Journal of Petrology Advance Access originally published online on October 3, 2007
Journal of Petrology 2007 48(11):2063-2091; doi:10.1093/petrology/egm050
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Development of a Continental Volcanic Field: Petrogenesis of Pre-caldera Intermediate and Silicic Rocks and Origin of the Bandelier Magmas, Jemez Mountains (New Mexico, USA)
1Department of Geological Sciences, University of Texas at Austin, Austin, TX 78712, USA
2Department of Geology, Washington State University, Pullman, WA 99164, USA
3Earth and Environmental Sciences, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
4Department of Geological Sciences, Central Washington University, Ellensburg, WA 98926, USA
5Department of Geological and Environmental Sciences, California State University, Chico, CA 95929, USA
6310 Garver Lane, White Rock, NM 87544, USA
RECEIVED JANUARY 20, 2006; ACCEPTED AUGUST 3, 2007
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONREGIONAL SETTING AND MAFIC...CHRONOLOGY AND CHARACTERISTICS...GEOCHEMISTRY OF INTERMEDIATE AND...PETROGENESISTEMPORAL EVOLUTION OF THE...SUPPLEMENTARY DATAREFERENCES |
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The Miocene–Quaternary Jemez Mountains volcanic field (JMVF) is the site of the Valles caldera and associated Bandelier Tuff. Caldera formation was preceded by > 10 Myr of volcanism dominated by intermediate composition rocks (57–70% SiO2) that contain components derived from the lithospheric mantle and Precambrian crust. Simple mixing between crust-dominated silicic melts and mantle-dominated mafic magmas, fractional crystallization, and assimilation accompanied by fractional crystallization are the principal mechanisms involved in the production of these intermediate lavas. A variety of isotopically distinct crustal sources were involved in magmatism between 13 and 6 Ma, but only one type (or two very similar types) of crust between 6 and 2 Ma. This long history constitutes a record of accommodation of mantle-derived magma in the crust by melting of country rock. The post-2 Ma Bandelier Tuff and associated rhyolites were, in contrast, generated by melting of hybridized crust in the form of buried, warm intrusive rocks associated with pre-6 Ma activity. Major shifts in the location, style and geochemical character of magmatism in the JMVF occur within a few million years after volcanic maxima and may correspond to pooling of magma at a new location in the crust following solidification of earlier magma chambers that acted as traps for basaltic replenishment.
KEY WORDS: crustal anatexis; fractional crystallization; Jemez Mountain Volcanic Field; Valles Caldera; radiogenic isotopes; trace elements
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONREGIONAL SETTING AND MAFIC...CHRONOLOGY AND CHARACTERISTICS...GEOCHEMISTRY OF INTERMEDIATE AND...PETROGENESISTEMPORAL EVOLUTION OF THE...SUPPLEMENTARY DATAREFERENCES |
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The Jemez Mountains region, situated on the western flank of the Rio Grande rift in northern New Mexico, contains a record of over 20 Myr of volcanism prior to the catastrophic eruptions of the rhyolitic Bandelier Tuff and formation of the Valles–Toledo caldera complex in two episodes at 1· 6 and 1· 2 Ma. The Jemez Mountains volcanic field (JMVF) is largely built of intermediate to silicic lavas, with dacites being especially prominent in the few million years preceding the rhyolitic caldera-forming eruptions. Prior studies have indicated a major role for crustal anatexis in the generation of the intermediate and silicic JMVF rocks (Gardner, 1985
; Singer & Kudo, 1986; Ellisor et al., 1996). The need for continuous basaltic input into the crust to sustain long-lived silicic centers has long been recognized, and the JMVF, like many silicic centers, may be regarded as ‘fundamentally basaltic’ (Hildreth, 1981). However, identification of specific mantle and crustal components, the processes by which they are blended, and their changing roles during the evolution of any one center remain problematic. For example, DePaolo et al. (1992) and Perry et al. (1993) noted, on the basis of Nd isotope ratios, that the Precambrian crustal contribution to JMVF magmatism decreased with time, reaching a minimum in the caldera-forming rhyolitic Bandelier Tuff, and inferred that fractional crystallization of mafic magma, with little accompanying assimilation, must have played a major role in the generation of the caldera-related rhyolites. The difficulty with this model is that the Valles caldera is the site of a pronounced gravity low (Segar, 1974; Nowell, 1996) and, although other geophysical data indicate a significant role for mafic underplating beneath the volcanic field (Ankeny et al., 1986; Steck et al., 1998; Aprea et al., 2002), there is no evidence for a large volume of mafic cumulate residue beneath the JMVF such as must accompany generation of many hundred cubic kilometers of high-silica rhyolite by fractional crystallization of basaltic magma. Crustal hybridization (Johnson et al., 1990; Riciputi & Johnson, 1990; Riciputi et al., 1995) is an alternative model, in which the crust gradually takes on the isotopic signature of the mantle as a result of repeated intrusion of mantle-derived magma during the lifetime of a volcanic field. Crustal melting late in the history of the field therefore produces magmas that have isotopic affinities with mantle rather than crust.
Recently, Wolff et al. (2005) showed that the lithospheric mantle has been a major source of basaltic magma throughout the history of the Jemez Mountains region and demonstrated the existence of two end-member parental magmas: (1) basanites and nephelinites derived from low-degree partial melting of lithospheric mantle with residual amphibole; (2) olivine tholeiites, which could be derived either by higher-degree partial melting of the same source or from asthenospheric mantle. Crustal contamination modified the strongly silica-undersaturated compositions to weakly ne-normative alkali basalts, hawaiites, and derivative basaltic andesites and mugearites that erupted alongside tholeiites through most of the lifetime of the volcanic field. In this paper, we extend the work of Wolff et al. (2005) to consider the petrogenesis of the intermediate and silicic pre-caldera lavas and tuffs during construction of a moderate-sized (
2000 km3), long-lived volcanic field that ultimately hosted rhyolitic caldera-forming eruptions of catastrophic magnitude. Together, the two studies are intended to provide a basis for future, more detailed studies of the temporal development of the JMVF, and to provide a foundation for understanding the petrogenesis of the climactic rhyolitic magmas.
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REGIONAL SETTING AND MAFIC MAGMATISM |
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TOPABSTRACTINTRODUCTIONREGIONAL SETTING AND MAFIC...CHRONOLOGY AND CHARACTERISTICS...GEOCHEMISTRY OF INTERMEDIATE AND...PETROGENESISTEMPORAL EVOLUTION OF THE...SUPPLEMENTARY DATAREFERENCES |
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The JMVF is perhaps best known for the work of R. L. Smith and associates (Smith, 1960
, 1979; Ross & Smith, 1961; Smith & Bailey, 1966) on the caldera-forming Bandelier Tuff and associated rhyolitic lavas. Caldera formation, however, was preceded by some 23 Myr of volcanic activity in the area. The JMVF is built on the western shoulder of the Española basin, one of the north–south-trending series of Cenozoic en echelon sedimentary basins that make up the Rio Grande rift (Fig. 1). The JMVF is located at the intersection of the rift and the Jemez lineament (Fig. 1). The lineament is a long-lived tectonic feature that originally formed during the Proterozoic assembly of North America by collision of the Yavapai and Mazatzal crustal provinces between 1· 6 and 1· 7 Ga, and is associated with a south-dipping lithospheric suture (Shaw & Karlstrom, 1999; Karlstrom et al., 2002; Magnani et al., 2004). An associated low-velocity region (Duecker et al., 2001) in the lithospheric mantle to the north of the suture extends between the Moho at 40–50 km depth and 120 km depth, the probable base of the lithosphere. This lithospheric mantle is the likely source of JMVF primitive mafic magmas [see the paper by Wolff et al. (2005), which also includes a more detailed description of regional geology and tectonics].
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Rifting in northern New Mexico began at
30 Ma and has continued episodically to the present (Aldrich, 1986; Gardner et al., 1986; Morgan et al., 1986). Following a period of quiescence from at least 18 to 13 Ma, extension reinitiated and continued for several million years, during which time much of the JMVF was constructed. A second period of reduced rate of extension between 7 and 4 Ma (Gardner et al., 1986) that coincides with a lull in volcanic activity, ended with the development of the Pajarito fault zone (Fig. 1), which marks an eastward shift of the western boundary of the Española basin from the earlier Cañada de Cochiti fault zone (Fig. 1). Currently, low-velocity mantle extends to the base of the crust beneath the rift axis, where any former lithospheric mantle is now presumably either geophysically indistinguishable from asthenosphere as a result of shearing and heating, or has been thermally eroded. A similar low-velocity zone exists at the base of the crust beneath the JMVF (Aprea et al., 2002; Steck et al., 1998). Temperatures may exceed 900°C in the lower crust beneath the rift (Baldridge et al., 1984; Clarkson & Reiter, 1984; Morgan & Golombek, 1984). This is sufficiently close to the range of crustal solidus temperatures that thermal perturbations caused by intrusions of mantle-derived primitive basaltic magma into the sub-rift crust are very likely to induce partial melting of the Proterozoic crustal rocks.
The JMVF is built on a substrate of Upper Paleozoic sedimentary strata that rest on the Proterozoic basement, which consists of granitoid and metavolcanic rocks (typically intermediate amphibolites) locally dated at 1· 62–1· 44 Ga (Eichelberger & Koch, 1979; Brookins & Laughlin, 1983; Laughlin et al., 1983). Metavolcanic and metasedimentary rocks are also exposed in uplifts around the borders of the Española basin. The Paleozoic succession is overlain by a veneer of Cenozoic sedimentary units, the uppermost of which is the eastward-thinning, rift-filling Santa Fe Group, which includes minor mafic lavas and tuffs (Bailey et al., 1969; Smith et al., 1970).
