Journal of Petrology | Volume 40 | Number 8 | Pages 1301-1320 | 1999
© Oxford University Press 1999
Pliocene Potassic Magmas from the Kings River Region, Sierra Nevada, California: Evidence for Melting of a Subduction-Modified Mantle
Department of Geological Sciences, University of Michigan 2534 C. C. Little Building, Ann Arbor, MI 48109–1063, USA
Received June 22, 1998; Revised typescript accepted February 2, 1999
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ABSTRACT |
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 TOP ABSTRACT IntroductionGeologic SettingTrace Element ChemistryMineral CompositionsPetrogenesisDiscussionAppendixReferences |
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During the Late Pliocene, absarokite and minette magmas (43–57 wt % SiO2) erupted along the western slope of the Sierra Nevada, California, within the Kings River drainage. The absarokites contain phenocrysts of olivine ± augite, whereas the minettes contain phlogopite + augite ± olivine; both are distinguished by a lack of feldspar phenocrysts. Pre-eruptive magmatic temperatures and pressures for a felsic and mafic minette are 1138 and 1144 (± 50)°C, and 12 and 16 (± 4) kbar, respectively. These magmas are characterized by extreme enrichments in the large ion lithophile elements (e.g. 1.9–8.1 wt % K2O, 1380–3719 ppm Ba), depletions in high field strength elements (Ba/NbPM of 7–33), and high oxygen fugacities (1–3 log units above the Ni–NiO buffer). Trace element ratios (e.g. Ba/Rb 20–100) are distinct from those observed for mid-ocean ridge basalt and ocean island basalt. Variations in K and Ba with respect to other incompatible elements require that phlogopite ± potassic amphibole was an important residual phase during magma generation. The buoyant ascent of the Kings River magmas through
40 km of sialic crust requires pre-eruptive volatile concentrations (H2O and F) >2 wt %. Volcanism probably was triggered as part of the regional response to Basin and Range extension, which resulted in asthenospheric upwelling and therefore higher heat flow to the subduction-modified lithosphere. KEY WORDS: absarokite; minette; metasomatism; phlogopite; potassic magmas
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Introduction |
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TOPABSTRACTIntroductionGeologic SettingTrace Element ChemistryMineral CompositionsPetrogenesisDiscussionAppendixReferences |
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A genetic relationship between potassic magmas and subduction zones has long been recognized (e.g. Johnson et al., 1978
; Peccerillo, 1985; Rogers & Hawkesworth, 1985). Trace element enrichments in large ion lithophile elements (LILE) and light rare earth elements (LREE) combined with relative depletions in the high field strength elements (HFSE) are common to arc lavas (e.g. Gill, 1981). The eruption of potassic and ultrapotassic magmas with an extreme arc trace element signature often post-dates active subduction and occurs synchronously with uplift, extension or strike-slip motion (e.g. Sloman, 1989). This relationship indicates that chemical heterogeneities, produced in the mantle during subduction via modal (the formation of new minerals) and/or cryptic (enrichment of pre-existing minerals in incompatible elements) metasomatism, can exist for substantial periods of time after subduction has ceased; subsequent tectonic or thermal events may then trigger further magma generation (Johnson et al., 1978; Roden, 1981; Rogers et al., 1987; Mauger, 1988; Sloman, 1989).
On the western slope of the Sierra Nevada, small volume eruptions of potassic and ultrapotassic lavas have occurred over the last 12 my (Dalrymple, 1963; Moore & Dodge, 1980, 1981; Van Kooten, 1980), broadly contemporaneous with Basin and Range extension and post-dating the long-lived subduction event that occurred off the western margin of North America between 220 and 80 Ma (Everden & Kistler, 1970; Chen & Moore, 1982; Saleeby et al., 1987). The geology and petrology of these lavas have been described by Moore & Dodge (1980, 1981), who included petrographic descriptions and whole-rock major element analyses of over 200 samples from both the eastern and western slopes of the Sierra Nevada, including 28 samples from the Kings River region. Potassic magmas from northwest of the Kings River, located in the Merced Quadrangle, were the subject of detailed petrographic, geochemical, and Sr and Pb isotope studies (Van Kooten, 1980, 1981).
The focus of the present work is the Kings River volcanic field in central Sierra Nevada, California, located 40 km south and east of the lavas studied by Van Kooten (1980). The Kings River lavas were erupted around 3 Ma (Moore & Dodge, 1980), post-dating active subduction off the western margin of North America by at least 7 my. The Kings River volcanic field broadly overlies the boundary between cratonic North America and allochthonous island arc terranes. Initially, Kistler & Peterman (1978) located the boundary further to the east, closer to the crest. Recent isotopic data (high 87Sr/86Sr, negative 
Nd; Ducea & Saleeby, 1998) on a Kings River volcanic rock and its mantle peridotite xenoliths suggest, however, that cratonic North American lithosphere underlies part of the Kings River volcanic field. The major and trace element compositions of these magmas are used in this study to infer characteristics of their mantle source region and the melting regime; in particular, the importance of compositional control by phlogopite ± potassic amphibole in the mantle source region is shown. The eruption of potassic lavas in the central Sierra Nevada can be understood in terms of the tectonic framework for the region over the last 10 my in which lithospheric thinning and/or asthenospheric upwelling has occurred in response to regional extension (Jones et al., 1994; Ducea & Saleeby, 1996; Fliedner et al., 1996; Wernicke et al., 1996).
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Geologic Setting |
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TOPABSTRACTIntroductionGeologic SettingTrace Element ChemistryMineral CompositionsPetrogenesisDiscussionAppendixReferences |
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Regional tectonics
During the Mesozoic and Cenozoic, the geologic history of the western United States was dominated by subduction of the Farallon plate beneath the western margin of North America. Plate reconstructions indicate that cessation of subduction was diachronous, beginning
30 Ma when the East Pacific Rise interacted with the continental margin of North America off Baja California and initiated the San Andreas transform fault (Atwater, 1970; Dickinson & Snyder, 1979; Severinghaus & Atwater, 1990). Progressive cessation of subduction occurred as the San Andreas transform subsequently lengthened both northward and southward from the original intersection point. By 10 Ma, subduction ceased in the region of central California (36–37°N, Atwater, 1970; Dickinson & Snyder, 1979; Severinghaus & Atwater, 1990).
