A Missing Piece of the Puzzle: Geochemical Analysis of the Lava Flows Within Stony Bay, Banks Peninsula, NZ. Lauren Pincus 1,2

1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 A Missing Piece of the Puzzle: Geochemical Analysis of the Lava Flows Within Stony Bay, Banks Peninsula, NZ Lauren Pincus 1,2 1 Department of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand 2 Department of Geology, Middlebury College, Middlebury, Vermont 05753, U.S.A. Abstract The creation of a detailed geologic map is critical to the understanding of any location on Earth. Geologic maps provide a wealth of information including rock and soil types, locations of faults, areas containing natural resources, and prediction of hazards such as landslides and volcanoes. The current geologic map, shown in Figure 1, (Sewal et al., 1993) of Banks Peninsula, a popular recreational destination located just south of Christchurch on the eastern side of the South Island of New Zealand, suggests that about half of the peninsula is made up of a single rock type (Johnson, 2012). However fieldwork conducted through the years has suggested a much higher level of complexity within this region, including the possibility of multiple volcanic sources for the lavas and eruption deposits (Johnson, 2012; Hampton et al., 2009). The objective of this study was to produce geochemical data through x-ray fluorescence spectroscopy on the major and trace elements in the lava flows of Stony Bay, Banks Peninsula. This data was used in conjunction with field observations to examine the vertical stratigraphy of observed lava flows on the east and west sides of Stony Bay. The flows of Stony Bay were found to be mafic in composition, ranging from 40-53% silica. Total alkali content was low, ranging from 2-7%. Results suggest multiple sources of magmatism in the bay due to a large 70 meters tall flow on the west side that is more mafic in composition (40% SiO 2 ) than all other flows sampled. Cyclic fluctuations in silica and alkali content observed in lava flows within the stratigraphy on the east side of the bay suggest the eruption of several batches of magma. This result agrees with previous findings of Johnson, 2012 for the style of volcanism in the Okains area of Banks Peninsula.

2 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Introduction Geochemical data is an invaluable tool to help decipher volcanic environments. While much can be gleamed about a volcano by studying its physical geology or the petrology of its deposits, the ratios of various elements within a rock sample can provide insight into its magmatic history as well as provide an age for when it was erupted. When rocks are aphanitic or contain few phenocrysts, geochemical analysis of major elements is the most accurate way to classify a rock. Geochemical analysis can also be used to correlate lava flows. Numerous studies have used geochemical techniques to help decipher the eruptive complexes resulting from intraplate volcanism throughout New Zealand, as well as for Banks Peninsula specifically. Banks Peninsula was developed mainly through the eruption of two composite shield volcanoes Akaroa (9.4-6.8 Ma) and Lyttleton (12.3-10.4 Ma) (Timm et al., 2009). Stony Bay (see Fig. 1) is a 0.4 km wide bay located on the north side of the peninsula. While recent research has focused on high resolution mapping of nearby areas such as Okains Bay (Johnson, 2012) and Lyttleton Harbor (Hampton et al., 2009), Stony Bay has yet to undergo any detailed geologic study. Field mapping done in February 2013 provided the first glimpse into the geology of this region. Multiple stacked lava flows were identified on both the east (Figure 2) and west (Figure 3) sides of the bay, as well as scoria deposits. While detailed field notes were taken, further geochemical evidence is needed to successfully correlate the lava flows identified on either side of the bay with the surrounding area. This paper aims to use geochemical data and field observations to study the vertical stratigraphy of the lava flows on either side of Stony Bay. This data may improve knowledge of the source of these deposits and the magmatic evolution of the volcanoes that produced them. In this study, x-ray fluorescence spectroscopy (XRF) was used to collect data on the major and trace elements within lava flow samples from Stony Bay. XRF analysis is a common method to collect geochemical stratigraphic data and has been used both on Banks Peninsula (Johnson, 2012; Coombs et al., 2008) and elsewhere in the world (Rhodes et al., 1996) successfully to provide insight into the stratigraphy of lava flows. This new geochemical data was combined with field notes taken on this region and used to interpret the stratigraphy of the lava flows, the volcanic source of the flows, and any observed changes in lava composition. This study then compares this data with similar high-resolution mapping and geochemical studies recently done, or currently under way in Okains Bay (Johnson, 2012) and Lavericks Bay.