Summary of the petrogenesis of the mafic lavas
The Santa Fe Group lavas and tuffs are the oldest volcanic rocks exposed in the area (Fig. 2). They form part of a more widespread, small-volume, late Oligocene to middle Miocene volcanic episode in the Española basin (Gibson et al., 1993; Woldegabriel et al., 2003), coincident with the first extensional phase, which pre-dates construction of the JMVF (Gardner et al., 1986; Fig. 2). Santa Fe Group volcanic rocks fall into two compositional groups: strongly silica-undersaturated nephelinites and basanites, which include primitive compositions (up to 16% MgO), and tholeiites and derivative quartz-normative basaltic andesites. Marked depletions in K relative to other incompatible elements among the silica-undersaturated lavas were attributed by Wolff et al. (2005) to residual amphibole in the lithospheric mantle source during partial melting. Although an asthenospheric source for the tholeiites cannot be ruled out, Wolff et al. (2005) argued that the tholeiitic parental magma is more probably derived from the same lithospheric mantle source material via a higher degree of partial melting involving the complete consumption of amphibole. A solely lithospheric source region for all JMVF parental mafic magmas is consistent with existing data [as reviewed by McMillan (1998)] indicating that the northern limit of asthenospheric contributions to magmatism in New Mexico lies south of the JMVF (Fig. 1). The actual source for JMVF mafic magmas is envisaged as ancient oceanic lithosphere associated with the Proterozoic suture zone (Wolff et al., 2005), which partially melted in association with rift extension. This is consistent with the geophysical evidence for shallow low-velocity mantle beneath the area at the present day (Duecker et al., 2001).
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Many mafic lavas of the JMVF (Paliza Canyon Formation, Cerros del Rio and El Alto basalts, and a few Lobato basalts: Figs 1 and 2) have distinctive incompatible trace element characteristics, with Th/(Nb,Ta) and La/(Nb,Ta) greater than Bulk Earth, but K/(Nb,Ta) similar to Bulk Earth, that do not correspond to any common globally recognized basalt type. Wolff et al. (2000
, 2005) showed that these compositions can be produced by contamination of K-depleted, silica-undersaturated magma with crustal melts having high K/(Nb,Ta) and proposed such an origin for the JMVF mafic rocks, with the implicit assumption that mantle partial melts produced during the early extensional phase are representative of those produced since 13 Ma. In this study, these rocks, which span several categories of formal classification (Fig. 3), are for convenience referred to as Type I mafic lavas. Tholeiites and derivative basaltic andesites are also crustally contaminated (Duncker et al., 1991; Wolff et al., 2000, 2005) and are referred to as Type II mafic lavas. We emphasize that all JMVF mafic magmas carry a component derived from continental crust.
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Type I and Type II JMVF mafic lavas contain variable proportions of Proterozoic granitoid crust (Wolff et al., 2005
) and consequently exhibit significant variations in Pb and Nd isotope ratios (206Pb/204Pb = 17·20 – 18·31; 143Nd/144Nd = 0·51244 – 0·51273). Sr isotope ratios, however, are low and relatively uniform (87Sr/86Sr = 0·7041 – 0·7048), despite high 87Sr/86Sr among potential contaminating lithologies. Wolff et al. (2005) compared the effects of contamination by low-87Sr/86Sr crust with assimilation of high-87Sr/86Sr granitoids by partial melting, with Sr retained in a feldspathic residue. Both models reproduced the isotopic characteristics of the rocks, but the lack of measurable Eu anomalies among most JMVF mafic lavas is inconsistent with a major role for plagioclase during petrogenesis. |
CHRONOLOGY AND CHARACTERISTICS OF INTERMEDIATE AND SILICIC ROCKS IN THE JMVF |
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TOPABSTRACTINTRODUCTIONREGIONAL SETTING AND MAFIC...CHRONOLOGY AND CHARACTERISTICS...GEOCHEMISTRY OF INTERMEDIATE AND...PETROGENESISTEMPORAL EVOLUTION OF THE...SUPPLEMENTARY DATAREFERENCES |
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The exact date of inception of volcanism in the JMVF proper is a largely semantic issue (Gardner et al., 1986
; Goff & Gardner, 2004); at some locations, there is a sharp contact between rift-filling Santa Fe Group sediments and a continuous volcanic section at 12–13 Ma (Goff et al., 1990), but elsewhere lavas dated as old as 14 Ma to as young as 9 Ma are interbedded with the sediments (Aldrich & Dethier, 1990). Overall, construction of the volcanic field was under way by 13 Ma, coincident with the onset of an episode of crustal extension in the adjacent Española basin (Gardner et al., 1986). The earliest rocks are basalts of the Paliza Canyon and Lobato Formations, and tuffs of the Canovas Canyon Rhyolite (Fig. 2).
The stratigraphy, geology, geochronology and petrography of the pre-caldera intermediate and silicic rocks are briefly described below; details of rocks with <57% SiO2 have been given by Wolff et al. (2005).
Revisions to the original JMVF stratigraphic scheme of Bailey et al. (1969) and Smith et al. (1970) have been made as new mapping and geochronological data have become available. Here we follow the formation stratigraphy of Bailey et al. (1969) and Smith et al. (1970) as modified by Gardner et al. (1986), but find the broader division into Keres and Polvadera Groups, made largely on geographical grounds, to be unhelpful. In particular, it is now clear that the Polvadera Group, as originally defined, includes rocks of widely disparate ages and origins. In contrast, the formations for the most part exhibit geological, geochronological, and petrological coherence. The Lobato Basalt, dominated by Type II tholeiites, and the El Alto basalt with both Type I and II mafic compositions, are omitted from the descriptions below because there are no samples from either with >57% SiO2 (see Wolff et al., 2005).
Paliza Canyon Formation
Exposures of the Paliza Canyon Formation are largely confined to the southern JMVF (Fig. 1), but on the basis of well data (Hulen et al., 1991), mapping (Smith et al., 1970; Gardner & Goff, 1996), and abundant lithic fragments in the Bandelier Tuff, it underlies the caldera and Tschicoma Formation in the north–central Jemez Mountains and has a total volume of 1000 km3. The Paliza Canyon Formation is volumetrically dominated by trachyandesites, trachydacites and dacites, with subordinate Type I and II mafic lavas, andesites and low-silica rhyolites (Fig. 3). Paliza Canyon flows, domes, tuffs and minor intrusives are petrographically diverse; Goff et al. (1990) recognized eight distinct varieties of andesite and dacite. On the basis of age (9·6 Ma, Goff et al., 1989) and chemical affinity, we include the dacite of Los Cerritos in the northeastern JMVF (Fig. 1), originally mapped as belonging to the Tschicoma Formation, in the Paliza Canyon Formation.
Bulk compositions are dominantly weakly alkaline, and thus most Paliza Canyon lavas in the range 57–63% SiO2 are classified as trachyandesite; the remaining samples are andesitic. Trachydacites and dacites are equally abundant in the range 63–69% SiO2 (Fig. 3). Na/K is variable and, whereas the mafic lavas are dominantly sodic (Wolff et al., 2005), the trachyandesites include both benmoreites and latites (sensu LeBas et al., 1986). Some minor adjustment of alkali contents may have occurred as a result of post-eruptive processes. Compared with Tschicoma Formation rocks of equivalent silica content, intermediate-composition Paliza Canyon rocks exhibit more scattered major element variations, higher TiO2 and K2O contents, and little systematic variation in alkali concentrations (Figs 3 and 4).
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Trachyandesites and andesites have the phenocryst assemblage plagioclase (An30–An60) + augite + hypersthene + opaque oxides ± hornblende ± olivine, and are characterized by disequilibrium textures: olivines invariably have reaction rims consisting of fine-grained clinopyroxene and opaque oxides, and plagioclase phenocrysts are commonly resorbed and exhibit complex zoning. Plagioclase (An16–53) is also the dominant phenocryst among the dacites, benmoreites and rhyodacites, with hornblende, biotite, clinopyroxene, opaque oxides and sparse apatite; glomeroporphyritic textures and crystal clots of plagioclase, augite, hypersthene and oxides are common.
Canovas Canyon Rhyolite
This formation consists of flows, domes, pumice fallout deposits, non-welded to densely welded and rheomorphic ignimbrites interbedded within the Paliza Canyon Formation, and minor rhyolitic intrusions. One tuff, dated at 12·4 Ma, locally forms the base of the JMVF (Gardner et al., 1986; Goff et al., 1990). Some Canovas Canyon units are low-silica rhyolites (69–72% SiO2, Fig. 3) that are petrographically very similar to, and compositionally overlap, the most silicic Paliza Canyon units that were mapped as ‘biotite dacite’ by Goff et al. (1990). The remainder are sparsely porphyritic high-silica rhyolites (76–78% SiO2) with phenocrysts of sanidine + quartz + opaque oxides ± plagioclase ± pyroxene ± hornblende ± biotite.
Bearhead Rhyolite
The high-silica rhyolite domes, flows, tuffs and minor intrusions of the Bearhead Rhyolite have been described in detail by Smith et al. (1991), Gay & Smith (1993), and Justet & Spell (2001). They are nearly aphyric and are similar to the Canovas Canyon high-silica rhyolites. The bulk of the Bearhead rhyolite post-dates the Paliza Canyon Formation at 7· 06–6·52 Ma, but two late flows have ages of 6·1 Ma (Justet & Spell, 2001). Some of the El Rechuelos rhyolite domes in the northern JMVF (see below) also fall into this age range.