Considerable attention has focused on determining the exact cause and timing of the uplift and the nature of the root of the Sierra Nevada block. Central to this question is why the Sierra Nevada is 1800 m higher than the Basin and Range province immediately to the east. Previously, workers concluded that the sialic crust beneath the Sierra Nevada is anomalously thick (55 km, Eaton, 1966; Bateman & Eaton, 1967; Prodehl, 1979; Pakiser & Brune, 1980; Mooney & Weaver, 1989), but more recent seismic surveys (Jones et al., 1994; Wernicke et al., 1996) and petrologic studies of crustal and mantle xenoliths (Mukhopadhyay & Manton, 1994; Ducea & Saleeby, 1996), indicate that the modern Sierra Nevada has a thin crust (35–40 km). Moreover, the maximum sialic crustal thickness of the modern Sierra Nevada (42 km) does not coincide with the highest point of elevation, but is offset 40 km to the west, beneath the central Sierra Nevada (i.e. the region of the Kings River).
Petrologic studies of xenolith suites entrained in 8–11 Ma lavas erupted in the central Sierra Nevada indicate that mafic crustal assemblages underlay the sialic crust and extended to depths of 65–70 km at that time (Mukhopadhyay & Manton, 1994; Ducea & Saleeby, 1996). In contrast, eclogitic xenoliths are absent in similar age lavas from the eastern Sierra Nevada, and only mantle peridotites are sampled from depths greater than 35 km (Mukhopadhyay & Manton, 1994; Ducea & Saleeby, 1996). If the eclogites formed by conversion of a gabbroic lower crust during cooling of the Mesozoic arc, then it appears that the lower, mafic (eclogitic) portion of the eastern Sierra Nevada crust was completely removed by 8 Ma. On the basis of the combined geophysical and xenolith evidence, a number of researchers (Ducea & Saleeby, 1996; Fliedner et al., 1996; Wernicke et al., 1996) support a model in which the present Sierra Nevada is supported by buoyancy forces derived from a low-density mantle, as originally proposed by Crough & Thompson (1977), and not an Airy-type crustal root. The emplacement of low-density mantle beneath the eastern Sierra Nevada probably was caused by lithospheric thinning, part of the regional response to Basin and Range extension in Neogene time (Jones et al., 1994). Lithospheric thinning, which may have led to or was caused by asthenospheric upwelling and/or eclogite removal (Ducea & Saleeby, 1996; Fliedner et al., 1996; Wernicke et al., 1996), may have resulted in higher heat flow to the subduction-modified lithosphere, causing melting and the generation of the potassic magmas.
Field description
Five distinct volcanic fields are recognized in east–central California: (1) San Joaquin–Kings; (2) Kern; (3) Coso; (4) Big Pine; (5) Mono Basin–Long Valley (Fig. 1; Moore & Dodge, 1980). Volcanism in the San Joaquin–Kings field occurred in the Miocene and Pliocene. Quaternary basaltic volcanism is confined to the eastern margin of the Sierra Nevada, represented by the Coso, Big Pine and Mono Basin–Long Valley volcanic fields.
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The Kings River portion of the San Joaquin–Kings volcanic field lies within the watershed of the Kings River (including the north, south and middle forks) along the western slope of the central Sierra Nevada, 65–80 km northeast of Fresno, California (Fig. 1). This area is encompassed by the Sierra National Forest, including the John Muir Wilderness Area, the Sequoia National Forest and the Kings Canyon National Park. Two K–Ar ages of 3.2 and 3.4 Ma were reported on basic lavas from the Kings River region by Moore & Dodge (1980)
; a third age of 4.5 Ma was obtained by these workers on a dacite plug. These ages are within the range of 1.0–11.4 Ma obtained for lavas from the San Joaquin River region located immediately to the north (Dalrymple, 1963; Moore & Dodge, 1980), and 3.4–3.6 Ma for the potassic and ultrapotassic lavas sampled by Van Kooten within the Merced Quadrangle between 37°20' and 37°38'N (Van Kooten, 1980, 1981).
In the Kings River region, 38 samples were collected between 36°45' and 37°00'N, and 118°42' and 119°03'W (Fig. 1), just west of the Sierra Nevada crest and within the National Forests. Some but not all of these are from outcrops sampled by Moore & Dodge (1980). All of the samples collected are visibly free of chlorite and zeolite alteration in thin section. The outcrops are isolated cinder cones and erosional flow remnants, with mostly the lower, non-vesicular portion of the flows now exposed. Field relations between flows are difficult to ascertain because of the high degree of erosion and glaciation. Most lavas erupted through the granitoids that form the Sierran batholith, a common association noted by Rock (1984) that is not surprising given that potassic magmas are often interpreted as partial melts of a subduction-modified mantle. A few lavas erupted onto pre-Cenozoic metamorphic rocks of the Sierran foothills. None of the lavas appear to be associated with major faults, in contrast to the Big Pine, Coso and Long Valley volcanic fields, which erupted along the eastern Sierran escarpment, and the Kern volcanic field, which erupted along the Kern Fault. The map in Fig. 1 shows sample localities and indicates the diversity of rock types. All sample localities are accessible by US Forest Service roads and trails (Appendix A).
Petrography and major element composition of lavas
Twenty-two of the 38 samples were chosen for analyses of major and trace elements, mineral modes and chemistry (Table 1). The dominant lava type is absarokite, which contains phenocrysts of olivine ± included chromite, with lesser amounts of augite, but lacks feldspar phenocrysts. Groundmass phases are predominantly augite and feldspars (plagioclase ± alkali feldspar ± anorthoclase ± hyalophane), along with leucite ± nepheline ± analcime ± olivine ± apatite ± titanomagnetite ± phlogopite and occasional pseudobrookite. Small amounts of glass were found in only one sample but could not be analyzed because of alteration. The absarokite lavas have a restricted range of SiO2 (45–51 wt %), exhibit a wide range in MgO contents (9–15 wt %), and have moderate to high K2O contents (2–6 wt %). The Na2O/K2O ratios for the absarokite lavas range from 0.3 to 1.7. All are normative in nepheline and/or leucite.