3 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 Geologic Background Banks Peninsula is one of several volcanic complexes in and around New Zealand associated with Cenozoic intraplate volcanism. The peninsula is located just south of Christchurch on the southeast side of the South Island, New Zealand. It is made up primarily of two volcanic complexes, Akaroa and Lyttleton, which formed simultaneously from two magmatic systems sourced from basaltic intraplate plumes (Hampton, 2010). While Stony Bay, the subject of this study is not located within the Lyttleton complex, knowledge of its eruptive and geochemical history can shed insight on the less studied, nearby Akaroa formation, and specifically on Stony Bay as well. The Lyttleton complex is made up of five overlapping cones each formed during an eruptive phase, or package, of the volcano (Hampton, 2010). The Lyttleton complex tended to erupt lavas of increasing crystalinity over time, particularly of feldspars, which contributed to explosive eruptions following effusive flows, as well as the development of domes and trachytic dykes (Hampton, 2010). Within this geochemical trend are cyclic fluctuations related to fractionation of the various eruptive packages (Hampton, 2010). The earliest eruptive packages were the McQueens Valley formation and Governors Bay formation (Barley and Weaver, 1988). A subject of debate amongst scientists is the age of these earliest formations, however it is agreed upon that the McQueens Valley formation erupted first in the area around the head of what is now Lyttleton Harbor (Barley and Weaver, 1988). These eruptions ranged in composition from high-potassium, subalkaline to silica-rich rhyolites (Barley and Weaver, 1988). The next formation, the Governors Bay formation, erupted with andesitic flows, dacite and rhyolite domes, and ignimbrites (Barley and Weaver, 1988). Geochemically, the Governors Bay formation tends to have higher concentrations of potassium and niobium than the McQueens Valley formation (Barley and Weaver, 1988). The main phase of volcanics, which are aged 11-9.7 Ma (Hampton and Cole, 2009), erupted next with transitional alkalic basaltic flows that ranged in composition from basalt through hawaiite to mugearite and benmoreite (Price et al., 1980). The later eruptive packages of Lyttleton such as the Diamond Harbor Group, aged 7.0-5.8 Ma, (Hampton and Cole, 2009) were alkalic but also transitioned slightly to theolitic (Price et al., 1980). The dykes in the Lyttleton complex range in composition from

4 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 basalt through trachyte and tend to be phenocryst-rich with up to 60% phenocrysts (Price et al., 1980). The Akaroa volcanic complex is the less studied of the two major volcanic complexes on Banks Peninsula. Stony Bay, the subject of this study is located within the Akaroa volcanic complex. While Stony Bay itself has never been studied, Price et al., 1980 did a geochemical study of the Akaroa area. They used X-ray fluorescence (XRF) to measure major element concentrations, mass spectroscopy to measure trace element concentrations, and a dispersive electron microprobe system to analyze the mineral content of samples. Their overall geochemical trends were decreasing TiO 2, FeO, MgO, CaO, and V with increased silica content of the rocks and increased Na 2 O, K 2 O, Pb, Th, U, Ba, Hf, and Nb with increased silica. The rocks sampled in the Akaroa complex were classified to be alkalic basalt and hawaiite flows, and trachytic dykes. They found that the flows of Akaroa had higher MgOx100/ (MgO + FeO), higher Na 2 O/K 2 O, and higher TiO 2 levels than those from the Lyttleton complex that had comparable SiO 2 values. Price et al. 2008 also found some general trends for Banks Peninsula as a whole. Their data suggested there are rocks that appear to be sourced from primitive mantle magma. They also found that the magma sources evolved geochemically over time due to partial melting so the lavas erupted later tend to be more alkalic and have lower MgO/FeO ratios than earlier lavas. They stated that the rocks on the peninsula were derived from the fractional removal of olivine, clinopyroxine, plagioclase feldspar, magnetite, and apatite from the basaltic magma as it ascended. Another set of geochemical studies done on Banks Peninsula by Timm et al. (2006, 2009, 2010) focused on the magma source for Banks Peninsula as a whole. They used XRF analysis for major elements and select trace elements Cr, Ni, Zr, and Sr concentrations. Ion-coupled-plasma spectroscopy (ICP-MS) was used to measure concentrations of the trace elements Rb, Ba, Y, Nb, Ta, Hf, U, Th, and Pb. Thermal ionization mass spectroscopy was utilized to analyze the isotope ratios of Sr-Nd-Pb. They found the rocks of Banks Peninsula to range in composition from basanite through tephrite to alkali baslat through perkaline rhyolite to theiite (Timm et al., 2006). Their trace element analysis identified the rocks to be enriched highly to moderately in incompatible elements with peaks at Nb and Ta. From this data, they interpreted Banks Peninsula rocks to have an ocean island basalt (OIB) type signature. According to Timm et al. this OIB type signature could have resulted from either crustal assimilation or mantle heterogeneity.

