JOURNAL OF PETROLOGY PAGE 1 of 38 doi:10.1093/petrology/egi048 Journal of Petrology Advance Access published June 3, 2005

The Stonyford Volcanic Complex: a Forearc Seamount in the Northern California Coast Ranges

JOHN W. SHERVAIS1*, MARCHELL M. ZOGLMAN SCHUMAN2y AND BARRY B. HANAN3

1DEPARTMENT OF GEOLOGY, UTAH STATE UNIVERSITY, 4505 OLD MAIN HILL, LOGAN, UT 84322-4505, USA 2DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF SOUTH CAROLINA, COLUMBIA, SC 29208, USA 3DEPARTMENT OF GEOLOGICAL SCIENCES, SAN DIEGO STATE UNIVERSITY, SAN DIEGO, CA 92182-1020, USA

RECEIVED APRIL 15, 2004; ACCEPTED APRIL 6, 2005

Jurassic age volcanic rocks of the Stonyford volcanic complex mantle wedge above the now defunct subduction zone produced (SFVC) comprise three distinct petrological groups based on their strongly depleted high-Al, low-Ti basalts that were partially fertil- whole-rock geochemistry: (1) oceanic tholeiites; (2) transitional ized with enriched, alkali basalt-type melts and slab-derived fluids. alkali basalts and glasses; (3) high-Al, low-Ti tholeiites. Major and trace element, and Sr–Nd–Pb isotopic data indicate that the oceanic tholeiites formed as low-degree partial melts of normal KEY WORDS: CRO; oceanic basalts; California mid-ocean ridge basalt (N-MORB)-source asthenosphere similar in isotope composition to the East Pacific Rise today; the alkalic lavas were derived from an enriched source similar to that of E-MORB. The high-Al, low-Ti lavas resemble second-stage INTRODUCTION melts of a depleted MORB-source asthenosphere that formed by Ophiolites have long been recognized as important ele- melting spinel lherzolite at low pressures. Trace element systematics ments of orogenic systems that indicate the formation and of the high-Al, low-Ti basalts show the influence of an enriched subsequent closure of oceanic basins (e.g. Moores, 1982). component, which overprints generally depleted trace element char- They are especially important in regions such as the acteristics. Tectonic discrimination diagrams show that the oceanic western Cordillera of North America, which comprises tholeiite and alkali suites are similar to present-day basalts generated in part a tectonic collage of accreted terranes, as well at mid-oceanic ridges. The high-Al, low-Ti suite resembles primitive as terranes formed by tectonic and magmatic processes arc basalts with an enriched, alkali basalt-like overprint. Isotopic in situ along the continental margin (Saleeby, 1992). data show the influence of recycled components in all three suites. The Coast Range ophiolite (CRO) of California is one The SFVC was constructed on a substrate of normal Coast Range of the most extensive middle Jurassic ophiolite terranes in ophiolite in an extensional forearc setting. The close juxtaposition of North America and has long been central to our under- the MORB-like olivine tholeiites with alkali and high-Al, low-Ti standing of Jurassic Cordilleran tectonics (e.g. Saleeby, basalts suggests derivation from a hybrid mantle source region that 1992; Fig. 1). None the less, its origin is controversial and included MORB-source asthenosphere, enriched oceanic astheno- three primary hypotheses have been advanced: (1) forma- sphere, and the depleted supra-subduction zone mantle wedge. We tion at a mid-ocean ridge spreading center at low paleo- propose that the SFVC formed in response to collision of a mid-ocean latitudes, and its subsequent rapid drift northward ridge spreading center with the Coast Range ophiolite subduction to collide with North America (Hopson et al., 1981; zone. Formation of a slab window beneath the forearc during Pessagno et al., 2000); (2) formation as a back-arc basin collision allowed the influx of ridge-derived magmas or the mantle above an east-facing volcanic arc that collided with North source of these magmas. Continued melting of the previously depleted America during the late Jurassic Nevadan orogeny

*Corresponding author. E-mail: [email protected] The Author 2005. Published by Oxford University Press. All yPresent address: 2623 Withington Peak Dr NE, Rio Rancho, rights reserved. For Permissions, please e-mail: journals.permissions@ NM 87144, USA. oupjournals.org JOURNAL OF PETROLOGY

° 124 W 122°

° 40 N Elder Creek 40° N

SV Stonyford

Harbin Springs Geyser Peak / Black Mountain Mt. St. Helena Healdsburg ° Mount Diablo 38 N 38° N Leona Rhyolite SF

Sierra Azul Del Puerto

SJV Tertiary Quinto Creek Modoc Plateau

Salinia SNF ° Llanada 36 N 36° N Great Valley Sequence

Coast Range Ophiolite SAF

Franciscan Complex Stanley Mtn Cuesta Ridge Sierra Nevada

Klamath terranes Point Sal 124° W 122° W 120° W

Fig. 1. Geological sketch map of California showing the location of Stonyford and other Coast Range ophiolite localities. SAF, San Andreas fault; SNF, Sur Nacimiento fault; SF, San Francisco; SV, Sacramento Valley; SJV, San Joaquin Valley. After Shervais et al. (2004).

2 SHERVAIS et al. THE STONYFORD VOLCANIC COMPLEX

(Dickinson et al., 1996; Godfrey & Klemperer, 1998; rocks of the Franciscan assemblage from unmetamorph- Ingersoll, 2000); (3) formation by forearc or intra-arc osed sediments of the Great Valley Series, and encom- rifting along the western margin of North America, in passes the arcuate outcrop belt of the SFVC (Fig. 2). response to nascent or renewed subduction of oceanic Franciscan schists in this area include metagreywacke plates beneath North America (Shervais & Kimbrough, and argillite (quartz–albite–lawsonite schists), pale green 1985a,1985b; Stern & Bloomer, 1992; Shervais, 2001). It metavolcanic rocks, and minor chert. These rocks, which is clear that not all of these models can be correct, correlate with the Yolla Bolly terrane of Blake et al. although there may be elements of truth in each. Most (1987), overlie chaotically sheared shale-matrix me´lange detailed studies of the CRO currently support the supra- of the Franciscan Central Belt to the north and SW. subduction zone (SSZ) model (Shervais & Kimbrough, The contact between the Franciscan schists and the 1985a, 1985b; Stern & Bloomer, 1992; Shervais et al., serpentinite-matrix me´lange is generally a high-angle 2004), but models based on back-arc basins and oceanic fault except along the northern border, where outcrop spreading centers still persist. patterns imply a low-angle fault contact (Fig. 2). The Stonyford volcanic complex (SFVC) is unique The serpentine-matrix me´lange may be a continuation within the CRO (Fig. 2). This seamount complex consists of the Round Mountain me´lange, which extends north to almost entirely of volcanic flows (pillow lava, sheet flows) the Elder Creek remnant of Coast Range ophiolite ( Jayko with subordinate diabase, hyaloclastite breccia, and et al., 1987; Huot & Maury, 2002). The me´lange matrix minor sedimentary intercalations of chert and limestone consists of strongly sheared and foliated serpentinite (Shervais & Kimbrough, 1987; Shervais & Hanan, 1989). schist that crops out along ridge crests and in canyons Other normal components of the ‘classic’ ophiolite series (Shervais et al., 2005b). Tectonic knockers include massive (cumulate mafic and ultramafic rocks, isotropic gabbros harzburgite (up to 5 km · 2 km in size), unmetamorph- and diorites, and sheeted dike complex) are found only as osed sedimentary rocks (coarse lithic wackes, gritstones, blocks in the serpentinite matrix me´lange that underlies and conglomerates—all rich in volcanic detritus), the volcanic complex. Most of the SFVC volcanic rocks metasediments (foliated greywacke, argillite, gritstone), show geochemical affinities to metavolcanic rocks of pale green metavolcanics, high-grade blocks of amphibol- the Franciscan assemblage and they are distinctly differ- ite and blueschist, fragments of the SFVC, and igneous ent from the majority of CRO volcanics (Shervais & rocks derived from the Coast Range ophiolite (wehrlite, Kimbrough, 1987; Shervais & Hanan, 1989; Shervais, clinopyroxenite, gabbro, diorite, quartz diorite, and ker- 1990; Shervais et al., 2004, 2005). atophyre). These blocks of CRO plutonic and volcanic In this paper we present field, petrological, geo- rocks are similar to those found near Elder Creek 60 km chemical, and isotopic data for rocks of the SFVC that north of this area (Shervais, 2001; Fig. 1). establish its relationship to the CRO, and have impli- The Great Valley Series here is a homoclinal sequence cations for formation of the CRO and its subsequent of mudstone or argillite with minor intercalated grey- tectonic evolution. These data also illuminate processes wacke and micritic limestone overlain by a coarse basaltic that may occur in active forearcs as a result of ridge– sandstone and the chert-rich Gravelly Ridge conglomer- trench interactions. ate (Brown, 1964). Buchia piochii of late Tithonian age are common throughout the section as high as the Gravelly Ridge conglomerate (Simpson-Seymore, 1999). The con- tact between the GVS and the serpentinite-matrix me´l- GEOLOGICAL RELATIONS ange is a high-angle reverse fault that dips steeply to the Overview west and places serpentinite over sediments of the GVS. The Stonyford volcanic complex (SFVC) is located in High-angle faults within the GVS that trend NW and the northern Coast Ranges of California approximately west are probably tear-faults related to earlier west- 120 miles north of San Francisco and 35 miles south of vergent thrusting of the GVS (e.g. Wentworth et al., the Elder Creek ophiolite, near the western margin of 1984; Glen, 1990). the Sacramento Valley (Fig. 1). The SFVC crops out in portions of four USGS 7Á50 quadrangles: Stonyford, St. Johns Mountain, Fout Springs, and Gilmore Peak. Structural relationships Mapping was conducted at a scale of 1:12 000 over two The SFVC is completely separated from metasediments field seasons, covering 15 km2 (Fig. 2). of the Franciscan complex, and from mudstones and The Stonyford area contains four main lithotectonic wackes of the Great Valley Series, by the serpentinite elements: (1) the SFVC; (2) low- to medium-grade schists me´lange that both underlies and overlies the massif. of the Franciscan assemblage; (3) mudstone, greywacke, Nearly all contacts between the serpentinite me´lange and conglomerate of the Great Valley Series; (4) a sheared and the adjacent units are high-angle faults that reflect serpentine-matrix me´lange that separates metamorphic Neogene compression in the Coast Ranges.

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Fig. 2. Geological map of the Stonyford volcanic complex (SFVC) and surrounding units. GVS, Great Valley series; SSP, sheared serpentinite me´lange; Harz, massive harzburgite; JKf, Franciscan assemblage; BDR, Black Diamond Ridge. Cherts A, B, C and D are the major horizons of siliceous sediment intercalated within the complex; Chert D is the Diversion Dam locality. Glass layers 1–8 are the glass horizons described by Shervais & Hanan (1981) and Shervais et al. (2004). These are the same chert and glass horizons as dated by Shervais et al. (2005a). Black star, town of Stonyford.

4 SHERVAIS et al. THE STONYFORD VOLCANIC COMPLEX

The volcanic complex is divided into at least three large hyaloclastite breccia, which only occurs high in the sec- blocks by high-angle cross faults that trend east–west or tion. The massive sheet flows range in thickness from SE–NW, and these blocks may be broken into smaller about 1 m to over 4 m thick, and in places are seen to blocks along smaller faults (Fig. 2). One large, SE–NW- be laterally extensive. Pillows are typically 0Á5–1 m across trending scissor fault has broken the largest block of and form long tubes that branch and bud. volcanic complex just south of Auk Auk Ridge. Move- ment on this fault was normal, with the NW end of the Hyaloclastite breccias upthrown block offset significantly more that the SE end, Hyaloclastite breccias form massive, unbedded layers causing the upthrown block to tilt some 15–20 to the SE which range from about 1 m thick to almost 100 m and exposing the low-angle contact between the serpent- thick, although layers 10–50 m thick are most common. inite me´lange and the SFVC on Auk Auk Ridge (Fig. 2). The hyaloclastite breccia layers are typically located near This fault and the relationships between the serpentinite the top of the section, and presumably formed as the me´lange and SFVC are well exposed by deep erosion seamount shoaled. Three layers crop out in Dry Creek along the south side of Auk Auk Ridge, making this the and four layers crop out between Dry Creek and Auk best area in which to study contact relations between the Auk Ridge (Glass, Fig. 2); other layers not shown in the volcanic complex and the me´lange. S–C crenulation fab- map are less extensive and generally do not contain fresh rics in the serpentinite matrix just west of Auk Auk Ridge glass. and near Hyphus Creek to the south suggest normal All of the layers shown in the map contain fresh glass shear sense on the low-angle contact with the volcanics and were described by Shervais & Hanan (1989). These (Dennis & Shervais, 1991). layers are intercalated with pillow lava, which allows us to The serpentinite-matrix me´lange beneath Auk Auk determine tops. The breccias contain centimeter-scale Ridge contains a wide variety of ophiolitic rocks similar rounded glass lapilli, along with 5–20% angular volcanic to those seen at Elder Creek (Shervais et al., 2004). These clasts up to 15 cm across in a matrix of glass shards and blocks include wehrlite, clinopyroxenite, gabbro, diorite, fragments. Pillow fragments, ‘finger pillow buds’, and quartz diorite, and keratophyre pillow lava, all distinct small isolated (<1⁄ m) pillows are common in some from rocks exposed within the SFVC. We interpret these 2 breccias, but most are dominated by glass lapilli and relations to suggest that the SFVC was built upon a shards. Small xenoliths of amphibolite are rare. These substrate of ‘normal’ Coast Range ophiolite prior to (or appear to represent fragments of the metamorphosed perhaps during) its structural dismemberment and basement upon which the seamount was built. The incorporation into the serpentinite me´lange (see below). hyaloclastite breccias were interpreted by Shervais & Hanan (1989) to represent submarine fire fountain depos- Field relations within the SFVC its, based on the dominance of rounded glass lapilli and The SFVC consists largely of pillowed and massive lava glass shards derived from round lapilli. Their occurrence flows with subordinate diabase, hyaloclastite breccia, and near the top of the volcanic section implies shoaling of the sedimentary intercalations (Shervais & Kimbrough, volcano and a lowering seawater confining pressure on 1987; Shervais & Hanan, 1989; Zoglman, 1991). Lensoid the magma. intercalations of red radiolarian chert and pink siliceous mudstone up to 1 km long and 50 m thick occur through- Chert out the section; limestone layers are smaller and less Thick, extensive chert deposits occur throughout the common. Although some folding is evident, most pillow SFVC; near the top of the section, pink siliceous mud- lavas dip at moderate angles (40–60) to the NE or east, stones are common. Limestone is scarce, forming small with tops in the same direction. Field relations suggest lenses <1 m thick. There are three thick chert lenses that that the volcanic complex represents a relatively intact are map-scale in size, which have been sampled extens- submarine volcano that has been tilted to the NE and ively for biostratigraphic age determinations (Shervais disrupted by high-angle faults. et al., 2004). These are labeled Chert A, Chert B, and Chert C in Fig. 2. A fourth layer, Chert D, crops out at Lavas the famous ‘Diversion Dam’ locality (Fig. 2). This layer Lava flows in the SFVC include both pillow lava and (and the intercalated pillow lava) has been extensively sheet flows, but massive sheet flows seem to the dominant affected by hydrothermal fluids that jasperized the flow type. The base of the seamount sequence, which lies chert, forming massive ochre-colored jasper; radiolaria to the SW and west, is dominated by massive flows are recrystallized so that they can no longer be identified of oceanic tholeiites, with intercalations of pillow lava. with confidence. Pillow lava becomes more common midway through Chert A, which crops out just north of Stony Creek, is the section and is commonly found intercalated with the thickest layer and also the most extensive, forming a

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mappable lens over 1 km long and up to 110 m thick, 3.5 OT Suite (a) including basalt intercalations. The chert in this layer is 3.0 manganiferous, and was exploited by a small Mn mining Alkali Suite operation. Chert B crops out above Dry Creek, very close 2.5 to the hyaloclastite breccias SFVG-1 to SFVG-4 2.0 Alkali Glass (Glass 1–4, Fig. 2). This chert horizon is about 20 m TiO2 thick and is well exposed along Black Diamond road. 1.5 High Al – Chert C crops out along Black Diamond Creek, near 1.0 Low Ti Suite the faulted margin of the complex, and in places it 0.5 appears to be in fault contact with basalt underlying it. Chert C includes both ribbon chert and pink siliceous 0.0 12 14 16 18 20 22 24 mudstone at least 4 m thick that contains rip-up clasts of the ribbon chert. Because of its faulted base a thickness Al2O3 could not be determined. 3.5