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The second most abundant lava type found in the Kings River area is minette (43–51 wt % SiO2), a type of lamprophyre that contains phenocrysts of phlogopite, augite, and/or olivine ± included chromite. Groundmass phases in the minette lavas are the same as those in the absarokite lavas. Two samples of felsic minette (55–56 wt % SiO2) were collected and contain a phenocryst assemblage that is similar to the mafic minettes. The K2O contents for both minette and felsic minette magmas are moderate to high (3.6–8.1 wt %), with higher K2O contents in minette than in absarokite lavas at comparable SiO2 contents. Low Na2O/K2O ratios are present in both the minettes (0.3–0.9) and felsic minettes (0.3–1.1). The minettes and felsic minettes are normative in nepheline and/or leucite. A fourth lava type, a high-K basaltic andesite [according to the classification of Gill & Whelan (1989)
], was found in only one sample locality. Disaggregated mineral fragments from the underlying granitoid rocks are found in most samples, although, on the basis of point counting, they comprise considerably less than 1% of the whole rock. Reaction between xenocrysts of quartz, plagioclase and alkali feldspar with the enclosing liquid is ubiquitous and involves partial melting of the xenocrysts and crystallization of clinopyroxene coronas. The preservation of the granitic xenocrysts suggests that eruption occurred relatively quickly after their incorporation. A single clot of orthopyroxene, clinopyroxene and olivine with mantle compositions was found in one sample. Many of the olivine crystals in the absarokites are evidently xenocrysts.
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Trace Element Chemistry |
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TOPABSTRACTIntroductionGeologic SettingTrace Element ChemistryMineral CompositionsPetrogenesisDiscussionAppendixReferences |
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The Kings River potassic lavas exhibit extreme enrichments in LILE, such as Rb, Ba and Sr, consistent with their elevated levels of K2O, and relative depletions in HFSE (Fig. 2a). There are only minor variations in the trace element pattern for absarokite, mafic and felsic minette lavas, predominantly in the relative enrichment or depletion of P and Zr. The degree of enrichment of Ba and K is in the order mafic minette > absarokite lavas > high-K basaltic andesite. The two felsic minette samples exhibit different degrees of enrichment: one has the greatest LILE enrichment, whereas the other is among the least enriched of all the Kings River lavas analyzed. All samples peak at Ba (200–640 times that of primitive mantle), with enrichments in LILE relative to HFSE resulting in ratios of Ba/NbPM [normalized to primitive mantle, Sun & McDonough (1989)
] of 7–33. There is no significant difference in the chondrite-normalized REE pattern between the magma types, all of which are steep from La to Tb; at Tb there is a variable change in slope resulting in intersecting patterns from Tb to Yb (Fig. 3). The LREE are enriched relative to heavy rare earth elements (HREE), with values of La/YbPM from 15 to 78.
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In Fig. 2b the trace element patterns of the Kings River magmas are contrasted with several other magma types. Normalized trace element compositions for the Kings River magmas are from one to four orders of magnitude greater than those for an average mid-ocean ridge basalt (MORB; Sun & McDonough, 1989
). Also shown are average compositions for ocean island basalts (OIB), Hawaiian nephelinites and kimberlites (Wedepohl & Muramatsu, 1979; Clague & Frey, 1982; Sun & McDonough, 1989). Although there is considerable overlap for many elements, the Kings River magmas have greater abundances of Rb, Ba, K and Sr, and a greater relative depletion in Nb than these other alkaline magmas. The trace element patterns of the Kings River magmas are similar to those found for the Merced Quadrangle potassic and ultrapotassic lavas located 40 km to the north, and only minor differences in elemental concentrations are evident (Fig. 2c; Van Kooten, 1980). The average composition of basalts from the Western Great Basin (WGB, Fig. 1; Fitton et al., 1991), has a pattern similar to and overlapping with those of the Kings River lavas for most elements; the major exceptions are the LILE Rb, K and Ba, for which the Kings River lavas have higher concentrations. |
Mineral Compositions |
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Olivine
Olivine is the most abundant and sometimes solephenocryst- and microphenocryst-size grain in the absarokites. It ranges up to 2.5 mm long, accounts for up to 18 vol. % of the lava, and frequently contains chromite inclusions. It is rare or often absent in the groundmass; modal analyses are presented in Table 1 and selected electron microprobe analyses of olivines are given in Table 2. The observed range of core compositions in the absarokite lavas is Fo91–77. Of the minettes, only one mafic and one felsic minette contain olivine grains; the range in core compositions for these samples is limited to Fo91 and Fo92–91, respectively. Within a single thin section, the compositions of phenocryst cores vary by 1–12% Fo, whereas rim compositions typically vary by only 3–4% Fo. The variation between cores and rims within a given sample ranges from 0 to 15% Fo. Core to rim zoning is commonly normal, with less frequent reverse zonation. Groundmass olivine is typically richer in iron than in phenocryst rims. However, in some samples, the groundmass olivines are more forsteritic than phenocryst rims, and in two samples, the groundmass olivines are more forsteritic than the phenocryst cores.