5 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 However, they stated that crustal assimilation was an unlikely cause because 50% of the crust would have to be assimilated by high-sio 2 rocks. Instead, they proposed a model of mantle heterogeneity where the mafic rocks on Banks Peninsula were derived from asthenospheric melts and the felsic rocks on Banks Peninsula derived from lithospheric melts possibly enriched by subduction-related fluids (Timm et al., 2006). They stated these conditions could have been created by a seismic low-velocity anomaly that has existed since the Cretaceous over 600 km beneath the crust stretching from West Antartica to Zealandia (Timm et al, 2010). This anomaly fed HIMU melt (high U/Pb mantle) into the upper mantle beneath Zealandia during the Cenozoic (Timm et al., 2010). This could have been the source of the OIB type signature in Banks Peninsula rocks (Timm et al., 2010). Banks Peninsula was not the sole location of intraplate volcanism during the Cenozoic. Gamble et al. (1986) and Sprung et al. (2006) among others studied some of the other regions of Cenozoic intraplate volcanism. Sprung et al. (2006) used XRF for major element analysis of samples, ICP-MS for trace element analysis, and MC-TIMS or MC-ICPMS for Hf-Nd-Sr-Pb isotope compositions of rocks collected in their study area, which included the North and South Islands of New Zealand, and the Chatham Islands. Their results for trace element analysis indicated that magma sources for intraplate volcanism in these different regions varied (Sprung et al., 2006). They found that the source for the Chatham Islands was HIMU melt, for the North Island was MORB (mid-ocean ridge basalt) dominated, and that the South Island volcanics fell within this spectrum (Sprung et al., 2006). Similar to Timm et al., they felt that HIMU melts were derived from a heterogenous lithospheric mantle and MORB melts were sourced from the asthenosphere (Sprung et al., 2006). To explain this range they proposed a model where the ratio of asthenosphere to lithosphere as the melt source is controlled by variations in lithospheric thickness as well as heat flow (Sprung et al., 2006). Different tectonic settings and rates of extension in the various areas of intraplate volcanism in and around New Zealand could have created these varied lithospheric thicknesses resulting in the different melt sources (Sprung et al., 2006).

6 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 Methods Comprehensive field mapping of the lava flows in Stony Bay was completed in February 2013. Forty samples of lava flows and other features were collected and detailed notes on their structure, texture, and composition put into a database. Twenty samples were selected for geochemical analysis based on their degree of weathering and importance within the stratigraphy. These samples were powdered using a rock crusher. Major and trace element concentrations were analyzed using a Philips PW2400 Sequential Wavelength Dispersive X-ray Fluorescence Spectrometer (Johnson 2012). Lab technicians at the University of Canterbury collected major and trace element data. For major element analysis, 1.3 g of powdered rock were fused with 6.98 g of Li 2 B 4 O 7 /Li 2 O/La 2 O 3 and a couple grains of NH 4 NO 3 at 1030 for minimum 15 minutes to create glass fusion beads (Johnson 2012). Pressed powder pellets were created for trace element analysis from 8 g of powdered rock and polyvinyl alcohol binder solution (Johnson 2012). The crushed rock and alcohol binder were combined and then pressed within a hardened steel die at 3000 psi for 10 seconds until a pellet was formed (Johnson 2012). The analysis of both types of elements was conduced using a rhodium tube set, which was set at 50kV/55mA for major elements and 60kV/46mA for trace elements (Johnson 2012). Results The major and trace element concentrations in the twenty flows sampled in Stony Bay are seen below in Table 1. The lava flows of Stony Bay sampled were mafic in composition. Their SiO 2 content ranged from 40-53% with most samples in the range of 46-52%. Total alkali content ranged from 2-7% with most in the range of 5-7%. Lavas enriched in silica tended to have higher total alkali concentrations as well (Figure 5). Most flows were hawaiite in composition but they ranged from foidite to mugearite (Figure 4). Major element analysis (Figure 5) indicated that with increased SiO 2, MgO, CaO, Fe 2 O 3, and TiO 2 tended to decrease linearly. K 2 O and Na 2 O had an increasing linear trend with higher silica concentrations. No distinguishable trend was seen for Al 2 O 3 and P 2 O 5. Trace element analysis (Figure 6) suggested an increasing linear trend for Zr, Ba, La, and Rb with higher SiO 2 levels. V tended to decrease linearly with increased silica. No significant trends were seen for Sr,

