Eruptive Cycles within Akaroa Lava Flows and Recognition...
Transcript of Eruptive Cycles within Akaroa Lava Flows and Recognition...
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Eruptive Cycles within Akaroa Lava Flows and Recognition of 1
Contemporaneous Explosive Eruptions 2
1,2Emma Goldrick and 1Samuel Hampton 3
1Department of Geological Sciences, University of Canterbury, Private Bag 4800, 4
Christchurch, New Zealand 5
2Department of Geology and Geophysics, Yale University, 210 Whitney Ave, New Haven, 6
CT 7
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ABSTRACT 9
Geochemical and stratigraphic study of the Akaroa Volcanic Complex can 10
reveal pre eruptive processes and inform eruption style. Volcanic deposits record 11
many details from their formational history, including source material, magma 12
chamber dynamics, and eruption style. Geochemistry can be used to show the 13
evolution of stacked lava flow composition as well as the beginning of new volcanic 14
cycles. All data must be taken into account when studying this complex, intraplate 15
system. Previous assumptions of a single vent, shield volcano will continue to be 16
revised as more evidence shows Akaroa as a large complex with many vents 17
erupting from shallow lava chambers sourced by a deep reservoir. This study 18
analyzes lava flow and ash deposits in the southeastern area of the volcano to create 19
an eruptive model recognizing contemporaneous explosive eruptions. 20
INTRODUCTION 21
Volcanic deposits on Banks Peninsula tell a geological history, reflecting the 22
processes that occurred deep below the surface millions of years ago. Banks 23
Peninsula offers a unique view on volcanism due to its intraplate nature, a less well 24
understood tectonic setting for volcanism. Geochemistry, mineral textures, and 25
stratigraphic sequencing of rock samples provide clues to reconstructing the nature 26
of past eruptions. The Akaroa Volcanic Complex on Banks Peninsula, NZ shows 27
clear, cyclic geochemical trends repeating all over the peninsula (Beckham 2015, 28
Johnson 2012). Beckham (2015) studied six eastern bays to create a shallow magma 29
chamber model. This model explains Akaroa’s the geochemical evolution from 30
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picritic basalt to trachyte and decreasing phenocryst percentage within successive 31
lava flows. 32
This study seeks to add three transects in the southeastern area of the 33
volcano (Figure 1) to the geochemical database, analyse the beginning stages of the 34
batch eruption sequence, and investigate Sewell et al.’s 1992 map of the peninsula. 35
GEOLOGICAL BACKGROUND 36
Banks Peninsula, located on New Zealand’s South Island east coast, consists 37
of two Miocene volcanic complexes. The Lyttelton Volcanic Complex (12.3-10.4 Ma) 38
is older but the younger Akaroa Volcanic Complex (9.4-6.8 Ma) constitutes two 39
thirds of the peninsula’s rocks (Weaver and Smith, 1989). The two complexes are 40
thought to be a result of two large lithospheric detachments, causing asthenospheric 41
upwelling and large intra-plate eruptions (Timm et al., 2009). Previously the 42
volcanoes were thought of as simple composite shield volcanoes fed by intraplate 43
dykes, but more recent studies have revealed a more complex picture (Sewell et al., 44
1992). 45
There is one 1:100000 scale geologic map of the peninsula created in 1992 46
and described by Sewell and Weaver in a series of guides. The map was created on 47
the assumption of a singular vent for both the Lyttleton and Akaroa complexes 48
(Sewell et al., 1992). Recent work undertaken by Hampton and Cole (2009) was able 49
to map 15 separate eruptive vents for the Lyttleton volcanic complex, showing how 50
much revision is needed in the Sewell (1992) map and formation units. 51
The Akaroa Volcanic Complex formation has been studied most recently by 52
Johnson (2012), Patel (2013), and Beckham (2015). The volcanics are 53
predominantly lava flows ranging in composition from picrite basalt to benmorite 54
(Beckham, 2015). Johnson (2012) correlated cycles of repeating geochemical 55
sequences spatially around different parts of the volcano, suggesting a model of 56
many eruptive vents similar to Hampton and Cole (2009). These batches of lavas are 57