Age of the SFVC 3.0 Alkali Shervais et al. (2005a) presented age data on the CRO in 2.5 Basalts OT Suite northern California, including several dates for the SFVC 2.0 and adjacent rocks. 40Ar–39Ar dates on volcanic glass TiO2 Alkali from the hyaloclastite breccias range from 163 Æ 0Á7to 1.5 Glass High Al – 164Á7 Æ 0Á8 Ma, a narrow time interval corresponding to 1.0 Low Ti Suite Bathonian on the time scale of Palfy et al. (2000). Cherts 0.5 intercalated with the volcanic rocks contain radiolarian (b) assemblages that imply a somewhat longer duration, ran- 0.0 ging from Bajocian in the older cherts to Kimmeridgian 0.5 1.0 1.5 2.0 2.5 3.0 3.5 or Tithonian in the youngest cherts. This corresponds to FeO*/MgO absolute ages ranging from c. 166–170 Ma near the base 3.5 to c. 150–155 Ma near the top on the time scale of Palfy OT Suite 3.0 et al. (2000). Alkali Suite Quartz diorite me´lange blocks that structurally under- 2.5 lie the SFVC yield U–Pb zircon concordia intercept ages 2.0 of 163Á5 Æ 3Á9 Ma and 164Á8 Æ 4Á8 Ma (all uncertainties TiO 2 Alkali Glass 2s). 207Pb/238U ages are similar to previously reported 1.5 OceanicTholeiite Alkali basalt zircon ages for the CRO, ranging from 163 to 166 Ma 1.0 Alkali glass (Shervais et al., 2005a). These ages are essentially the High Al – HALT suite same as the volcanic glass and chert ages reported 0.5 Low Ti Suite (c) above, but may range to somewhat older ages. All of 0.0 the age data reported for the SFVC by Shervais et al. 0 10 20 30 40 50 (2005a) correspond to previously reported ages for the Nb ppm CRO, and cherts within the SFVC preserve the same Fig. 3. Geochemical definition of magma suites: (a) TiO vs Al O ; (b) fauna variations as observed in cherts of the CRO. 2 2 3 TiO2 vs FeO/MgO; (c) TiO2 vs Nb. Oceanic tholeiites are high in TiO2 at a range of FeO/MgO values, alkali basalts and glasses are high in Nb, whereas the HALT suite is low in TiO2 and high in Al2O3. PETROGRAPHY AND MINERALOGY Volcanic rocks of the SFVC comprise three distinct pet- rological groups based on their petrographic character- These distinctions are illustrated in Fig. 3, which istics and whole-rock geochemistry: (1) oceanic tholeiites compares their TiO2, and Al2O3, FeO*/MgO and Nb with 1Á9–3Á3 wt % TiO2, 14 ppm Nb, Zr/Y <4, contents. Oceanic tholeiites form most of the complex Ti/V ¼ 27–32, and La/Smn ¼ 0Á67–0Á9; (2) transitional and are found at all levels of exposure. Alkali basalts alkali basalts and glasses with 1Á7–2Á9 wt % TiO2, and basaltic glass are most common near the top of 10–44 ppm Nb, Zr/Y ¼ 2Á4–7Á8, Ti/V ¼ 44–58, and the seamount complex as interbedded pillow lava– La/Smn ¼ 1Á1–2Á4; (3) high-Al, low-Ti tholeiites with hyaloclastite breccia sequences, but are also found <1Á5 wt % TiO2, 16 ppm Nb (most <10 ppm Nb), beneath a thick chert lens low in the section, and Zr/Y ¼ 0Á8–7, Ti/V ¼ 15–28, La/Smn ¼ 0Á34–1Á8, commonly form pillows lower in the section. High-Al, and 15–24 wt % Al2O3. low-Ti (HALT) lavas occur sporadically throughout

6 SHERVAIS et al. THE STONYFORD VOLCANIC COMPLEX

(a) (b)

(c) (d)

(e) (f)

Fig. 4. Photomicrographs of SFVC basalts. (a, b) Oceanic tholeiite basalt SF-10; plagioclase laths with intergranular to hyalophitic clinopyroxene, opaque oxides, and altered glass; field of view is 2Á8 mm, (a) plane-polarized light, (b) crossed Nicols. (c, d) Alkali basalt SFV-66-2; plagioclase laths with intergranular clinopyroxene and opaque oxides; field of view 4Á6 mm, (c) plane-polarized light, (d) crossed Nicols. (e, f ) High-Al, low-Ti basalt SFV-111-1; abundant plagioclase laths with intersertal glass; minor clinopyroxene and opaque oxides; field of view 2Á8 mm, (e) plane-polarized light, (f ) crossed Nicols. the complex, and are also interbedded with hyaloclastites ilmenite (Fig. 4a and b). Chemical modes calculated using of alkali basalt. Some high-Al basalts are plagioclase the least-squares mixing program Genmix (Le Maitre, megaphyric, but most are aphyric, so their high Al2O3 1981) show that average modal abundances of these contents cannot be explained by simple plagioclase rocks are 50% plagioclase, 38% clinopyroxene, 5Á5% accumulation. titanomagnetite, 3Á6% olivine, and 2Á3% ilmenite. Clinopyroxene forms colorless, non-pleochroic, granu- lar to sub-ophitic grains of augite (Wo En , mean Petrography 30–40 36–53 Wo36 En46; mg-number 72) (Fig. 4). The pyroxenes are Oceanic tholeiites fresh and homogeneous, and range in size from 0Á01 mm Oceanic tholeiites are the most abundant of the three to 1Á0 mm in diameter. Alteration of pyroxenes is to basalt suites. They crop out as black to reddish brown chlorite, magnetite, actinolite, and epidote. Plagioclase pillows and massive flows that form red to brick red soils. feldspar forms euhedral to subhedral laths, 0Á05 mm to These basalts are predominantly aphyric or microphyric 1Á25 mm long ranging in composition from An27 to An69; with equigranular to intergranular groundmass consist- larger phenocrysts range from An61 to An91 (Fig. 6). ing of plagioclase, clinopyroxene, titanomagnetite, and Secondary mineral replacement may occur along the

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Di Hd Stonyford Pyroxenes Di Hd

Di Hd

(a) Tholeiite series (b) Alkali basalt En series En (c) HALT series

En Fs Fig. 5. Pyroxene quadilateral plots for the oceanic tholeiite, alkali basalt, and HALT suites. Oceanic tholeiite pyroxenes are augites, alkali basalt pyroxenes are salitic, and HALT pyroxenes are magnesian diopsides. edges of the grains or along the twin planes within the An An crystal. Many samples contain euhedral feldspar with Stonyford little or no alteration, whereas other samples show Feldspars An (a) complete alteration and replacement of the feldspar (c) by Ca-zeolites, albite, and prehnite. Pseudomorphs of OT olivine were identified in a few samples. They occur as series (b) microphenocrysts and represent <1% of the modal abundance of the sample. The grains are 0Á05–0Á3mm in size, with complete replacement by chlorite and clay minerals. Titanomagnetite occurs as granular crystals ranging in size from 0Á005 mm to 0Á7 mm, found in HALT series the intergranular spaces between the plagioclase and Ab pyroxene grains. Ab Or Alkali basalts Ab Alkali basalt Or Alkali basalts are the second most common volcanic suite Fig. 6. Plagioclase Ab–Or–An ternary plots for the oceanic tholeiite in the SFVC. They are most common towards the top of suite (a), alkali basalt suite (b), and HALT suite (c). the volcanic sequence, but are found at all stratigraphic levels and underlie the lowermost chert lens in the sea- (<0Á75 mm long) is subhedral to anhedral, with composi- mount. The average chemical mode of the alkali basalts, tions ranging from An24 to An70 where fresh, and

8 SHERVAIS et al. THE STONYFORD VOLCANIC COMPLEX

basaltic glass, with <2% modal microphenocrysts of times of 20–100 s, using natural and synthetic mineral olivine, plagioclase, and Cr-spinel; no clinopyroxene standards. Analyses were corrected for instrumental drift, microphenocrysts were observed. All microphenocrysts deadtime, and electron beam/matrix effects using the are euhedral and show no indications of resorption phi–rho–Z correction procedure provided with the or textural disequilibrium. Plagioclase (An72–80)and Cameca microprobe automation system (Pouchou & Cr-spinel remain relatively unaltered, but the olivine Pichoir, 1991). Relative accuracy of the analyses, based is typically replaced by smectites. Rare relict olivine is upon comparison between measured and published Fo85–87 in composition (Shervais & Hanan, 1989). compositions of the standards, is 1–2% for oxide concentrations >1 wt % and 10% for oxide concentra- tions <1 wt %. Mineral compositions are presented in High-Al, low-Ti (HALT) basalts Tables 1–3. These basalts include both plagioclase-megaphyric lavas that have accumulated feldspar and aphyric lavas with no feldspar accumulation. The Genmix-calculated chemical Pyroxene modes are 5–7% olivine, 60–77% plagioclase, 8–30% Clinopyroxene is ubiquitous in all volcanic rocks of the pyroxene, 5–7% titanomagnetite, and 0–1% ilmenite. SFVC except for volcanic glass, where it is unknown Plagioclase phenocrysts compose anywhere from 2% to (Table 1). Pyroxene is typically unaffected by 30% of the mode in the porphyritic basalts. The ground- low-temperature hydrothermal alteration and its com- mass of the plagioclase-phyric rocks is typically inter- positions reflect primary magmatic compositions. Clino- granular feldspar and pyroxene with one sample pyroxene varies from augite in the oceanic tholeiite series containing 10% devitrified glass. The aphyric samples (Wo3040 En36–53) to titanian diopside or salite in the alkali exhibit textures ranging from equigranular to sub-ophitic basalt series (Wo31–46 En24–57); pyroxenes in the HALT and poikilitic. series are magnesian augites (Wo31–45 En38–65). Pyroxene Clinopyroxene forms subhedral granular to sub-ophitic quadrilateral compositions are shown in Fig. 5. grains of magnesian augite (Wo31–45 En38–65; mean Wo39 Pyroxene minor element contents vary to some extent En50) with an average mg-number of 78 (Fig. 5), which with both fractionation [mg-number ¼ 100 · Mg/(Mg þ range in size from 0Á01 mm to 2 mm. Feldspar pheno- Fe2þ)] and magma series (Fig. 7). Pyroxenes in the HALT crysts in the HALT suite range up to 5 mm across with series are the most primitive, with mg-numbers ranging compositions in the range An70–92Á5 (Fig. 6). The less from 70 to 87 in most cases, and Cr2O3 contents as high altered samples have euhedral plagioclase phenocrysts as 1Á5%. Pyroxenes of the oceanic tholeiite series and and lack zoning or albitic rims. More altered samples alkali basalt series have lower mg-numbers and generally contain phenocrysts with albitic rims or anhedral lower Cr2O3, although some oceanic basalt Cpx has grains completely altered to clay minerals. Some pheno- Cr2O3 as high as 0Á8 wt % (Fig. 7). TiO2 and Na2O crysts contain altered glass inclusions. The ragged edges contents increase with decreasing mg-number in all ser- of anhedral grains appear to have been resorbed as the ies, and increase between series from HALT to oceanic remaining melt crystallized. Groundmass plagioclase tholeiite to alkali basalt. Alumina shows little correlation (<0Á5 mm long) comprises 50% of the groundmass in with either mg-number or magma series; some of this the porphyritic samples, with compositions that range scatter may relate to cooling rate in groundmass pyrox- from An31 to An79. In the aphyric basalts, plagioclase ene (e.g. Coish & Taylor, 1979). accounts for 65% of the mode with an average com- Crystal chemical substitutions among non- position of An65. Plagioclase

9 Table 1: Representative pyroxene analyses from the Stonyford volcanic complex, California, by electron microprobe

Sample: SFV-84-1 SFV-124-1 SFV-65-1 SFV-65-1 SFV-18-5 SFV-29-1 SFV-29-1 SFV-92-1 SFV-91-1 SFV-7-1 SFV-7-1 SFV-8-1 SFZ 8-1 SFV-110A SFV-110A SFV-37-2 SFV-37-2 SFV-87-1 Suite:OTOTOTOTOTOTOTOTOTOTOTOTOTOTOTOTOTOT

SiO2 52.05 49.53 51.16 50.85 50.91 52.40 52.74 52.08 52.16 51.22 50.31 51.95 51.12 51.35 49.91 49.97 51.02 50.81

TiO2 0.70 1.54 0.90 0.89 0.82 0.57 0.47 0.62 0.71 0.79 1.02 0.63 0.76 0.81 1.29 1.31 0.81 0.88

Al2O3 2.31 4.30 2.53 2.57 3.17 1.36 1.49 1.82 2.58 3.12 4.00 2.21 2.91 2.24 4.60 3.28 2.70 2.47 FeO 8.17 10.03 10.09 9.18 8.21 10.62 8.96 10.01 8.59 9.77 9.94 8.00 8.17 11.03 9.83 9.17 8.30 11.61 MnO 0.330 0.211 0.281 0.186 0.894 1.144 0.933 1.061 0.21 0.275 0.184 0.234 0.118 0.365 0.154 0.233 0.204 0.35 MgO 17.00 15.41 15.91 15.17 16.47 17.71 18.16 16.92 18.02 17.53 16.33 16.76 16.23 16.96 15.93 15.32 16.24 16.60 CaO 18.83 17.87 17.53 19.77 18.40 16.25 17.20 17.22 17.66 16.17 16.77 19.36 19.07 16.58 17.78 19.05 19.16 16.62

Na2O0.24 0.27 0.24 0.30 0.28 0.18 0.19 0.22 0.19 0.20 0.22 0.24 0.23 0.22 0.28 0.31 0.22 0.23 PETROLOGY OF JOURNAL

Cr2O3 0.120 0.107 0.087 0.105 0.694 0.011 0.061 0.062 0.09 0.162 0.239 0.155 0.166 0.077 0.369 0.418 0.358 0.06 Sum 99.75 99.26 98.73 99.02 99.85 100.25 100.22 100.02 100.21 99.22 99.02 99.53 98.78 99.66 100.14 99.05 99.03 99.63 Wo 37.134.935.239.736.232.233.934.334.931.332.538.237.832.534.037.937.933.3 En 49.547.647.845.049.850.751.849.351.452.350.348.748.549.449.046.548.347.9 10 mg-no. 78.773.273.874.678.174.878.375.178.976.274.578.978.073.374.374.977.771.8

Sample: SFV-85-1 SFV-66-2 SFV-66-2 SFV-65-2 SFV-89-2 SFV-89-2 SFV-46-1 SFV-10-6 SFV-10-6 SFV-10-6 SFV-10-6 SFV-10-6 SFV-10-8 SFV-10-8 SFV-10-8 SFV-10-8 SFV-10-8 SFV-10-8 Suite:ABABABABABABABABABABABABABABABABABAB

SiO2 42.99 47.25 46.65 45.21 51.00 50.25 47.63 46.73 48.41 46.04 48.46 47.72 43.11 44.10 42.61 43.30 42.66 45.19

TiO2 5.34 2.61 2.94 3.31 0.69 0.83 2.11 2.92 1.53 3.03 1.72 2.28 4.17 3.92 4.05 4.28 4.75 3.25

Al2O3 8.79 4.63 5.39 7.54 4.19 5.06 5.46 6.05 2.76 6.44 3.32 3.97 6.00 5.77 6.96 6.56 6.13 7.30 FeO 14.02 9.26 8.55 7.92 6.36 6.19 8.39 9.41 15.72 9.07 12.58 11.84 13.21 13.74 12.41 13.03 13.60 11.38 MnO 0.304 0.240 0.192 0.831 0.667 0.648 0.853 0.212 0.439 0.249 0.365 0.255 0.33 0.24 0.21 0.25 0.23 0.24 MgO 6.13 12.33 12.53 12.00 16.89 16.08 13.22 12.24 10.08 12.20 11.41 11.70 9.53 9.69 9.80 9.61 9.25 9.93 CaO 19.91 21.40 21.61 22.58 19.67 20.21 21.61 21.39 19.84 21.66 20.88 21.05 21.24 20.79 21.45 21.35 20.93 20.48

Na2O1.50 0.43 0.45 0.43 0.20 0.24 0.38 0.45 0.40 0.43 0.39 0.42 0.51 0.52 0.57 0.53 0.54 1.03

Cr2O3 0.035 0.000 0.039 0.038 0.257 0.201 0.058 0.000 0.003 0.001 0.079 0.089 0.04 0.03 0.05 0.04 0.05 0.00 Sum 99.21 98.14 98.35 99.86 99.93 99.71 99.71 99.40 99.19 99.13 99.23 99.34 98.13 98.81 98.12 98.96 98.15 99.00 Wo 45.744.144.145.138.039.243.244.742.945.044.344.445.744.545.945.745.345.5 En 23.839.440.440.051.250.041.938.630.438.834.435.530.630.931.630.930.033.2 mg-no. 43.870.472.373.082.582.273.869.953.370.661.863.856.355.758.556.854.860.9 SHERVAIS

Sample: SFV-88-1 SFV-111-1 SFV-111-1 SFV-111-1 SFV-122-1 SFV-106-1 SFV-106-1 SFV-47-1 SFV-18-1 SFV-18-1 SFV-37-1 SFV-37-1 SFV-112-1 SFV-112-1 SFV-SFZ-49-1 SFV-SFZ-49-1 SFV-123-1 SFV-123-1 tal et Suite HALT HALT HALT HALT HALT HALT HALT HALT HALT HALT HALT HALT HALT HALT HALT HALT HALT HALT .