7 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 Cr, Ni, Ga, Y, Nb, Nd, Ce, Pb or Th. Differences in trace element chemistry between most samples, with a few notable exceptions, were generally very small. On the east side of the bay cyclic fluctuations between picrite basalt and hawaiite were observed in three separate groupings. Units H (47.6%), G (49.0%), and F (49.3%) (see Figure 2 for stratigraphic location) recorded an increasing SiO 2 trend up the stratigraphy. Unit I, located beneath unit H had a much higher SiO 2 concentration (51.%). Units S (51.3%) and I (51.7%) located near to each other in the stratigraphy had similarly increasing silica concentrations in the younging direction. A third grouping of this type was units L (46.8%), N (47.2%), and P (47.9%) also located near to each other in the stratigraphy. Each of these groupings also had increasing total alkali concentration up the stratigraphy as well. On the west side of the bay only two flows, unit AJ and AH had chemical compositions similar to the flows on the east side of the bay. Unit AI, a red ash horizon had significantly different major and trace element chemistry than all other samples. It had elevated Al 2 O 3 (24%, typical 17-19%) and Fe 2 O 3 (17.7%, typical 11-13%). It was depleted compared to other samples in CaO (0.79%, typical 4-9%), total alkali (2%, typical 5-7%), and P 2 O 5 (0.24%, typical 0.65-1.1%). For trace elements it was enriched in Ga (47 ppm, typical 22-26 ppm), Zr (427 ppm, typical 225-380 ppm), and Pb (17 ppm, typical <1-8.4 ppm). It was depleted in Sr (161 ppm, typical 600-900 ppm) and La (7.2 ppm, typical 25-59 ppm). The predominant feature of the west side of Stony Bay is a single massive flow approximately 70 meters tall. This flow, unit AB/AG, differed from all other samples as well. It had a much lower SiO 2 content (40%) compared with the typical 46-52% observed for the other flows. It was enriched in MgO (11%, typical 2-4%), and CaO (11% typical 5-9%). It was depleted in Al 2 O 3 compared to most other samples (12%, typical 17-19%). For trace elements it was enriched in V (~275 ppm, typical 64-193 ppm), Cr (~200 ppm, typical <3-26 ppm), Ni (186 ppm, typical <3-5 ppm), and Sr (~1080 ppm, typical 600-900 ppm). Discussion Most of these lavas fit into the eruptive scheme proposed by Johnson 2012. Silica content ranged between 46-52. Total alkali content rose with silica concentration. Typical MgO content was around 11, Ca 5-9, Al203 17-19%. These similar chemical compositions suggest a shared

8 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 magma source for most of the samples in Stony Bay and many in Okains Bay sampled by Johnson in 2012 (Johnson, 2012). The cyclic fluctuations in silica and alkali content seen on the east side of the bay for units H-G-F, S-I, and L-N-P are also similar to Johnson s findings. Johnson hypothesized that these were related to fractional crystallization processes in the source magma (Johnson 2012). The magma over time undergoes increased crystallization increasing crystal content of the flow and enriching it in SiO 2 and alkali content and transitioning the flows from picrite basalt to hawaiite and mugearite (Johnson 2012). Eventually the source magma becomes too viscous to erupt until a new more mafic and more viscous injection of fresh magma decreases the viscosity enough for a new eruption. This is signaled geochemically by a decrease in SiO 2 and alkali content as you move up the stratigraphy between two adjacent flows (Johnson 2012). Further evidence for this process in the samples for the east side of Stony Bay comes from the petrology which indicates increased phenocryst content within the successive lavas of each eruptive package (Patel 2013). Since each successive batch begins with a lava flow of similar picrite basalt composition it is probable that these eruptive packages are from the same magma source (Johnson 2012). It is difficult to accurately measure concentrations of trace elements using XRF due to the poor resolution of this method for trace element analysis. Therefore in this study, variation in trace elements was not used as a singular distinguishing feature for any sample. If large differences of two significant figures or more were seen for a trace element in conjunction with differences in major element chemistry then the lavas were viewed as distinct. The first outlier suggested by major and trace element analysis was unit AI, a red ash horizon located between two flows on the west side of the bay. It is possible that unit AI differs from the other samples due to it being derived from a separate magmatic source (Johnson 2012). Alternatively, the sample could have been chemically altered from weathering over time. Unit AI was located within a zone of highly weathered lavas so this is the probable cause for alkali, and calcium depletion. The sample was not different from the other lavas in its silica content, an element more resistant to weathering, which suggests it was originally of a similar composition to the other flows. The second significant outlier was unit AB/AG, a 70 meter high lava flow that makes up the majority of the west side of Stony Bay. This lava was found to be of a much lower silica

