic rocks are characterized to some extent by features considered diagnostic of subduction-related magmas (Fig. 13; Pearce, 1982). For example, relative to MORB, basaltic andesites have strong enrichments in large ion lithophile elements (LILE; Sr, K, Rb, Ba, and Th) and depletions in the high field strength elements (HFSE) Ta, Nb, and Ti. The HFSE Zr and Hf, however, are not depleted relative to MORB. High-K2O basaltic rocks from the shoshonitic Sunlight volcano, eastern AVP (Fig. 1), are further enriched in LILE, but not HSFE, relative to Washburn basaltic andesites (Fig. 13b). Least evolved (MgO 5·93 wt %; Meen & Eggler, 1987) high-alumina tholeiitic basaltic andesite from Independence volcano, eastern AVP, shows variable enrichments and depletions in LILE relative to Washburn basaltic andesites, although with the exception of Ti, HFSE are depleted. Plots for Washburn andesitic and dacitic rocks are enriched in both LILE and Zr and Hf relative to basaltic andesitic rocks, although they are depleted in Ti, Y, and Yb.
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Sr and Nd isotopes
Initial 87Sr/86Sr and 143Nd/144Nd isotopic ratios corrected for 53 my of in situ growth of radiogenic Sr and Nd for nine Washburn rocks are presented in Table 2 and illustrated in Fig. 14. Relative to ‘bulk Earth’ all rocks have high Sr and low Nd isotopic ratios and thus plot within the ‘enriched’ quadrant in Fig. 14a. The data define a negatively correlated array extending from a field defined by ultramafic xenoliths brought up in Eocene alkalic magmas in the Crazy Mountains, 150 km north of Washburn volcano, to fields defined by Wyoming Province Archean granulite-facies rocks brought up as xenoliths in Cenozoic magmas (Fig. 14a; Leeman et al., 1985; Dudas et al., 1987) and Archean supracrustal amphibolites and granitoids that dominate the exposed basement near Washburn volcano (Meen, 1987a; Wooden & Mueller, 1988). Washburn mafic rocks have slightly higher 87Sr/86Sr ratios but similar 143Nd/144Nd ratios compared with mafic rocks from Independence and Sunlight volcanoes in the eastern belt of the AVP (Meen & Eggler, 1987; T. C. Feeley, unpublished data, 2001). Although the Washburn rocks have MORB-normalized trace element patterns characterized by depletions in Ta and Nb (Fig. 13), they plot well below the field defined by modern subduction-related basalts because of their low Nd isotopic ratios. This feature is characteristic of many early- to late-Cenozoic magmatic rocks in the Wyoming Province and is considered to reflect derivation from or interaction with ancient subcontinental lithospheric mantle (e.g. Fraser et al., 1985; Dudas et al., 1987; Meen & Eggler, 1987; Scambos, 1991; MacDonald et al., 1992; O’Brien et al., 1995).
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The Nd and Sr isotopic data indicate that crustal material was important in the genesis of Washburn magmas, and differences in crustal composition may provide an explanation for the contrasts between the types of dacitic magmas described previously. For example, although values for all rock types overlap, there are fairly well defined positively and negatively correlated arrays between SiO2 content and Sr and Nd isotopic compositions, respectively (Fig. 14b and c). In addition, at least one type 1 dacitic rock has a significantly higher Sr isotopic ratio than the single analyzed type 2 dacitic rock. This feature is consistent with the relatively high Rb/Sr ratios for type 1 dacitic rocks as a whole (e.g. Fig. 11). Together these relationships imply that the continental crust beneath Washburn volcano is isotopically heterogeneous but remained isolated for significant periods of time to allow for greater in situ growth of radiogenic Sr in Rb-enriched sources for type 1 dacitic rocks. It is also noteworthy that Washburn basaltic andesitic and andesitic rocks contained within the corunal band in Fig. 10a have variable Nd and Sr isotopic compositions that correlate with SiO2. Such relationships are typical of many other volcanic suites resulting from assimilation of or mixing with continental crust and they preclude an origin for Washburn basaltic andesites by closed-system processes. Accordingly, although diagrams such as Fig. 10a may be useful for identifying magmas having experienced some fractional crystallization during their genesis, they do not uniquely preclude the possibility that these magmas have also experienced contamination by continental crust (see Meen, 1985; Meen & Eggler, 1987).
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PETROGENESIS OF WASHBURN VOLCANO MAGMAS |
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TOPABSTRACTINTRODUCTIONTECTONIC AND GEOLOGIC SETTINGGEOCHRONOLOGYPETROGRAPHY AND SILICATE MINERAL...CHEMICAL COMPOSITIONSPETROGENESIS OF WASHBURN VOLCANO...DISCUSSIONCONCLUSIONSAPPENDIX: ANALYTICAL METHODSREFERENCES |
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In the following sections several models for the origin of compositional diversity in the Washburn system are evaluated. In these models we use the geochemical and isotopic data in Table 2, published data available for potential crustal source components, and standard expressions for (1) trace element and isotopic evolution during Rayleigh fractionation of crystals from magma, (2) simultaneous fractional crystallization and wall assimilation (DePaolo, 1981
), and (3) incremental batch melting of crustal rocks. As discussed below, there are two significant limitations on the models.
The first limitation is that there is no direct evidence for the composition of crustal rocks beneath Washburn volcano. Published geochemical data are available for rocks from nearby areas, including information on the compositions of supracrustal igneous and metamorphic rocks in the Beartooth Mountains and Wyoming Province deep-crustal granulite-facies xenoliths (Mueller et al., 1982, 1983, 1985; Wooden et al., 1982; Leeman et al., 1985; Dudas et al., 1987; Meen, 1987a; Wooden & Mueller, 1988; Collerson et al., 1989; Joswiak, 1992). Only small, partial datasets exist for compositions of the granulite-facies xenoliths, however. The models utilizing data for these lithologies are therefore not rigorously constrained because trace element and isotopic calculations are based on data from different sources and localities. Specifically, the trace element calculations utilize data from Joswiak (1992), whereas the isotopic calculations utilize data from Leeman et al. (1985) because these papers contain the most comprehensive trace element and isotopic datasets, respectively, for deep-crustal lithologies in the Wyoming Province. The models do, however, place constraints on processes and sources responsible for the origin of compositional diversity in the Washburn magmatic system.