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Pyroxenes
Augite is an abundant microphenocryst and groundmass phase in all samples but is rarely a phenocryst phase. Augites are aluminous, titanian and ferrian (Table 2), with concentric zoning common in both phenocrysts and microphenocrysts, and contain substantial nonquadrilateral components. Ferric iron substitution is high in some samples, with up to 7.5 wt % Fe2O3 (Fe3+/FeT up to 100%). The high Fe3+/FeT in some of the Kings River augites probably reflects a high Fe3+/FeT in the liquids (Cooper et al., 1995
), which may be related to the high fO2 of the magmas and to their high whole-rock alkali contents, especially K, which stabilize Fe3+ over Fe2+ at any given fO2 (Sack et al., 1980; Kilinc et al., 1983; Kress & Carmichael, 1991). Clinopyroxene coronas around quartz and alkali feldspar xenocrysts are distinguished compositionally, either by a lack of Aliv and lower ferric iron contents, or more rarely by high Na and Fe3+ contents (Table 2). Orthopyroxene was found in only two samples. A single clot of orthopyroxene, zoned from En89Fs10Wo1 to En87Fs11Wo3, is surrounded by olivine (zoned from Fo92 to Fo85) and clinopyroxene (En50Fs6Wo44) in TD6b. The clinopyroxene is compositionally similar to rim compositions of phenocryst-size clinopyroxene in this sample. Some of the augite phenocrysts in this sample have inclusions of orthopyroxene in the core region, which probably represent orthopyroxene xenocrysts that have almost completely reacted with the liquid. The high Mg content of the orthopyroxene and olivine suggests they were derived from a mantle peridotite xenolith entrained in the minette magma, variably equilibrated with the minette and enclosed by clinopyroxene. Orthopyroxene (En86Fs12Wo2) was found in a second sample (PM2) as inclusions in olivine (Fo84–81).
Phlogopite
Phlogopite phenocrysts (up to 0.8 mm long) and microphenocrysts are found in both mafic and felsic minettelavas; groundmass phlogopite frequently occurs in both minette and absarokite lavas. Representative analyses of phenocryst and groundmass compositions are given in Table 2. Analysis of phlogopite phenocrysts and microphenocrysts was possible in only two samples because of their decomposition. Phenocryst cores and rims have similar compositions and contain substantial amounts of Ti, Ba, Cr and Ni. In addition, F contents are high, up to 6.3 wt %, occupying up to two-thirds of the hydroxyl site. Groundmass phlogopites have substantially higher Ti and Ba contents than the phenocrysts, with negligible Cr and Ni; the F contents overlap between phenocrysts and groundmass.
Spinel
Chromite is a frequent inclusion in phenocrysts of olivine but is only rarely found in augite. Titanian magnetite occurs as a groundmass phase in all absarokite and minette lavas and as inclusions along the rims of olivine. Pseudobrookite was found in three samples (D1, D3 and TD16a) as discrete groundmass crystals coexisting with titanian magnetite. Although the equilibrium coexistence of titanian magnetite and pseudobrookite (without the presence of ilmenite) is rare, similar occurrences have been found in the Merced Quadrangle potassic lavas (Van Kooten, 1980), other Sierra Nevada potassic magmas (Moore & Dodge, 1980), potassic lavas from western Mexico (Righter et al., 1995) and highly oxidized Hawaiian gabbros (Johnson et al., 1985). In phase equilibrium experiments at 1 bar on augite minette, Righter & Carmichael (1996) found titanian magnetite and pseudobrookite together without ilmenite over a wide range of oxygen fugacities from the Ni–NiO (NNO) to the hematite–magnetite (HM) buffer. Van Kooten (1980) suggested that the presence of an additional element such as V may favor the coexistence of pseudobrookite with titanian magnetite. High Mg contents also may enhance the stability of the assemblage pseudobrookite–magnetite (Frost & Lindsley, 1991; Righter & Carmichael, 1996). The Kings River pseudobrookites and magnetites have high contents of V and Mg (Table 2).
Feldspars and feldspathoids
Feldspar is notably absent as a phenocryst and microphenocryst in all samples collected from the Kings River region, including those with 56–58 wt % SiO2, although it is ubiquitous in the groundmass of both the absarokite and minette lavas. In most absarokite lavas the groundmass phase is predominantly plagioclase with lesser amounts of alkali feldspar ± anorthoclase ± hyalophane (up to 8.8 wt % BaO), although a few samples contain little to no plagioclase. The minette lavas contain alkali feldspar or anorthoclase with hyalophane. Appreciable amounts of SrO and Fe2O3 are found in all feldspars in both absarokite and minette lavas (up to 1.8 wt % for both).
Feldspathoids are present in the groundmass of some absarokite and minette lavas. Leucite is the most common feldspathoid, with nepheline occurring less frequently. Rare analcime is found as round groundmass grains and appears to have formed by replacement of leucite. Leucite and nepheline are characterized by significant substitution of Fe2O3 (up to 1.8 and 1.5 wt %, respectively).
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Petrogenesis |
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TOPABSTRACTIntroductionGeologic SettingTrace Element ChemistryMineral CompositionsPetrogenesisDiscussionAppendixReferences |
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An evaluation of crustal contamination
The presence of xenocrysts of quartz and alkali feldspar in the Kings River lavas indicates some interaction between the erupting lavas and the crust. However, the modal percentage of these xenocrysts is extremely low, <1% of the rock (as determined by point counting). Furthermore, neither K, nor Ba, nor Rb correlate with SiO2 or Al2O3 in bulk-rock compositions, as might be expected if contamination by granitic crust were responsible for the elevated LILE contents (Fig. 4). Indeed, the sample with the lowest SiO2 content (43.2 wt %, PM3) has one of the highest Ba contents (3719 ppm). Sr and Pb isotope data are available for the potassic and ultrapotassic Merced Quadrangle lavas that contain similarly high alkali contents (Van Kooten, 1980
, 1981). In a diagram of 1/Sr vs 87Sr/86Sr and 1/Pb vs Pb isotopic composition, the data for the Merced Quadrangle lavas scatter, inconsistent with mixing between homogeneous mantle and crustal sources. In addition, the ranges in Sr and Pb isotopic composition of the lavas overlap those of mantle xenoliths entrained in the Merced lavas and are narrow despite considerable variation in Sr and Pb concentrations, indicating that assimilation of isotopically variable crustal material does not control the chemical variability observed by Van Kooten (1980, 1981). He concluded that the trace element and isotopic data are characteristic of the mantle source region, and the presence of mantle xenoliths in some of the Merced Quadrangle lavas suggests that they ascended through the crust rapidly. The similarity in trace element concentrations between the Merced Quadrangle and Kings River lavas suggests that the high abundances of K2O and other LILE in the latter are also related to the mantle source region and are not caused by crustal contamination.