Tschicoma Formation
The Tschicoma Formation, with a volume of nearly 500 km3 (Gardner et al., 1986), dominates the northern JMVF. Most of this volume is dacite, with lesser amounts of trachyandesite, andesite and occasional rhyolite (Fig. 3). Most of the exposed Tschicoma Formation lies in the sector from north to east of the Valles Caldera (Fig. 1). Excluding the La Grulla Plateau volcanics (see below), ages range from 6·9 to 2·7 Ma, with most of the dacites having been erupted between 5 and 2·7 Ma (Dalrymple et al., 1967; Leudke & Smith, 1978; Gardner & Goff, 1984; Goff et al., 1989; Woldegabriel et al., 2001; Goff & Gardner, 2004). Although early eruptions of the Tschicoma Formation coincide with a period of relative tectonic inactivity along the Jemez lineament and Rio Grande rift (Gardner & Goff, 1984; Aldrich, 1986), the large dacite domes in the NE JMVF, including Cerro Rubio, post-date the onset of renewed extension at 4 Ma.
The degree of preservation of the thick Tschicoma lavas allows use of topography as a mapping aid in identifying different volcanic flows and eruptive centers. Dacitic flows, up to 6·5 km in length in the case of Mesa de la Gallina (Fig. 1), originate from as many as 70 domes or eruptive centers that dominate the northern Jemez Mountains. Although there are a few crystal-poor to aphyric lavas, Tschicoma Formation dacites are typically coarsely porphyritic, containing 15–20% phenocrysts of plagioclase ± clinopyroxene + resorbed orthopyroxene ± hornblende ± biotite + opaque oxides, sometimes accompanied by rounded quartz xenocrysts with fine-grained haloes of clinopyroxene. Plagioclase is frequently resorbed and hornblende phenocrysts have dehydration rims. Mafic enclaves, up to 25 cm in diameter, of basaltic andesite, andesite and trachyandesite compositionally similar to Cerros del Rio lavas are common in the coarsely porphyritic dacites.
We include the two Cerro Rubio quartz latites, located in the Toledo Embayment and dated by K–Ar at 2·18 ± 0·09 Ma and 3·59 ± 0·36 Ma (Heiken et al., 1986), in the Tschicoma Formation on the basis of similar age, lithology and chemistry. Cerro Grande (Fig. 1) is a rhyolite with abundant dacitic enclaves that are chemically identical to the bulk of the Tschicoma dacites.
The La Grulla Plateau extends northward from the Valles Caldera (Fig. 1). It is geographically and tectonically separated from the rest of the Tschicoma Formation by a Bandelier Tuff-filled paleo-valley (Mesa del Medio) coinciding with a splay of the Canada de Cochiti fault zone (Fig. 1). The fault zone is a major down-to-the-east rift-bounding structure that has been active since at least 10 Ma. The plateau is formed of sparsely phyric andesite lavas capped by domes and flows of dacite and trachydacite (Singer & Kudo, 1986). The La Grulla Plateau suite has been dated at 7·9–7·4 Ma (Singer & Kudo, 1986), up to 1 Myr older than the oldest ages reported for the rest of the Tschicoma Formation and overlapping with late Paliza Canyon activity (Fig. 2). The La Grulla Plateau rocks share major element, trace element and isotopic characteristics with both the Paliza Canyon Formation and the main pile of Tschicoma rocks to the east of the fault zone (Figs 3 and 5–7), but also have some distinct isotopic features (Fig. 7c). At least one dacite dome contains mafic enclaves, chemically similar to those in the more easterly Tschicoma dacites.
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Puye Formation
The Puye Formation is an alluvial fan derived from Tschicoma Formation lavas on the eastern flank of the JMVF (Fig. 1) and covers a region of
200 km2 with a total thickness of at least 110 m and volume of >15 km3 (Waresback & Turbeville, 1990). Conglomeratic beds of the Puye Formation contain clasts that represent Tschicoma lavas that may no longer be preserved, having been removed by erosion and/or obliterated by later caldera-forming eruptions. Primary pyroclastic fallout units equivalent to Tschicoma dacite lavas are interbedded throughout the sequence, and the 2·53 ± 0·1 Ma Puye ignimbrite (Waresback & Turbeville, 1990), chemically identical to the Cerro Rubio dacites (Table 1), is prominently exposed in the middle of the Puye Formation section in medial exposures. The dominant cobble type in conglomeratic beds above the ignimbrite also closely resembles Cerro Rubio dacite.
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High-silica rhyolite pyroclastic fallout deposits occur near the top of the Puye Formation; one is correlated with the 1· 85 Ma San Diego Canyon ignimbrites (Turbeville & Self, 1988
) that represent the first eruptions from the Bandelier magma system.
El Rechuelos Rhyolites
The El Rechuelos Rhyolite consists of six domes in the northern JMVF (Smith et al., 1970). Loeffler et al. (1988) dated these centers and found that they represent three periods of rhyolitic volcanism at 7·5, 5–6, and 2 Ma (Fig. 2). The two older groups are similar in age to Bearhead Rhyolite (7·1–6·1 Ma) but are chemically more similar to Tschicoma rhyolites than to the Bearhead main group lavas of Justet & Spell (2001). Three domes, located along the east flank of Polvadera Peak, are dated at 2 Ma (Loeffler et al., 1988), consistent with the original stratigraphic definition (Bailey et al., 1969). They are high-silica rhyolites with sparse microphenocrysts of sanidine, sodic plagioclase and quartz.
Cerros del Rio volcanic field
This dominantly mafic volcanic field, with both Type I and II mafic compositions, is exposed in the axis of the Española basin, to the east of the JMVF (Fig. 1). Domes and flows of benmoreite (trachyandesite with Na2O – 2·0 
K2O) with up to 63% SiO2 occur in the northern part of the field (Duncker et al., 1991). These lavas temporally overlap and post-date late Tschicoma activity and form a compositional continuum with Tschicoma dacites, their enclaves, and the dominant mafic lavas of the Cerros del Rio (Wolff et al., 2005). Duncker et al. (1991) found that 143Nd/144Nd systematically decreases, and 
18O increases, with increasing silica content among the Cerros del Rio lavas, and concluded that the more silicic (>57% SiO2) flows are related to the mafic magmas of the field through mixing with silicic melts derived from regional crust, or assimilation–fractional crystallization of Type I mafic magma with continental crust as the assimilant, or some combination of the two processes.
San Diego Canyon ignimbrites
These two high-silica (75–78% SiO2) rhyolitic ignimbrites were erupted at 1· 85 Ma and are now found beneath the Bandelier Tuff in upper Cañon San Diego (Fig. 1), and in wells drilled inside the caldera (Hulen et al., 1991). A correlative pumice fallout deposit occurs interbedded with sediments in the uppermost part of the Puye Formation (Turbeville & Self, 1988). All three units show strong geochemical affinities with the Bandelier Tuff and are regarded as early products of the Bandelier magma system (Spell et al., 1990). They are much smaller than the climactic Bandelier ignimbrites, with probable volumes <10 km3 (Turbeville & Self, 1988).
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GEOCHEMISTRY OF INTERMEDIATE AND SILICIC ROCKS IN THE JMVF |
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TOPABSTRACTINTRODUCTIONREGIONAL SETTING AND MAFIC...CHRONOLOGY AND CHARACTERISTICS...GEOCHEMISTRY OF INTERMEDIATE AND...PETROGENESISTEMPORAL EVOLUTION OF THE...SUPPLEMENTARY DATAREFERENCES |
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Representative chemical compositions, including all samples that were analyzed for Sr, Nd and Pb isotopes, are given in Table 1. The complete set of chemical analyses used in this study (n = 196) can be found in Electronic Appendix 1 (available for downloading from http://www.petrology.oxfordjournals.org) and the data are plotted in Figs 3–5
. Analytical procedures for major and trace elements by X-ray fluorescence (XRF), trace elements by quadrupole inductively coupled plasma mass spectrometry (ICP-MS), and Sr, Nd and Pb isotope ratios by thermal ionization mass spectrometry (TIMS) and multicollector (MC)-ICP-MS, plus analyses of Type I and II mafic lavas and regional Precambrian crustal rocks referred to in subsequent sections of the paper have been reported by Wolff et al. (2005). It should be noted that, as a result of difficulties experienced during analysis, we do not report 207Pb/204Pb ratios for the JM93- series of samples. 87Sr/86Sr values are age-corrected using ICP-MS Rb and Sr data. Most samples have low Rb/Sr, thus most corrections are trivial (< 0·00005), but there are errors associated with Rb and Sr precision and with uncertain sample ages in many cases; a potential age range of 8–12 Ma has been assumed for each Paliza Canyon sample. Similarly, where ages for Tschicoma and El Rechuelos Formation lavas were unknown a potential age range of 6·9–2·7 Ma is assumed (7·9–7· 4 Ma for La Grulla Plateau lavas). Calculated (87Sr/86Sr)i values, with errors, are reported in Table 1. Initial 87Sr/86Sr values for the two Canovas Canyon high-silica rhyolites were rejected because they have experienced post-eruptive alteration, and were thus open to Rb and Sr exchange since eruption. Bearhead Rhyolite samples are fresh, and consequently (87Sr/86Sr)i values are reported in Table 1, although with large errors; the ages of these samples are accurately known (Justet & Spell, 2001), but significant imprecision arises from uncertainties associated with Rb and, especially, Sr contents in the trace element analyses of these high-Rb/Sr samples.