The second limitation is that Washburn mafic rocks have mg-numbers (= 100[Mg/(Mg + Fe*)]) <64 and Ni <175 ppm. Thus, none represent primary mantle melts. Furthermore, primary mantle melts are generally unknown from the AVP. Meen & Eggler (1987) and Meen (1985) argued on the basis of plots of K2O vs K2O/MgO (e.g. Fig. 10a) that rocks from the central portion of the Independence stock represent uncontaminated mantle-derived melts related solely by fractional crystallization. In support of this conclusion they presented Sr (0·70453–70475) and Nd (0·51182–0·51196) isotopic data for these rocks virtually constant across the compositional spectrum (basaltic andesitic to high-K dacitic). This conclusion is now unclear, however, because Meen (1985) and Meen & Eggler (1987) calculated initial isotopic ratios for the Independence rocks using an age of 84 Ma. Recently reported 40Ar/39Ar data on mineral separates clearly show that the age of the Independence stock is 49·96–48·50 Ma (Harlan et al., 1996). Using these ages the calculated initial Sr (0·70459–0·70508) and Nd (0·51167–0·51192) isotopic data for the Independence stock show analytically significant variations and modest correlations exist with parameters such as K2O, SiO2, and 1/Sr.
To address the lack of primary melts erupted at Washburn volcano, as a first step in the modeling we calculated a hypothetical primary mantle melt composition to use as a starting point. The composition of the primary melt was estimated by assuming that (1) it has 400 ppm Ni (Sato, 1977; Frey et al., 1978; Wilson, 1989) and (2) a high Ni, but olivine-free (to avoid cumulate effects) andesitic rock (MW9729) is a simple two-component mixture of an evolved dacitic magma (OP9881) and the primary melt. Using the proportions of the endmembers predicted by this procedure (68:32) yields a basalt with 49·3 wt % SiO2, 14·6 wt % MgO, and an mg-number of 68, which are not unreasonable estimates for primary or primitive calc-alkaline magmas that were in equilibrium with mantle peridotite (Gill, 1981; Foden, 1983; DeBari & Sleep, 1991).
In Fig. 13 the MORB-normalized trace element pattern of the model primary melt is compared with plots for several other mafic rocks in the AVP and Challis–Absaroka volcanic episode (e.g. Figs 1 and 2). These include subalkaline basalts from the Challis volcanic field (Norman & Mertzman, 1991), the Colville igneous complex (Morris et al., 2000), and the Crazy Mountains, Montana (Dudas, 1991; Fig. 13c); a high-alumina tholeiitic basaltic andesite from Independence volcano (Meen & Eggler, 1987); a potassic trachy-basalt from Sunlight volcano, eastern AVP (T. C. Feeley, unpublished data, 2001); and an alkaline basalt (malignite) from the Crazy Mountains (Fig. 13d).
In general, the trace element pattern for the model Washburn basalt is similar to patterns for the subalkaline basalts from the Colville igneous complex, the Challis volcanic field, and Crazy Mountains, suggesting melting of similar sources (Fig. 13c). Only minor differences in most elemental concentrations are evident, the major exception being Nb, for which the other basalts have significantly higher concentrations. Patterns for the other mafic rocks show more pronounced differences from the Washburn model primary basalt pattern and may derive from more disparate sources (Fig. 13d). The Sunlight basalt has nearly identical Nb, Zr, Ti, and Y contents to the model Washburn basalt, but LILE are greater by as much as a factor of three. The Independence basaltic andesite has lower concentrations of K, Rb, and Th, but concentrations of Sr, Ba, and P that exceed those in the Washburn basalt by a factor of two. The alkaline basalt from the Crazy Mountains is enriched in nearly all trace elements compared with the subalkalic rocks.
Origin of intermediate composition magmas
To illustrate processes responsible for producing the spectrum of intermediate composition magmas at Washburn volcano, Fig. 15 presents several calculated differentiation models as a function of trace element variations vs Sc. For these models Sc was selected as a trace element index of differentiation because there is a well-correlated decrease with increase in SiO2, and Sc emphasizes the role of clinopyroxene, the most abundant mafic silicate phase in the rocks.
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The continuous curves labeled ‘fx’ originating from the model primary basaltic magma in Fig. 15 are calculated fractional crystallization trends assuming a mineral assemblage of 40% clinopyroxene, 30% olivine, 29% plagioclase, and 1% Fe–Ti oxides, and intermediate values for the range of basalt mineral–melt partition coefficients presented by Rollinson (1993). This plagioclase-poor, augite-rich phenocryst assemblage is characteristic of crystallization under conditions of elevated pressure following the experimental work of Sisson & Grove (1993). We do not illustrate nor do we consider fractionation of modal proportions of phenocrysts as a suitable process because all rocks contain low-pressure, plagioclase-rich mineral assemblages that if removed would increase Sc and deplete Sr contents in residual liquids, opposite to the depletion in Sc and increase in Sr contents with increasing SiO2 observed for the suite. In addition, the increasingly positive Eu anomalies and decreasing HREE with increasing SiO2 also argue strongly against any process involving shallow pressure fractionation.
The model curves in Fig. 15 indicate that small amounts of fractionation (e.g. F = amount original magma remaining >0·8) followed by mixing with diverse composition dacitic magmas (thick, short-dashed lines) can replicate the range of compositions observed. This process is successful in accounting for the contrasting wedge-shaped data fields in Fig. 15 defined by large variations in Ni and Cr contents for basaltic andesitic rocks on the one hand, and large variations in LILE and Y for dacitic rocks on the other. Figure 16 tests the validity of the mixing model for the andesitic magmas using ratio–ratio variation diagrams and corresponding companion plots following the method of Langmuir et al. (1978). Excluding type 1 dacitic rocks, the diffuse, but hyperbolic form of the data array is consistent with a mixing relationship between diverse composition mafic and silicic magmas to produce the andesitic magmas (Langmuir et al., 1978). For example, in Fig. 16a four calculated possible mixing arrays are illustrated. Curves 1a and 1b illustrate mixing between the most primitive sample in the Washburn suite (MW9713) with average compositions of type 2 and type 3 dacites (diamonds). Curves 2a and 2b illustrate mixing with a slightly more evolved basaltic andesite (MW9716). The close correspondence of the data to the calculated curves supports the mixing model and implies that type 1 dacitic magmas were not utilized as silicic endmembers. Figure 16b and c are companion plots in which the ratios in Fig. 16a are plotted against a ratio of the denominators (see Langmuir et al., 1978). The consistency of the mixing relationship is substantiated by the linear variation of the data (excluding type 1 dacites) in these diagrams (Langmuir et al., 1978).