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Evidence for olivine accumulation
Several lines of evidence point to the accumulation of olivine in most of the Kings River absarokite samples collected; heterogeneity within a lava flow may occur but was not addressed owing to the high degree of erosion and glaciation. First, the whole-rock MgO concentration is strongly correlated with the modal abundance of olivine but displays no correlation with the most forsteritic olivine core composition (Fig. 4a). Second, the MgO concentration in the bulk rock is strongly correlated with whole-rock concentrations of Ni (a compatible trace element in olivine) and Cr (present in chromite inclusions in olivine; Fig. 4a). Third, the whole-rock MgO and SiO2 concentrations are inversely correlated at <10 wt % MgO, but show no correlation between 10 and 15 wt % MgO. Petrographic features, such as groundmass olivine more forsteritic than phenocryst rims, a large variation in phenocryst core composition, and reversals in composition, also suggest olivine accumulation. These chemical and petrographic features suggest that all but five samples (BcM1, TD5, TD10, TD15 and TD16a) underwent olivine accumulation. However, consideration of Fe–Mg exchange equilibria between olivine and melt (e.g. Roeder & Emslie, 1970
; Ulmer, 1989; Snyder & Carmichael, 1992) indicates that the olivine in sample BcM1 could be in equilibrium with the host liquid only if the fO2 was below the iron–wüstite buffer; the olivine therefore must not be phenocrysts of this liquid. Thus only four of the absarokite samples selected for study may represent liquid compositions (TD5, TD10, TD15 and TD16a). The minettes TD3c, TD3e and PM3 may also represent liquid compositions, although PM3 may have incorporated minor amounts of olivine (phenocryst olivine is only 1.3 vol. %). Because olivine has very low incompatible trace element abundances, its incorporation results in a small diluting effect, and variations in the trace element ratios remain unaffected.
Oxygen fugacity
Heterogeneity in the oxidation state of the mantle has been widely documented, on the basis of analyses of mantle xenoliths and mantle-derived lavas. Metasomatism has been proposed as an important mechanism for increasing the oxygen fugacity of the mantle over time (e.g. Bryndzia et al., 1989; Canil et al., 1990; Wood et al., 1990; Ballhaus, 1993). The oxygen fugacity of basaltic lavas can be estimated using whole-rock Fe2O3/FeO values and the experimentally calibrated relationship involving temperature, oxygen fugacity and bulk composition (e.g. Kress & Carmichael, 1991). Moore et al. (1995) showed that this model can be applied to hydrous silicate liquids, as dissolved water has no measurable effect on the oxidation state of iron in a silicate melt. Ferrous iron concentrations were measured by titration for samples TD5, TD10, TD15 and TD16a (the only Kings River absarokite lavas that show no evidence for olivine accumulation and therefore may represent liquid compositions) and the augite minette sample TD3e (Table 1). The derived estimates of oxygen fugacity relative to NNO (
NNO = logfO2[magma] – logfO2[Ni–NiO buffer]) range between + 0.8 and + 3.3 NNO for the absarokites and +1.8 NNO for the minette. Post-eruptive weathering and alteration of bulk ferric/ferrous ratios do not appear to be a significant concern for the absarokite lavas because their phenocryst and groundmass olivines do not display iddingsite rims. These values are similar to those determined for absarokite and olivine minette magmas from the western Mexico continental arc (+ 1 to + 3 NNO; Lange & Carmichael, 1991; Carmichael et al., 1996), and are considerably higher those determined for either MORB or Hawaiian basalts (–2 to –5 NNO; Christie et al., 1986). These results suggest that the mantle source for the Kings River magmas is 3–7 log units more oxidizing than asthenospheric mantle, probably a result of pervasive metasomatism associated with subduction.
Pre-eruptive volatile contents, temperatures and pressures
The importance of volatile content in producing the variation in rock type in the Kings River region can be seen by comparing sample TD16a, an absarokite, with sample TD3c, a minette: these samples are very similar in their SiO2 and K2O contents (Table 1), yet phlogopite occurs as phenocrysts in TD3c but is confined to the groundmass in TD16a. The difference in their phenocryst assemblage probably reflects differences in their respective water and/or fluorine concentrations, with the minette magma containing higher concentrations, allowing phlogopite to be stable. Phase equilibrium experiments on mafic and felsic minettes show that water contents between 3 and 5 wt % are sufficient to stabilize phlogopite in potassic liquids over a wide range of pressures (1–20 kbar; Esperança & Holloway, 1986, 1987; Righter & Carmichael, 1996). Water contents not only control the stability of phlogopite but may also affect the crystallization of plagioclase. The absarokite and minette lavas are characterized by a lack of plagioclase phenocrysts and microphenocrysts. Experiments on a variety of liquid compositions have shown that water is an effective agent for the suppression of early crystallization of plagioclase as well as pyroxene (Eggler, 1972; Sisson & Grove, 1993).
Fluorine is another volatile component that can stabilize phlogopite and appears to have been important in some magmas. The felsic minette, D3, contains phlogopite phenocryst cores with up to 6 wt % F (2/3 of the hydroxyl site; Table 2); the mafic minette TD3c contains 1.5 wt % F in its phenocryst cores. Although the pre-eruptive, whole-rock fluorine contents of the Kings River magmas are not known, the high fluorine contents in the phlogopite phenocrysts suggest that water contents in these particular magmas could have been lower than 3 wt %, the minimum water content needed to stabilize phlogopite (Esperança & Holloway, 1986, 1987; Righter & Carmichael, 1996). Because F stabilizes phlogopite to low pressure (1 bar), F-rich phlogopite is preferentially preserved compared with more hydrous varieties. Thus the high F contents of the analyzed phlogopite grains may reflect selective decomposition of more hydrous phlogopites, and not a source region that is uniformly high in F.