Major elements
Variations among major elements (Figs 3 and 4) show some systematic differences between the various JMVF formations. Paliza Canyon Formation dacites have a significantly greater range in total alkali content than the Tschicoma Formation dacites from the NE and east JMVF (6·3–8·9% and 6·3–7·1% respectively at 65% SiO2, Fig. 3) and, with some exceptions, are enriched in K2O relative to Tschicoma Formation lavas between 60 and 70 wt% SiO2 (Fig. 4). However, Tschicoma dacites from the La Grulla Plateau show a similar range to that of the Paliza Canyon Formation dacites. For most major oxides, the compositions of Paliza Canyon lavas typically overlap with those of the Tschicoma dacites at constant SiO2. Paliza Canyon Formation lavas are nonetheless skewed to slightly higher TiO2 and lower MgO over most of the compositional range, and to lower CaO among the dacites, without any clear systematic contrasts between the La Grulla and the NE to east groups of Tshicoma lavas (Fig. 4). With the exception of alkalis, these differences are not apparent at the mafic end of the compositional range. Trachyandesites from the Cerros del Rio tend to have slightly elevated Na2O and lower TiO2 contents, but are otherwise chemically similar to mafic enclaves recovered from Tschicoma Formation dacites (Table 1). In general, major element variations among the NE to east Tschicoma lavas tend be more linear than among Paliza Canyon flows. For example, in the MgO vs SiO2 plot (Fig. 4c), the Paliza Canyon lavas have an overall larger range in MgO despite generally lower concentrations relative to Tschicoma Formation lavas such that the maximum MgO at a given SiO2 concentration in Paliza Canyon Formation lavas is equivalent to that in Tschicoma Formation lavas.
Rhyolites of all ages (Canovas Canyon, Bearhead, Tschicoma, El Rechuelos, and San Diego Canyon Formations) range from 70 to 79 wt% SiO2 and formations are essentially indistinguishable based solely on major element contents (Figs 3 and 4).
Trace elements
Strontium
Strontium concentrations vary by over two orders of magnitude, from 8 to 1380 ppm. In all formations, Sr concentrations decrease with increasing SiO2 (Fig. 5a). As with major elements, Paliza Canyon Formation lavas exhibit larger variability in Sr concentration at constant SiO2 (e.g. Sr 960–1286 ppm at 58% SiO2; Sr 114–506 ppm at 69% SiO2) than do NE to east Tschicoma rocks, which have a range in Sr content of less than 150 ppm at constant SiO2 (Fig. 5a). Mafic enclaves recovered from the Tschicoma Formation exhibit a large range in Sr concentration from 640 to 1380 ppm. Cerros del Rio trachyandesites have relatively restricted Sr abundances, varying from 775 to 921 ppm. Low-silica rhyolites have varying concentrations of Sr that are comparable with those for dacites of the Paliza Canyon and Tschicoma Formations. High-silica rhyolites are extremely depleted in Sr, with concentrations 
84 ppm.
Barium
The overall range in Ba concentrations for the Paliza Canyon and NE to east Tschicoma lavas is less than a factor of two (913–1562 ppm). Most flows from these two formations have Ba concentrations between 1100 and 1300 ppm with only very slight enrichment between 60 and 70% SiO2, and high-silica rhyolites are Ba-depleted (Fig. 5b). Ba concentrations in La Grulla Plateau lavas are generally equivalent to those for the rest of the Tschicoma Formation lavas, although a few dacites have extremely high Ba concentrations (3450–4000 ppm; Fig. 5b). Excluding one anomalously low-Ba sample, Ba concentrations in Cerros del Rio trachyandesites increase over a restricted range of SiO2, a pattern distinctly different from trends observed in either the Paliza Canyon or Tschicoma Formation.
Rubidium
Below 65 wt% SiO2, Rb concentrations show little systematic covariation with SiO2; concentrations range from 22 to 85 ppm (Fig. 5c). As with Ba, there is little difference between Paliza Canyon Formation and Tschicoma Formation lavas. Above 65 wt% SiO2, Rb concentrations increase as silica increases, with the high-silica rhyolites containing the greatest concentration of Rb (up to 173 ppm). Cerros del Rio trachyandesites show little variation in Rb, with concentrations varying from 26 to 40 ppm.
Niobium and zirconium
In Paliza Canyon Formation lavas, Nb and Zr concentrations show large variations at constant SiO2 (Fig. 5d and e). At 58 wt% SiO2, Nb and Zr concentrations are clustered at 35 ppm Nb and 300 ppm Zr. As SiO2 increases, both Nb and Zr fan out to higher and lower concentrations (13–64 ppm and 123–539 ppm, respectively).
In distinct contrast, Nb in Tschicoma Formation lavas with <70% SiO2 (including those of the La Grulla Plateau), mafic enclaves, and Cerros del Rio trachyandesites consistently decreases with increasing SiO2. Tschicoma Formation dacites further form distinct high-Nb and low-Nb groups (19 and 11 ppm Nb respectively at 68·5% SiO2; Fig. 5d). These Nb groupings are geographically restricted; the low-Nb group lavas are found on the eastern side of the JMVF, exemplified by Pajarito Mt. and Cerro Rubio dome samples (Fig. 1), and conglomerate clasts and pumice from the Puye ignimbrite of Turbeville et al. (1989). The high-Nb group dominates the NE Tschicoma dacites, exemplified by Tschicoma Mt. and Polvadera Peak lavas. La Grulla Plateau lavas cover the range in Nb observed in both the high- and low-Nb groups.
Zr concentrations in the NE to east Tschicoma lavas, like Nb, exhibit an overall decrease with increasing SiO2 but unlike Nb do not fall into two distinct groups among the dacites; instead, there is little systematic variation in Zr between 65 and 69% SiO2. Zr in the La Grulla lavas is highly variable, and therefore more similar to the Paliza Canyon than to the NE to east Tschicoma Formation.
Nb and Zr are highly variable in the rhyolites, with some tendency for different formations to fall into distinct groups; for example, the Bearhead rhyolites are notably low in Zr and Nb compared with the rest of the JMVF.
Lead
Pb concentrations in JMVF lavas on average increase from 12 ppm to 24 ppm with increasing SiO2 over the entire compositional range (Fig. 5f). Little systematic variation is evident within each formation. Tschicoma Formation lavas from the La Grulla Plateau have the greatest overall variation in Pb, ranging from 11 to 31 ppm at 67% SiO2; however, the average concentration of 17 ppm is similar to that of both the Paliza Canyon and Tschicoma Formation lavas.
Thorium and uranium
U, and to a greater extent, Th appears to behave similarly to Nb and Zr. In general, the variability in U and Th concentrations increases with increasing SiO2 in Paliza Canyon Formation lavas (Fig. 5g and h). U and Th concentrations broadly decrease with increasing SiO2 in Tschicoma and Puye formation samples. Overall Tschicoma and Puye Formation samples have a more restricted range in U and Th concentrations (1–3·4 ppm and 4·5–13·3 ppm respectively, excluding La Grulla Plateau lavas) than Paliza Canyon Formation lavas (1·2–6·6 ppm U and 4·4–23·7 ppm Th).
Bearhead Formation rhyolites are generally depleted in both U and Th relative to other high-silica rhyolites and have concentrations similar to those in Paliza Canyon Formation dacites.
Rare earth elements
Rare earth elements (REE) are highly variable in all JMVF formations (see Ce and Yb vs SiO2; Fig. 5i and j). As with Nb, Zr, U and Th, the Paliza Canyon Formation has the greatest overall variability in REE concentrations. REE in Tschicoma Formation lavas and mafic enclaves and Puye clasts define a broad negative correlation with increasing SiO2, although the high-Ba La Grulla Plateau lavas (Fig. 5b) are also light REE (LREE) enriched with Ce concentrations up to 150 ppm (Fig. 5i). Cerros del Rio trachyandesites fall within the broad range of compositions observed from Tschicoma lavas. In high-silica rhyolites, LREE concentrations span the entire range exhibited by the other JMVF formations. However, most high-silica rhyolites have much lower LREE/HREE (heavy REE) ratios than less evolved compositions (Table 1; Electronic Appendix 1).
JMVF lavas with less than 67 wt% SiO2 lack a significant Eu anomaly, with a Eu/Eu* ratio between 1· 0 and 0·8, identical to the range observed in the ‘Type I’ mafic lavas (Fig. 5k; Wolff et al., 2005). Eu is depleted with respect to Sm and Gd in several of the low-silica rhyolites and, like Sr and Ba, shows extreme depletion among the high-silica rhyolites.
Radiogenic isotopes
Lead isotopes
Pb isotope ratios in Paliza Canyon lavas do not correlate with SiO2 content (Fig. 6a and b); 206Pb/204Pb = 17·24 – 18·18 and 208Pb/204Pb = 36·95 – 37·87, within the ranges reported by Wolff et al. (2005) for the JMVF mafic lavas. In contrast, 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb in Tschicoma lavas, excluding the La Grulla Plateau, are negatively correlated with SiO2 at <70% SiO2. La Grulla Plateau lavas have more radiogenic Pb signatures than other Tschicoma rocks. Mafic enclaves from Tschicoma lavas have Pb isotope ratios (206Pb/204Pb = 17·60 – 17·88, 208Pb/204Pb = 37·45 – 37·69) that encompass the small range in the least evolved (63% SiO2) Tschicoma lavas and Cerros del Rio trachyandesites (Fig. 6a and b). Rhyolitic lavas and tuffs overall have a large range in Pb isotope ratios, with 206Pb/204Pb varying from 17·50 to 18·07 and 208Pb/204Pb from 36·95 to 37·73, similar to the overall variability observed in the Paliza Canyon lavas (Fig. 6a and b). The San Diego Canyon ignimbrites and caldera-forming Bandelier Tuff (206Pb/204Pb = 17·79 – 18·02, 208Pb/204Pb = 37·52 – 37·87; Wolff & Ramos, 2003, and unpublished data) overlap the radiogenic ends of the ranges seen in the Paliza Canyon lavas and earlier rhyolites, but are more radiogenic than most Tschicoma dacites.