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It is also likely that assimilation of crustal rocks occurred during fractional crystallization of the mafic parental magmas, as these processes are inferred to be physically coupled (Taylor, 1980) and probably accounts for the absence of data points in direct proximity to the model closed-system fractionation curves in Fig. 15 and the isotopic diversity among the basaltic andesitic rocks. As a first-order approximation of the effects of assimilation plus fractional crystallization, we constructed model curves using the method of DePaolo (1981), the crystallizing assemblage discussed above, and crustal assimilants comparable in composition with the average of supracrustal Archean granitoids (curve afc 1) and amphibolites (curve afc 2) in the Beartooth Mountains [data of Meen (1987a)] and deep-crustal granulite-facies xenoliths in the Wyoming Province (curve afc 3). The r value (ratio of rates of assimilation to crystallization; see DePaolo, 1981) assumed during all models is 0·8; this value is considered to simulate differentiation under mid- to deep-crustal conditions where the crust is at an elevated temperature and olivine, clinopyroxene, and plagioclase are multiply saturated (Reiners et al., 1995). Additionally, we included 10% garnet in the fractionating assemblage for the deep-crustal assimilation model, to investigate assimilation at high pressures where garnet, with elevated partition coefficients for Sc (2·6–3·7; Irving & Frey, 1978; Hauri et al., 1994) and Cr (nine; Johnson, 1998), is an important residual phase during crustal melting.
The model assimilation–fractional crystallization curves suggest, like the closed-system trends, that only small amounts (F > 0·9) of differentiation of a potentially primary magma are necessary to produce the range of Ni and Cr contents in parental mafic liquids required in the subsequent two-component mixing models. The most important differences between the supracrustal (curves afc 1 and afc 2) and deep-crustal (curve afc 3) assimilation models is that LILE such as Rb and Ba are elevated more rapidly in the former whereas Y contents are depleted in the latter. However, none of the models illustrated in Fig. 15 can independently reproduce the unique compositional features of the Washburn suite. More complex mathematical expressions for the assimilation–fractional crystallization process have also been considered (Spera & Bohrson, 2001). These also are unable to independently reproduce the roughly linear trends for many elements and the high Cr and Ni contents of the andesitic and dacitic rocks. Together these features virtually require mixing between mafic and silicic magmas to produce intermediate composition magmas (McMillan & Dungan, 1984, 1986; Bacon, 1986).
The relationships illustrated in Fig. 15 do not uniquely constrain the crustal lithologies assimilated because of the limited amounts of differentiation of the Washburn parental magmas. Therefore, to better constrain the identity of crustal lithologies potentially assimilated during differentiation of Washburn parental magmas, assimilation plus fractional crystallization calculations were carried out with Sr and Nd isotopic data. The results of a variety of calculations are shown in Fig. 17, which represent the best of a family of models in which compositions of the assimilants were chosen to be consistent with known crustal lithologies. These calculations produce realistic constraints on possible crustal assimilants because Nd is highly incompatible during differentiation of mafic magmas and the Sr bulk distribution is controlled primarily by plagioclase.
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Assimilation plus fractional crystallization calculations using the average of supracrustal Archean granitoids (curve afc 1; Fig. 17) and amphibolites (curve afc 2) in the Beartooth Mountains cannot produce the 87Sr/86Sr vs 143Nd/144Nd trend for Washburn basaltic andesitic rocks. Published data for these crustal rocks (Meen, 1987a) indicate elevated Sr isotopic ratios and bulk Sr contents, which produce strong enrichments in 87Sr/86Sr relative to 143Nd/144Nd during the early stages of differentiation. Instead, the large range in Nd isotopic compositions at roughly constant 87Sr/86Sr ratios of Washburn basaltic andesitic magmas requires assimilation of crustal rocks with low 87Sr/86Sr and 143Nd/144Nd ratios. Rocks with these features occur as granulite xenoliths brought to the surface in late Cenozoic basaltic magmas of the Snake River Plain in the Spencer–Kilgore area 125 km to the SW of Washburn volcano (afc 3; Leeman et al., 1985). Assimilation of rocks with these isotopic features results in little change in Sr isotopic ratios, although Nd isotopic ratios are lowered drastically (afc 3; Fig. 17). This process produces a range of compositions that are appropriate for mixing with Washburn dacitic magmas to produce the intermediate composition magmas. Therefore, a model where Washburn parental magmas undergo combined assimilation–fractional crystallization in the deep crust followed by mixing with silicic melts is consistent with the model calculations performed here.
Origin of Washburn dacitic magmas
The conclusion that Washburn intermediate composition rocks are products of mixing between mafic and silicic magmas does not preclude an origin for the dacitic magmas by a fractionation-dominated liquid line of descent from less evolved magmas, as several workers have proposed mechanisms capable of producing fractionation-generated composition gaps in calc-alkaline systems (e.g. Grove & Donnelly-Nolan, 1986; Brophy, 1991). Various geochemical criteria indicate that the dacitic magmas in the Washburn suite (and hence, the silicic mixing endmembers) are, however, not direct fractionation products of basaltic andesitic magmas at present exposed at the volcano. The strongest and most obvious evidence against a closed-system process is that dacitic rocks have lower Nd and higher Sr isotopic ratios than those of most less evolved rocks. Thus, if fractional crystallization produced the dacitic magmas, it was accompanied by crustal assimilation.