The phlogopite phenocrysts in the minette magmas can further be used to estimate pre-eruptive temperatures and pressures, on the basis of the partitioning of TiO2 and BaO between biotite and liquid. 
decreases with increasing temperature and is not sensitive to pressure or oxygen fugacity (Esperança & Holloway, 1987; Guo & Green, 1990; Righter & Carmichael, 1996). Using the empirical calibration presented by Righter & Carmichael (1996), and approximating the liquid composition by subtracting the TiO2 content apportioned in each phenocryst phase from the bulk-rock composition, pre-eruptive temperatures of 1138 and 1144 (± 50)°C were calculated for the felsic and mafic minette magmas, D3 and TD3c, respectively. Analysis of phlogopite phenocrysts and microphenocrysts in other minette samples was not possible because of their decomposition. Similarly, 
decreases with increasing pressure (Esperança & Holloway, 1987; Guo & Green, 1990; Righter & Carmichael, 1996). Calculated pressures range between 12 and 16 (± 4) kbar and are relatively insensitive to water activity. For example, an increase in water activity from 0.25 to 1.0 results in only a 2 kbar change in the calculated pressure. Assuming an integrated density of 2.8 g/cm3 at the base of the crust (see below; Fig. 7), a density of 3.28 g/cm3 for the lithospheric mantle (e.g. Fliedner et al., 1996), and a maximum thickness for the sialic crust of 42 km in the Kings River region (Fliedner et al., 1996), these pressures correspond to depths of 44 and 56 km (± 15) km, respectively.
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Discussion |
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TOPABSTRACTIntroductionGeologic SettingTrace Element ChemistryMineral CompositionsPetrogenesisDiscussionAppendixReferences |
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The mantle source region and the LILE
The Kings River magmas are characterized by high LILE (e.g. K, Rb, Ba) contents but minimal quantities (<5.6 vol. %) of phlogopite; these chemical features, therefore, are not caused by phlogopite accumulation but reflect concentrations in the liquid. The asthenospheric and continental lithospheric mantle are both potential source regions for the Kings River magmas. The continental lithospheric mantle, chemically isolated from the underlying convecting mantle, may have unique trace element and isotopic characteristics owing to its interaction with fluids and melts ascending from the asthenosphere and/or driven off from subducting slabs over hundreds of millions of years (e.g. Menzies et al., 1987a
, 1987b; McDonough, 1990). Studies of basic magmatism in the western USA have shown a progression from magmas derived from the continental lithosphere to magmas with a significant asthenosphere component as Basin and Range extension progressed (Fitton et al., 1988, 1991; Kempton et al., 1991). To generate high concentrations in LILE from an unmetasomatized, asthenospheric mantle, either very small degrees of partial melting or, possibly, melting at high pressures (Carmichael et al., 1977) must be invoked. A source within the unmetasomatized asthenosphere can effectively be ruled out, however, by comparing the Kings River lavas with a wide variety of MORB and OIB magmas, which, despite differences in the inferred degrees of partial melting and depth of melting, have identical Ba/Rb, Rb/Cs and Nb/U ratios (12, 80, and 47, respectively; Hofmann, 1986; Hofmann et al., 1986; Sun & McDonough, 1989). In the Kings River magmas, these ratios are highly variable and distinctly different from those observed for MORB and OIB (Ba/Rb 20–100, Rb/Cs 35–77, and Nb/U 5–31), implying a different mantle source.
Evidence that the Kings River mantle source underwent both cryptic and modal metasomatism, which involved the formation of phlogopite ± potassic amphibole, is found in plots of La vs La/K and La/Ba for the Kings River and Merced Quadrangle magmas (Fig. 5). The element La is incompatible even in phlogopite- and amphibole-bearing assemblages and therefore is a variable that is sensitive to the degree of partial melting, with lower concentrations of La correlating with higher degrees of partial melting. The elements K and Ba have similar degrees of incompatibility as La in clinopyroxene, but are more compatible than La in phlogopite and potassic amphibole. If the concentration ratio of two incompatible elements is independent of concentration, it is argued that the bulk distribution coefficients of the two elements are the same for a source that has homogeneous La concentration (Minster & Allègre, 1978; Ormerod et al., 1991).
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The increase in the La/K and La/Ba ratios for the Kings River and Merced Quadrangle magmas with increasing La (Fig. 5) requires that the bulk partition coefficients for K and Ba exceed that of La (Minster & Allègre, 1978
; Ormerod et al., 1991). Because the inferred crystallizing assemblages of the potassic lavas do not yield bulk partition coefficients that generate an increase in La/K with increasing La during fractional crystallization, the positive correlation suggests that a potassic phase is present in the mantle source region, probably phlogopite and potassic amphibole. Similar plots using Th as the incompatible element (not shown) and K, Ba as well as Rb as the elements of interest yield the same results. This conclusion is consistent with the occurrence of phlogopite- and amphibole-bearing mantle xenoliths found entrained in some Merced Quadrangle flows as well as in volcanic pipes in the central Sierra Nevada (Van Kooten, 1980, 1981; Dodge et al., 1988; Mukhopadhyay & Manton, 1994). There does not appear to be a kink in the trends at low concentrations of La which would result from phlogopite completely melting out at higher degrees of partial melting; similarly, no kinks are evident in plots of La vs Ba/Rb and Ba/K. It appears, therefore, that a K-bearing phase remained a residual for all the Kings River magmas. The significant degree of scatter in Fig. 5 (low r2) suggests that source compositions or melting relationships are not identical for each erupted magma.
Although the concept of slab-derived fluids as a metasomatic agent in the subarc mantle wedge has been widely accepted for some time, the relative contributions of pelagic sediments, altered basalt and peridotite are difficult to characterize. A notable feature of the LILE enrichment in the Kings River lavas is the high Ba/Rb ratio (20–100) in comparison with island arc basalts (e.g. <30; McCulloch & Gamble, 1991). A number of models have described island-arc petrogenesis; a common feature is minimization of the sediment contribution, to account for their low Ba/Rb ratio (e.g. McCulloch & Gamble, 1991; Stolper & Newman, 1994; Tatsumi & Kogiso, 1997). Pelagic sediments have high Ba/Rb ratios (135; McCulloch & Gamble, 1991, and references therein). In addition, barite deposits, such as those associated with biological communities along the Peru margin, are not uniformly distributed along the ocean floor (Torres et al., 1996; Aquilina et al., 1997) and therefore may contribute to variable Ba contents in slab-derived fluids. The elevated Ba/Rb ratio in the Kings River lavas suggests a significant contribution from pelagic and/or barite sediments to the slab-derived fluid that metasomatized the mantle source region. Alternatively, high Ba/Rb ratios may be caused by incorporation of small degree partial melts from a foundering lower crust (Ducea & Saleeby, 1996, 1998).