Strontium isotopes
Paliza Canyon lavas have a relatively restricted range in initial 87Sr/86Sr from 0·70392 to 0·70490 (Fig. 6c). The bulk of the Tschicoma lavas and mafic enclaves also have a narrow compositional range (0·70413–0·70490). However, a Tschicoma Formation rhyolitic lava is slightly enriched in 87Sr/86Sri at 0·7051, and lavas from the La Grulla Plateau [87Sr/86Sri = 0·70496 – 0·70626, similar to the range reported by Singer & Kudo (1986) of 0·7051–0·7069] are significantly more radiogenic than the rest of the Tschicoma and Paliza Canyon flows (Fig. 6c). An El Rechuelos rhyolite has 87Sr/86Sri = 0·70520, similar to that of the Tschicoma Formation rhyolite and consistent with previously reported values (87Sr/86Sri = 0·7050 – 0·70566; Loeffler et al., 1988). Bearhead high-silica rhyolites have the highest 87Sr/86Sr of the pre-caldera JMVF volcanics, varying from 0·7068 to 0·7080. The exceptionally Sr-depleted San Diego Canyon and Bandelier Tuff high-silica rhyolites range to still higher values of (87Sr/86Sr)i, attributed by Wolff et al. (1999) and Wolff & Ramos (2003) to trivial amounts of wall-rock contamination following rhyolite genesis. The least radiogenic Bandelier (87Sr/86Sr)i ratios of 0·7041–0·7056 (Skuba and Wolff, 1990; Wolff et al., 1999; Fig. 6c) correspond to the bulk of the Paliza Canyon and Tschicoma lavas.
Neodymium isotopes
As with Pb and Sr isotopes, 143Nd/144Nd ratios in Paliza Canyon Formation lavas show poor correlation with silica content, with a range from 0·51240 to 0·51272 (Fig. 6d). The 143Nd/144Nd of Tschicoma Formation lavas (including La Grulla Plateau) and mafic enclaves decreases systematically with increasing SiO2 from 0·51267 (57·4 wt% SiO2) to 0·51231 (68·6 wt% SiO2). Bearhead rhyolites are isotopically distinct from Canovas Canyon rhyolites, with a narrow 143Nd/144Nd range of 0·51238–0·51236. El Rechuelos, Canovas Canyon, and Tschicoma Formation rhyolites have a wider range in 143Nd/144Nd from 0·51245 to 0·51257. San Diego Canyon and Bandelier Tuff high-silica rhyolites (143Nd/144Nd = 0·51249 – 0·51266; Skuba, 1990) show a strong similarity to the bulk of the Paliza Canyon rocks (Fig. 6d).
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TOPABSTRACTINTRODUCTIONREGIONAL SETTING AND MAFIC...CHRONOLOGY AND CHARACTERISTICS...GEOCHEMISTRY OF INTERMEDIATE AND...PETROGENESISTEMPORAL EVOLUTION OF THE...SUPPLEMENTARY DATAREFERENCES |
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Identification of components
Intermediate and silicic lavas and tuffs make up >90% of the volume of the JMVF, and the diversity of their major element, trace element and isotopic compositions attests to continual magma–crust interaction throughout the lifetime of the volcanic field. Several petrogenic models, including crustal melting, simple mixing, fractional crystallization, and energy constrained assimilation–fractional crystallization (EC-AFC; Bohrson & Spera, 2001
, 2003; Spera & Bohrson, 2001) may be invoked to explain this diversity. To test different models against the geochemical dataset, some assumptions regarding the choice of starting compositions must be made. Below, we outline a rationale for selection of basaltic and crustal end-member compositions used in petrogenetic modeling. In addition to the basaltic and crustal end-members, some rhyolites and trachyandesites have been used in mixing calculations: a Tschicoma rhyolite (MR00-1), Bearhead rhyolite (JM9373-2), Cerros del Rio benmoreites (E6-19a and E6-8a) and Paliza Canyon Formation trachyandesites (JM9384 and JM93190). End-member compositions used in various models are reported in Table 2 and model parameters for EC-AFC and fractional crystallization simulations are provided in Table 3.
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Type I basalts
The abundances of, and ratios between, many major and trace elements in intermediate magmas form continua with those of the Type I mafic lavas but are dissimilar to Type II tholeiites (Figs 3, 4f, g, 5 and 7). High concentrations of P, Sr, Ce, Nb, Zr, U, and Th in Type I basaltic lavas coincide with those in the evolved trachyandesites (i.e.
57–60 wt% SiO2) and are distinctly enriched relative to Type II basaltic lavas (Figs 4f and 5). Type I mafic lavas are therefore considered to represent the mafic parent or end-member involved in petrogenesis of JMVF intermediate and silicic magmas. A representative Type I Cerros del Rio hawaiite (H I-5; Table 2) is used for modeling purposes.
Precambrian crust
Wolff et al. (2005) evaluated regional basement lithologies as representatives of the crustal component(s) in JMVF mafic lavas and found that, perhaps not surprisingly, Precambrian crystalline rocks from beneath the volcanic field itself provide the best approximation of silicic compositions required for generation of the isotopic and trace element characteristics of both types of mafic lavas. The Precambrian rocks are found as rare amphibolite and granitoid lithic fragments in the Otowi Member of the Bandelier Tuff (Eichelberger & Koch, 1979). Wolff et al. (2005) showed that, with the exception of Sr isotopes, isotopic and trace element variations among JMVF mafic lavas could be accounted for by interaction of primitive nephelinite–basanite and tholeiitic magmas with these granitoids. For Nd and Pb isotopes and incompatible trace elements, either simple mixing of bulk granitoid or partial melting accompanied by mixing with primitive magmas (modeled as EC-AFC; Bohrson and Spera, 2001, 2003) reproduces the observed compositions of the mafic lavas. However, Wolff et al. (2005) identified a Sr–(Pb, Nd) paradox, characterized by low and uniform Sr isotope ratios relative to large variability in Pb and Nd isotopes. Wolff et al. (2005) showed that the Sr isotope characteristics could be explained by crustal melt production in the presence of residual feldspar (DSr = 2), thus suppressing 87Sr/86Sr variations in the resulting contaminated basalts, but also noted that this model is difficult to reconcile with the lack of a measurable Eu anomaly in the mafic lavas. A second possibility is that the actual assimilant is characterized by low 87Sr/86Sr, which has been previously invoked for contaminated basalts in the Rio Grande rift region (Dungan et al., 1986; McMillan & Dungan, 1988; Duncker et al., 1991), but is otherwise isotopically similar to its surface equivalents. Low-87Sr/86Sr crust is in fact found regionally (Wolff et al., 2005), but has inappropriate Pb isotope ratios for components in many JMVF lavas.
JMVF lavas with SiO2 between 57 and 67% also lack Eu anomalies (Fig. 5k), and we now consider Eu–Sr coupling in more detail. Eu partitioning between feldspar and silicate melt is strongly dependent upon oxygen fugacity (Drake, 1975; Wilke & Behrens, 1999), because Eu2+ is expected to behave similarly to Sr2+, whereas Eu3+ should behave similarly to Sm and Gd. In oxidized magmas where Eu is mostly present as Eu3+, the relative abundance of Eu is not sensitive to the presence of plagioclase. Nine iron–titanium oxide pairs from two representative Paliza Canyon trachyandesites, which fulfill textural and compositional (Bacon & Hirschmann, 1988) equilibrium criteria, yield T–fO2 results that cluster close to the nickel–nickel oxide (NNO) buffer. Regression of the plagioclase–liquid Eu partitioning experimental results of Drake (1975) for basaltic compositions and Wilke & Behrens (1999) for tonalitic compositions predicts D(Eu, plag/liq) values of 0·4 and 1· 0 respectively at NNO. D(Sm, plag/liq) and D(Gd, plag/liq) values from the same experiments are 0·1. Hence, a large role for plagioclase during magma genesis under oxidation conditions approximating the NNO buffer should be accompanied by development of a Eu anomaly; the Paliza Canyon magmas are not so oxidized that Sr and Eu are strongly decoupled. Therefore, we conclude that the 87Sr/86Sr values of the lithic fragments do not represent those of the assimilant(s) involved in petrogenesis.
The lithic fragments themselves yield some evidence that their 87Sr/86Sr values have been modified since crystallization. The Nd model age for this portion of the North American continent is 1· 7–1· 8 Ga (Bennett & DePaolo, 1987), which presumably represents the time of extraction from the mantle and hence a maximum possible crystallization age. Consistent with this model age, basement rocks in the vicinity of the JMVF have crystallization ages of 1· 62–1· 44 Ga (Brookins & Laughlin, 1983). Attempts to calculate initial 87Sr/86Sr ratios for the granitoid lithic fragments using measured Rb/Sr and a minimum age of 1· 4 Ga yield unreasonably low values (< 0·69), hence these rocks have been open to Rb and/or Sr since crystallization at >1· 4 Ga. A variety of metasomatic and metamorphic processes could be responsible, but it is noteworthy that simple bulk mixing (see below) of the lithic compositions with Type I mafic magma satisfies the compositions of the 67–68% SiO2 dacites (representing 70–80% of the crustal end-member; Fig. 7) for all trace elements except Rb (Fig. 5). We conclude that the Bandelier granitoid lithic fragments are, in fact, good representatives of the Precambrian crust that is involved in Tschicoma Formation magmatism and mafic magma genesis for most geochemical tracers except Rb and 87Sr/86Sr.