Geochemical characteristics of Washburn dacitic magmas are also difficult to reconcile with an origin by assimilation of crustal rocks plus fractional crystallization of mafic magmas. The strongest evidence against an assimilation process is the high contents of compatible trace elements (e.g. Cr 159–42 ppm; Ni 63–25 ppm) and the large ranges in incompatible trace element contents in dacitic rocks from Washburn volcano. Together these features conflict with an origin by Rayleigh-type fractionation coupled with assimilation as this process generates coupled exponential enrichments and depletions, respectively, in these trace element groups. It is possible that low contents of compatible trace elements in some dacitic rocks from Mt. Washburn in part reflect extensive fractionation. However, model assimilation curves in Fig. 15 predict irreconcilable incompatible and compatible trace element contents using any reasonable crustal contaminant. As described above, the simplest interpretation of the low contents of compatible trace elements is that they reflect mixing with mafic endmember magmas slightly more fractionated than those for the SW Washburn Range suite. Thus, all constraints together indicate that the net result of fractionation plus assimilation at Washburn volcano is not to produce highly evolved magmas, but to contribute to the genesis of a spectrum of mafic magma types and the formation of multiple mixing lines. The main mixing trends are identified by the majority of rocks from the SW Washburn Range suite with anomalously high MgO, Ni, and Cr contents on the one hand, and rocks from Mt. Washburn with lower contents of these elements on the other (Fig. 15).
Crustal melting, restite unmixing, magma mixing, and fractional crystallization
Geochemical characteristics (as discussed above) of the Washburn dacitic magmas are hard to reconcile by a fractionation- or assimilation-dominated process, adding credence to a crustal melting hypothesis for their origin. Assuming this to be correct, the origin of the roughly linear trends for individual groups of dacitic rocks in some Harker plots (e.g. Al2O3; Fig. 9) may reflect several processes, including crystal fractionation, restite unmixing, mixing with less evolved magmas, and melting processes occurring within source regions.
Mixing between crustally derived silicic melts and restite assemblages as a petrogenetic process is typically reserved for coarse-grained, felsic plutonic rocks (e.g. White & Chappell, 1977). For rocks in the Absaroka Volcanic Province this process was discussed by Meen & Eggler (1989) for granitoids associated with the Independence volcanic center. In their study of the Independence granitoids, Meen & Eggler (1989) argued that phenocryst assemblages represent restite retained from zones of crustal melting and silicic melt production. Although broadly similar in composition to the Independence granitoids, chemical variations of the Washburn dacites probably do not reflect this process, because of the lack of petrographic evidence that the phenocryst assemblages represent non-magmatic, residual assemblages, such as highly corroded mineral phases and high-pressure, high-temperature, anhydrous minerals (Clemens, 1989). Furthermore, dacitic magmas at Washburn volcano with similar major element compositions have wide ranges in modal phenocryst proportions, inconsistent with a restite unmixing process (e.g. Fig. 5).
Restite unmixing is also unlikely on the basis of chemical variations. For illustration, following a similar (but less rigorous) procedure to that employed by Meen & Eggler (1989), we estimated SiO2 contents of model crustal melts by extrapolating linear regression lines for MgO, Ni, and Cr contents of the SW Washburn Range and Mt. Washburn dacites to zero intercepts on Harker diagrams (Figs 9 and 10). Using the average SiO2 content for each group as predicted by the intercepts, contents of other major elements in model crustal melts were estimated by extrapolation of regression lines (not shown) to these values. Table 7 presents the model crustal melt compositions and the results of least-squares mixing calculations involving the melts and phenocryst phases in the Washburn dacitic rocks. The results indicate that dacitic magmas broadly similar in composition (
r2 = 1·3–1·5) to the Washburn dacitic rocks can be produced by phenocryst addition. However, the models require similar quantities of amphibole (14–15 wt %) for suites of dacitic rocks from the SW Washburn Range and Mt Washburn, which cannot be reconciled with the different trends in Ni and Cr in Figs 11 and 14 for these two suites and the large variation in modal amphibole in the dacitic rocks.
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Rather than restite unmixing, some of the geochemical variations for the Washburn dacitic rocks may reflect limited fractional crystallization of intermediate to silicic composition magmas following magma mixing. In particular, crystal fractionation may have been involved in the petrogenesis of SW Washburn dacitic rocks because they have Al2O3 (and P2O5) contents that decrease slightly with increasing SiO2 (Fig. 9). Least-squares mass balance calculations allow such a process in that they predict small sums of the squared residuals for major elements and plagioclase-dominated, low-pressure phenocryst assemblages similar to those found in the SW Washburn dacitic rocks (Table 8). It is also notable that the trend for these rocks in a plot of Al2O3 vs MgO (Fig. 18) extends off the main mixing trend for the Washburn suite as a whole. This indicates that if fractional crystallization occurred, it followed mixing of mafic magmas with silicic crustal melts compositionally similar to dacitic rocks from Mt. Washburn.
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Constraints on crustal source materials
Sr and Nd isotopic ratios of most Washburn andesitic and dacitic rocks project toward or plot within fields formed by Wyoming Province deep-crustal granulite-facies rocks brought up as xenoliths in Cenozoic magmas (Fig. 14a; Leeman et al., 1985; Dudas et al., 1987). A straightforward interpretation of the isotope data is that the sources for the silicic magmas are similar in composition to the ancient deep-crustal rocks. For type 1 dacitic rocks, supracrustal granitoids and amphibolites in the Beartooth Mountains might also provide suitable crustal sources, as the isotopic composition of the single analyzed sample lies at the edge of the field formed by the compiled data of Meen (1987a) and Wooden & Mueller (1988) in Fig. 14.
Trace element compositions of the dacitic rocks provide additional evidence that source rocks of the dacitic magmas were similar in composition to Wyoming Province granulite-facies xenoliths. Figure 19 illustrates variations in Th contents of Washburn dacitic magmas relative to Zr contents and Zr/Th ratios. Also illustrated in Fig. 19 are data for Wyoming Province deep-crustal granulite-facies xenoliths. The Washburn dacitic rocks have low Th contents and a large range in Zr/Th ratios. Partial melting of Wyoming Province granulite-facies lithologies can yield melts with these trace element features. As an illustration, Fig. 19 shows batch partial melting curves originating from an arbitrarily chosen composition within the fields defined by the xenoliths (e.g. Zr 75 ppm; Th 1·25 ppm). In calculating these curves the residue of anatexis was considered to contain clinopyroxene but not plagioclase, in accordance with the positive Eu anomalies and relatively flat MREE to HREE patterns of the dacitic rocks (Fig. 12). The close correspondence between the model partial melting curves and the compositions of many of the dacitic rocks indicates that lithologies with compositions similar to Wyoming Province granulite-facies xenoliths are viable source rocks. It should be noted, however, that the large range in Zr/Th, Sr/Y, and 87Sr/86Sr ratios (Figs 13, 15 and 18) and other compositional features of the silicic magmas cannot be produced by melting of a single, homogeneous crustal source. Melting of heterogeneous deep crust is required to produce the several distinct types of silicic crustal melt present at Washburn volcano, as represented by the diversity of exposed dacitic rocks (Fig. 11).