SiO2 52.79 50.56 50.96 51.26 50.88 52.34 52.54 47.13 50.16 50.20 50.69 51.15 51.35 52.58 52.16 53.11 53.28 53.24 H TNFR OCNCCOMPLEX VOLCANIC STONYFORD THE

TiO2 0.33 0.89 1.02 0.89 0.42 0.30 0.41 2.21 0.83 0.78 0.44 0.51 0.58 0.35 0.33 0.32 0.23 0.22

Al2O3 2.34 4.09 3.58 3.61 2.19 2.03 1.89 5.06 4.64 4.51 3.14 2.93 3.16 1.65 2.64 1.73 1.56 1.49 FeO 4.91 6.63 7.00 7.35 10.30 4.88 6.19 10.40 6.39 7.87 8.89 7.43 8.38 9.50 6.29 6.58 6.29 6.60

11 MnO 0.146 0.204 0.220 0.198 0.27 0.141 0.221 0.267 0.661 0.834 0.925 0.790 0.857 0.940 0.638 0.653 0.645 0.691 MgO 18.42 15.88 15.91 15.92 16.58 17.84 18.10 12.19 15.60 15.07 14.93 16.20 17.16 18.86 16.71 17.71 19.33 19.38 CaO 20.31 20.78 20.80 20.60 18.12 20.59 19.60 20.84 20.90 20.25 20.02 19.88 18.34 16.15 20.66 20.04 18.59 18.29

Na2O0.18 0.24 0.22 0.27 0.16 0.20 0.19 0.34 0.22 0.19 0.26 0.17 0.22 0.16 0.23 0.22 0.15 0.13

Cr2O3 0.576 0.109 0.073 0.005 0.07 0.641 0.071 0.210 0.429 0.403 0.262 0.123 0.045 0.000 0.503 0.001 0.163 0.083 Sum 100.00 99.38 99.80 100.10 98.97 98.96 99.21 98.65 99.82 100.11 99.55 99.19 100.09 100.19 100.16 100.36 100.23 100.11 Wo 39.240.740.840.336.140.438.442.240.839.639.939.335.531.340.639.335.935.4 En 52.948.047.547.447.451.751.739.148.146.745.048.250.653.549.150.254.254.3 mg-no. 87.081.080.279.474.186.783.967.781.377.474.979.578.578.082.682.884.683.9

OT, oceanic tholeiite suite; AB, alkali basalt suite; HALT, high-Al, low-Ti suite; Wo, wollastonite component; En, enstatite component (corrected for non-quadrilateral components); mg-number, 100 · molar Mg/(Mg þ Fe). Table 2: Representative plagioclase analyses from the Stonyford volcanic complex, California, by electron microprobe

Sample: SFV-114-1 SFV-114-1 SFV-124-1 SFV-124-1 SFV-SFZ-84-1 SFV-SFZ-84-1 SFV-65-1 SFV-65-1 SFV-18-5 SFV-18-5 SFV-29-1 SFV-92-1 SFV-92-1 SFV-91-1 SFV-91-1 SFV-SFZ-58-1 SFV-110A SFV-37-2 SFV-7-1 Suite:OTOTOTOTOTOTOTOTOTOTOTOTOTOTOTOTOTOTOT

SiO2 52.86 52.88 52.08 53.43 63.47 53.20 50.33 53.25 52.92 51.66 33.56 45.72 49.62 46.89 46.98 52.99 68.26 58.84 54.34 Al2O3 29.45 29.27 29.37 28.40 19.75 29.04 31.18 29.02 29.59 30.85 46.65 34.96 31.39 32.39 33.66 28.71 20.81 25.24 27.95 Fe2O3 1.06 0.70 0.99 1.03 3.63 0.98 0.49 0.62 0.72 0.56 0.46 0.36 0.60 0.48 0.43 0.90 0.01 0.74 1.28 MgO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 CaO 12.84 12.65 12.97 12.21 1.69 12.38 14.41 11.91 12.97 14.14 17.41 18.88 15.71 16.52 17.47 11.73 0.87 7.37 11.60

Na2O4.08 3.97 3.90 4.41 9.86 4.34 3.12 4.49 3.92 3.40 1.81 1.01 2.50 2.06 1.62 4.69 9.66 6.71 4.55 K2O0.17 0.04 0.02 0.05 0.36 0.06 0.15 0.25 0.27 0.20 0.00 0.02 0.02 0.02 0.02 0.01 0.03 0.12 0.03 Sum 100.46 99.52 99.32 99.55 98.75 99.99 99.68 99.53 100.39 100.81 99.90 100.95 99.85 98.37 100.19 99.03 99.65 99.01 99.75 An 62.87 63.60 64.65 60.27 8.47 60.96 71.22 58.62 63.66 68.88 84.13 91.04 77.53 81.50 85.53 58.00 4.74 37.51 58.38 Or 1.02 0.26 0.13 0.32 2.15 0.36 0.87 1.44 1.55 1.18 0.01 0.13 0.13 0.10 0.11 0.06 0.21 0.70 0.17

Sample: SFV-8-1 SFV-8-1 SFV-8-1 SFV-9-1 SFV-85-1 SFV-85-1 SFV-66-2 SFV-66-2 SFV-65-2 SFV-89-2 SFV-46-1 SFV-46-1 SFV-10-8 SFV-10-6 Glass Glass Glass SFV-106-1 SFV-106-1 PETROLOGY OF JOURNAL Suite: OT OT OT OT AB AB AB AB AB AB AB AB AB AB Glass Glass Glass HALT HALT

SiO2 52.51 53.24 55.06 54.60 64.65 55.10 53.32 52.02 47.54 50.57 68.32 52.21 64.39 63.91 49.282 49.19 49.008 46.43 46.23 Al2O3 29.24 28.51 27.17 27.82 21.88 28.38 29.19 30.31 33.50 30.98 21.21 29.32 20.00 19.24 32.042 32.057 32.89 34.17 34.16 ...... 12 Fe2O3 0 82 0 92 1 16 1 22 0 03 0 89 0 93 0 79 0 41 0 67 0 32 0 81 0 46 0 17 0 394 0 409 0 441 0 42 0 42 MgO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.194 0.179 0.174 0.00 0.00 CaO 12.67 12.27 10.66 11.03 2.35 11.16 12.44 13.78 17.53 14.76 1.75 12.36 0.24 0.06 15.576 15.679 15.92 17.94 18.09

Na2O3.91 4.20 4.95 4.96 9.70 4.81 4.14 3.42 1.68 3.07 9.98 4.39 1.70 1.15 2.269 2.239 2.11.36 1.36 K2O0.03 0.04 0.09 0.09 0.04 0.44 0.06 0.03 0.03 0.03 0.04 0.01 12.33 14.52 0.20.104 0.095 0.01 0.02 Sum 99.18 99.18 99.09 99.72 98.65 100.78 100.08 100.35 100.68 100.08 101.62 99.09 99.11 99.04 99.96 99.86 100.63 100.32 100.28 An 64.05 61.57 54.05 54.86 11.76 54.74 62.18 68.90 85.13 72.49 8.83 60.85 1.30 0.30 78.20 78.97 80.27 87.90 87.91 Or 0.21 0.26 0.53 0.54 0.22 2.56 0.38 0.16 0.15 0.18 0.24 0.05 81.61 88.99 1.195 0.621 0.571 0.06 0.09

Sample: SFV-106-1 SFV-106-1 SFV-111-1 SFV-111-1 SFV-122-1 SFV-122-1 SFV-122-1 SFV-88-1 SFV-88-1 SFV-37-1 SFV-94-1 SFV-94-1 SFV-94-1 SFV-112-1 SFV-SFZ-49-1 SFV-SFZ-49-1 SFV-123-1 SFV-123-1 SFV-123-1 Suite: HALT HALT HALT HALT HALT HALT HALT HALT HALT HALT HALT HALT HALT HALT HALT HALT HALT HALT HALT

SiO2 51.51 46.11 55.03 54.37 45.83 45.59 49.46 44.76 44.73 50.18 46.14 46.76 49.92 53.04 52.38 54.00 50.70 50.52 52.86 Al2O3 30.32 33.90 26.67 27.65 34.69 34.21 31.85 35.30 34.95 31.38 34.78 34.02 30.95 29.32 30.37 29.09 31.36 31.53 29.88 Fe2O3 0.73 0.51 1.18 1.26 0.39 0.39 0.88 0.38 0.32 0.63 0.45 0.38 0.76 1.07 0.84 0.96 0.63 0.67 0.83 MgO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 CaO 13.73 17.78 9.96 11.05 18.50 18.22 15.82 18.82 18.78 15.76 18.52 18.22 15.13 13.19 13.52 12.21 15.32 15.49 13.09

Na2O3.45 1.51 5.51 5.02 1.10 1.11 2.40 0.83 0.86 2.57 1.18 1.31 2.85 3.86 3.66 4.41 2.76 2.79 3.90 K2O0.10 0.02 0.09 0.08 0.00 0.02 0.02 0.01 0.01 0.00 0.01 0.01 0.02 0.07 0.11 0.15 0.03 0.02 0.14 Sum 99.83 99.83 98.43 99.43 100.51 99.54 100.43 100.09 99.65 100.51 101.07 100.71 99.62 100.54 100.87 100.82 100.80 101.02 100.70 An 68.33 86.53 49.70 54.64 90.27 89.96 78.38 92.54 92.31 77.23 89.66 88.40 74.53 65.13 66.71 59.95 75.31 75.33 64.45 Or 0.61 0.14 0.51 0.47 0.00 0.13 0.09 0.05 0.08 0.02 0.04 0.07 0.10 0.41 0.64 0.89 0.18 0.10 0.80

OT, oceanic tholeiite suite; AB, alkali basalt suite; Glass, alkali basalt volcanic glass; HALT, high-Al, low-Ti suite; An, anorthite component; Or, orthoclase component. Table 3: Representative spinel analyses from the Stonyford volcanic complex, California, by electron microprobe

Sample: SFVG3x3 SFVG3 SFVG3 SFVG3 SFVG3 SFVG3 SFVG2 SFVG2 SFVG2 SFVG2 SFVG2 SFVG2 SFVG2 SFVG2 SHERVAIS

TiO2 0.89 1.10 1.15 1.02 1.04 1.10 1.05 1.06 1.05 1.09 1.10 1.12 0.94 1.03

Al2O3 34.70 34.05 33.84 35.21 37.41 34.78 33.98 34.94 35.20 34.98 35.64 36.35 35.44 34.53

Fe2O3 6.42 8.00 7.80 6.92 6.50 7.03 7.12 6.95 7.02 7.58 7.59 7.84 6.60 7.61

FeO 13.08 13.42 13.43 13.14 13.18 13.11 13.22 12.92 13.32 13.05 13.34 12.96 13.18 12.82 al et ...... MnO 0 18 0 16 0 16 0 17 0 15 0 23 0 14 0 16 0 19 0 21 0 23 0 20 0 16 0 22 . MgO 16.28 16.11 16.11 16.26 16.62 16.14 16.22 16.44 16.27 16.37 16.28 16.65 16.33 16.44

CaO 0.03 0.00 0.04 0.08 0.03 0.13 0.14 0.13 0.05 0.09 0.03 0.04 0.02 0.06 COMPLEX VOLCANIC STONYFORD THE

Cr2O3 28.24 26.94 27.26 26.53 24.94 26.35 27.97 26.95 26.80 26.30 25.58 24.69 27.11 26.95 Sum 99.85 99.82 99.79 99.39 99.91 98.93 99.92 99.62 99.94 99.73 99.82 99.87 99.83 99.70

13 Atomic formula units per 32 oxygens Ti 0.1530 0.1910 0.2000 0.1770 0.1780 0.1930 0.1830 0.1830 0.1810 0.1880 0.1900 0.1930 0.1620 0.1780 Al 9.4220 9.2860 9.2390 9.5790 10.032 9.5170 9.2520 9.4860 9.5370 9.4950 9.6510 9.7920 9.5960 9.3880 Fe3þ 1.1130 1.3930 1.3600 1.2010 1.1130 1.2280 1.2380 1.2050 1.2140 1.3140 1.3110 1.3480 1.1410 1.3210 Fe2þ 2.5200 2.5970 2.6020 2.5370 2.5070 2.5460 2.5540 2.4890 2.5600 2.5140 2.5640 2.4770 2.5320 2.4730 Mn 0.0350 0.0320 0.0320 0.0330 0.0290 0.0440 0.0270 0.0320 0.0370 0.0400 0.0450 0.0380 0.0320 0.0430 Mg 5.5920 5.5570 5.5620 5.5940 5.6350 5.5860 5.5850 5.6460 5.5760 5.6200 5.5750 5.6710 5.5900 5.6530 Ca0.0060 0.0000 0.0090 0.0210 0.0080 0.0320 0.0340 0.0330 0.0130 0.0220 0.0070 0.0090 0.0060 0.0140 Cr5.1450 4.9290 4.9940 4.8420 4.4860 4.8370 5.1100 4.9090 4.8710 4.7890 4.6470 4.4610 4.9240 4.9160 Sum 23.995 23.996 24.000 23.998 23.999 24.000 24.001 23.999 23.997 24.004 24.000 23.997 23.998 23.997 Mg/(Mg þ Fe2þ)68.64 67.88 67.86 68.52 68.96 68.32 68.39 69.13 68.22 68.75 68.12 69.28 68.56 69.20 Cr/(Cr þ Al) 35.32 34.67 35.09 33.58 30.90 33.70 35.58 34.10 33.81 33.53 32.50 31.30 33.91 34.37 Fe3/R3 7.10 8.92 8.72 7.69 7.12 7.88 7.94 7.72 7.77 8.42 8.40 8.64 7.29 8.45 Table 3: continued

Sample: SFVG2 SFVG2 SFVG2 SFVG1 SFVG3 SFVG2 SFVG2 SFVG2 SFVG2 SFVG2 SFVG3x3 SFVG3 SFV-65-2 SFV-18-1

TiO2 1.02 1.03 1.05 1.05 1.26 1.02 1.05 1.05 1.14 1.04 0.86 0.84 1.26 0.34

Al2O3 34.30 35.45 34.71 34.77 35.24 34.66 34.76 35.26 37.62 33.35 34.93 34.78 31.95 29.99

Fe2O3 6.82 7.02 7.62 7.51 6.73 6.96 7.45 6.62 7.03 7.44 6.74 6.99 9.79 6.05 FeO 13.35 13.41 13.04 13.34 13.40 13.20 13.09 13.34 12.98 12.86 12.86 12.44 12.94 12.01 MnO 0.17 0.16 0.14 0.09 0.23 0.19 0.22 0.20 0.13 0.26 0.18 0.16 2.22 1.73 MgO 16.09 16.23 16.31 16.15 16.12 16.18 16.33 16.17 16.77 16.21 16.39 16.51 14.15 15.35 ORA FPETROLOGY OF JOURNAL CaO 0.03 0.01 0.05 0.07 0.21 0.09 0.09 0.12 0.03 0.03 0.04 0.16 0.00 0.00

Cr2O3 27.77 26.46 26.43 26.35 25.88 27.15 26.87 26.75 23.88 28.14 27.64 27.20 27.27 36.13 Sum 99.57 99.83 99.40 99.41 99.13 99.51 99.93 99.57 99.62 99.36 99.67 99.15 99.58 101.60 Atomic formula units per 32 oxygens

14 Ti 0.1780 0.1770 0.1820 0.1830 0.2200 0.1770 0.1810 0.1830 0.1940 0.1810 0.1490 0.1470 0.2200 0.0600 Al 9.3630 9.6050 9.4590 9.4810 9.6130 9.4460 9.4310 9.5820 10.093 9.1460 9.4860 9.4800 8.8800 8.2100 Fe3þ 1.1880 1.2140 1.3250 1.3080 1.1720 1.2120 1.2900 1.1490 1.2040 1.3030 1.1680 1.2170 1.7400 1.0600 Fe2þ 2.5860 2.5780 2.5220 2.5820 2.5950 2.5530 2.5210 2.5720 2.4720 2.5030 2.4780 2.4070 2.5500 2.3400 Mn 0.0330 0.0300 0.0280 0.0180 0.0450 0.0370 0.0430 0.0390 0.0240 0.0510 0.0350 0.0310 0.4400 0.3400 Mg 5.5550 5.5610 5.6220 5.5710 5.5620 5.5750 5.6050 5.5580 5.6900 5.6210 5.6290 5.6920 4.9700 5.3200 Ca0.0080 0.0020 0.0110 0.0180 0.0530 0.0210 0.0220 0.0290 0.0060 0.0080 0.0110 0.0400 0.0000 0.0000 Cr5.0860 4.8090 4.8320 4.8210 4.7370 4.9650 4.8920 4.8770 4.2990 5.1760 5.0350 4.9740 5.0900 6.6400 Sum 23.999 23.996 23.999 23.999 24.007 24.006 24.005 24.007 23.995 23.997 24.001 24.002 23.890 23.970 Mg/(Mg þ Fe2þ)67.96 68.07 68.80 68.18 67.81 68.28 68.61 68.04 69.51 68.76 69.14 70.01 66.10 69.45 Cr/(Cr þ Al) 35.20 33.36 33.81 33.71 33.01 34.45 34.15 33.73 29.87 36.14 34.67 34.41 36.63 44.71 Fe3/R3 7.60 7.77 8.48 8.38 7.55 7.76 8.26 7.36 7.72 8.34 7.44 7.77 11.08 6.66

SFVB3x3, basaltic clast in breccia from location 3; SFVG, alkali basalt glass suite; SFV-65-2 and SFV-18-1, oceanic tholeiite suite. SHERVAIS et al. THE STONYFORD VOLCANIC COMPLEX

1.5 10.0 (a) Tholeiite Cpx (b) Alkali Cpx 8.0 1.0 HALT Cpx wt%

wt% 6.0 3 3 O O 2

2 4.0 0.5 Al Cr 2.0

0.0 0.0 90 80 70 60 50 40 90 80 70 60 50 40 mg# mg# 6.0 1.5 (c) 5.0 (d) Alkali 4.0 1.0

wt% 3.0 2 O wt%

Tholeiite 2 Alkali TiO 2.0 Na 0.5

1.0 Tholeiite HALT HALT 0.0 0.0 90 80 70 60 50 40 90 80 70 60 50 40 mg# mg#

Fig. 7. Pyroxene mg-number vs (a) Cr2O3, (b) Al2O3, (c) TiO2, and (d) Na2O. Pyroxenes of the HALT suite have the highest mg-number and Cr, whereas all suites show a similar range in Al2O3 contents. TiO2 and Na2O increase with decreasing mg-number, in response to crystal fractionation; these oxides also increase between suites, from HALT to oceanic tholeiite to alkali basalt, reflecting primary magmatic compositions. show trends towards jadeite (Fig. 8c and d). Thus, acmite Plagioclase phenocrysts with compositions more calcic and jadeite are not important components in the oceanic than An80 are found in all magma series (Fig. 6). These tholeiite or HALT magma series, and form minor com- phenocrysts appear to be out of equilibrium with their ponents in the alkali basalt series. This suggests that host magmas, and may represent relict phenocrysts pre- sodium must be charge balanced by other coupled subsi- served from a more primitive magma that was mixed IV titutions, such as NaTiAl SiO6, as proposed by with a more evolved magma prior to eruption. Such Schweitzer et al. (1979). It is also consistent with the mixing events are common in oceanic magma systems observed absence of blueschist-facies mineral phases in (e.g. Stakes et al., 1984). any of the SFVC rocks studied to date. Olivine Feldspars Olivine is no longer present in the basalts, but pseudo- Feldspar is the most abundant of the major phases, with morphs of chlorite and clays after olivine are found. modes ranging from 50% to 75%. Primary feldspar com- Relict olivine of Fo85–87 composition is found within positions are commonly preserved, but feldspars are some glass lapilli (Shervais & Hanan, 1989), but it is more likely than pyroxene to be altered to lower- rare. These olivines form euhedral prisms that are in temperature assemblages. Plagioclase phenocrysts in all equilibrium with the host glass and are clearly cognate series range from An60 to An92 whereas groundmass phenocrysts (Table 1). plagioclase is typically more sodic, ranging from An24 to An70 (An79 in the HALT series; Fig. 6; Table 2). Oxides Groundmass plagioclase and the rims of phenocrysts The predominant accessory oxide phase is titanomag- are commonly ‘albitized’, with clear plagioclase