9 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 concentration (40%) than other samples which had 46-52% silica content. This lava flow also did not have similar chemical composition to any samples collected by past research by Timm et al. 2009, Hartung 2011, Dorsey 1988, or Johnson 2012. The flow also differed from other samples in many major and trace elements, suggesting it is probably derived from a separate magma source (Johnson 2012). It is surprising that a low silica flow created a thick deposit. Typically more mafic flows are less viscous and thus generate thinner deposits. Further research could be conducted to investigate the possible source of this thickening as well as if flows of this type are seen elsewhere on the peninsula. On the east side of the bay, a unit called mystery unit X appears to be structurally similar to unit AB/AG. Geochemical analysis of this unit could help correlate the two sides of the bay if the two flows match and provide insight into possible paleotopography and controls on flow deposition for Akaroa Volcano. References Barley, M.E., Weaver, S.D., and De Laeter, J.R., 1988, Strontium isotope composition and geochronology of intermediate-silicic volcanics, Mt Somers and Banks Peninsula, New Zealand: New Zealand Journal of Geology & Geophysics, v. 31, p. 197-206. Coombs, D. S., Adams, C. J., Roser, B. P., & Reay, A. 2008. Geochronology and geochemistry of the Dunedin Volcanic Group, eastern Otago, New Zealand. New Zealand Journal of Geology and Geophysics, 51(3), 195-218. Dorsey, C.J. 1988. The geology and geochemistry of Akaroa volcano, Banks Peninsula, New Zealand. University of Canterbury. Geological Sciences. Gamble, J.A., Morris, P.A., and Adams, C.J., 1986, The geology, petrology and geochemistry of Cenozoic volcanic rocks from the Campbell Plateau and Chatham Rise. Late Cenozoic Volcanism in New Zealand, Vol.23, p. 344-365. Hartung, E. 2011. Early magmatism and the formation of a daly gap at Akaroa shield volcano, New Zealand. University of Canterbury. Hampton, S.J., 2010, Growth, Structure and Evolution the Lyttelton Volcanic Complex, Banks Peninsula, New Zealand [Ph.D. thesis]: Christchurch, New Zealand, University of Canterbury. Geological Sciences,. Hampton, S.J., and Cole, J.W., 2009, Lyttelton Volcano, Banks Peninsula, New Zealand:

10 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 Primary volcanic landforms and eruptive centre identification: Geomorphology, v. 104, p. 284-298. Hawkesworth, C.J., and Gallagher, K., 1993, Mantle hotspots, plumes and regional tectonics as causes of intraplate magmatism: Terra Nova, v. 5, p. 552-559. Hoernle, K., White, J.D.L., van den Bogaard, P., Hauff, F., Coombs, D.S., Werner, R., Timm, C., Garbe-Schönberg, D., Reay, A., and Cooper, A.F., 2006, Cenozoic intraplate volcanism on New Zealand: Upwelling induced by lithospheric removal: Earth and Planetary Science Letters, v. 248, p. 335-352. Johnson, J. 2012. Insights into the magmatic evolution of Akaroa Volcano from the geochemistry of volcanic deposits in Okains Bay, New Zealand. University of Canterbury Patel, S. 2013. On the subsurface evolution of batch lavas at Stony Bay, New Zealand: A study in plagioclase phenocryst development. University of Canterbury Price, R.C., and Taylor, S.R., 1980, Petrology and geochemistry of the Banks Peninsula volcanoes, South Island, New Zealand: Contributions to Mineralogy and Petrology, v. 72, p. 1-18. Rhodes, J. M. (1996). Geochemical stratigraphy of lava flows sampled by the Hawaii Scientific Drilling Project. Journal of Geophysical Research, 101(B5), 11729-11746. Sewell, R., 1990, Petrological evolution of Miocene continental intraplate volcanics of Banks Peninsula, New Zealand: Bulletin- New Mexico Bureau of Mines & Mineral Resources, v. 131, p. 238-. Sewell, R.J., 1985, The volcanic geology and geochemistry of central Banks Peninsula and relationships to Lyttelton and Akaroa volcanoes [Ph.D. thesis]: Christchurch, New Zealand, University of Canterbury. Geology. Sprung, P., Schuth, S., Münker, C., and Hoke, L., 2007, Intraplate volcanism in New Zealand: The role of fossil plume material and variable lithospheric properties: Contributions to Mineralogy and Petrology, v. 153, p. 669-687. Timm, C., Hoernle, K., Hauff, F., van den Bogaard, P., and Weaver, S., 2006, A mantle origin for the enriched signature in basalts from banks Peninsula, New Zealand: Geochimica Et Cosmochimica Acta, v. 70, no. 18,. Timm, C., Hoernle, K., van den Bogaard, P., Bindeman, I., and Weaver, S., 2009, Geochemical evolution of intraplate volcanism at Banks Peninsula, New Zealand: Interaction between asthenospheric and lithospheric melts: Journal of Petrology, v. 50, p. 989-1023. Timm, C., Hoernle, K., Werner, R., Hauff, F., den Bogaard, P.v., White, J., Mortimer, N.,

11 334 335 336 337 Figures and Garbe-Schönberg, D., 2010, Temporal and geochemical evolution of the Cenozoic intraplate volcanism of Zealandia: Earth-Science Reviews, v. 98, p. 38-64. 338 339 340 341 342 343 Figure 1. The current geologic map of Banks Peninsula, modified from Johnson (2012) and Hampton et al. (2009). Stony Bay, the research area of this study is highlighted by a blue box. The results of this study will help to improve the resolution of this map for the Stony Bay area.

12 344 345 346 Figure 2. The east side of Stony Bay with important lava flows for this study colored in. Mystery unit X is in pink, unit F in green, unit G in red, unit H in blue and unit I in yellow. A single eruptive package with increased SiO2 wt% as you move up the stratigraphy is seen for units H 347 (47.6%), G (49.0%), and F (49.3%). Unit I is from a separate batch eruption so has much higher 349 correlated with unit AB/AG on the west side of the bay. 348 350 351 SiO2 concentration (51.6%) than unit H right above it. Mystery unit X may be able to be

13 352 353 354 355 356 357 358 Figure 3. The west side of Stony Bay with lava flows important for this study colored in. Unit AB/AG is in green, unit AI in pink, unit AH in blue, and unit AJ in red. Units AH and AJ have similar chemical composition to flows on the east side. Unit AI is a red ash horizon which is alkali depleted. Unit AB/AG is a 70 m tall flow with a much lower SiO2 weight percent (40%) than any other flows in the region.

14 359 360 361 362 363 364 Figure 4. Total alkali-silica (TAS) classification diagram modeled after Cox et al., 1979 for all 20 samples collected from Stony Bay. Most samples lay on a trend from picrite basalt to hawaiite to mugearite. The major outliers were unit AI (pink diamond) an ash horizon with depleted total alkali concentration and unit AB/AG (green triangles), a nephel lava flow from the west side of the bay.

15 365 366 367 368 369 Figure 5. Variation diagrams for selected major elements versus SiO 2. Data collected via XRF analysis.

16 370 371 372 373 Figure 6. Variation diagrams for selected trace elements versus SiO 2. Data collected via XRF analysis.