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Alternative sources for the dacitic magmas include Archean supracrustal amphibolites and granitoids that dominate the exposed basement near Washburn volcano. The trace element characteristics of the dacitic magmas are not easily reconcilable with partial melting of these lithologies, however. Most of the supracrustal rocks have Th contents that are too high to serve as appropriate sources unless some process reduced Th contents in the partial melts relative to the source lithologies (Fig. 19). The most efficient mechanism to lower Th contents is zircon retention in the source. Retention of zircon (or any other phase) during melting of the supracrustal lithologies cannot directly yield magmas similar in composition to the Washburn dacitic rocks because the latter have elevated Zr/Th ratios. Retention of zircon during crustal melting lowers Zr/Th ratios and Th contents of melts relative to source rocks (Fig. 19b). In contrast, Wyoming Province granulite-facies xenoliths have low Th contents and an extensive range in Zr/Th ratios that span those of the Washburn dacitic rocks.
In summary, crystal fractionation and crustal contamination of mafic magmas followed by mixing are important in controlling compositional trends of the Washburn rocks, but jointly cannot give rise to all of the geochemical variations observed. Generation of diverse silicic crustal melts is additionally required to explain the trace element and Sr isotopic characteristics of the dacitic rocks. The compositional diversity of the dacitic magmas probably reflects the lithologic heterogeneity and thickness of the Archean Wyoming Province crust and it contrasts with magmatic systems developed in young thin crust, particularly those associated with subduction. In continental margin subduction systems such as the southern Andes, silicic magmas are typically more uniform in trace element and isotopic ratios than associated mafic magmas (Gerlach et al., 1988; Hickey-Vargas et al., 1989; McMillan et al., 1989; Tormey et al., 1995; Feeley et al., 1998). Although in some cases this results from little crustal involvement following generation of heterogeneous mantle-derived melts (e.g. Hickey-Vargas et al., 1989), to a large extent it is probably a function of the limited compositional and isotopic contrast between the parental magmas and the young, subarc crust (Davidson et al., 1987; Hildreth & Moorbath, 1988; Feeley, 1993). In contrast, in ancient terrains having experienced complex evolutionary histories, including multiple deformation events and magmatic episodes, the crustal column may be heterogeneous over relatively small length scales and capable of producing at single eruptive centers silicic melts showing much greater compositional diversity than associated mantle melts.
Mantle source considerations
Depletions of HFSE and enrichments of LILE relative to neighboring elements in diagrams such as Fig. 13 are widely considered diagnostic of magmas derived from subduction processes [e.g. Thirlwall et al. (1994) and references therein]. A critical question is whether the ‘arc’ signature present in Washburn trace element patterns, and developed to varying degrees in nearly all AVP mafic rocks (Fig. 13), is inherited from the mantle source, or whether it reflects crustal contamination, as the continental crust is also depleted in HFSE relative to LILE (Weaver & Tarney, 1984; Taylor & McLennan, 1985; Wilson, 1989). This question is particularly relevant in the case of the AVP because the tectonic setting of the field is unclear. Rocks of the AVP were emplaced nearly 700 km from the Eocene trench and those who have advocated a subduction origin for the field have had to invoke complex subduction geometries, including shallow-angle subduction, imbricate subduction, and downward buckling of subducted oceanic lithosphere along an axis normal to the trend of the magmatic belt (e.g. Coney & Reynolds, 1977; Lipman, 1980; Humphreys, 1995).
To address the problem of whether the elevated LILE/HFSE ratios in Washburn rocks reflect that of the source or crustal contamination, or both, Fig. 20 plots Ba/Nb ratios of Washburn rocks vs SiO2 contents (e.g. Nelson & Davidson, 1993). For the suite as a whole, Ba/Nb ratios increase as a function of differentiation and compositions of the basaltic andesites project back toward compositions of primitive island arc basalts, although they greatly exceed those of primitive ocean-island basalt (OIB) and MORB, and OIB-like late Cenozoic basaltic magmas that are common in the nearby Basin and Range Province to the south (e.g. Fitton et al., 1988). Moreover, bulk Ba and Nb contents for all Washburn rocks plot well within the orogenic andesite field of Gill (1981; Fig. 20). We interpret these relationships to indicate that the high Ba/Nb ratios of Washburn mafic rocks are a characteristic of the mantle source, and that this signature was amplified in more differentiated rocks by crustal contamination. Dudas (1991) and Hooper et al. (1995) arrived at similar conclusions for Eocene magmatic rocks in the Crazy Mountains and Pacific Northwest, respectively. Subsequent to the study of Hooper et al. (1995), Morris & Hooper (1997) and Morris et al. (2000) considered a comparable ‘arc’ signature in the geochemistry of rocks from the Colville complex to be entirely crustal in origin and therefore unrelated to contemporaneous subduction processes. This conclusion largely stems from the lack of strong Nb depletions in parental mafic samples present in the complex (e.g. Fig. 13c).
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The considerations above imply a subduction origin or subduction-modified sources for Washburn parental magmas. An interesting feature of AVP volcanic rocks in this regard is that mafic rocks from the shoshonitic Sunlight volcano have higher Ba/Nb ratios than Washburn rocks at equivalent bulk compositions (Fig. 20). Further, although Ba contents are significantly elevated in Sunlight basalts, Nb contents are comparable with those in Washburn mafic rocks, implying similar degrees of melting in the source. It is unknown if mafic rocks from the shoshonitic Independence volcano share the same characteristics, as Nb data are not available for the suite. The Independence rocks probably have high Ba/Nb ratios, however, as Ta and Ba contents are depleted and enriched, respectively, relative to Washburn basaltic andesites (Fig. 13b). These relationships are consistent with variable degrees of source metasomatism by LILE-rich fluids as suggested by Tatsumi et al. (1986) and summarized by Hawkesworth et al. (1993). As LILE are particularly mobile in hydrous fluids, especially relative to HFSE (Tatsumi et al., 1986; Brenan & Watson, 1991), the elevated LILE/HFSE values in mafic rocks from eastern AVP shoshonitic centers are tentatively consistent with higher degrees of source metasomatism by LILE-rich fluids toward the east.