15 JOURNAL OF PETROLOGY

0.20 0.30 (a) (b) 0.25 Acmite 0.15 Ti/Al = 1/2 0.20 afu

0.10 3+ 0.15 Tschermakites Ti afu Ti 0.10 0.05 Tholeiite Cpx Alkali Cpx

Ti+Cr+Fe 0.05 HALT Cpx 0.00 0.00 0.0 0.2 0.4 0.0 0.2 0.4 AlIV afu AlIV afu 0.14 0.14 (c) Acmite 0.12 Acmite 0.12 (d) 0.10 0.10 Jadeite 0.08 0.08 0.06 0.06 Na afu 0.04 Na afu 0.04 0.02 0.02 0.00 0.00 0.00 0.04 0.08 0.00 0.04 0.08 0.12 0.16 3+ AlVI afu Fe afu Fig. 8. Pyroxene non-quadrilateral components, plotted to highlight crystal chemical effects in pyroxene substitutions: (a) Ti a.f.u. vs AlIV a.f.u.; (b) Ti þ Cr þ Fe3þ a.f.u. vs AlIV a.f.u.; (c) Na a.f.u. vs AlVI a.f.u.; (d) Na a.f.u. vs Fe3þa.f.u. The strong correlation among Ti, Cr, and Fe3þ vs AlIV implies that Ca-tschermakite molecules (with AlIV substitutions) are the dominant non-quadrilateral components. The poor correlation of Na vs AlVI and Fe3þ shows that neither jadeite nor acmite is an important substitution. throughout the crystallization sequence. Titanomagnetite quartz–chlorite breccias and metavolcanics of the appears at the very end of crystallization in the high-Al, Mid-Atlantic Ridge. Olivines are commonly altered, but low-Ti basalts. Ilmenite within the alkali basalts occurs as pyroxene and plagioclase are typically preserved. As a an early crystallizing phase, whereas titanomagnetite result, whole-rock geochemistry is only marginally appears in the later stages of crystallization. Ilmenite affected by low-temperature metamorphism. contains 2Á8–4Á7% MnO. Cr-spinel is rare in the volcanic rocks studied here, but relatively common in the volcanic glasses (Table 3). WHOLE-ROCK CHEMISTRY These spinels are all titaniferous magnesiochromites, Analytical methods which indicates alkali magmatic affinities (e.g. Sigurdsson & Schilling, 1976). One spinel analyzed from an alkali Whole-rock major element analyses were determined by basalt is also a titaniferous magnesiochromite. One spinel a fused glass bead technique for 10 major elements [SiO2, was analyzed from a high-Al, low-Ti basalt and found to TiO2,Al2O3, FeO* (total iron as FeO), MnO, MgO, be a chromian spinel. Spinel was not found in oceanic CaO, Na2O, K2O, and P2O5] using the Cameca SX-50 tholeiite series. electron microprobe at the University of South Carolina and natural glass standards (Shervais et al., 1990). Whole- rock powder was fused in an electric induction furnace Secondary phases with a nitrogen atmosphere using molybdenum foil boats; Secondary phases formed in response to low-temperature the resulting glass chips were mounted on glass micro- metamorphism include chlorite, albite, titanite, quartz, probe mounts and polished prior to analysis. The fused epidote, prehnite, pumpellyite, zeolites, smectites, and glass beads were analyzed using the same conditions calcite; actinolite is rare. Chlorites are aluminous, similar as for mineral analysis and standards selected for glass to chlorites in Franciscan metabasalts, and distinct from analysis. The natural Juan de Fuca glass VG2 was used

16 SHERVAIS et al. THE STONYFORD VOLCANIC COMPLEX

for most elements, supplemented by mineral standards concentrations being attributed to low-temperature for minor elements. Na was analyzed first to minimize hydrothermal alteration of the primary basalts. However, effects of diffusion. Repeated analyses of USGS whole- the volcanic glasses show no such effect, indicating that rock standards show that analytical uncertainty is <2% their concentrations of 3–4 wt % alkalis are primary relative for major elements and <5–10% relative for and probably represent the highest values expected in the minor elements (i.e. elements with concentrations of unaltered protoliths. Two alkali basalts with the highest <1 wt %; Shervais et al., 1990). total alkali contents (5Á4–7Á0 wt %) also have extremely Selected trace elements (Nb, Zr, Y, Sr, Rb, Zn, Cu, Ni, low CaO (<6Á2 wt %) and Cr (100 ppm), and the high- Cr, V, Sc, Ba) were analyzed by X-ray fluorescence est observed MgO (8Á6 wt %) concentrations (Fig. 9); (XRF ) spectrometry using a Phillips PW 1400 XRF sys- one sample also has extremely high Ba (6400 ppm). This tem equipped with a Rh tube. Powdered sample (6–8 g) combination of contrasting refractory and evolved char- was combined with 4–6 drops of a 2% polyvinyl alcohol acteristics suggests that these two samples have been solution, pressed into pellets, and dried overnight at seriously compromised by low-temperature hydro- 110C. Calibration was based on 12 well-characterized thermal alteration, including the exchange of Ca in the USGS rock standards. Additional trace elements [Th, Hf, protolith for Mg and alkalis in seawater. Most samples do Ta, Co, and the rare earth elements (REE)] were determ- not show these effects and may be used to reconstruct ined by instrumental neutron activation analysis (INAA) primary magma evolution. for 40 samples at the Oregon State University Radiation Center. Samples were irradiated in the OSU TRIGA reactor and counted following the procedures outlined Trace elements by Laul (1986). Analytical uncertainties are typically The oceanic tholeiite and alkali suites are generally 2–5% relative for most elements, except Nd (10%). higher in high field strength elements (HFSE; e.g. Zr, Major and trace element data are presented in Table 4. Nb) and lower in the plagiophile elements (Sr, Rb) and refractory elements (Cr, Ni). In contrast, the HALT suite is characterized by higher concentrations of refractory Major elements and plagiophile elements, and lower HFSE (Fig. 3). In Oceanic tholeiites of the SFVC are characterized by detail, the alkali glasses have higher Cr and Ni (compar- high FeO* (>12 wt %) and TiO2 concentrations able with the HALT basalts) than the alkali basalts, con- (>2 wt %) that increase with decreasing MgO, and by sistent with their high MgO concentrations, and all of the low CaO (8–12 wt %) and Al2O3 (12–16 wt %) con- volcanic suites have variable Rb concentrations, consist- centrations that decrease with decreasing MgO (Fig. 9). ent with their variable alkali concentrations and modest The alkali basalts and glasses have TiO2 concentrations low-temperature mobilization of these elements. Nb con- similar to the oceanic tholeiites, but they are lower in centrations are generally much higher in the alkali basalts FeO* and higher in Al2O3, all of which increase or remain and glasses (10–44 ppm) than in comparable oceanic constant with decreasing MgO (Fig. 9; all data recalcu- tholeiites (14 ppm) and HALT basalts (10 ppm). lated anhydrous). Sc concentrations are highest in the oceanic tholeiite The HALT basalts are characterized by high Al2O3 suite (34–55 ppm, with most >40 ppm), suggesting deriva- (15–24 wt %) and CaO concentrations (10–18 wt %), tion from a source rich in calcic pyroxene. Sc concentra- and by low TiO2 (<1Á5 wt %) and FeO* (10 wt %) con- tions are lower in the alkali basalts (Sc 17–35 ppm) and centrations that decrease with decreasing MgO (Fig. 9). glasses (Sc 28–31 ppm), suggesting that they were formed CaO concentrations increase, and alumina concentra- by larger degrees of partial melting that eliminated calcic tions are relatively constant with fractionation. Because pyroxene in their source residues. The HALT basalts also they have low FeO* at relatively high MgO, FeO*/MgO have relatively low Sc concentrations (25–38 ppm) that ratios in the HALT suite are low for a given MgO con- imply either higher degrees of partial melting or deriva- centration, in contrast to the oceanic tholeiite suite, which tion from a source in which a previous melting event extend to higher FeO*/MgO ratios (Fig. 3). This implies removed the low melting components such as calcic pyr- formation of the HALT suite by partial melting of a source oxene and the trace elements they contained. This latter region that was more refractory than that sampled by the hypothesis is consistent with the high refractory element tholeiitic suite. The extremely high concentrations of contents (Mg, Cr, Ni) of the HALT series. Al2O3 (>19 wt % in some samples) may be attributed to The REE concentrations are shown in Fig. 10 normal- plagioclase accumulation, but the overall high Al2O3 con- ized to the average C1 chondrite of Sun & McDonough centrations in the aphyric and sparsely phyric samples (1989). Oceanic tholeiites of the SFVC are characterized must represent the actual liquid composition. by light REE (LREE)-depleted patterns (La/Smn ¼ The alkali elements, Na2O þ K2O, range from 0Á67–0Á9) that parallel normal MORB (N-MORB; 2 wt % to as high as 7 wt %, with most of the higher Sun & McDonough, 1989) but with concentrations two

17 Table 4: Whole-rock analyses from the Stonyford volcanic complex, California

Sample: SFV-1-1 SFV-1-2 SFV-5-1 SFV-6-1 SFV-6-2 SFV-7-1 SFV-7-2 SFV-8-1 SFV-9-1 SFV-12-1 SF-10-1 SF-12-4 SF-29-1 SFV-37-2 SFV-Z58-1 SFV-Z84-1 SFV-29-1 Suite: OT OT OT OT OT OT OT OT OT OT OT OT OT OT OT OT OT

wt %

SiO2 48.42 50.73 49.30 50.23 49.65 49.15 49.42 50.54 51.60 49.67 49.55 49.76 48.09 50.81 51.52 48.45 50.10

TiO2 2.91 2.93 3.03 2.18 2.18 2.42 2.26 2.31 3.02 3.13 2.98 2.93 2.06 2.49 2.10 2.27 2.34

Al2O3 13.00 14.18 12.93 14.13 13.71 13.89 13.31 14.10 14.59 12.96 12.85 13.43 15.92 12.81 14.22 13.37 14.16 FeO* 15.29 13.89 15.24 12.74 12.48 13.75 12.87 10.52 12.00 14.82 15.79 13.93 9.14 12.76 11.66 12.41 12.96 MnO 0.35 0.31 0.34 0.34 0.30 0.29 0.25 0.29 0.19 0.35 0.23 0.26 0.18 0.20 0.16 0.34 0.22 MgO 6.07 5.13 5.58 6.50 7.28 6.41 6.38 5.78 5.73 5.21 5.45 5.35 7.98 5.64 5.99 5.95 5.85 CaO 10.49 10.00 10.52 10.96 11.54 11.18 11.48 12.47 8.82 10.73 7.64 10.81 10.93 9.43 9.24 14.13 9.73

Na2O2.72 2.86 2.38 2.75 2.54 2.62 3.00 2.86 3.92 2.59 3.60 2.01 1.73 4.55 3.78 2.57 3.86 ORA FPETROLOGY OF JOURNAL K2O0.158 0.142 0.070 0.216 0.233 0.152 0.149 0.142 0.347 0.080 0.228 0.134 2.962 0.163 0.805 0.140 0.276

P2O5 0.081 0.046 0.005 0.010 0.020 0.020 0.024 0.070 0.126 0.100 0.310 0.298 0.277 0.224 0.163 0.190 0.174 Sum 99.49 100.22 99.40 100.03 99.94 99.89 99.15 99.07 100.34 99.54 98.64 98.91 99.26 99.08 99.65 99.82 99.69 LOI 1.10 1.45 1.23 1.50 0.93 1.37 1.34 4.82 3.52 1.13 2.58 2.12 3.08 1.71 1.73 2.79 2.32 18 ppm Ti 16246 16786 16906 13129 12350 14148 12470 12410 14928 18884 17869 17580 12326 14936 12592 13609 14035 Nb 14 11 10 6 6 13 6 3 7 14 12 7 7 1 8 9 1 Zr 183 202 193 147 145 160 148 147 188 215 194 210 207 177 124 134 90 Hf n.a. n.a. n.a. 4.14 3.81 4.17 3.80 n.a. 5.42 6.34 5.20 5.76 5.51 n.a. n.a. n.a. n.a. Ta n.a. n.a. n.a. 0.36 0.32 0.38 0.29 n.a. 0.40 0.50 0.97 0.42 0.72 n.a. n.a. n.a. n.a. Y 5956644853584950598068716956524718 Sr 119 122 106 127 131 125 168 113 213 110 247 132 197 158 214 143 61 Th n.a. n.a. n.a. <1.2 <1.3 <1 <0.99 n.a. <0.9 <1.40.20 <0.48 <1 n.a. n.a. n.a. n.a. Rb 0.55.05.04.00.50.51.00.510.00.516.01.01.00.523.07.00.5 Zn 128 136 134 120 138 117 114 135 158 146 125 123 171 218 117 106 82 Cu 807871841219010178966284627082285565 Ni 36 39 40 42 45 43 47 47 50 41 23 45 49 71 40 51 49 Cr 76 82 101 80 77 79 78 101 147 109 72 147 135 56 119 103 68 V 518 524 515 435 415 453 417 427 452 549 566 637 477 498 388 432 412 Sc 47 55 54 55 50 52 51 58 44 45 54 52 48 40 39 40 45 Co n.a. n.a. n.a. 65 69 65 60 n.a. 55 59 61 73 52 n.a. n.a. n.a. n.a. Ba 117 128 128 104 102 111 89 86 109 136 72 50 55 47 131 47 115 La n.a. n.a. n.a. 5.10 4.84 5.58 5.13 n.a. 6.21 7.28 6.98 7.02 7.17 n.a. n.a. n.a. n.a. Sample: SFV-1-1 SFV-1-2 SFV-5-1 SFV-6-1 SFV-6-2 SFV-7-1 SFV-7-2 SFV-8-1 SFV-9-1 SFV-12-1 SF-10-1 SF-12-4 SF-29-1 SFV-37-2 SFV-Z58-1 SFV-Z84-1 SFV-29-1 Suite: OT OT OT OT OT OT OT OT OT OT OT OT OT OT OT OT OT

Ce n.a. n.a. n.a. 15.03 15.88 17.31 15.58 n.a. 19.18 18.70 17.42 21.01 20.38 n.a. n.a. n.a. n.a. Nd n.a. n.a. n.a. 16.32 13.69 15.99 12.31 n.a. 16.34 17.89 15.92 20.86 17.79 n.a. n.a. n.a. n.a. Sm n.a. n.a. n.a. 4.97 4.81 5.32 4.91 n.a. 5.94 7.11 6.32 6.55 6.67 n.a. n.a. n.a. n.a. Eu n.a. n.a. n.a. 1.73 1.78 1.91 1.70 n.a. 2.10 2.33 2.03 2.31 2.33 n.a. n.a. n.a. n.a. Tb n.a. n.a. n.a. 1.25 1.24 1.17 1.10 n.a. 1.40 1.71 1.35 1.51 1.60 n.a. n.a. n.a. n.a. SHERVAIS Yb n.a. n.a. n.a. 4.41 4.49 5.20 4.52 n.a. 4.97 6.39 5.78 5.94 6.13 n.a. n.a. n.a. n.a. Lu n.a. n.a. n.a. 0.72 0.71 0.81 0.69 n.a. 0.76 0.94 0.93 0.93 0.91 n.a. n.a. n.a. n.a.