Southern JMVF: Paliza Canyon Formation lavas with 57–69% SiO2
No single petrogenetic model can explain the compositional diversity, for example the increasing diversity of Nb, Zr and REE concentrations with increasing SiO2, among lavas of the Paliza Canyon Formation (Figs 5 and 6). Given the longevity of the Paliza Canyon magmatic system(s), this is perhaps not surprising. In the following discussion, we consider the roles of fractional crystallization, incorporation of continental crust (including mixing with crustal melts), and mixing between mafic and silicic magmas.
Fractional crystallization
The increase of Zr and Nb concentrations with increasing SiO2 among some Paliza Canyon Formation lavas is consistent with fractional crystallization (Fig. 5; Table 3). A 50–55% crystallization of a plagioclase–clinopyroxene–orthopyroxene–oxide assemblage (0·8:0·11:0·06:0·03) from a trachyandesite bulk composition (JM93190) reproduces the highest Nb and Zr concentrations measured in some Paliza Canyon trachydacites (Fig. 5d and e), which have isotopic compositions that lie within the range defined by the Type I mafic lavas. In the same group of lavas, Ba/Nb and K/Nb overlap with Type I mafic lavas and vary little with increasing SiO2 and Nb concentrations (Figs 7 and 9). Ba/Nb and K/Nb are expected to remain approximately constant during fractional crystallization that does not involve potassium feldspar or biotite, and hence are good discriminators between crustal assimilation and fractional crystallization in JMVF trachyandesites and trachydacites.
Although zircon fractionation will strongly influence Zr concentrations, most andesitic and dacitic liquids are expected to be zircon undersaturated (Watson & Harrison, 1983; Miller et al., 2003). Zircon saturation temperatures calculated after Miller et al. (2003), for a range of Paliza Canyon and Tschicoma Formation lavas with <68% SiO2, vary from 750°C to 850°C, lower than magmatic temperatures calculated from Fe–Ti oxides (850–1050°C). It is possible that zircon saturation could have been achieved in some high-Zr, high-SiO2 (68–71 wt%) Paliza Canyon compositions. However, Nb and Zr show very similar behavior among lavas with <72% SiO2 (Fig. 5), and there is no Nb-phase analogous to zircon that could precipitate from these liquids and cause a decrease in Nb concentrations. The two elements are decoupled in high-silica rhyolites of all ages, with Zr showing relative depletion (Fig. 5), as expected when zircon joins the fractionating assemblage. We conclude that zircon fractionation did not occur during petrogenesis of the lavas with 57–68% SiO2. Therefore, the approximate coincidence of the fractional crystallization curve with the boundary of the data distribution for Zr, Nb and several other trace elements (Fig. 5) strongly suggests that fractional crystallization represents a limiting case for the Paliza Canyon intermediate rocks.
Type I Paliza Canyon mafic lavas have Pb/Ce, which in JMVF mafic lavas is a sensitive indicator of open-system processes, up to 0·15 (Wolff et al., 2005). It is significant, therefore, that Pb/Ce is elevated above this value among some of the dacites and trachydacites that we have identified as dominantly the products of fractional crystallization (Fig. 9b). Among intermediate magmas, preferential removal of Ce during fractionation may occur as a result of separation of phases for which DCe > DPb, such as hornblende and apatite. Pb/Ce may be increased by up to 50% because of fractionation of a hornblende- and apatite-bearing assemblage over the interval 57–70% SiO2, and values of Pb/Ce up to 0·25 are consistent with fractionational crystallization.
Open-system processes
Trace element abundances in the bulk of the Paliza Canyon intermediate lavas deviate from the predictions of closed-system fractional crystallization. In particular, Zr and Nb and other incompatible elements either remain approximately constant or show a sharp decrease in concentration with increasing SiO2 (Fig. 5), whereas K/Nb, Ba/Nb, and Pb/Ce all increase (Figs 7 and 9). Lavas with the lowest Nb and highest K/Nb, Ba/Nb and Pb/Ce also have low 206Pb/204Pb and can be modeled as simple mixtures of mafic magma with low-206Pb/204Pb, low-Nb crust as represented by the Otowi lithic fragments (Figs 5–9). Their petrogenesis is thus similar to that of Tschicoma Formation andesites, trachyandesites and dacites (Fig. 7; see next section).
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A significant subset of lavas exhibit increasing Pb/Ce with decreasing Nb content whereas 206Pb/204Pb remains within the range of the Type I basalts, and Nb and SiO2 show little correlation. The latter observation suggests a petrogenesis involving both magma–crust interaction and fractional crystallization, such as assimilation-fractional crystallization (AFC). EC-AFC simulations (Figs 8 and 10) are successful in modeling some of these compositions, but no single composition in the suite of Otowi xenoliths has the combination of high Pb/Ce, appropriate Pb isotope ratios, and low Ba/Nb to be a suitable contaminant for all cases. Of course, it is possible that an appropriate crustal composition for generating the Paliza Canyon intermediate rocks is not represented in our sample suite, especially given the highly heterogeneous nature of the Proterozoic basement (Magnani et al., 2004
).
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Further possibilities are interaction between mafic magma and isotopically similar crust, perhaps hybridized crust produced earlier in the history of the volcanic field, or mixing between mafic and rhyolitic magma. Within the limitations of our dataset, the high-206Pb/204Pb, high-Pb/Ce subset of Paliza Canyon andesites and dacites may be most closely modeled through fractional crystallization or as mixtures between mafic lavas and broadly contemporaneous rhyolites, particularly Bearhead Formation and early Tschicoma Formation rhyolites (Figs 6 and 8–10
), as well as Otowi granitoid lithic fragments. Textural evidence for disequilibrium among phenocrysts and groundmass, as described above, also supports magma mixing as a significant process in Paliza Canyon Formation lavas. It should be noted that Pb/Ce may be very high in rhyolitic magma as a result of fractionation of LREE-rich phases such as chevkinite and allanite. In contrast, the combination of low 206Pb/204Pb with low Pb/Ce (Fig. 10) in a few Paliza Canyon (and Tschicoma Formation) lavas may simply be the result of changing the Pb/Ce ratio during melting of granitic basement. Melting of all the Ce-bearing phases with Pb still retained in residual feldspars or simply having an assimilant with a higher proportion of allanite would be two possible scenarios.
The petrological and geochemical complexity of the Paliza Canyon lavas calls for more detailed study to answer the questions raised above, but it is clear that there is a major role for interaction of mafic magma (itself carrying a significant crustal component; Wolff et al., 2005) with Proterozoic crust. At least three distinct crustal reservoirs must be involved to account for the respective isotopic uniqueness of the Bearhead, Canovas Canyon, and Tschicoma Formation–El Rechuelos rhyolites (Fig. 8; see also ‘Rhyolites’ section below) and the granitoid component, invoked by Wolff et al. (2005) as a contaminant of the basalts. We may recall that Bearhead and Canovas Canyon Formation rhyolites are erupted either contemporaneously with, or immediately following, the eruption of the Paliza Canyon Formation lavas, and that early El Rechuelos and Tschicoma Formation rhyolites temporally overlapped with the Paliza Canyon Formation (Loeffler et al., 1988). This requires consumption of at least four different types of crust beneath the volcanic field, of the order of hundreds of cubic kilometers, during magma genesis. Given the longevity of the Paliza Canyon Formation (at least 6 Myr; Fig. 2) the diversity is perhaps not surprising, although most of the geochemical variation we observe was developed in a much shorter period. The detailed geochronology of the Paliza Canyon Formation is imperfectly known, although many dates cluster at 8·5 ± 1 Ma (Goff et al., 1990; Justet, 2003). Most of our samples for which an eruptive age can be established either directly or by stratigraphic bracketing fall in this range, hence it appears that several magma chambers, or a large complex system allowing different crustal lithologies to be melted, were established in the crust beneath the volcanic field during this 2 Myr period.
North to east JMVF: Tschicoma and Puye Formations, and Cerros del Rio trachyandesites
Mafic enclaves in Tschicoma Formation dacites have the same geochemical traits as Cerros del Rio lavas in the range 53–60% SiO2, with the characteristic low K/Nb and high Th/Nb and La/Nb of Type I basalts (Wolff et al., 2005). The most silicic Cerros del Rio lavas likewise resemble Tschicoma lavas of the same silica content (63% SiO2). The contemporaneity of Cerros del Rio and later Tschicoma Formation activity (Fig. 2), the presence of mafic enclaves with trace element concentrations and ratios identical to Cerros del Rio benmoreites, and the compositional overlap between the Tschicoma Formation and Cerros del Rio lavas (Figs 3–6) strongly indicates that the Tschicoma Formation and Cerros del Rio lavas are genetically related.
Tschicoma Formation andesites and dacites and Cerros del Rio benmoreites exhibit variations in incompatible trace elements (especially high field strength elements; HFSE) and radiogenic isotopes that are relatively well correlated with SiO2 (Figs 4–6). Systematic variation of isotopic ratios (especially Nd and Pb; Fig. 6) with increasing SiO2 and near-ubiquitous disequilibrium textures (see descriptions above) indicate a significant role for mixing and crustal assimilation in the generation of Tschicoma Formation lavas. In addition, the lack of divergence in trace element or isotopic compositions with increasing SiO2, in contrast to Paliza Canyon Formation lavas, indicates a lack of diversity in petrogenetic mechanisms and among the crustal end-member(s) involved. 207Pb/204Pb vs 206Pb/204Pb of Tschicoma Formation lavas, as well as Otowi basement xenoliths, define a linear covariation that intercepts the Stacey–Kramers terrestrial Pb growth curve at 1· 8 Ga (Fig. 11), which is the Nd model age for this part of the North American craton (Bennett & DePaolo, 1987). This observation is consistent with the model that less evolved Tschicoma lavas are assimilating Proterozoic granitic crust, compositionally similar to the Otowi granitoid xenoliths.