In terms of the tectonic significance of this interpretation, an apparent paradox is immediately obvious if the across-strike K2O enrichment in the AVP in part derives from mantle processes involving contemporaneous slab dehydration. For example, if the AVP does indeed represent a subduction-related continental volcanic arc, one might also expect sources further inland to be less affected by metasomatism, as seen in many modern arcs (e.g. Hickey-Vargas et al., 1989; Davidson & de Silva, 1995). Further, Sr and Nd isotopic compositions of all AVP rocks plot well below the field defined by modern subduction-related basalts as a result of their low Nd isotopic ratios (Fig. 14). These features indicate that although an arc-like component may be present in AVP magmas, basalt generation may have been triggered in previously metasomatized lithospheric mantle by other heating or decompression mechanisms proposed for regional magma generation during the mid-Eocene in the northwestern USA (Dudas, 1991; Morris et al., 2000).
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DISCUSSION |
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Summary: evolution of the Washburn volcano magmatic system
Integration of eruptive stratigraphy with petrologic modeling has been proven to be a useful technique in unraveling petrologic processes operating at long-lived continental magmatic centers (Dungan et al., 2001
). At Washburn volcano the geochronologic, petrologic, and geochemical data can be used to construct a model for the evolution of the system with the following components:
- (1) primitive mantle-derived basaltic magmas fractionated clinopyroxene-rich mineral assemblages and assimilated partial melts of conduit walls at mid- to deep-crustal levels.
- (2) Contaminated mafic magmas stalled in the mid- to deep crust, where they melted rocks with compositions similar to Archean granulite-facies rocks and mixed thoroughly with the resultant melts. This stage is, of course, analogous to the MASH (mixing, assimilation, storage, homogenization) process of Hildreth & Moorbath (1988)
and it produced the dominant volume of andesitic and basaltic andesitic rocks present on Mt. Washburn and explains the relative scarcity of either basaltic or dacitic rocks.
- (3) Hybrid magmas ascended to shallow crustal reservoirs, where they crystallized and in some cases fractionated small amounts of low-pressure mineral assemblages dominated by plagioclase.
- (4) Repeated injections of hybrid, intermediate composition magma into the bases of the shallow chambers caused minor oscillatory and reverse zoning in any resident phenocrysts.
- (2) Contaminated mafic magmas stalled in the mid- to deep crust, where they melted rocks with compositions similar to Archean granulite-facies rocks and mixed thoroughly with the resultant melts. This stage is, of course, analogous to the MASH (mixing, assimilation, storage, homogenization) process of Hildreth & Moorbath (1988)
The scenario for rocks in the SW Washburn Range differs from that for rocks on Mt. Washburn in that the amount of fractionation of the mafic magmas was generally less and the silicic magmas experienced small degrees of shallow-level fractionation. Moreover, because the SW Washburn suite is a bimodal assemblage of basaltic andesitic and dacitic rocks it appears that the extent of homogenization was also less. All of these features may reflect progressive growth of a crustal magmatic system as a result of repeated basaltic injections. For example, the composition gap during the early stages of magmatism is interpreted to reflect that mafic magmas were progressively mixing with silicic crustal melts, but that the proportion of silicic melt initially was not great enough to produce the full spectrum of hybrids. Small proportions of crustally derived melt during the early stages of magmatic activity may indicate an initially cold, dense crustal column. This process is envisioned as one in which early batches of mafic magma were emplaced into the deep crust, but after limited fractionation and mixing with crustal melts the magmas obtained sufficiently low densities to promote further ascent. The process of basalt emplacement and crystallization occurred repeatedly, leading to large-scale heating of the crust. Higher temperatures in the crust rapidly increased the proportion of silicic melt (Huppert & Sparks, 1988), decreased the density of the crustal column, and increased the residence time for basaltic magmas. Collectively, these processes induced greater degrees of high-pressure fractionation, contamination, and homogenization with silicic crustal melts. At all temporal stages in the evolution of Washburn volcano the hybrid magmas stalled in shallow magma chambers before eruption. This episode is documented by growth of new, plagioclase-rich mineral assemblages in equilibrium with the hybrid liquids. In the case of the SW Washburn system where early batches of silicic magma were probably small and the crust relatively cold, convective velocities within the shallow chambers may have been sufficiently low to permit fractionation of resident phenocrysts.
Petrologic significance
In light of the foregoing discussion one of the more intriguing aspects of the Washburn suite is that although bulk compositions strongly suggest that the intermediate composition rocks are hybrids, compositional and modal data for phenocrysts show only limited evidence for mixing in the form of mineral–melt disequilibria or coexisting high- and low-temperature phenocryst assemblages (e.g. Figs 5–8). In this regard the rocks resemble the ‘cryptic hybrids’ of Dungan (1987) and Kerr et al. (1999). Dungan (1987) suggested that the most likely condition for generating cryptic hybrids involves blending of mineralogically similar, low-viscosity tholeiitic mafic magmas, although he also proposed a more general model applicable to calc-alkaline and bimodal systems where endmember magmas differ markedly in solidus temperatures, densities, viscosities, and mineral assemblages. Important elements of the model include incremental mixing of superheated, crystal-poor magmas long before eruption. Although Dungan (1987) did not further identify specific magmatic environments where these conditions are optimized, he noted that the main requirement is that sufficient time must exist following mixing for the hybrid magmas to achieve textural and mineralogical equilibrium before eruption.