Sample: SFV-65-1 SFV-87-1 SFV-18-5 SFV-28-1 SFV-92-1 SFV-110A-1 SFV-114-1 SFV-114-2 SFV-91-1 SFV-124-1 SFV-154-1 SFV-164-1 SFV-35-2 SFVB-3X3 SFVB-1X1 SFVP-1 SFVP-2 tal et Suite: OT OT OT OT OT OT OT OT OT OT OT OT OT Xeno Xeno AB AB .

wt % COMPLEX VOLCANIC STONYFORD THE

SiO2 51.03 51.58 49.56 50.05 49.03 50.96 49.38 49.89 48.94 49.02 47.02 49.93 53.11 49.06 44.92 49.10 49.69

TiO2 3.07 3.17 3.25 2.20 2.65 2.30 3.11 1.85 2.03 2.56 2.28 1.96 2.86 1.90 1.46 2.56 2.42

Al2O3 12.87 12.73 13.82 14.15 13.80 13.42 13.15 14.09 15.57 13.74 13.32 12.61 15.22 19.77 15.63 17.35 16.12

19 FeO* 14.53 15.24 14.06 12.12 14.01 11.64 15.32 11.52 10.13 13.73 13.52 12.69 10.36 6.56 16.73 10.96 10.04 MnO 0.46 0.20 0.28 0.26 0.28 0.22 0.38 0.23 0.21 0.27 0.17 0.28 0.18 0.09 0.29 0.22 0.18 MgO 5.09 4.68 5.29 6.49 6.17 6.34 5.05 5.90 7.98 6.10 5.58 7.69 3.26 5.09 6.85 4.90 5.36 CaO 7.47 7.95 8.76 11.51 9.80 9.75 10.99 13.16 12.69 10.38 15.62 11.73 9.86 14.05 10.52 8.68 10.26

Na2O4.15 3.77 3.35 2.59 3.62 3.80 2.43 2.23 2.64 3.23 2.45 3.33 5.43 2.61 2.70 4.54 4.49

K2O0.275 0.161 0.418 0.111 0.132 0.951 0.122 0.133 0.071 0.193 0.078 0.398 0.051 0.59 0.16 0.840 0.710

P2O5 0.306 0.282 0.316 0.201 0.265 0.192 0.326 0.170 0.031 0.193 0.219 0.183 0.123 0.27 0.25 0.440 0.390 Sum 99.24 99.76 99.09 99.68 99.75 99.58 100.25 99.18 100.29 99.43 100.27 100.80 100.45 99.99 99.51 99.59 99.66 LOI 0.86 1.76 0.95 2.57 3.48 2.63 2.03 1.51 4.62 3.43 2.33 3.10 0.47 3.35 3.72 ppm Ti 18385 19030 19485 13192 15859 13768 18639 11068 12153 15370 13657 11750 17138 11391 8723 15347 14508 Nb101496104 84856598n.a.2921 Zr 197 216 239 124 163 186 200 126 114 163 163 124 169 145 n.a. 226 214 Hf 5.64 5.50 n.a. n.a. 4.54 n.a. 5.23 n.a. 3.44 4.28 n.a. n.a. n.a. 2.80 n.a. 4.65 4.18 Ta 0.30 0.49 n.a. n.a. 0.40 n.a. 0.39 n.a. 0.31 0.43 n.a. n.a. n.a. 0.88 n.a. 1.50 1.60 Y 6673815668577743396451426428n.a.4834 Sr 220 198 222 155 202 492 167 119 145 251 54 399 111 398 n.a. 462 421 Th <1 <1.2 n.a. n.a. <0.96 n.a. <0.93 n.a. <1.1 <0.96 n.a. n.a. n.a. 0.93 n.a. 1.77 1.81 Rb 12.010.015.05.06.021.012.03.010.09.02.08.05.012.4 n.a. 22.821.8 Zn 138 164 149 114 128 117 167 104 94 121 108 107 133 109 n.a. 173 163 Table 4: continued

Sample: SFV-65-1 SFV-87-1 SFV-18-5 SFV-28-1 SFV-92-1 SFV-110A-1 SFV-114-1 SFV-114-2 SFV-91-1 SFV-124-1 SFV-154-1 SFV-164-1 SFV-35-2 SFVB-3X3 SFVB-1X1 SFVP-1 SFVP-2 Suite: OT OT OT OT OT OT OT OT OT OT OT OT OT Xeno Xeno AB AB

Cu 5152385457564373725849803753n.a.3416 Ni 26 38 49 55 37 54 29 54 59 47 48 53 46 152 n.a. 82 35 Cr 35 45 131 125 54 105 42 153 184 98 143 169 64 214 n.a. 42 45 V 527 561 560 460 490 433 584 428 398 475 446 407 499 263 n.a. 300 296 Sc 36 35 34 35 36 41 41 46 35 38 42 41 43 30 n.a. 32 34 Co 65 78 n.a. n.a. 75 n.a. 68 n.a. 53 63 n.a. n.a. n.a. 32 n.a. 58 51 Ba 194 189 222 61 105 3186 150 102 41 108 122 150 109 148 n.a. 237 197 La 6.79 7.17 n.a. n.a. 5.65 n.a. 6.21 n.a. 4.45 5.71 n.a. n.a. n.a. 10.75 n.a. 17.76 17.20 Ce23.98 23.01 n.a. n.a. 18.28 n.a. 22.54 n.a. 11.73 17.67 n.a. n.a. n.a. 32.14 n.a. 51.83 59.49 ORA FPETROLOGY OF JOURNAL Nd 20.22 19.91 n.a. n.a. 14.65 n.a. 19.52 n.a. 13.92 15.99 n.a. n.a. n.a. 13.17 n.a. 21.19 22.51 Sm 6.70 6.28 n.a. n.a. 5.51 n.a. 6.58 n.a. 4.25 5.39 n.a. n.a. n.a. 3.32 n.a. 5.69 5.44 Eu 2.29 2.24 n.a. n.a. 1.89 n.a. 2.11 n.a. 1.64 1.87 n.a. n.a. n.a. 1.18 n.a. 1.98 1.91 Tb 1.36 1.60 n.a. n.a. 1.32 n.a. 1.54 n.a. 0.89 1.36 n.a. n.a. n.a. 0.61 n.a. 0.99 1.01

20 Yb 6.25 5.61 n.a. n.a. 4.88 n.a. 6.09 n.a. 3.73 5.20 n.a. n.a. n.a. 1.81 n.a. 2.84 2.73 Lu 0.88 0.77 n.a. n.a. 0.71 n.a. 0.87 n.a. 0.54 0.70 n.a. n.a. n.a. 0.30 n.a. 0.41 0.39

Sample: SFV-10-6 SFV-10-7 SFV-10-8 SFV-18-4 SFV-46-1 SFV-65-2 SFV-66-2 SFV-85-1 SFV-89-1 SFVG-1 SFVG-2 SFVG-3 SFVG-5 SFVG-4 SFVG-7 SFVG-8 SFV-88-1 Suite: AB AB AB AB AB AB AB AB AB Glass Glass Glass Glass Glass Glass Glass HALT

wt %

SiO2 49.27 48.93 46.15 49.53 49.77 50.38 47.41 46.88 50.57 49.10 48.30 48.70 48.11 48.84 48.36 48.63 51.11

TiO2 2.66 2.27 2.75 2.11 2.33 2.10 2.27 1.98 2.84 2.00 2.00 1.99 2.02 2.12 2.07 1.78 0.94

Al2O3 15.13 15.74 14.21 17.05 16.79 17.60 17.34 17.58 16.37 17.77 17.50 17.40 17.29 17.59 17.13 17.36 18.84 FeO* 10.97 10.78 10.87 8.86 11.09 8.30 6.71 6.49 10.85 8.49 8.58 8.47 8.31 8.50 8.41 8.68 6.91 MnO 0.26 0.33 0.91 1.58 0.15 1.04 0.15 0.09 0.22 0.13 0.16 0.18 0.13 0.13 0.14 0.19 0.09 MgO 6.97 7.44 7.16 8.56 8.59 6.30 4.49 2.29 4.72 7.80 7.78 7.82 7.28 6.99 7.00 7.85 6.65 CaO 8.98 8.62 11.10 4.17 6.26 8.86 16.21 19.15 9.06 11.50 10.93 10.91 10.54 10.57 11.55 11.29 10.77

Na2O3.50 3.70 3.55 1.89 5.03 4.47 3.45 4.16 4.34 2.59 2.76 3.15 3.24 2.53 2.09 2.72 3.94

K2O1.340 1.540 1.420 5.162 0.336 0.610 1.120 1.026 0.897 0.370 0.820 0.800 0.920 0.960 0.436 0.587 0.547

P2O5 0.240 0.200 0.250 0.385 0.291 0.320 0.333 0.396 0.520 0.252 0.300 0.293 0.300 0.200 0.179 0.085 0.170 Sum 99.32 99.44 98.36 99.29 100.65 99.98 99.48 100.04 100.39 100.00 99.13 99.71 98.14 98.42 97.36 99.16 99.88 LOI 3.22 4.48 5.15 3.98 6.68 9.22 3.57 2.80 2.83 4.31 2.79 3.87 Sample: SFV-10-6 SFV-10-7 SFV-10-8 SFV-18-4 SFV-46-1 SFV-65-2 SFV-66-2 SFV-85-1 SFV-89-1 SFVG-1 SFVG-2 SFVG-3 SFVG-5 SFVG-4 SFVG-7 SFVG-8 SFV-88-1 Suite: AB AB AB AB AB AB AB AB AB Glass Glass Glass Glass Glass Glass Glass HALT

ppm Ti 14208 12769 12889 12620 13985 12590 13606 11873 17041 12000 11997 11930 12110 12684 12396 10679 5650 Nb 27 22 22 32 38 33 17 26 44 22 17 13 16 n.a. n.a. n.a. 8 Zr 209 196 195 216 353 176 167 165 292 193 196 194 212 n.a. n.a. n.a. 91 Hf 4.45 4.12 4.37 n.a. 5.50 4.32 n.a. n.a. 6.18 3.70 3.95 3.75 4.30 n.a. n.a. n.a. 1.61 Ta 1.39 1.26 1.35 n.a. 1.82 1.92 n.a. n.a. 2.09 1.17 1.23 1.13 1.48 n.a. n.a. n.a. 0.33 Y 39333136453523245231343229n.a.n.a.n.a.21 SHERVAIS Sr 233 186 340 259 642 346 314 83 184 800 812 968 669 n.a. n.a. n.a. 568 Th 1.26 1.35 1.43 n.a. 2.37 2.47 n.a. n.a. 2.96 1.26 1.42 1.24 1.75 n.a. n.a. n.a. 0.54 Rb 23.031.038.096.015.011.015.020.015.014.410.39.418.5 n.a. n.a. n.a. 15.0 Zn 128 118 122 76 77 66 63 75 106 72 86 88 104 n.a. n.a. n.a. 61 al et

Cu 84827231325045413565666766n.a.n.a.n.a.79 . Ni 31 49 42 89 48 92 91 49 24 131 130 140 109 n.a. n.a. n.a. 77 H TNFR OCNCCOMPLEX VOLCANIC STONYFORD THE Cr 59 58 50 100 61 243 181 203 29 208 210 218 186 n.a. n.a. n.a. 324 V 300 281 290 235 268 238 283 205 380 252 263 260 255 n.a. n.a. n.a. 208 Sc 35 34 31 17 19 23 32 34 26 30 30 28 31 n.a. n.a. n.a. 30

21 Co 46 45 45 n.a. 59 45 n.a. n.a. 51 36 38 38 34 n.a. n.a. n.a. 65 Ba 974 1415 2689 6428 178 274 520 192 290 236 269 353 285 n.a. n.a. n.a. 31 La 16.28 14.73 15.24 n.a. 23.40 19.60 n.a. n.a. 24.44 15.17 14.56 13.82 16.92 n.a. n.a. n.a. 4.46 Ce49.66 47.01 45.79 n.a. 52.93 43.61 n.a. n.a. 59.96 30.94 31.42 30.29 36.71 n.a. n.a. n.a. 10.29 Nd 19.63 21.01 22.69 n.a. 28.86 21.98 n.a. n.a. 33.75 14.98 16.48 18.46 17.68 n.a. n.a. n.a. 6.98 Sm 5.53 4.96 5.06 n.a. 7.29 5.15 n.a. n.a. 7.44 4.46 4.51 4.37 4.82 n.a. n.a. n.a. 2.01 Eu 1.92 1.76 1.81 n.a. 2.35 1.79 n.a. n.a. 2.38 1.49 1.56 1.49 1.60 n.a. n.a. n.a. 0.81 Tb 1.03 0.92 0.93 n.a. 1.24 0.89 n.a. n.a. 1.32 0.71 0.73 0.73 0.75 n.a. n.a. n.a. 0.53 Yb 2.70 2.49 2.62 n.a. 3.55 2.73 n.a. n.a. 4.26 2.42 2.45 2.58 2.56 n.a. n.a. n.a. 1.94 Lu 0.43 0.42 0.40 n.a. 0.51 0.36 n.a. n.a. 0.65 0.39 0.34 0.38 0.39 n.a. n.a. n.a. 0.27

Sample: SFV-37-1 SFV-Z47-1 SFV-69-1 SFV-94-1 SFV-112-1 SFV-111-1 SFV-47-1 SFV-Z49-1 SFV-122-1 SFV-18-1 SFV-106-1 SFV-123-1 SFV-151-1 SFV-160-1 SFV-89-2 Suite: HALT HALT HALT HALT HALT HALT HALT HALT HALT HALT HALT HALT HALT HALT HALT

wt %

SiO2 49.23 48.02 48.61 50.94 50.16 49.21 49.80 48.94 50.80 46.79 50.76 52.09 49.07 51.59 51.46

TiO2 0.67 0.56 0.80 1.30 1.00 0.83 1.38 1.02 0.54 0.75 0.93 0.89 0.83 1.51 1.74

Al2O3 18.24 16.90 18.31 17.81 17.99 18.97 17.92 16.31 20.89 23.43 18.28 15.17 19.10 14.02 15.17 FeO* 7.93 9.10 7.38 8.09 9.05 7.67 7.01 8.85 6.11 7.99 8.25 10.55 7.90 11.12 9.48 MnO 0.12 0.54 0.13 0.12 0.19 0.13 0.14 0.18 0.13 0.15 0.13 0.18 0.17 0.23 0.23 MgO 6.62 4.46 6.64 5.84 6.30 6.63 4.60 7.32 5.81 4.20 6.95 7.66 6.61 7.29 6.47 CaO 13.56 18.10 12.56 13.43 11.32 13.70 14.41 13.31 11.94 12.01 10.07 10.04 13.50 11.14 10.43 Table 4: continued

Sample: SFV-37-1 SFV-Z47-1 SFV-69-1 SFV-94-1 SFV-112-1 SFV-111-1 SFV-47-1 SFV-Z49-1 SFV-122-1 SFV-18-1 SFV-106-1 SFV-123-1 SFV-151-1 SFV-160-1 SFV-89-2 Suite: HALT HALT HALT HALT HALT HALT HALT HALT HALT HALT HALT HALT HALT HALT HALT

Na2O2.75 2.05 3.37 2.68 2.91 2.69 4.36 3.13 3.16 3.59 3.60 3.69 2.71 3.69 4.01

K2O0.671 0.040 0.220 0.081 0.971 0.260 0.464 0.281 0.680 0.794 0.960 0.420 0.369 0.207 0.705

P2O5 0.080 0.020 0.060 0.220 0.051 0.290 0.210 0.080 0.140 0.040 0.210 0.110 0.080 0.115 0.191 Sum 99.83 99.80 98.08 100.39 99.95 100.29 100.20 99.42 100.09 99.75 100.05 100.69 100.34 100.90 99.90 LOI 3.46 4.74 4.74 1.05 2.79 2.51 4.76 3.93 3.48 0.00 2.54 2.73 3.09 2.68 2.09 ppm Ti 4020 3374 4796 7807 6000 4976 8281 6128 3237 4521 5575 5336 5003 9042 10441 Nb33 872 9 71 1 9161 5 3 9 Zr 16 10 80 61 73 135 118 54 46 128 121 69 52 95 56 Hf 1.03 n.a. n.a. 2.26 n.a. 3.25 2.44 n.a. 0.79 n.a. 2.93 1.16 n.a. n.a. 1.42 ORA FPETROLOGY OF JOURNAL Ta 0.16 n.a. n.a. 0.24 n.a. 0.48 0.26 n.a. 0.09 n.a. 0.78 <0.26 n.a. n.a. 0.33 Y 191221202637302917183027213623 Sr 115 68 473 134 460 245 425 287 411 918 404 487 360 195 234 Th <0.87 n.a. n.a. <0.78 n.a. <1.2 <1.1 n.a. <0.69 n.a. 0.81 <0.66 n.a. n.a. 0.40

22 Rb 26.06.07.08.029.023.017.010.014.032.016.018.07.04.06.0 Zn 55 65 57 68 61 81 83 58 41 53 55 86 61 92 54 Cu393977696079647170836581888570 Ni 111 158 69 107 56 50 230 71 64 103 61 60 75 59 72 Cr 372 354 311 391 199 115 408 354 286 481 238 99 301 212 276 V 215 206 200 265 262 319 291 245 175 232 200 302 246 397 205 Sc 29 25 26 31 26 38 34 30 27 34 21 38 38 49 27 Co 77 n.a. n.a. 57 n.a. 61 86 n.a. 44 n.a. 54 64 n.a. n.a. 55 Ba 5 n.d. n.d. n.d. 642 175 128 n.d. 51 560 180 35 80 91 24 La 0.75 n.a. n.a. 4.45 n.a. 6.89 4.25 n.a. 1.18 n.a. 8.25 1.08 n.a. n.a. 3.33 Ce <14 n.a. n.a. 13.02 n.a. 19.31 13.13 n.a. 3.68 n.a. 18.19 2.84 n.a. n.a. 8.22 Nd <51 n.a. n.a. <33 n.a. 14.82 10.41 n.a. 3.15 n.a. 11.57 <26 n.a. n.a. <25 Sm 1.40 n.a. n.a. 2.75 n.a. 3.94 3.16 n.a. 1.10 n.a. 2.92 1.87 n.a. n.a. 1.87 Eu 0.54 n.a. n.a. 1.08 n.a. 1.45 1.14 n.a. 0.50 n.a. 1.01 0.72 n.a. n.a. 0.76 Tb 0.46 n.a. n.a. 0.65 n.a. 0.83 0.75 n.a. 0.33 n.a. 0.77 0.56 n.a. n.a. 0.52 Yb 1.91 n.a. n.a. 2.41 n.a. 3.35 2.74 n.a. 1.47 n.a. 2.68 2.48 n.a. n.a. 2.00 Lu 0.26 n.a. n.a. 0.39 n.a. 0.48 0.38 n.a. 0.20 n.a. 0.37 0.35 n.a. n.a. 0.29