sourced from multiple shallow magma chambers beneath the peninsula, fed by a 58
much deeper reservoir linked to the lithospheric detachment (Johnson, 2012, Timm 59
et al., 2009). This gives the whole complex similar geochemical trends while having 60
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varying evolutions depending on the individual vent and shallow chamber. 61
Replenishing events begin the eruptive cycle with picrite basalt containing altered 62
crystal inclusions and over time to become more aphyric evolved lavas (Beckham, 63
2015). These altered crystals are the remnants from the past cycle’s unerupted 64
magma, essentially having new cycles start off by clearing out the shallow magma 65
chamber (Beckham, 2015). 66
METHODS 67
Five transects were taken in southeastern bays in Banks Peninsula 68
(Haylock’s, Damon’s and Flea Bay South, North and North East) resulting in 49 69
samples. Fieldwork was conducted in July 2015 and overseen by Frontiers Abroad. 70
Samples were taken in stratigraphic order, working either from the top to the 71
bottom of ridgelines or from the bottom to top. 72
Whole-rock geochemical data obtained from X-Ray Fluorescence and 73
Diffraction (XRF) was organized based on stratigraphy and plotted in IgPet2012 to 74
identify rock type, magma batches, and trace element variation within picritic 75
basalts. Rock type was determined through the Total Alkali-Silica Diagram. Element 76
and compound trends were plotted to define the boundaries of individual batches 77
and spatially mapped onto the study area (Figures 2-4). Thin sections were 78
photographed under normal and cross polarized light to highlight textural features. 79
Mineral textures were analysed according to the Beckham (2015) Petrographic 80
Guide (Figure 6) based off of Viccaro et al. 2010. 81
Shankle (2015) sampled 17 ash beds across Banks Peninsula and obtained 82
XRF/XRD data and conducted thorough thin section microscopy analysis. 83
RESULTS 84
Substantial, stratigraphic geochemical data was obtained for three of the five 85
transects (Haylock’s Bay, Damon’s Bay and Flea Bay South) taken in southeastern 86
Banks Peninsula. 37 data points were organized vertically by compounds and 87
elements to show geochemical variations within batches (Figures 2-4). Thin sections 88
were compared under normal and cross polarized light. Batches with three or more 89
sequential samples were analysed under thin section microscopy, looking for trends 90
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and patterns within the matrix and mineral composition as the batch evolves 91
(Figures 7-9). 92
DISCUSSION 93
UNIT DISTINCTION IN AKAROA COMPLEX 94
In 1992 the New Zealand Geological Survey created the first 1:100000 scale 95
map of Banks Peninsula. In this map there is a clear distinction between two 96
formations in the study area as seen in Figure 10. These two formations are the 97
French Hill Formation (light green) and the Te Oka Formation (dark green). As 98
listed in “Geology of the Akaroa West Area”, the Te Oka Formation unconformably 99
overlays the French Hill formation and is marked by a shift from porphyritic varying 100
basalts to steeply dipping aphyric hawaiite flows. Two transects collected by 101
Frontiers Abroad students crossed over the dividing line between the two 102
formations and two stayed completely within the French Hill Formation, as shown 103
by red lines in Figure 10. 104
A spider plot using Pearce 1983 of trace elements in picritic basalts from the 105
four transects was created in IgPet2012 (Figure 11). There appears to be no 106
significant variation between the trace elements in varying transects or within the 107
two transects that cross the formation’s boundary (Damon’s Bay and Flea Bay 108
South). Field observations recorded by Frontiers Abroad students showed no sign of 109
an unconformity within the Damon’s Bay or Flea Bay South transects. In addition, 110
Sewell & Weaver (1990) state Te Oka Formation rock type is exclusively hawaiite. 111
Total Alkali- Silica Diagrams created in IgPet2012 list rock types from these 112
transects as varying from picrite basalt to hawaiite. 113
These results indicate that a large-scale revision must take place in analysing 114
the formation units of the Akaroa Volcanic Complex. The origin of the distinction 115