The petrogenetic model developed in this paper implies that generation of cryptic hybrids in compositionally diverse calc-alkaline systems may be facilitated in deep-crustal zones of differentiation. At Washburn volcano juxtaposition of compositionally diverse magmas appears to have occurred in deep chambers repeatedly fluxed by high-temperature, relatively primitive mafic magmas. In such environments heating of the crust may be substantial, resulting in production of partial melts that are effectively liquid as a result of nearly complete resorption of any entrained pre-existing crystals (Watson, 1982; Tsuchiyama, 1985; Huppert & Sparks, 1988; Koyaguchi & Kaneko, 1999). Furthermore, as proposed by Hildreth & Moorbath (1988) and suggested by the stratigraphic succession documented in this paper, mixing in deep-crustal magma chambers may be incremental and involve repeated blending of fractionated mantle-derived mafic magmas, silicic crustal melts, and evolved magmas which themselves are hybrids formed during earlier injection–mixing episodes. In this manner, endmember magmas progressively converge toward intermediate compositions that can readily homogenize. Subsequent ascent and crystallization of hybrid magmas in shallow chambers will overprint these earlier mixing episodes.
In contrast to the Washburn system, many calc-alkaline suites contain dramatic mineralogic and petrographic evidence for mixing, particularly where homogenization is incomplete and compositionally banded tephra or commingled mafic inclusions occur within more silicic hosts (Wilcox, 1999). In these systems magma interaction appears to have occurred rapidly during injection of basaltic magma into the bases of shallow silicic chambers (Blake et al., 1965; Eichelberger, 1975; Anderson, 1976; Bacon, 1986; Coombs et al., 2000). There is also abundant evidence in these systems that basaltic injection is responsible for triggering eruption, thereby arresting re-equilibration of high- and low-temperature phenocryst assemblages within the new hybrid magma (Pallister et al., 1992; Feeley & Sharp, 1996). At Washburn volcano injections into shallow chambers may also have triggered eruptions. These injections, however, were probably hybrid intermediate magmas compositionally similar to resident magmas, thereby producing little discernible textural or mineralogical disequilibrium in the erupted lavas.
Implications for magmagenesis in the AVP
Recognition of the Washburn magmas as cryptic hybrids provides new insight into magmatic processes operating in the AVP. An outstanding question concerning the origin of magmatic rocks in the AVP is the extent to which western belt calc-alkaline magmas interacted with the continental crust. At other magmatic fields in the ‘Challis arc’ (Fig. 1), a magma mixing origin has been demonstrated for calc-alkaline rocks (e.g. Morris & Hooper, 1997; Morris et al., 2000). However, before this study, a simple closed-system history was assumed for the petrogenesis of the AVP calc-alkaline magmas because no convincing evidence had been presented to indicate crustal involvement during differentiation. Indeed, the simple phenocryst assemblages of the rocks were cited as strong evidence for differentiation by closed-system fractional crystallization. This facet was established over a century ago in the classic study of Iddings (1891) on AVP calc-alkaline rocks exposed at the Electric Peak–Sepulcher Mountain eruptive center (Fig. 1), which produced rocks temporally, compositionally, and petrographically similar to those at Washburn volcano (Lindsay & Feeley, 1999). At this center Iddings (1891) utilized optical and chemical techniques available at the time to argue for textural and compositional equilibrium between phenocrysts and whole rocks, and correlated changes in phenocryst modes with differentiation nearly identical to those documented here. In his view these features suggested that: ‘the chemical differences of igneous rocks are the result of a chemical differentiation of a general magma’ (Iddings, 1891). Subsequent petrologic studies based on limited chemical datasets maintained this opinion and further argued that differentiation of AVP calc-alkaline magmas involved simple fractional crystallization of plagioclase-rich assemblages (Schultz, 1962; Peterman et al., 1970; Love et al., 1976; LaPointe, 1977).
The significance of the open-system petrogenetic model advocated here lies not only in the new insight it provides into magma generation processes in the AVP, but perhaps more importantly in the constraints it places on mechanisms responsible for producing calc-alkaline suites in the western belt and shoshonitic suites in the eastern belt. Meen (1987b) used experimental data for shoshonitic rocks at the eastern belt Independence volcano to argue that parental magmas for both suites may be produced from mantle-derived subalkaline basaltic magmas without interaction with crustal rocks. In this scenario, shoshonitic differentiation trends result from high-pressure (10 kbar) fractional crystallization of subalkaline basaltic magmas, whereas calc-alkaline magma series are generated from shallow pressure, anhydrous fractional crystallization involving significant olivine in the crystallizing assemblage. The effect of the latter process is to rapidly enrich SiO2 contents and deplete MgO contents relative to K2O and produce relatively linear, shallow arrays comparable with those in Figs 9 and 10b for the Washburn suite. Lending uncertainty to this interpretation, however, is the recognition that the calc-alkaline trend may also be produced by crustal contamination of anhydrous mafic magmas evolving along high-pressure liquid lines of descent [see fig. 11 of Meen (1987b)].
As outlined above, before this study no convincing petrologic evidence existed to indicate that crustal involvement was important in the generation of magmas at calc-alkaline centers in the AVP, allowing for a simple ‘increasing depth to magma chamber relationship’ to explain the eastward transition from calc-alkaline to shoshonitic differentiation trends in the AVP. The compositional relationships among rocks at Washburn volcano indicate, however, that AVP calc-alkaline andesitic magmas evolved mainly by magma mixing and that differentiation of parental magmas involved fractional crystallization plus one or more open-system processes such as mixing and assimilation of crust. All Washburn rocks thus appear to contain a crustal component and none can reasonably be inferred to be direct differentiates of primary mantle-derived magmas, as has been suggested for magmas parental to shoshonites at Independence volcano (e.g. Fig. 10; Meen, 1987b; Meen & Eggler, 1987). Differences in the location of crustal partial melting or sources during silicic melt generation also contributed to the development of compositional diversity at Washburn volcano. Given these results, magma mixing, partial melting of the crust, and contamination of parental magmas must therefore be considered as important in either generating or influencing differences between calc-alkaline and shoshonitic suites in the AVP. In this regard we do not dismiss crystal fractionation as trivial, but the variation from rocks that follow calc-alkaline trends in the west to those that follow shoshonitic trends in the east cannot simply reflect higher pressures of fractionation to the east in Moho-level magma chambers in the absence of crustal interaction. Although the origin of the across-strike K2O trend is still unclear, further progress into better understanding the roles of open- vs closed-system processes in the AVP is anticipated with additional geochemical studies of individual centers.
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CONCLUSIONS |
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- (1) Magmatic rocks at Washburn volcano range in age from 55 to 52 Ma and form a medium- to high-K, calc-alkaline suite ranging in composition from basaltic andesitic through dacitic.