Major elements were determined by fused bead electron microprobe analysis, trace elements by XRF analysis (Nb, Zr, Y, Sr, Rb, Zn, Cu, Ni, Cr, V, Sc, Ba) or INAA (REE, Th, Ta, Hf). OT, oceanic tholeiite suite; AB, alkali basalt suite; glass, alkali basalt volcanic glass suite; HALT, high-Al, low-Ti suite; n.a., not analyzed or below detection limit. Details of analyses in text. SHERVAIS et al. THE STONYFORD VOLCANIC COMPLEX

16 8 7 14 Tholeiite (e) (a) Alkali O 6 Alkali Glass 2 12 HALT 5 4 10

O + K 3 FeO* 2 8 2

Na 1 6 0 9 8 7 6 5 4 3 2 9 8 7 6 5 4 3 2 3.5 400 3.0 (b) 350 (f)

2 300 2.5 250

TiO 2.0 200 150 1.5 Zr ppm 100 1.0 50 0.5 0 9 8 7 6 5 4 3 2 9 8 7 6 5 4 3 2 20 50 18 (c) (g) 16 40 14 30 12

CaO 10 20 8 10 6 4 0 9 8 7 6 5 4 3 2 9 8 7 6 5 4 3 2 24 500 22 (d) 400 (h) 3 20 O 300 2 18 Al 200 16 Cr ppm Nb ppm 14 100 12 0 9 8 7 6 5 4 3 2 9 8 7 6 5 4 3 2 MgO MgO

Fig. 9. MgO variation diagrams showing major and trace element variations as a function of MgO content: (a) FeO*; (b) TiO2; (c) CaO; (d) Al2O3; (e) Na2O þ K2O; (f ) Zr ppm; (g) Nb ppm; (h) Cr ppm. to three times higher than N-MORB (La  16–24 times oceanic tholeiite series and the alkali basalt–glass series, chondrite). Patterns for the middle REE (MREE) and and cannot be related to either of these series by any heavy REE (HREE) are relatively flat, with a slight neg- simple magmatic processes. ative Eu anomaly (Fig. 10a). Alkali basalts and glasses are Multi-element variation diagrams of incompatible enriched in the LREE (La/Smn ¼ 1Á1–2Á4) and have and compatible elements normalized to the average patterns that slope continuously from La to Lu N-MORB of Sun & McDonough (1989) are presented (Fig. 10b). They are more enriched than the average E- in Fig. 11. Elements are arranged in this diagram from MORB (enriched MORB) of Sun & McDonough (1989) those most incompatible in refractory mantle assemblages but less enriched than the average ocean island basalt on the left to those most compatible with refractory (OIB), with La concentrations 5Á5–9Á8 times N-MORB. mantle assemblages on the right (Thompson et al., 1983; The HALT basalts have a wide range of REE patterns, Sun & McDonough, 1989). Oceanic tholeiites are char- from LREE-depleted to LREE-enriched (La/Smn ¼ acterized by abundance patterns that parallel N-MORB 0Á34–1Á8), and La concentrations that range from 0Á3to for the less incompatible elements (La to Na), are relat- 3Á3 times N-MORB (Fig. 10c). The REE patterns of these ively depleted in the more compatible elements (Fe to Ni), samples are distinct from those of both the more common and highly enriched in the most incompatible elements

23 JOURNAL OF PETROLOGY

(a) Stonyford 100 (a) Oceanic Tholeiites 100.0 Oceanic Tholeiite

10.0

10 1.0 Sample/MORB REE / C1 chondrite 0.1 1 Rb Th Ta La Sr Nd Hf Eu Tb Yb Na Fe Co Ni La Ce Nd Sm Eu Tb Yb Lu Y Ba Nb K Ce P Zr Sm Ti Y Lu Zn Sc Cr

100 (b) Alkali Basalts and Glass (b) Stonyford 100.0 Alkali basalts, glass

10.0 10 1.0 Sample/MORB REE / C1 chondrite 0.1 1 Rb Th Ta La Sr Nd Hf Eu Tb Yb Na Fe Co Ni La Ce Nd SmEu Tb Yb Lu Y Ba Nb K Ce P Zr Sm Ti Y Lu Zn Sc Cr

100 (c) Hi-Al/Low Ti suite (c) Stonyford HALT 100.0 (High Al, Low Ti)

10.0 10 1.0 Sample/MORB REE / C1 chondrite 0.1 1 Rb Th Ta La Sr Nd Hf Eu Tb Yb Na Fe Co Ni La Ce Nd Sm Eu Tb Yb Lu Y Ba Nb K Ce P Zr Sm Ti Y Lu Zn Sc Cr

Fig. 10. Chondrite-normalized plots of the rare earth elements in Fig. 11. MORB-normalized trace element variation diagrams of volcanic rocks from the Stonyford volcanic complex: (a) oceanic tho- volcanic rocks from the Stonyford volcanic complex: (a) oceanic tho- leiites; (b) alkali basalts and glasses; (c) high-Al, low-Ti (HALT). Shown leiites; (b) alkali basalts and glasses; (c) high-Al, low-Ti (HALT). for comparison are average N-MORB (long dash), E-MORB (short dash), and OIB (dash–dot) of Sun & McDonough (1989).

from an enriched source similar to that proposed for (Rb to K; Fig. 11a). Phosphorus is depleted in a few E-MORB and OIB. This source is presumably a mixture samples, all of which were affected by strong saprolitic of N-MORB source asthenosphere with plume source alteration. Abundance patterns of the SFVC ocean island mantle, similar to that observed along the Reykjanes tholeiite suite suggest derivation from a mantle source Ridge south of Iceland (Sun et al., 1975). dominated by MORB-source asthenosphere, somewhat The HALT basalts are characterized by abundance enriched by a low-melt component (high Nb, Ta) and patterns for the less incompatible (Zr to Na) and compat- possibly a subduction component (high Rb, Ba), although ible (Zn to Ni) elements that are parallel to or depleted this may reflect later hydrothermal alteration. relative to N-MORB, with the more incompatible ele- Alkali basalts and glasses are characterized by abund- ments ranging from depleted to strongly enriched relative ance patterns that are enriched relative to N-MORB for to N-MORB (Fig. 11c). These patterns are similar to all elements more incompatible than Y; they are depleted those observed in forearc ophiolite lavas, with strong in the more compatible elements relative to N-MORB enrichment in the lithophile low field strength elements (Fig. 11b). A few samples show positive spikes in Rb, Ba, superimposed on less incompatible REE and high field K, and Sr that may be attributed to low-temperature strength elements that are more depleted than N-MORB alteration, but most display smooth patterns that decrease (Metcalf et al., 2000). In contrast to this simple scenario, systematically from Rb to Sc, consistent with derivation however, the HALT basalts are also enriched in the more

24 SHERVAIS et al. THE STONYFORD VOLCANIC COMPLEX

30 are presented in Table 5. The measured Sr, Nd, and Pb isotope compositions have been adjusted for radiogenic 25 "Depleted decay to their initial isotope ratios at 164 Ma (Shervais MORB" et al., 2005a) using the XRF and INAA data (Table 4) 20 for Rb, Sr, Nd and Sm and isotope dilution data for U, High Al – Th and Pb elemental abundances (Table 5). Co-variation Low Ti Suite 143 144 87 86 143 diagrams for Nd/ Nd164 Ma– Sr/ Sr164 Ma, Nd/ 15 144 206 204 87 86 Nd164 Ma– Pb/ Pb164 Ma, and Sr/ Sr164 Ma– 206 204 Y/Nb N-MORB Pb/ Pb164 Ma are shown in Fig. 13; co-variation 10 diagrams for the Pb isotopes are shown in Fig. 14. Oceanic Tholeiite Alkali basalt The volcanic glasses are free from visible alteration. All 5 Alkali glass of the basalts have been affected to some extent by low- "Plume" HALT suite temperature alteration at sub-greenschist-facies condi- 0 tions, but the extent of this alteration is generally minor. 0 20 40 60 80 100 Olivine is typically replaced by chlorite–smectite and Zr/Nb plagioclase may have albitic rims, but in general plagio- clase, pyroxene, and oxides all retain their primary com- Fig. 12. Y/Nb vs Zr/Nb for volcanic rocks from the Stonyford positions. All samples were carefully handpicked, to avoid volcanic complex. Oceanic tholeiites lie between N-MORB and OIB (plume), and alkali basalts cluster near OIB (plume) ratios, whereas the alteration, secondary vein minerals, and vesicle fillings, HALT series has ratios that range from more depleted than MORB to and then rinsed in dilute acids to remove pollution that plume-like. may have taken place during preparation. incompatible ‘high field strength’ elements Nb and Ta as Results well, suggesting source enrichment similar to that Basalts of the oceanic tholeiite suite have slightly more observed in the alkali suite. depleted Nd, Sr, and Pb isotope signatures relative to the The nature of the enrichments seen in the MORB- 143 144 alkali basalts and HALT lavas. The Nd/ Nd164 Ma normalized trace element diagrams can be examined 87 86 (0Á512899–0Á512906) and Sr/ Sr (0Á70232– using incompatible element ratio plots in which each 164 Ma 0Á70296) initial ratios of the oceanic tholeiite samples ratio has the same denominator, so that mixing of sources show little variation, and fall within the range of Nd results in a linear array, and fractional crystallization has and Sr initial isotope ratios reported for Mesozoic (130– no effect. A plot of Zr/Nb vs Y/Nb (Fig. 12) shows 151 Ma) MORB from Deep Sea Drilling Project (DSDP) mixing between an enriched OIB or hotspot-source man- sites in the western Pacific (Janney & Castillo, 1997). The tle (low Zr/Nb and Y/Nb) and depleted MORB-source 143 144 alkali basalts have Nd/ Nd164 Ma (0Á512268) and mantle (high Zr/Nb and Y/Nb). The SFVC alkali basalt 87 86 Sr/ Sr (0Á70337–0Á70394) initial ratios that are series plots near the enriched source composition, 164 Ma enriched relative to the western Pacific Jurassic MORB whereas the SFVC oceanic tholeiite series forms an and to the modern, zero-age East Pacific Ridge (EPR) array that ranges from somewhat higher than plume- MORB field (Hanan & Schilling, 1989; Janney & like source ratios to MORB-like ratios (Fig. 12). In con- Castillo, 1997; Fig. 13a). The HALT sample SFV 37-1 trast, the HALT series forms an array that ranges from has an unusually high Rb/Sr ratio of 0Á23 and very low low ratios, similar to the alkali basalts, to ratios that are 87 86 Sr/ Sr of 0Á70294. much higher than MORB. These super-MORB ratios 164 Ma The Pb isotope initial ratios are more radiogenic than must reflect melting of a source that was depleted in Nb the 130–151 Ma Pacific MORB reported by Janney & relative to Zr and Y, prior to the addition of an enriched, Castillo (1997) except for two 132 Ma DSDP leg 17, site plume-like component. 206 204 166 basalts that have similar Pb/ Pb164 Ma initial ratios (Fig. 14). Whereas the SFV lavas have 206Pb/ 204 Pb164 Ma ratios within the range of the zero-age EPR ISOTOPE GEOCHEMISTRY and Easter microplate region MORB, the 207Pb/ 204 208 204 Analytical methods Pb164 Ma and Pb/ Pb164 Ma ratios are relatively 206 204 Thirteen whole-rock and glass samples were selected for higher for the Pb/ Pb164 Ma, and trend toward and Pb, Sr and Nd isotope analysis and for determination of overlap with the field for Pacific ocean-floor sediment the Pb, U and Th elemental abundances following meth- (Fig. 14). The trend toward oceanic sediment is also 206 204 87 86 ods similar to those described by Hanan & Schilling apparent in the Pb/ Pb164 Ma vs Sr/ Sr164 Ma 143 144 (1989) and Graham et al. (1998). The Pb, Sr and Nd and Nd/ Nd164 Ma isotope variation diagrams isotopes and the Pb, U and Th elemental abundances (Fig. 13b and c). The lack of significant alteration and

25 Table 5: Isotopic composition and concentration of Pb, U, and Th in selected lavas, glasses, and mafic xenolith in breccia

No. Sample Suite Pb U Th Measured Measured Measured Measured Measured Initial Initial Initial Initial Initial (ppm) (ppm) (ppm) 87 Sr/86 Sr 143 Nd/144 Nd 206 Pb/204 Pb 207 Pb/204 Pb 208 Pb/204 Pb 87Sr/86Sr 143 Nd/144 Nd 206 Pb/204 Pb 207 Pb/204 Pb 208 Pb/204 Pb

1 SFV-91-1 OT 0.36 0.20 0.25 0.702768 0.513096 19.105 15.588 38.165 0.702320 0.512899 18.213 15.544 37.795 2 SFV-124-1 OT 0.34 0.12 0.26 0.703195 0.513124 19.048 15.553 38.236 0.702962 0.512906 18.487 15.525 37.831 3 SFV-Z84-1 OT 0.29 0.10 0.22 0.702824 0.513112 18.967 15.539 38.253 0.702506 0.512902 18.428 15.512 37.846 4 SFVG-1 Glass 1.60 0.40 1.36 0.703731 0.512945 18.791 15.531 38.381 0.703614 0.512753 18.387 15.511 37.930 5 SFVG-2 Glass 2.27 0.42 1.39 0.703782 0.513048 18.785 15.564 38.422 0.703700 0.512871 18.485 15.549 38.097 6 SFVG-3 Glass 1.90 0.40 1.20 0.703920 0.512945 18.853 15.544 38.355 0.703857 0.512792 18.516 15.527 38.021

7 SFVG-3B Glass 1.72 0.39 1.10 0.703829 0.512967 18.811 15.556 38.403 0.703766 0.512814 18.439 15.538 38.063 PETROLOGY OF JOURNAL 8 SFVG-5 Glass 1.96 0.50 1.64 0.703557 0.512926 18.919 15.550 38.541 0.703377 0.512750 18.501 15.529 38.097 9 SFVP-2 AB 1.44 0.41 1.66 0.703893 18.934 15.535 38.498 0.703556 18.471 15.512 37.884 10 SFV-10-6 AB 0.89 0.49 1.49 0.704586 0.512450 19.418 15.587 39.060 0.703944 0.512268 18.519 15.543 38.157 11 SFV-65-2 AB 3.00 0.86 2.45 0.703578 19.030 15.587 38.732 0.703371 18.559 15.564 38.295 26 12 SFV-89-2 HALT 0.43 0.11 0.30 0.703724 0.513063 19.099 15.633 38.673 0.703557 0.512921 18.673 15.612 38.296 13 SFV-37-1 HALT 0.04 0.07 0.01 0.704409 0.512983 21.189 15.909 38.490 0.702939 0.512773 18.302 15.767 38.333 14 SFVB-1X1 Xeno 4.81 0.02 0.04 0.703817 18.326 15.625 37.966 0.703817 18.320 15.625 37.961

All Pb isotope ratios are normalized on the basis of replicate measurements of NIST SRM981, using the values of Todt et al. (1984). The Pb, U, and Th concentrations were determined by isotope dilution (ID) using a triple spike enriched in 208 Pb, 235 U, and 230 Th. Mass discrimination of U and Th was monitored using NIST U050. Pb was loaded on a loop type Re filament with silica gel and phosphoric acid. The Pb isotopic compositions (IC) for numbers 48 were determined by thermal ionization mass spectrometry (TIMS) at the University of Rhode Island on a Micromass 30B instrument and for numbers 13, 914 at the San Diego State University (SDSU) on a VG-SECTOR 54 instrument (multi-collector static mode). The discrimination factors averaged 0.97 Æ 0.006 (2s/Hn) ‰ per mass unit for the Micromass 30B and 0.84 Æ 0.001 (2s/Hn) ‰ per mass unit for the SECTOR 54 instrument. The Pb, U, Th concentrations for samples 48 were determined by TIMS at the SDSU on the AVCO 30.5 cm single-collector system and for numbers 13, 914 on the SECTOR 54. Total uncertainties in the Pb isotope ratios are <0.05% per a.m.u., computed by error propagation of both sample and standard analyses. The Pb blanks were <100 pg for the ICand <150 pg for the ID procedures, and are negligible. Uncertainties for U, Th and Pb concentrations are considered to be 0.5%. Nd and Sr isotope analyses were performed on the SDSU VG-SECTOR 54 mass spectrometer. Samples were analyzed as metal species. Sr was loaded on a Re loop type filament with a slurry of Ta2O5 in phosphoric acid, and data were collected in multi-dynamic mode. Repeated measurements of 87 Sr/86 Sr for NIST SRM987 at SDSU gave 0.710273 Æ 0.000003 (2s/Hn), n ¼ 78; sample measurements have been normalized to 88Sr/86Sr ¼ 0.1194 and referenced to a value of 0.71025 for the standard. Nd was loaded with an HClH3PO4 mixture onto a Re side filament using the triple filament technique, and data were collected in multi-dynamic mode. Repeated measurements of 143 Nd/144 Nd for the La Jolla Nd standard gave 0.511841 Æ 0.000002 (2s/Hn), n ¼ 41; sample measurements have been normalized to 146 Nd/144 Nd ¼ 0.7219 and referenced to a value of 0.511858 for the standard. Procedural blanks for Sr and Nd are <50 pg and are negligible for the analyses reported here. The initial ratios at 164 Ma were calculated using the U, Th, and Pb concentrations (this table), the Rb, Sr, Sm, and Nd 11 10 10 12 concentrations from Table 4, and the decay constants l230Th ¼ 4.9475 · 10 , l238 U ¼ 1.55125 · 10 , l235 U ¼ 9.8485 · 10 , l147Sm ¼ 6.54 · 10 , and l87Rb ¼ 1.43 · 1011. SHERVAIS et al. THE STONYFORD VOLCANIC COMPLEX