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Among the dacites, there appear to be two slightly different end-member compositions at 68% SiO2. The first has Nb
11 ppm, 143Nd/144Nd 0·51235, and 206Pb/204Pb 17·1 (‘eastern dacite’, exemplified by Pajarito Mt. and Cerro Rubio; Figs 1 and 5). This composition is also seen in the Puye ignimbrite of Turbeville et al. (1989) and in conglomerate cobbles from the Puye fan above the level of the ignimbrite, and hence may be volumetrically more significant than is suggested by the present size of the dome remnants in the east (Fig. 1). The second dacite composition has Nb 20 ppm, 143Nd/144Nd 0·51250, and 206Pb/204Pb 17·5 (‘northern dacite’, exemplified by Tschicoma Mt., the Mesa Gallina flow, and Polvadera Peak; Figs 1 and 5). Simple mixing relations (Figs 5 and 6) suggest that the northern dacite composition is the more important end-member for most of the intermediate to silicic Tschicoma and Cerros del Rio magmas. The two distinct dacite types cannot themselves be related to each other by simple mixing with mafic magma and may instead represent melts derived from slightly different crustal assimilants (Figs 5d and 6). The eastern dacite type may require a crustal end-member slightly less radiogenic in Pb (206Pb/204Pb 17·0) than is present among the Otowi basement xenoliths and may indicate a crustal component not present in the current suite of basement samples.
Simple bulk-mixing of a Cerros del Rio benmoreite E6-8B (30–60%) and Otowi lithic fragments CCL-1 and 18L-3 (Table 2) can reproduce much of the trace element and isotopic variability of Tschicoma dacites (Figs 5–9). Eastern dacites, with low Nb and high Ba/Nb, culminating with Cerro Rubio lavas, require up to 60–70% crust mixed with Cerros del Rio benmoreite (Fig. 7). These high crustal proportions are supported by the Pb isotope data; Cerro Rubio Pb isotopic ratios are nearly identical to those of the inferred basement (Fig. 6). The evolved Cerros del Rio benmoreites have already incorporated up to 25% crust to generate the characteristic trace element patterns (Wolff et al., 2005) such that higher-silica dacites are dominantly crustal melts. That the dacites are dominated by the products of near-complete crustal melting represents renewed consumption of crust by magma at <4 Ma following the hiatus since the Paliza Canyon Formation volcanic maximum at about 8·5 Ma. Taking into account the 25% crustal contribution to the Cerros del Rio benmoreites and assuming a conservative average of 50% crustal component and 50% benmoreite for the bulk of the Tshicoma Formation dacites, over 300 km3 of granitoid crust must be involved in the generation of the Tschicoma Formation lavas.
NW JMVF: Tschicoma Formation, La Grulla Plateau
The La Grulla Plateau lavas temporally overlap late Paliza Canyon Formation activity (Singer & Kudo, 1986) and share geochemical characteristics with both the north and east Tschicoma Formation and Paliza Canyon lavas. Singer & Kudo (1986) noted a positive correlation between 87Sr/86Sr and SiO2 among La Grulla lavas (Fig. 6c), and also that La Grulla plateau lavas have higher 87Sr/86Sr at a given SiO2 content relative to Tschicoma Formation lavas to the east of Mesa del Media and the Cañada de Cochiti fault zone (Fig. 1). Our limited La Grulla isotopic data are consistent with this pattern and 143Nd/144Nd is negatively correlated with 87Sr/86Sr and silica as expected. La Grulla lavas resemble the Tschicoma Formation in 143Nd/144Nd and Nb content, but Pb isotope ratios resemble the high-206Pb/204Pb subset of Paliza Canyon Formation lavas (Fig. 6), and one La Grulla plateau rhyodacite has some Nd isotopic affinities with Bearhead rhyolite (Figs 6 and 8).
Singer & Kudo (1986) modeled 87Sr/86Sr variations in the La Grulla lavas by AFC using a Lobato basalt as the parent and an upper crustal assimilant with highly radiogenic 87Sr/86Sr (0·74). Nothing in our data contradicts this model, although mixing between intermediate magma and silicic liquid resembling Bearhead Rhyolite (albeit with higher 206Pb/204Pb) is equally plausible. Clearly, more work is required on the La Grulla lavas. Nonetheless, the salient feature emerges that the consumed crust is probably distinct from both that in the main Tschicoma Formation east of the Cañada de Cochiti fault zone and that in Paliza Canyon Formation lavas.
A striking feature of some La Grulla Plateau lavas is their enrichment in Ba, which reaches concentrations up to 4000 ppm in lavas from Cerro Pavo (Fig. 1) yielding extremely high Ba/Nb ratios (> 160; Fig. 7). The same lavas are LREE-enriched with La up to 140 ppm (Table 1), roughly twice that of comparable Tschicoma Formation dacites, and higher La/Sm (13·6–14·5) than the rest of the La Grulla Plateau suite (7·7–9·9). Distinctive LREE and Ba enrichments in lavas from a single small center may point to the involvement of heterogeneous crust in the genesis of the La Grulla suite; Singer & Kudo (1986) also noted the ‘outlier’ character of this chemical type.
Rhyolites
JMVF rhyolites record extensive separation of feldspar in their very low Sr, Ba and Eu/Eu* (Fig. 4). The depletion in Ba requires that at least some of the fractionating assemblage is alkali feldspar. Despite such clear indications, high-silica rhyolites (>75% SiO2) present a special problem for geochemically based petrogenetic interpretation. The high probability of multiple saturation with small amounts of accessory phases (e.g. allanite, chevkinite, monazite, zircon) complicates understanding the inheritance of large ion lithophile elements (LILE), HFSE and REE from a parental magma. For example, Zr is highly variable at near-constant silica content in JMVF rhyolites, probably as a result, in part, of fractionation of variable amounts of zircon consequent upon a range of zircon solubilities and the actual physical separation of the phase.
A further problem is that the very low Sr concentrations put magmatic 87Sr/86Sr values at the mercy of contamination by very small quantities of wallrock that are otherwise difficult to detect (Wolff et al., 1999). In the case of older rhyolites, very high Rb/Sr ratios, themselves subject to change during weathering, lead to large errors in age corrections. Nd and Pb isotopes are more reliable guides to the sources of rhyolitic liquids because, relative to Sr, high concentrations of Nd and Pb render these systems insensitive to minor amounts of late-stage contamination (Wolff & Ramos, 2003). Uncertainty about effective partition coefficients, especially for Nd, plus the numerous possible process-dependent responses of magma to contamination (e.g. Bohrson & Spera, 2003), make it difficult to identify precise petrogenetic pathways. Some indications can nonetheless be gained from the positions of rocks and potential source materials on isotope–isotope plots.
Nd and Pb isotope ratios of JMVF rhyolites, including the Bandelier Tuff, show no systematic variation with the ages of their formations. However, individual formations do exhibit restricted isotopic ranges (Fig. 6). Canovas Canyon high-silica rhyolites and one Paliza Canyon rhyodacite have unusually low 208Pb/204Pb (Fig. 8a) and cannot be simple fractionates of the Paliza Canyon andesites, trachyandesites and dacites with which they are interbedded. Several rocks, distinct from the Otowi lithic fragments, which may represent sub-JMVF basement (Wolff et al., 2005) also have low 208Pb/204Pb, especially Taos Range amphibolites (Fig. 8). If these represent one component in the Canovas Canyon rhyolites, Pb–Pb relations indicate that a second component lies among the low-206Pb/204Pb group of Paliza Canyon Formation mafic to intermediate magmas (Fig. 8a). Otowi lithic fragments, with low 206Pb/204Pb ratios, are ruled out by 208Pb/204Pb–143Nd/144Nd relations (Fig. 8c). Alternatively, the Canovas Canyon rhyolites may be derived by partial melting of a composition not represented among the basement suite. Bearhead Formation rhyolites and the La Grulla plateau rhyodacite form another group that may contain an additional crustal component, with relatively low 143Nd/144Nd at high 206Pb/204Pb and 208Pb/204Pb. This group is distinct from the broad trend defined by non-rhyolitic Paliza Canyon and Tschicoma Formation rocks, and the Otowi amphibolite xenolith CCL-2 is a candidate for the crustal component (Fig. 8b and c). For both Canovas Canyon and Bearhead rhyolites, the exact petrogenic processes by which high-silica rhyolite is derived from a mixture of basement partial melt and mafic or intermediate magma are not well constrained; however, it may be significant that each is accompanied by a rhyodacite with broadly similar isotopic composition that may represent a parent for the respective high-silica rhyolites.
The Pb and Nd isotope compositions of the San Diego Canyon and Bandelier ignimbrites coincide with the middle of the ‘JMVF isotopic array’, although they more closely resemble the Paliza Canyon Formation lavas rather than the immediately preceding Tschicoma Formation (Figs 6 and 8). The caldera-forming Bandelier magmas therefore overlap with JMVF mafic lavas in Pb–Nd isotope space. Nonetheless, the suggestion that this is due to fractionation from basaltic magma (Perry et al., 1993) seems hard to sustain in light of the fact that no isotopically similar intermediate magmas, with the exception of Cerros del Rio benmoreites located 30 km away in the rift axis, were erupted from the JMVF during the previous 5 Myr.