- (2) Mineral textural and composition features along with whole-rock compositional data can be interpreted as reflecting near-equilibrium crystallization of observed phenocryst phases.
- (3) Stratigraphic relationships, along with petrogenetic modeling of trace element and Sr and Nd isotopic data, demonstrate that the Washburn rocks were produced by progressive mixing of variably fractionated and contaminated mantle-derived melts and heterogeneous silicic crustal melts.
- (4) Nd and Sr isotopic compositions along with trace element data indicate that silicic melts in the Washburn system are derived from deep-crustal rocks broadly similar in composition to granulite-facies xenoliths in the Wyoming Province. Data for basaltic andesitic rocks indicate that the ‘subduction’ signature is of mantle origin, although it may not be related to active subduction.
- (5) Compositional and petrographic diversity in the Washburn suite were produced by homogenization of mafic and silicic crustal melts in deep-crustal magma chambers followed by ascent and growth of new plagioclase-rich mineral assemblages in equilibrium with the hybrid liquids. Many calc-alkaline magmas may therefore be cryptic hybrids recognizable only on chemical grounds, particularly in systems where mixing precedes and is widely separated from crystallization in space and time.
- (6) Given the role ascribed to crustal processes at Washburn volcano, partial melting of the crust, crustal contamination of parental magmas, and magma mixing must play some role in either generating or influencing across-strike K2O trends in the AVP. The variation between rocks that follow calc-alkaline trends in the west to those that follow shoshonitic trends in the east cannot simply reflect higher pressures of fractionation to the east in Moho-level magma chambers in the absence of crustal interaction.
- (2) Mineral textural and composition features along with whole-rock compositional data can be interpreted as reflecting near-equilibrium crystallization of observed phenocryst phases.
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APPENDIX: ANALYTICAL METHODS |
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40Ar/39Ar analytical procedures
The 40Ar/39Ar analyses were carried out at the Université de Lausanne. Samples together with the standards were irradiated for 20 MWH in the central thimble position of the US Geological Survey Triga reactor in Denver, Colorado (Dalrymple et al., 1981
). All analyses were made using a low blank, double vacuum resistance furnace and metal extraction line connected to a MAP 215-50 mass spectrometer using an electron multiplier. The samples were incrementally heated in the furnace and the gas was expanded and purified using activated Zr/Ti/Al getters and a metal cold finger maintained at liquid nitrogen temperatures. Linear time zero regressions were fitted to data collected from eight scans over the mass range 40–36. Peak heights above backgrounds were corrected for mass discrimination, isotopic decay and interfering Ca-, K- and Cl-derived isotopes of Ar. Blanks were measured at temperature and subtracted from the sample signal. For mass 40, blank values ranged from 4 x 10-15 moles below 1350°C to 9 x 10-15 moles at 1650°C. Blank values for masses 36–39 were below 2 x 10-17 moles for all temperatures. Isotopic production ratios for the Triga reactor were determined from analyses of irradiated CaF2 and K2SO4 and the following values have been used in the calculations: 36Ar/37Ar(Ca) = 0·000264 ± 0·0000017; 39Ar/37Ar(Ca) = 0·000673 ± 0·0000037; 40Ar/39Ar(K) = 0·00568 ± 0·004. Correction for the neutron flux was determined using the standard MMHB-1, assuming an age of 520·4 Ma (Samson & Alexander, 1987). A mass discrimination correction of 1·008 a.m.u. was determined by online measurement of air and was applied to the data. Age plateaux were calculated for samples in which three or more consecutive steps yielded statistically indistinguishable ages at the 95% confidence level and which collectively made up at least 50% of all 39Ar released from the sample. The linear regressions used for determining the isochron values were calculated following the method of York (1969).
Whole-rock and mineral geochemistry analytical procedures
Compositions of silicate mineral phases in Washburn rocks are given in Tables 3–6. Mineral compositions were determined at the Université de Lausanne, on a Cameca SX50 electron microprobe using ZAF on-line data reduction and matrix correction procedures. A 15 kV accelerating voltage was used with a 10 nA specimen beam current for plagioclase and 20 nA for all other minerals.
Major element data, trace element data, modal data, and Sr and Nd isotopic ratios for Washburn igneous rocks are given in Table 2. All samples were pulverized in a shatterbox, tungsten carbide for XRF and isotopic ratio determinations, and alumina for INAA. Major and trace analyses by XRF were determined at the Geoanalytical Laboratory at Washington State University following the procedure described by Johnson et al. (1999). INAA was performed at the Oregon State University Triga reactor facility following the method of Laul (1979). Estimated precision on the INAA analyses is better than 5% (1 standard deviation) for Sc, Co, Cs, La, Sm, Eu, Tb, Yb, Lu, Hf, and Ta, and 7–12% for Ce, Nd, and U. Sr and Nd isotopic ratios were determined at the Keck Center for Isotope Geochemistry at the University of California, Los Angeles, by TIMS, following the procedure described by Feeley & Davidson (1993). Precision on the Sr and Nd isotopic determinations is generally better than ±0·00002 and ±0·00001, respectively. All initial ratios were calculated assuming an age of 53 Ma. Modal data were determined by point counting 1000–1200 points per thin section, with phenocrysts defined as >0·3 mm in the longest dimension.
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ACKNOWLEDGEMENTS |
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This work was supported by NSF grant EAR-9725287 to T.C.F. and GSA Penrose and Sigma Xi grants to C.R.L. The Radiation Center at Oregon State University provided the instrumental neutron activation analyses. The authors thank François Bussy for assistance with the microprobe, Megan O’Connor for able and cheerful field assistance, and Yellowstone National Park personnel for permission to obtain samples from the field area and hospitality. Discussions with Margaret Hiza helped to stimulate many of the ideas presented in this paper. Helpful reviews by Peter Hooper, Andrew Kerr, George Morris, Marjorie Wilson, and Jim Meen on earlier versions of this paper are appreciated, as is the editorial work of Marjorie Wilson and Alastair Lumsden.
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FOOTNOTES |
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Extended dataset can be found at http://www.petrology.oupjournals.org
*Corresponding author. Telephone: 406/994-6917. Fax: 406/994-6923. E-mail: tfeeley{at}montana.edu
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