0.5132 (a) (b) Pacific MORB Pacific MORB

0.5128 Nd 144

Nd/ Oceanic Sediment

143 0.5124

Oceanic Sediment

0.5120 0.702 0.704 0.706 0.708 0.710 0.712 (c) 87Sr/86Sr 0.710

Pacific Mesozoic MORB EPR MORB

Sr 0.708 Easter Microplate MORB Olivine Tholeiite 86 Oceanic Sediment Alkali Basalt Glasses Alkali Basalt Sr/ 87 0.706 Hi-Al Tholeiite Pacific Ocean Sediment

0.704

B Pacific MOR 0.702 17.0 17.5 18.0 18.5 19.0 19.5 206Pb/204Pb

143 144 87 86 Fig. 13. Sr–Nd–Pb isotopic composition of volcanic rocks from the Stonyford volcanic complex: (a) Nd/ Nd164 Ma vs Sr/ Sr164 Ma; 143 144 206 204 87 86 206 204 (b) Nd/ Nd164 Ma vs Pb/ Pb; (c) Sr/ Sr164 Ma vs Pb/ Pb. Shown for comparison are isotopic compositions of unaltered ‘zero-age’ EPR MORB (Hanan & Graham, 1996), Mesozoic MORB ( Janney & Castillo, 1997), ‘zero-age’ plume-polluted MORB from the Easter microplate (Hanan & Schilling, 1989), and oceanic sediment (Ben Othman et al., 1989; McLennan et al., 1990; Hemming & McLennan, 2001) from the Pacific region. Black star, amphibolite xenolith SFVB-1x1. the tendency for the alkali basalts to fall out of the range basalt, compared with 2Á7Æ 0Á345 in fresh Pacific MORB for Pacific MORB, toward the field of oceanic sediment, (White, 1993). The lack of significant alteration in the in Nd–Sr–Pb isotope multi-isotope diagrams are con- basalts suggests that the high 238U/204Pb, relative to sistent with a recycled sediment component in the MORB, is a source feature and not due to sea-floor mantle source. alteration-related U uptake. However, the very low The measured 238U/204Pb ratios are 21–35 in OIB, 232Th/238U and high 238U/204Pb of the HALT basalt 12–35 in alkalic basalt whole rocks, and 112 in the HALT SFV-37-1 is suspect. When this basalt is age corrected its basalt. These ranges in 238U/204Pb are elevated relative Pb and Sr isotope compositions appear to be overcorrec- to the 12Á7 Æ 5Á78 range reported for unaltered Pacific ted for U and Rb decay in the 206Pb/204Pb and 87Sr/86Sr MORB (White, 1993). The measured 232Th/238U vs 206Pb/204Pb diagrams (Fig. 13). The limited alteration ratios in the whole rocks are 1Á3–2Á4inOIB,2Á9–4Á2in of the HALT basalt argues against seawater alteration alkalic basalt whole rocks, and 0Á17–2Á8 in the HALT as an explanation, and suggests that assimilation of a

27 JOURNAL OF PETROLOGY

39.50 were used to calculate the extent of crystal fractionation (a) within each volcanic suite, using the program Genmix (Le Maitre, 1981). Fractionation was determined by mix- Oceanic Sediment ing average phenocrysts compositions with an evolved magma composition and determining the proportions 38.50 of each required to create the most primitive sample

Pb composition.

204 The oceanic tholeiite suite ranges in MgO from 8 wt % to 3Á2 wt %, with FeO*/MgO ratios of 1Á3–3Á3 (Fig. 9). Pb/ Using sample SFV-91-1 (7Á98 wt % MgO) as the parent

208 acific MORB 37.50 P magma, 71% fractional crystallization is needed to create the evolved sample SFV-114-1 (5Á0 wt % MgO). The fractionating phase assemblage is plagioclase (52%), pyroxene (37%), olivine (10%), and ilmenite (1Á4%). The sum of the square of the residuals is 0Á15, suggesting a 36.50 robust fit to the data. The fractionating assemblage is 15.85 close to the average chemical mode for this suite (plagio- (b) clase 50%, pyroxene 38%, olivine 4%, and oxides 8%),

nt but with more fractionating olivine and less fractionating 15.75 edime oxides, which supports the conclusion that this calcula- S tion provides a good approximation of the actual frac- Pb Oceanic tionating assemblage. 204 15.65 The alkali basalts and glasses range in MgO from 7Á8to 2Á2 wt %, with FeO*/MgO ratios of 1Á0–3Á2, although Pb/ most samples have MgO >4Á5 wt % (Fig. 9). Using glass 207 15.55 SFVG-5 (7Á28 wt % MgO) as the parent magma, 40% B fractional crystallization is needed to create the evolved M sample SFVP-1 (4Á9 wt % MgO). The fractionating phase 15.45 Pacific MOR assemblage is plagioclase (52%), pyroxene (31%), olivine (16%), and ilmenite (1%). The sum of the square of the residuals is 0Á73, suggesting a reasonable fit to the data. 15.35 The fractionating assemblage is close to the average 17.0 17.5 18.0 18.5 19.0 19.5 chemical mode for this suite (plagioclase 53%, pyroxene 206Pb/204Pb 29%, olivine 15%, and oxides 3%). The high proportion of olivine in the fractionating assemblage, compared with Fig. 14. Pb isotope correlation plots showing the isotopic composi- the ocean island tholeiite suite and the HALT suite, tion of volcanic rocks from the Stonyford volcanic complex: explains why the alkali basalts require almost half the 208 204 206 204 207 204 (a) Pb/ Pb164 Ma vs Pb/ Pb164 Ma; (b) Pb/ Pb164 Ma vs 206 204 fractional crystallization of the other suites to reduce Pb/ Pb164 Ma. Shown for comparison are isotopic compositions of unaltered ‘zero-age’ EPR MORB (Hanan & Graham, 1996), Meso- MgO concentration by the same amount as seen in the zoic MORB (Janney & Castillo, 1997), ‘zero-age’ plume-polluted other suites. MORB from the Easter microplate (Hanan & Schilling, 1989), and The high-Al, low-Ti basalts range in MgO from 7Á6to oceanic sediment (Ben Othman et al., 1989; Hemming & McLennan, 2001) from the Pacific region. (Symbols are as in Fig. 13.) 4Á2 wt %, with FeO*/MgO ratios of 1Á0–1Á9 (Fig. 9). Using sample SFV-123-1 (7Á66 wt % MgO) as the parent magma, 83% fractional crystallization is needed to sea-floor sediment (Æ seawater) component may have create the evolved sample SFV-Z47-1 (4Á5 wt % MgO). been a factor. The fractionating phase assemblage is plagioclase (65%), pyroxene (20%), olivine (8%), titanomagnetite (6%), and DISCUSSION ilmenite (1%). The sum of the square of the residuals is 1Á35, suggesting a poor fit to the data. The fractionating Petrogenesis assemblage is close to the average chemical mode for Major element fractionation modeling this suite (plagioclase 68%, pyroxene 21%, olivine 6%, Each of the volcanic suites studied here displays system- and oxides 7%). The proportion of plagioclase in the atic linear trends in elemental variation diagrams that fractionation assemblage of the HALT suite is consistent may result from fractional crystallization of more with its high Al2O3 and CaO concentrations, and primitive parental magmas. Least-squares mixing models low concentrations of mafic components (FeO*, TiO2).

28 SHERVAIS et al. THE STONYFORD VOLCANIC COMPLEX

The poor fits obtained for this suite imply that these The N-MORB source composition was taken from samples are not related by simple fractional crystalliza- McKenzie & O’Nions (1995), the E-MORB source com- tion alone. position was taken from Mertz et al. (2001), calculated as In summary, each of the magmatic suites studied here the N-MORB source þ 8% metasomatic melt (0Á3% displays major element chemical variations that are fractional melt of MORB source; Mertz et al., 2001). consistent with fractional crystallization of distinct The depleted N-MORB source was calculated by sub- phenocryst assemblages from parent magmas that are tracting a 1% melt fraction from the N-MORB source also distinct in their chemical compositions. There is composition of McKenzie & O’Nions (1995). Melting no indication that any of these suites can be related to calculations were carried out using the non-modal batch one another by any simple magmatic processes, a melting equation; results are presented in Fig. 15 normal- conclusion that is supported by their trace element sys- ized to the primitive mantle composition of McKenzie & tematics as well. We conclude that although these magma O’Nions (1995). suites may share a common or related mantle source The oceanic tholeiite suite provides good fits at 3–5% region, they have remained largely independent of one partial melting of a spinel lherzolite, N-MORB source another since separation from their mantle source (Fig. 15a). The LREE are slightly depleted relative to the regions. This requires that each magma series exploited HREE, and the HFSE Ti, Zr, Hf, Nb, and Ta are its own plumbing system to reach the surface, and, enriched relative to the HREE. Other models (not because these suites are intercalated at all levels of the shown) show that the oceanic tholeiite suite cannot be volcano, that these plumbing systems may have been formed by melting an E-MORB type source, regardless active concurrently. Similar conclusions have been of the mantle mode used. In addition, the alkali basalt– reached for the plumbing systems of large Hawaiian glass suite cannot be derived from melting an N-MORB volcanoes, which also erupt distinct magmas that show composition source: the LREE/HREE ratios of the no signs of chemical commingling (e.g. Wright, 1971; model melts reflect those of the source (that is, LREE- Tilling et al., 1987). depleted) except for melt fractions <1%, but even here the LREE/HREE ratio of the model melt is too low to match the alkali basalt–glass suite; other elements show Trace element modeling equally poor fits (Fig. 15b). The alkali basalt–glass suite is Incompatible trace elements in the SFVC volcanic suites well matched by models that melt an E-MORB composi- exhibit limited enrichment in response to fractionation tion source at relatively shallow depths (spinel lherzolite (decreasing MgO), with maximum fractionation factors modes). Good fits are obtained for all elements at 10– of 40–60% fractional crystallization. This compares 15% melting; a more enriched source would result in well with fractionation factors based on major element smaller melt fractions (Fig. 15c). Models that use the modeling of 40–80% fractional crystallization (see above). same source composition but with a garnet lherzolite In detail, the enrichment in trace elements between mode show poor fits with data: LREE/HREE ratios are potential parent–daughter pairs commonly implies too high in the model melts, resulting in REE patterns different amounts of fractionation than the major ele- that cross the alkali basalt–glass REE patterns, and HFSE ment models, with the major elements indicating less contents that are too high (Fig. 15d). fractionation than the trace elements. This suggests that The HALT suite shows a wide range of incompatible much of the variation in trace element concentrations trace element concentrations that cannot be fitted by any results from differences in the parent magmas within simple models. The most depleted HALT samples cannot each suite, and is not controlled exclusively by fractional be modeled as melts of spinel lherzolite N-MORB source crystallization. at <20% partial melting (Fig. 15e). More enriched Differences within each suite in incompatible trace samples match the model REE patterns approximately element concentrations must be due in part to different at 3–5% melting, but results for K, Sr, and the HFSE are degrees of partial melting of similar source regions. In poor. Melting of a depleted N-MORB source works contrast, differences between the three suites defined here somewhat better: REE patterns match approximately at (oceanic tholeiite, alkali basalt, high-Al/low-Ti) probably 7–15% melting, but results for K, Sr, and the HFSE are represent partial melting of distinct source regions that still poor (Fig. 15f ). The highly depleted nature of the differ significantly in their trace element characteristics. HALT source is supported by the Nb/Zr/Y systematics To model these variations, we have constructed a series (Fig. 12) and by their high mg-number and Cr content; of melting models featuring spinel or garnet lherzolite these data also suggest that the depleted melts may have modes, and chemical compositions that reflect the mixed with an enriched component similar to the alkali source regions of N-MORB, E-MORB, and a depleted basalt–glass suite to achieve the observed range in N-MORB source. Partition coefficients were taken from incompatible element concentrations. If we use the Arth (1976) and McKenzie & O’Nions (1991, 1995). depleted HALT samples as potential ‘primary melts’,

29 JOURNAL OF PETROLOGY

1000 N-MORB (a) Spinel Lherzolite 100

10

1% melt 3% melt 1 5% melt 7.5% melt SFVC Sample/Primitive Mantle Sample/Primitive 10% melt 15% melt Ocean Island Tholeiite 0.1 La Nd Eu Tb Ho Tm LuSr Sc Zr Nb Th Ce Sm Gd Dy Er Yb K Y Ti Hf Ta 1000 N-MORB (b) Spinel Lherzolite 100

10

1% melt 3% melt 1 5% melt 7.5% melt SFVC Sample/Primitive Mantle Sample/Primitive 10% melt Alkali & Glass 15% melt 0.1 La Nd Eu Tb Ho Tm LuSr Sc Zr Nb Th Ce Sm Gd Dy Er Yb K Y Ti Hf Ta 1000 E-MORB (c) Spinel Lherzolite 100

10

3% melt 1 5% melt 7.5% melt 10% melt SFVC Sample/Primitive Mantle Sample/Primitive 15% melt Alkali & Glass 20% melt 0.1 LaNd Eu Tb Ho Tm LuSr Sc Zr Nb Th Ce Sm Gd Dy Er Yb K Y Ti Hf Ta

Fig. 15. Multi-element melting models based on calculated primary source compositions and partition coefficient data compiled by McKenzie & O’Nions (1991, 1995): (a) N-MORB source composition with spinel lherzolite mineralogy, with SFVC oceanic tholeiites for comparison; (b) N-MORB source composition with spinel lherzolite mineralogy, with SFVC alkali basalts for comparison; (c) E-MORB source composition with spinel lherzolite mineralogy, with SFVC alkali basalts for comparison; (d) E-MORB source composition with garnet lherzolite mineralogy, with SFVC alkali basalts for comparison; (e) N-MORB source composition with spinel lherzolite mineralogy, with SFVC HALT basalts for comparison; (f ) depleted N-MORB source composition (N-MORB minus 1% melt fraction) with spinel lherzolite mineralogy, with SFVC HALT basalts for comparison. In all panels actual data for each suite are superimposed in grey. (See text for discussion.)

30 SHERVAIS et al. THE STONYFORD VOLCANIC COMPLEX

1000 E-MORB (d) Garnet Lherzolite 100

10

1% melt 3% melt 1 5% melt 7.5% melt SFVC

Sample/Primitive Mantle Sample/Primitive 10% melt 15% melt Alkali & Glass 0.1 LaNd Eu Tb Ho Tm LuSr Sc Zr Nb Th Ce Sm Gd Dy Er Yb K Y Ti Hf Ta 1000 N-MORB Spinel Lherzolite (e) 100

10

1% melt 3% melt 1 5% melt 7.5% melt SFVC

Sample/Primitive Mantle Sample/Primitive 10% melt HALT 15% melt 0.1 LaNd Eu Tb Ho Tm LuSr Sc Zr Nb Th Ce Sm Gd Dy Er Yb K Y Ti Hf Ta 1000 1% melt (f) Depleted N-MORB 3% melt 5% melt Spinel Lherzolite 7.5% melt 100 10% melt 15% melt 20% melt 10

1 SFVC Sample/Primitive Mantle Sample/Primitive HALT 0.1 LaNd Eu Tb Ho Tm LuSr Sc Zr Nb Th Ce Sm Gd Dy Er Yb K Y Ti Hf Ta Fig. 15. Continued. we can reproduce the trace element concentrations of the HALT melts, and supports inferences drawn from their more enriched HALT samples by mixing the ‘primary major and trace element systematics for enrichment of a melts’ with up to 50% alkali basalt–glass melts. This previously depleted source region by an enriched OIB mixing model provides the best fit to the enriched melt component. The radiogenic isotope signature of the

31 JOURNAL OF PETROLOGY

alkali basalts requires a recycled sediment component in possibly deposited by cold, circulating seawater sealed the mantle source. off the stratigraphically lower horizons. The calcite In summary, the oceanic tholeiite suite represents low- cement virtually blocked off penetration of hydrothermal degree partial melts (3–5%) of an N-MORB astheno- fluids coming from depth or from seawater circulating sphere source region at relatively shallow levels (spinel down from the surface. After filling of remaining voids by lherzolite facies). The alkali basalt–glass suite represents chlorite, calcite, and analcime, the last stage of meta- moderate degrees of melting (10–15%) of an E-MORB– morphism involved further replacement of glass lapilli OIB-like source that was physically distinct from the by Ca-zeolites. Textures of the secondary minerals show oceanic tholeiite source. Finally, the HALT suite appears that deposition of these phases occurred early, prior to to represent second-stage melts from a depleted MORB extensive alteration of the volcanic glass and preser- source that have been variably enriched by melts similar ving the glass from extensive replacement (Shervais & to the alkali basalt suite. Hanan, 1989).