The conclusion that the mantle source region for the Kings River potassic magmas contained residual phlogopite ± potassic amphibole contrasts to what can be inferred for the source region of the Big Pine basalts, which erupted along the eastern Sierra Nevada escarpment (Fig. 1). Ormerod et al. (1991) used plots of Rb vs Rb/i (where i is an element of interest) to conclude that a K-bearing phase was not residual in the Big Pine mantle source during melt generation. This difference is also suggested by comparing the La/K and La/Ba ratios of the lavas from these two regions; these ratios do not vary with La concentration in Big Pine as they do in the Kings River lavas (Fig. 5). The difference between the two lava suites is consistent with relatively higher temperatures beneath eastern Sierra Nevada during the Pliocene and Quaternary than beneath the central Sierra Nevada in the Miocene (Ducea & Saleeby, 1996, 1998). Higher temperatures would lead to larger degrees of partial melting for the Big Pine source region, which would cause refractory phases such as phlogopite and/or potassic amphibole to completely melt out. The retention of these phases during melting would be enhanced in the regions of the Kings River source with high F content (i.e. more refractory; Foley, 1991; Vukodinovic & Edgar, 1993; Edgar & Pizzolato, 1995), inferred from the high F content of some of the cores of phlogopite phenocrysts. Furthermore, the Big Pine lavas have high La/K and La/Ba ratios at low La concentrations, not low ratios as would be expected if these lavas were derived from melting of a source region identical to that of the Kings River lavas but at larger degrees of partial melting. This suggests some difference, in mineralogy or chemistry, between the two regions; distance to the paleotrench is a possible factor.
The mantle source region and the HFSE
The depletion of HFSE relative to its neighboring element is recognized as a fingerprint of subduction processes [see Thirlwall et al. (1994) and references therein]. Hofmann and others (Hofmann, 1986; Hofmann et al., 1986) have demonstrated that the Nb/U ratio is constant in both MORB and OIB (47), indicating that Nb and U have nearly identical mantle mineral–melt partition coefficients. Thus, partial melting, either in the slab or wedge, cannot fractionate Nb from U. The Nb vs Nb/U is plotted in Fig. 6 for the Kings River magmas. In all cases, the Nb/U ratios for both the absarokites and minettes are lower than in MORB and OIB. The disparity between the Nb/U ratios observed in the Kings River magmas and MORB and OIBs cannot be explained by repeated melt extraction from the mantle wedge alone. The presence of residual rutile during dehydration of the subducting slab can result in fractionation of HFSE (Ryerson & Watson, 1987). Brenan et al. (1994) have shown that rutile–fluid partition coefficients for the HFSE (Nb, Ta, Hf, Zr) are uniformly large (>100), whereas values for other large radius cations (U, Th) are small. Brenan et al. (1995) concluded that depletions of Nb relative to U did not occur during fluid generation.
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The Kings River magmas exhibit relative HFSE depletions, and low concentrations of some HFSE such as Nb in comparison with OIB (Fig. 2b). However, the absolute concentration of Nb is fairly high in comparison with island arc suites (e.g. 0.4–2.1 ppm, McCulloch & Gamble, 1991
) but similar to that in minettes from a variety of regions (14–24 ppm, Thompson et al., 1989; 10–47 ppm, Wallace & Carmichael, 1989; 32 ppm, Conticelli & Peccerillo, 1990). In addition to variations in melt concentration owing to different degrees of partial melting, a possibility is that phlogopite and amphibole may act as a reservoir for Nb. Ion-microprobe studies of these phases in mantle xenoliths indicate that they can be an important host for Nb in K-rich mantle rocks (O'Reilly et al., 1991; Ionov & Hofmann, 1995). Ionov & Hofmann (1995) analyzed coexisting phlogopite and clinopyroxene in one xenolith; phlogopite Nb contents were 40 times higher than that of the clinopyroxene, even though the bulk trace element pattern for this sample had a relative Nb depletion. In the Kings River absarokites and minettes, the presence of phlogopite does appear to exert some control over the HFSE, as was found for similar composition lavas from Mascota, Mexico (Carmichael et al., 1996). Figure 4 shows that Ti is positively correlated with Ba, suggesting that phlogopite, in which both Ti and Ba are compatible (Esperança & Holloway, 1987; Guo & Green, 1990; Adam et al., 1993; Foley et al., 1994; Righter & Carmichael, 1996), is a dominant phase in controlling the abundances of these elements. A positive correlation is also observed between Ba (and Ti) with Nb (Fig. 4) and Ta for the minettes; however, the data scatter for the absarokites, suggesting that the distribution of Nb and Ta in the absarokites is not controlled simply by phlogopite in the source. Similarly, La/Nb, La/Ta and Sm/Hf ratios increase with increasing La/Ba, although there is more scatter for the absarokites than the minettes. The data for the Kings River minettes in terms of La vs La/Nb and La/Ta define a positive slope, suggesting that the bulk partition coefficient for Nb is greater than that for La; the data scatter for the absarokites (Fig. 5). The HFSE Zr shows both a positive and negative anomaly (Fig. 2a). A broad correlation between Zr and K (and Ba) suggests that a Zr-bearing phase(s) is associated with phlogopite ± potassic amphibole, with its variable contribution to the partial melt responsible for the variable anomaly. In contrast to the LILE, there does not appear to be any difference between the HFSE pattern in Kings River and Big Pine lavas (Ormerod et al., 1991).