17 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 Table 1. Major and trace element concentrations of 20 Stony Bay samples analyzed via XRF. Table continued on page 17. Sample SB- AB SB- AB2 SB- AG SB- AH SB- AI SB- AJ SB-D SB-F SB-G SB-H Major elements (wt% determined by XRF) SiO 2 40.98 40.11 40.10 51.87 50.15 48.29 48.12 49.38 49.00 47.65 TiO 2 3.53 3.59 3.70 2.26 3.48 2.84 3.12 2.59 2.99 3.12 Al 2 O 3 12.25 12.36 12.26 18.52 24.06 19.40 17.18 17.53 17.06 17.62 Fe 2 O 3 14.20 14.46 14.87 11.66 17.70 11.13 13.24 12.30 13.11 13.09 MnO 0.197 0.199 0.200 0.160 0.185 0.153 0.199 0.197 0.174 0.173 MgO 11.22 11.41 10.97 2.03 1.41 3.35 3.87 3.46 3.58 4.09 CaO 10.91 11.13 11.39 5.39 0.79 9.03 7.60 7.12 7.20 8.50 Na 2 O 4.74 4.80 4.32 4.97 1.44 3.96 4.16 4.45 4.38 3.75 K 2 O 1.15 1.11 1.26 2.32 0.55 1.29 1.63 1.95 1.75 1.36 P 2 O 5 0.82 0.84 0.94 0.83 0.24 0.54 0.89 1.03 0.77 0.66 Trace elements (ppm; determined by XRF) V 270 271 284 70 113 156 131 97 181 180 Cr 198 205 201 <3 8 12 <3 <3 8 27 Ni 186 187 187 4 7 15 4 4 16 45 Zn 124 126 131 114 127 89 134 128 120 108 Ga 25 23 23 23 26 47 21 25 23 25 Rb 22 21 27 59 48 24 32 41 43 26 Sr 1073 1063 1099 721 161 901 978 785 691 754 Y 30 30 31 44 34 30 38 44 40 31 Zr 273 272 290 379 427 225 302 364 270 261 Nb 83 81 86 89 102 57 82 92 64 65 Ba 322 312 354 580 317 384 427 555 396 335 La 25 25 36 50 7 26 32 59 38 25 Ce 90 87 103 104 84 69 88 110 82 75 Nd 28 50 57 47 47 29 38 37 54 32 Pb 3 1 <1 4 17 <1 2 2 6 2 Th 8 4 4 7 9 12 4 5 7 8

18 389 390 Table 1. Continued Sample SB-I SB-J SB-L SB-M SB- N1 SB-P SB-S SB-V SB- W SB-U Major elements (wt% determined by XRF) SiO 2 51.69 47.27 46.80 51.05 47.15 47.93 51.34 46.98 52.54 48.25 TiO 2 2.26 3.44 3.46 2.50 3.11 3.24 2.39 3.15 2.13 2.94 Al 2 O 3 17.02 19.81 17.24 17.95 17.39 18.86 17.00 18.28 18.54 17.08 Fe 2 O 3 12.39 13.73 13.98 12.51 12.78 13.13 12.55 12.47 11.49 13.00 MnO 0.197 0.203 0.175 0.165 0.181 0.150 0.147 0.173 0.165 0.203 MgO 2.29 2.44 4.12 1.85 4.91 3.00 2.30 3.98 1.91 3.88 CaO 5.49 7.00 8.37 5.72 8.90 7.83 5.69 9.25 4.85 7.61 Na 2 O 5.25 3.89 3.62 4.84 3.65 3.78 5.29 3.74 5.18 4.40 K 2 O 2.39 1.50 1.42 2.32 1.29 1.41 2.29 1.33 2.44 1.68 P 2 O 5 1.04 0.71 0.82 1.14 0.65 0.68 0.99 0.65 0.75 0.95 Trace elements (ppm; determined by XRF) V 101 187 193 92 172 187 108 185 64 104 Cr <3 15 25 <3 22 18 <3 22 <3 <3 Ni 15 30 33 4 50 45 6 34 8 <3 Zn 140 125 128 133 97 104 143 103 114 121 Ga 26 26 24 26 23 23 25 23 25 25 Rb 62 26 26 56 25 27 58 25 55 33 Sr 614 714 798 662 763 777 640 791 661 800 Y 46 34 35 39 33 30 47 33 40 41 Zr 352 268 278 379 250 258 337 247 397 315 Nb 82 68 75 94 62 65 79 62 92 84 Ba 536 379 412 634 322 376 496 323 658 447 La 44 35 37 51 36 25 50 28 56 40 Ce 103 88 84 94 81 65 101 69 108 100 Nd 47 33 38 45 30 35 61 32 51 28 Pb 8 <1 <1 5 2 <1 7 2 7 2 9 9 6 5 10 6 4 8 4 11 8