Metamorphism in the SFVC Vein minerals and vesicle fillings Replacement of minerals in basalt Vein assemblages and vesicle fillings are common in the Metamorphic assemblages found in the SFVC are typical SFVC. Calcite, prehnite, and Ca-zeolites (thomsonite, of low-temperature ocean-floor metamorphism. Meta- laumontite) constitute the most common vein fillings; morphic phases include chlorite, albite, epidote, prehnite, quartz, analcime, and epidote are less common. Larger calcite, titanite, Ca-zeolites, clay minerals, and quartz; veins are commonly lined by Ca-zeolites along their actinolite also occurs in some samples. Plagioclase laths margins and filled with calcite in their cores. Thin veins may be albitized, especially along their margins, and of pyrite cross-cut veins of prehnite, calcite, and cores may be replaced by epidote, prehnite, calcite, Ca-zeolites. In general, the veins and vesicle fillings are Ca-zeolites, or clay minerals. Albitization is commonly consistent with sub-greenschist-, zeolite- or prehnite– associated with the alteration of primary magnetite actinolite-facies metamorphism. or ilmenite to magnetite–titanite intergrowths. Olivine phenocrysts are rare and when found are totally replaced Metamorphic conditions by green chlorite and smectite, except in the volcanic Typical metamorphic mineral assemblages include glass. The pyroxenes remain relatively fresh and show albite, chlorite, prehnite, calcite, epidote, and titanite, alteration only along grain edges and fractures. The Æ Ca-zeolites (laumontite, thomsonite, heulandite), pyroxenes alter to chlorite, actinolite, epidote, and analcime, and actinolite. Minerals indicative of blueschist- titanite. facies metamorphism have not been observed. Meta- Despite the widespread occurrence of secondary min- morphic mineral assemblages typically found in the erals in volcanic rocks of the SFVC, preservation of SFVC represent low temperatures and pressures of the primary minerals is extensive and shows that circulation zeolite to prehnite–actinolite subfacies. Analcime depos- of hydrothermal fluids was not pervasive and that water/ ition and heulandite replacement of glass within the rock ratios were low. The most extensive alteration of hyaloclastite layers probably occurred at temperatures SFVC volcanic rocks is found along Stony Creek in the that did not exceed 100C (Shervais & Hanan, 1989). western part of the complex. Here, large tracts of pillow Zeolite minerals are stable only until 300C and lava have been extensively replaced by clay minerals, <0Á1 GPa pressure. In general, rocks low in the section calcite, and chlorite (‘brownstone’). Chert horizons strati- appear to have been metamorphosed at somewhat higher graphically above this area of extensive clay-alteration temperatures (300C) than rocks near the top of the are jasperized to massive, red and ochre-colored chalced- section (<100C). Based on these conditions, it seems ony. We interpret these relations to document the loca- unlikely that the SFVC was ever emplaced within a sub- tion of an extensive submarine hydrothermal vent system duction zone; in fact, the conditions of metamorphism that was active prior to disruption and emplacement of observed are consistent with burial beneath sedimentary the complex. rocks of the Great Valley Series.

Hydrothermal alteration of the hyaloclastite breccias Tectonic setting of the SFVC Secondary mineral assemblages in the hyaloclastite brec- The SFVC contains three geochemically and petrologic- cias include calcite, analcime, chlorite, and chlorite– ally distinct magma series, each of which provides a smectite intergrowths (Shervais & Hanan, 1989). Initial somewhat different view of the paleo-tectonic setting in alteration began with replacement of some glass lapilli which the complex formed. The discrimination plots by smectite–chlorite intergrowths. Calcite cementation of Leterrier et al. (1982) use pyroxene minor element

32 SHERVAIS et al. THE STONYFORD VOLCANIC COMPLEX

chemistry to distinguish alkali basalts, non-orogenic 0.20 basalts (MORB, OIB), and orogenic basalts (arc tholeiite, calc-alkaline; Fig. 16). These plots show that pyroxenes of OT Cpx (a) the alkali suite are indeed alkaline in composition, Alkali Cpx whereas pyroxenes of the oceanic tholeiite and HALT 0.15 HALT Cpx suites are sub-alkaline (Fig. 16a). The sub-alkaline pyrox- enes are further divided into non-orogenic (all of the Alkaline oceanic tholeiite suite pyroxenes plus some HALT suite Ti 0.10 pyroxenes) and orogenic (the remaining HALT suite afu pyroxenes; Fig. 16b). Finally, the HALT suite pyroxenes straddle the dividing line between calc-alkaline and tho- Sub-alkaline leiitic (Fig. 16c). The calc-alkaline character of the HALT 0.05 suite is confirmed by MgO variation plots, which show that basalts of the HALT suite do not increase in FeO* and TiO2 with fractionation, consistent with calc-alkaline fractionation trends (Fig. 9). 0.00 These three petrological series appear to represent 0.5 0.6 0.7 0.8 0.9 1.0 1.1 two distinct tectonic settings. The Ti–Zr plot of Ca+Na afu Pearce & Cann (1973) shows that the oceanic tholeiite and alkali suites are oceanic in character, but are more 0.06 Non-Orogenic (b) enriched in these elements than the limited data used to define this plot (Fig. 17a). The HALT suite has charac- teristics that overlap the arc tholeiite, calc-alkaline, and primitive MORB fields, with a significant gap 0.04 between them and the other suites (Fig. 17a). The Ti–V Ti+Cr plot of Shervais (1982) confirms these observations, and afu further shows that the Stonyford oceanic tholeiite and 0.02 alkali suites are distinct: the Stonyford oceanic tholeiite suite has Ti/V ratios of 25–38, similar to Orogenic MORB, whereas the alkali suite has Ti/V ratios of 45– 0.00 54, similar to alkali basalts erupted in both continental 0.5 0.6 0.7 0.8 0.9 1.0 and oceanic settings (Fig. 16b). In contrast, the HALT Ca afu suite has Ti/V ratios that range from 15 to 30, overlap- ping the trends of both island arc volcanic suites and 0.04 depleted MORB suites (Fig. 17b). Similar trends are (c) Calc-alkaline seen in back-arc basin basalts, which vary from arc-like 0.03 to oceanic in character. A plot of Ta vs Hf (Fig. 17c) shows that the oceanic tholeiite suite has Hf/Ta ratios similar to N-MORB Ti afu 0.02 (15), but with much higher concentrations, consistent with small fractions of partial melting; this is consistent Tholeiitic with the partial melting models presented earlier (Fig. 15). 0.01 The alkali basalt series appears to lie on a mixing trend between E-MORB (Hf/Ta  4) and OIB (Hf/Ta  3); this trend suggests that a relatively enriched 0.00 OIB-like source containing a recycled oceanic sediment 0.00 0.10 0.20 0.30 component is needed to form these basalts (Fig. 17c). Al total afu Finally, the HALT series basalts have Hf/Ta ratios that lie between N-MORB and E-MORB, but with concen- Fig. 16. Clinopyroxene discrimination plots of Leterrier et al. (1982): (a) Ti a.f.u. vs Ca þ Na a.f.u., showing discrimination of alkali pyrox- trations that range from less than half to more than enes from sub-alkaline pyroxenes; (b) Ti þ Cr a.f.u. vs Ca a.f.u., double these components (Fig. 17c). These trends are showing discrimination of non-orogenic pyroxenes (MORB) from oro- consistent with depletion of the HALT source mantle, genic pyroxenes; (c) Ti a.f.u. vs Altot a.f.u., showing discrimination of calc-alkaline pyroxenes from tholeiitic pyroxenes. Pyroxenes from the followed by variable re-enrichment with a com- alkali basalts are alkaline; pyroxenes from the oceanic tholeiites are ponent similar in trace element character to the alkali subalkaline, non-orogenic; whereas pyroxenes from the HALT series basalt suite. are orogenic, including both calc-alkaline and tholeiitic.

33 JOURNAL OF PETROLOGY

20000 Complex of MacPherson (1983). However, as noted by Ti (a) MacPherson & Phipps (1985), these units are distinct and OT Suite unrelated. The Snow Mountain Volcanic Complex con- 15000 ppm Alkali Suite tains incipient blueschist metamorphic phases and lies Oceanic Tholeiite west of the serpentinite belt that marks the Coast Range 10000 MORB Alkali basalt Alkali glass fault (MacPherson, 1983; MacPherson & Phipps, 1985). All High Al – HALT In contrast, the SFVC contains unaltered volcanic glass, 5000 Low Ti Suite lacks incipient blueschist metamorphic phases, lies Arc entirely within the Coast Range fault serpentinite belt, Calc-alkaline Zr ppm 0 Tholeiite and is closely associated with (but is not directly overlain 050 100 150 200 250 300 350 400 by) sediments of the Jurassic Knoxville formation 700 (Shervais & Hanan, 1989; Zoglman, 1991). The SFVC V (b) also contains radiolarian chert intercalations with faunal 600 ppm assemblages that are similar to those found at other CRO 10 OT Suite 20 locations, but distinct from faunal assemblages derived 500 ARC from Franciscan cherts (Shervais et al., 2005a). Thus, the Snow Mountain Volcanic Complex is clearly part of 400 the Franciscan assemblage, whereas the SFVC is best MORB 50 300 regarded as part of the CRO (contrary to our former Alkaline correlation of both units with the Franciscan assemblage, e.g. Shervais & Kimbrough, 1987; Shervais & Hanan, 200 Alkali Suite 1989; Shervais, 1990). This distinction is important High Al – 100 Low Ti Suite because it has significant tectonic implications for the Ti ppm origin and evolution of the CRO. 0 0 5000 10000 15000 20000 8 Hf Plume/OIB Petrotectonic model for the SFVC and ppm N-MORB 2.9 (c) its relationship to Jurassic orogeny 7 15.5 E-MORB 4.3 in California 6 OT Suite The SFVC apparently overlies disrupted ophiolite similar to that found at Elder Creek and other Coast Range 5 ophiolite localities, and recently published age dates 4 show that it is slightly younger than the underlying Alkali Suite ophiolite (Shervais et al., 2005a). Previously published 3 geochemical data on these localities, along with data in 2 unpublished theses, show that the CRO represents a supra-subduction zone ophiolite (Shervais & Kimbrough, 1 High Al – 1985; Beaman, 1989; Shervais, 1990, 2001; Giaramita Low Ti Suite Ta ppm et al., 1998; Evarts et al., 1999; Snow, 2002; Shervais et al., 0 2004, 2005b). At Stonyford, remnants of ophiolitic 0.0 0.5 1.0 1.5 2.0 2.5 3.0 rocks similar to those found at Elder Creek and other Fig. 17. Tectonic discrimination plots for volcanic rocks from the CRO localities form knockers in the serpentinite matrix Stonyford volcanic complex: (a) Ti ppm vs Zr ppm plot of Pearce & Cann (1973); oceanic tholeiites and alkali basalts are oceanic, whereas me´lange that underlies the volcanic complex. These the HALT series is transitional MORB–arc; (b) Ti ppm vs V ppm plot knockers include wehrlite and pyroxenite cumulates, of Shervais (1982); oceanic tholeiites are MORB-like, alkali basalts cumulate and isotropic gabbro, diorite and quartz diorite, are alkaline, whereas the HALT series rocks straddle the boundary and volcanic rocks. Their position beneath the SFVC between oceanic and arc volcanics; (c) Ta ppm vs Hf ppm; oceanic tholeiites have Hf/Ta ratios similar to N-MORB but at higher con- suggests that they originally formed the substrate upon centrations; alkali basalts lie on mixing trend between E-MORB and which the volcanic complex was erupted. plume; HALT series has concentrations that range from less than This conclusion is reinforced by radiolarian N-MORB to more than E-MORB. assemblages found in chert intercalations in the SFVC (Murchey, cited by Shervais et al., 2005a). Radiolaria Relationship of the SFVC to the Franciscan found in the large lower chert lens above Stony Creek assemblage in California (near the base of the complex) represent the same Shervais & Kimbrough (1985a, 1987) did not distinguish assemblages as found in radiolarian chert that sits on between the SFVC and the Snow Mountain Volcanic top of CRO elsewhere. Radiolaria found in the upper

34 SHERVAIS et al. THE STONYFORD VOLCANIC COMPLEX

(a) Formation of CRO above subduction zone as ridgecrest approaches trench Supra-subduction CRO: Arc tholeiites, Boninites, Seamount Active Calc-alkaline volcanics, (e.g., St. John′s quartz diorite intrusions Spreading Mountain) Center

Zone of Melting: New Asthenosphere Flows into Previously Depleted Melt Zone

Plume-polluted Pacific Basin Asthenosphere

(b) Collision and partial subduction of ridgecrest opens slab window beneath CRO

Volcanoes, dikes of oceanic tholeiite, alkali basalt, and high-Al, low-Ti basalt form Stonyford seamount High T Metamorphism on substrate of partially dismembered CRO. of Oceanic Crust

Subduction of ridgecrest; Influx of melt and mantle from MORB, OIB source Asthenosphere, Continued melting of SSZ wedge

Delaminated slab Sinks back into mantle

(c) Renewed subduction forms Franciscan complex as locus of melting shifts to Sierras

Accretion of Coast Range Great Valley Sierra Arc Franciscan Ophiolite Fore-arc Basin Complex

Shallow Underthrusting of Young Buoyant Crust

Zone of Melting Moves under Sierra Arc

Fig. 18. Model for origin of the Stonyford volcanic complex and related rocks of the Coast Range ophiolite, California, after Shervais (2001): (a) formation of the Coast Range ophiolite as a supra-subduction ophiolite above sinking slab during the middle Jurassic, as spreading center approaches; (b) collision and partial subduction of the spreading ridge, which opens up a slab window allowing mass transfer of material (melts, mantle?) from beneath the former spreading center into the mantle wedge above the subduction zone; mixing of melts from the former source of the spreading center with melts of the refractory mantle wedge above the subduction zone; shallow subduction of the buoyant crust near the spreading center results in uplift and deformation of the forearc and arc; (c) renewed shallow subduction of buoyant oceanic crust formed west of the former spreading center continues deformation, accumulation of the Franciscan assemblage in response to uplift and erosion of arc and forearc. chert lens above Dry Creek (near the top of the complex) being deposited elsewhere, after formation of the main correlate with radiolarian cherts that overlie the older ophiolite sequence and prior to deposition of the Great cherts at other CRO localities (e.g. Hagstrum & Valley Series. Murchey, 1996). These observations require that the Where rocks with oceanic affinities are found at other bulk of the SFVC was erupted onto its CRO substrate CRO localities they always post-date formation of the at the same time that ash-rich radiolarian chert was main ophiolite (Shervais, 2001; Shervais et al., 2004).

35 JOURNAL OF PETROLOGY

These include dikes with MORB-like geochemistry that (1993) found a reversal in faunal ages for accreted intrude older SSZ ophiolite assemblages at Elder Creek, terranes in the Klamaths and the Sierras, such that res- Black Mountain, Mount Diablo, Del Puerto, and Cuesta idence time on the ocean floor (age of oceanic basement Ridge (Giaramita et al., 1998; Evarts et al., 1999; Shervais, or oldest chert deposited on it versus time of arrival at the 2001; Snow, 2002), inclusions of MORB glass in continental margin) of these accreted terranes became the Leona rhyolite (Shervais et al., 2004), and flows of progressively shorter in the early Jurassic with a min- MORB pillows that underlie chert and overlie SSZ imum at c. 160 Ma, followed by progressively longer volcanics at Cuesta Ridge (Snow, 2002). All of these residence times in the late Jurassic to early Cretaceous. occurrences suggest that after formation of the main They proposed that this minimum in residence time ophiolite suites (arc tholeiite gabbros and volcanics, (<10 Myr) corresponds to a ridge collision event, wehrlite–pyroxenite intrusions and boninitic lavas, and followed by a long period of dominantly transcurrent diorite–quartz diorite plutons and calc-alkaline volcanics) relative motion (Murchey & Blake, 1993). Ward (1995) in an SSZ setting, the SSZ ophiolites interacted with showed that apparent polar wander paths for North a spreading ridge erupting N-MORB, E-MORB, and America display an abrupt change at c. 160 Ma from alkali basalts. longitudinal drift to rapid northward drift at the J2 There are two situations where this is likely to occur: cusp, a change that he attributed to a ridge collision (1) where a back-arc basin spreading center propagates and subsequent reordering of plate motions in response through an active arc setting, moving the active arc sea- to the end of coupled motion between North America ward and leaving a remnant arc behind (Karig, 1982); and the subducting plate. Finally, there is the change in (2) collision of a spreading center with the subduction kinematics of the Cordilleran margin shortly after the zone, where the spreading center is partly overridden Nevadan phase of compression, from convergent in the by the actively extending forearc (Shervais, 2001; middle Jurassic to sinistral transtension–transpression in Shervais et al., 2004, 2005a). We prefer the latter explana- the late Jurassic and early Cretaceous. tion for several reasons. First, there are no arc rocks to the west that could represent either the active seaward arc or a remnant arc. This geometric problem persists regard- ACKNOWLEDGEMENTS less of which subduction polarity is assumed. Second, This paper would not have been possible without the the rock series that form the main ophiolite assemblages pioneering work and insights of Cliff Hopson, who intro- of the CRO are consistent with formation in a rapidly duced us (Shervais, Hanan) to the Coast Range ophiolite extending forearc setting. These include the boninitic and who has provided the inspiration for our continued volcanic suites that are common at many CRO localities, work there. This research was supported by NSF and the equally common wehrlite–pyroxenite intrusive grants EAR8816398 and EAR9018721 (Shervais) and series. Harzburgites and dunites associated with the CRO EAR9018275 (Kimbrough and Hanan). Geological map- at Stonyford contain depleted Cr-spinel (cr-numbers up ping of the Stonyford ophiolite formed part of a Master’s to 74) that are characteristic of depleted forearcs parental thesis by Marchell Zoglman Schuman (Zoglman, 1991) to boninitic lavas (Shervais et al., 2005b). at the University of South Carolina. Instrumental Our preferred model is shown in Fig. 18: (a) formation neutron activation analyses were provided through the of the CRO above a nascent subduction zone, as Oregon State University Reactor Sharing program documented by previous studies; ( b) collision of an active (Robert J. Walker, analyst). Thoughtful reviews by spreading center with this subduction zone, allowing Melanie Barnes, Dieter Mertz, and Reinhard Werner influx of heterogeneous MORB-source mantle and greatly improved the manuscript and are gratefully melts derived from that mantle into the previously acknowledged. depleted supra-subduction mantle wedge; (c) finally, renewed subduction and shallow underthrusting to form the Franciscan complex and cause deformation associ- REFERENCES ated with the Nevadan orogeny in the Sierra foothills (e.g. Arth, J. G. (1976). Behavior of trace elements during magmatic Shervais, 2001; Shervais et al., 2004). The geochemical processes; a summary of theoretical models and their applications. data presented here and elsewhere, and the age data Journal of Research of the US Geological Survey 4(1), 41–47. presented by Shervais et al. (2005a) contradict models Beaman, B. J. (1989). 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