Thus, there appears to be no systematic chemical pattern among JMVF rhyolites as a whole. This, and the possible status of the Canovas Canyon and Bearhead Formation rhyolites as the crustal component in some of the intermediate magmas, is more consistent with a model of heterogeneous rhyolite production under the influence of mafic intrusion than it is with an origin by fractional crystallization. In the case of the Bandelier Tuff and associated rhyolites, Nd and Pb isotope data indicate that the likely protolith consists wholly of buried intrusions associated with Paliza Canyon volcanism. Because such intrusions crystallize from crustally contaminated, mantle-derived magma, they constitute hybridized crust (sensu Riciputi & Johnson, 1990; Riciputi et al., 1995). Based on mixing models and erupted volumes of the Paliza Canyon (1000 km3) and Tschicoma Formations (500 km3), making the conservative assumption of no cognate cumulates and that the volume deficit created by eruptions was compensated by magma, the minimum volume of hybridized crust is of the order of 1500 km3, and is likely to be much greater. Furthermore, such young intrusions were probaby still warm, reducing the thermal penalty associated with their rejuvenation. The much greater volume of Bandelier Tuff than either Canovas Canyon or Bearhead rhyolite, both of which involved melting of previously untapped volumes of Precambrian crust, may thus be explained.
Summary of petrogenesis
Geochemical variations among pre-caldera JMVF rocks permit the following conclusions.
- Mafic lavas produced throughout the history of the volcanic field carry a crustal component that has the overall composition of Precambrian granitoid rocks excavated by the Bandelier Tuff eruptions (Wolff et al., 2005).
- Granitoids at depth may have lower Rb contents and 87Sr/86Sr than extant lithic fragments which, at least for the JMVF, solves the Sr–(Nd, Pb) paradox.
- The same granitoids experienced near-complete melting to produce the large Tschicoma dacite domes of the north and east JMVF, which carry only a minor component of mantle-derived basalt. Magma of the same composition mixed with Type I basalt to produce the intermediate lavas and tuffs of the main Tschicoma Formation and Cerros del Rio, and some intermediate lavas of the Paliza Canyon Formation.
- Many Paliza Canyon, Canovas Canyon, and Bearhead Formation samples and La Grulla Plateau volcanic rocks in the range 63–73% SiO2 carry crustal component(s) that are isotopically distinct from the Bandelier lithic fragments; at least three additional crustal components are required. Petrogenetic processes were similarly diverse and involved fractional crystallization and AFC in addition to simple mixing of mafic magmas with crustal melts. These lavas therefore record earlier melting events in the crust that peaked at 8·5 Ma. Mantle and crustal components were apparently blended in a complex magmatic system with more than one storage chamber.
- JMVF rhyolites have isotopic compositions that are distinct from contemporaneous or immediately preceding intermediate volcanic rocks, and there is no direct line-of-descent (by FC or AFC) of rhyolites from more primitive compositions. Instead, rhyolites appear to be generated by melting of both pre-existing and hybridized crust. The Bandelier Tuff and associated caldera-related rhyolites originated through rejuvenation of intrusions associated with Paliza Canyon Formation magmatism, whereas older JMVF rhyolites are isotopically distinct and in part represent melting of ‘pristine’ Proterozoic crust. The reduced thermal penalty associated with remelting young, warm intrusions may in part explain the larger volume of the caldera-related rhyolites.
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Pre-caldera volcanic formations of the JMVF have distinctive isotopic compositions and compositional variations with each new major volcanic phase, consistent with consumption of new volumes of crust. This belies the apparent overall simple evolution of the three successive major formations: the andesite–trachyandesite-dominated Paliza Canyon Formation (1000 km3, 13–7 Ma), through the dacite-dominated Tschicoma Formation (500 km3, 7–2·2 Ma), to the high-silica rhyolite caldera-related San Diego Canyon ignimbrites and Bandelier Tuff (> 600 km3, < 2 Ma). The caldera rhyolites do not mark a new isotopic excursion, but are more similar to the Paliza Canyon Formation (>5 Myr older) than to the Tschicoma Formation (mostly <3 Myr older), and are probably derived wholly through partial remelting of intrusive rocks, consisting of crust–mantle hybrids, related to the Paliza Canyon Formation. The temporal geochemical record does not support any simple liquid-line-of-descent model, with or without minor accompanying contamination, for the ultimate generation of large volumes of rhyolite.
Throughout the history of the JMVF, an important petrogenetic mechanism for the production of magmas in the range 57–70% SiO2 appears to have been mixing between mafic liquids and crustal melts, particularly in the case of the Tschicoma Formation. Heating of crust above its solidus by mafic magma implies partial crystallization of the latter, and there is additionally a significant role for fractional crystallization in Paliza Canyon petrogenesis. It is very likely, therefore, that significant volumes of cumulates cognate with the Paliza Canyon Formation, and perhaps also the Tschicoma, exist at depth. A lack of evidence for caldera formation or subsidence on the volume scale of the two formations suggests volume compensation of erupted magma by new liquid. Complex plutons resulting from the assembly of cumulates, non-erupted, and recharged magma now make up much of the crust beneath the JMVF, which consists isotopically of a crust–mantle mixture. Melting of significant volumes of pristine Precambrian crust effectively ceased between 2 and 3 Ma, after the generation of the late Tschicoma dacites.
The tempo of JMVF magmatism is controlled by Rio Grande rift tectonic activity (Gardner et al., 1986), which is fully consistent with a lithospheric origin for the mafic magmas (Wolff et al., 2005). Significant rift-related extension at 10–7 Ma (Gardner et al., 1986; Self et al., 1986) coincides with the Paliza Canyon volcanic maximum at 8·5 ± 1 Ma. It has been previously stated that Tschicoma dacitic volcanism was associated with a tectonic lull from 7 to 4 Ma (Gardner & Goff, 1984; Gardner et al., 1986; Self et al., 1986), but the major dacite domes such as Tschicoma Mountain and Polvadera Peak, representing significant crustal melting, erupted after the onset of renewed extension at about 4 Ma. The dacite domes contain mafic enclaves, and this period of activity continued after 2·7 Ma in the adjacent rift-floor Cerros del Rio field until 2·3 Ma (Woldegabriel et al., 1996), with mafic and intermediate lavas that chemically and isotopically overlap the Tschicoma dacites and their enclaves. Hence it is clear that the major Tschicoma dacites are ‘fundamentally basaltic’ and represent advective heat transfer from mantle to crust. Cerros del Rio vents migrated westward between 2·7 and 2·3 Ma (Woldegabriel et al., 1996; Dethier, 1997), and we therefore speculate that repeated intrusion of basalt beneath the central JMVF after 2·3 Ma ultimately led to the formation of the Bandelier magmatic system, which first vented at 1· 85 Ma. A continuing role for mafic magma in powering the JMVF volcanic system is hinted at by an andesite lava between the two Bandelier tuffs (Smith et al., 1970) and a basaltic andesite component among the products of the most recent rhyolitic eruption from the caldera (Wolff & Gardner, 1995).
Major transitions in JMVF activity therefore occur on timescales of 105–106 years, but petrological patterns and styles of activity, once established, seem to persist for a few million years. These patterns may then relate to sites of melt lodgement and maximum heat transport into the crust that act as traps for later rising magma. Major transitions in style may correspond to times when the system is starved of mantle input, hence the crust becomes sufficiently rigid to permit, when activity is renewed, mantle-derived magma to rise and lodge at a new location in the still-warm crustal column. A similar result may occur where a lateral shift in the focus of intrusion occurs; this may be the case for the La Grulla Plateau suite, which is tectonically, geographically and chemically distinct from the rest of the Tschicoma and Paliza Canyon Formations. If this model is generally valid, it then becomes difficult to predict at what stage in its development an individual continental intermediate volcanic field may enter a major rhyolitic, caldera-forming phase.
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SUPPLEMENTARY DATA |
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TOPABSTRACTINTRODUCTIONREGIONAL SETTING AND MAFIC...CHRONOLOGY AND CHARACTERISTICS...GEOCHEMISTRY OF INTERMEDIATE AND...PETROGENESISTEMPORAL EVOLUTION OF THE...SUPPLEMENTARY DATAREFERENCES |
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Supplementary data for this paper are available at Journal of Petrology online.
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ACKNOWLEDGEMENTS |
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We wish to thank the many who have contributed to this paper in tangible and intangible ways over several years, including Rob Creaser, Steve Self, Fraser Goff, Philip Kyle, Terry Spell, Scott Baldridge, Jon Davidson, Phil Leat, Bob Thompson, Mike Dungan, Stephen Moorbath, Anita Grunder, Jack Flannery, Katherine Romanak (née Duncker), Bruce Turbeville, Damon Waresback, Steve Balsley, Dave Kuentz, Marty Horn, Wade Aubin, Lee Winters, Pam Hartman, Keith Brunstad, and Geoff Cook, for discussion, data, and/or assistance in field and laboratory. Diane Johnson Cornelius, Rick Conrey and Charles Knaack of the WSU Geoanalytical Laboratory generated many of the data presented here. This manuscript has been substantially improved by reviews from Wendy Bohrson, Nancy McMillan, Calvin Barnes and Thomas Vogel. None of the aforementioned individuals carry any responsibility for the conclusions presented herein. Research in the JMVF has been supported by Associated Western Universities, Inc., the US Department of Energy, and the National Science Foundation, most recently under EAR-9909700 to J.A.W.
*Corresponding author. Telephone: 1-512-471-8054. Fax: 1-512-471-9425. E-mail: rowem{at}mail.utexas.edu
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