Melting in the source region
The variability in the source composition and/or melting relationships for the Kings River lavas can be understood in terms of the multiple pathways through which heterogeneity is introduced during mantle metasomatism. First, slab-derived fluids percolating through the subarc mantle wedge may stabilize phlogopite ± potassic amphibole ubiquitously within the host rock or in the form of localized networks and veins, depending on the depth, the availability of H2O and K2O (Peacock, 1990), and the stress field in the lithosphere. Second, metasomatism of the wall-rock surrounding vein contacts may result in geochemical gradients in the peridotite mantle (e.g. Irving, 1980; Griffin et al., 1984; Nielson et al., 1991; O'Reilly et al., 1991). Furthermore, complex reactions between melt and wallrock may lead to significant changes in melt composition within a vein system (e.g. Wulff-Pedersen et al., 1996). Additional complexity is introduced because of the time scale over which the infiltration of fluids and melts may have occurred in the lithospheric mantle of the Sierra Nevada. The subduction of the Farallon plate off the Pacific coast provided the opportunity for metasomatism between 220 and 80 Ma. In addition, Beard & Glazner (1995) provided evidence from mantle xenoliths entrained in Big Pine lavas that metasomatism of Sierran lithosphere may date back to 800 Ma, associated with continental rifting. Distinct pulses of fluid and/or melt over that time period are likely to result in the formation of hydrous phases with very different trace element abundances (e.g. Griffin et al., 1988).
Experiments by Hirose & Kawamoto (1995) demonstrate that in the presence of a small amount of water (<1 wt %), the solidus temperature of lherzolite drops from 1250°C to <1100°C and is accompanied by significant increases in the degree of partial melting. Given the temperatures calculated for the Kings River minette magmas (1140 ± 50°C) and the availability of water, it is expected that the non-hydrous peridotite mantle adjacent to the phlogopite- and potassic amphibole-bearing assemblage in the lithospheric mantle underwent some partial melting. Foley (1992), describing the mechanism of vein-plus-wall-rock melting, pointed out that the presence of solid solutions, particularly fluorine in phlogopite and amphibole, causes the melting interval of the vein to expand and overlap with that of the surrounding peridotite. Initial melts will be dominated by the vein component, whereas later melts will contain greater concentrations of a wall-rock component. Moreover, melting temperatures of fluoro-phlogopite and fluoro-richterite are higher than their hydrous counterparts and may be residual after small degrees of partial melting of hydrous veins (Foley, 1991; Vukodinovic & Edgar, 1993; Edgar & Pizzolato, 1995). The high F contents in the cores of phlogopite phenocrysts suggest that the minette liquids may be derived from second-stage melting of a fluorine-rich assemblage.
Tectonic implications
Several conclusions can be drawn regarding the nature of the subarc lithospheric mantle beneath the central Sierra Nevada on the basis of this study. Equilibration depths recorded by the minette magmas are between 44 and 56 (± 15) km and therefore at the depth where the eclogite portion of the central Sierran lithosphere is inferred to have existed, on the basis of studies of xenoliths entrained within 8–11 Ma lavas (Dodge et al., 1988; Mukhopadhyay & Manton, 1994; Ducea & Saleeby, 1996). This implies that the minette magmas ascended from their veined peridotite source region and then stalled to re-equilibrate within the dense eclogite layer; however, this seems unreasonable given the high volatile abundances inferred for these magmas (see below). A more likely scenario is that the equilibration depth recorded by the magmas represents that of their mantle source and that they ascended rapidly to the surface after their generation. This implies that between 8–11 and 3–4 Ma, the age of the Kings River lavas, the eclogite layer was removed. This possibility was suggested by Ducea & Saleeby (1996) on the basis of their observation that garnet is present in xenoliths entrained in 8–11 Ma lavas but is absent in lavas of 3–4 Ma in the central Sierra Nevada, and by Zandt & Ruppert (1996) on the basis of geophysical evidence.
Melt migration through sialic crust
The questions regarding the thickness of the Sierran crust have direct implications for evaluating the role of sialic crust as a barrier to the migration of mantle-derived melts. Thick crustal roots, such as those in the Andes, can act as both density and thermal filters to the migration of most mantle-derived melts. Although a thick, Airy-type crustal root is not believed to be present beneath the Sierra Nevada, the 35–40 km thickness of sialic crust beneath the central Sierra Nevada (Fliedner et al., 1996) might present a density barrier to some primary mantle melts. Magma buoyancy is enhanced by high liquid H2O, K2O and F contents (Lange & Carmichael, 1987; Dingwell et al., 1993; Ochs & Lange, 1997), all of which may be a factor for the Kings River magmas. The eruptability of magmas is described by contrasting calculated liquid densities with an integrated crustal density (Fig. 7a); the latter incorporates the density of the entire overlying crustal block and therefore is the best reflection of the density contrast actually encountered by a melt at any given depth. Calculated liquid densities are shown for the most primitive minette lava erupted in the Kings River volcanic field (PM3) and indicate that a dry mafic melt, even with the high K2O content of this minette, would be more dense than the sialic crust; a minimum of at least 2 wt % H2O is needed for this melt to become buoyant at 40 km depth. Although the thermal expansivity and compressibility of the fluorine component is not well known, an estimate of the effect of dissolved fluorine on melt density can be made using the data of Dingwell et al. (1993). The results show that, in the absence of water, >8 wt % F would be required to render the minette liquid buoyant with respect to the Sierran crust (Fig. 7b). This indicates that F alone cannot provide the buoyancy necessary to ensure eruption; H2O must also be present. These general results support the earlier conclusions that the potassic magmas of the Kings River volcanic field are not only characterized by high oxidation states and extreme LILE enrichments, but also contained substantial H2O and F concentrations, thus ensuring their eruptability through the Sierran batholith.
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Appendix |
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TOPABSTRACTIntroductionGeologic SettingTrace Element ChemistryMineral CompositionsPetrogenesisDiscussionAppendixReferences |
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Acknowledgements |
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J. Tangeman and K. Shipley are thanked for assistance in the field. C. E. Henderson is thanked for assistance with the electron microprobe. Reviews of this manuscript and/or a previous version by Ian Carmichael, M. Ducea, E. J. Essene, A. N. Halliday, J. R. O'Neil, N. Rogers and W. Bohrson are gratefully acknowledged. Support for field work was provided by Sigma Xi and Scott Turner awards to S.N.F. The support of the NSF (EAR-92-19070) is also acknowledged.
* Corresponding author. Telephone: 734-764-7421. Fax: 734-763-4690. e-mail: becky{at}umich.edu
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