between the Te Oka and the French Hill formation must be questioned. With new 116
research indicating the complex’s formation came from multiple vents it would be 117
wise to create a new map of the whole volcano, as local formations may appear 118
depending on their spatial relation to varying vents. This map has already begun 119
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through the collaboration of many generations of Frontiers Abroad students, but 120
should be opened to all geologists wishing to join. 121
ERUPTIVE MODEL 122
Batches of evolving geochemistry within bay transects are consistent with 123
Beckham's (2015) shallow magma chamber model designed for eastern bays in 124
Banks Peninsula. Transects in Figures 2-4 show clear, cyclic geochemical evolution 125
where increasing SiO2 corresponds to increasing K2O, MgO, P2O5 and Zr and 126
decreasing Al2O3, FeO, V, and Sr. The consistency within these transect's 127
geochemistry implies the southeastern bays went through magma recharge events 128
(Figure 12). It is possible one magma chamber was not shared by all the samples, 129
but it is likely the same primitive source underlay the volcano. 130
Thin sections show evolution of melt through mineral percentages 131
and textures. As magma is recharged into the shallow chamber it mixes 132
leftoverphenocrysts, partially melting or refractionating crystals, leading to a 133
plethora of textures and higher mineral percentages seen in the early picrite 134
basalts (Figures 7-9). Sieving, patchy cores, and resorbed rims all support 135
thehypothesis of an intrusive, hot melt signalling the beginning of a new 136
batch and altering existing crystal cumulate mush. As the source for the 137
injectiondepletes, mixing stops and flows become more aphyric as well as evolve 138
intohigher silica lavas. In the southeastern bays the lavas evolved from picrite 139
basalt to hawaiite to mugearite. However, before these lavas even erupted, another 140
important stage in the batch cycle occurs. 141
Shankle (2015) found that ash deposits on Banks Peninsula are almost 142
exclusively sourced from early stage picritic magma. This would indicate that 143
explosive, ash producing eruptions only occurred early in the eruption sequence 144
when the newly injected magma was still primitive. The ash deposits sampled were 145
almost all found stratigraphically below picrite basalt flows (Figure 13). As a result a 146
collaborative model has been developed to explain the evolving nature of the 147
Akaroa Volcano going from explosive to effusive in eruption style. 148
Eruptive Model Stages 149
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1) A capped vent traps volatiles within the vent and the attached shallow magma 150
chamber, crystal cumulates sit at the bottom of the chamber, leftover from the last 151
magma batch 152
2) A new injection of magma into the chamber begins to mix the crystals, partially 153
melting and recrystallizing them, as well as increasing the pressure within the 154
chamber and conduit as volatile bubbles grow and coalesce 155
3) The cap fails at a critical pressure resulting in an explosive eruption driven by the 156
rapid expansion of volatiles, depositing layers of ash – immediately followed by 157
continued explosive activity, erupting picritic basalt containing the sieved and 158
resorbed minerals phenocrysts 159
4) The magma injection is over and explosive eruptions become less frequent, 160
magma within the chamber begins to fractionate new crystals which sink to the 161
bottom without the mixing the injection had provided 162
5) Lava erupted becomes more evolved and aphyric over time, from picrite basalt to 163
hawaiite to mugearite, and fractionated crystals build up on the bottom of the 164
magma chamber 165
6) In the final stages the evolved, aphyric lava is erupted effusively and the conduit 166
closes signifying the end of the injected lava batch and returning to stage 1 167
CONCLUSIONS 168
The shallow magma chamber model for Akaroa developed by Johnson (2012) 169
and refined by Beckham (2015) applies to the sampled southeastern bays: 170
repeating geochemical and thin section textural evolutionary trends matched 171
those described in Beckham (2015) 172
Formation of Akaroa complex in the southeastern area of the volcano 173
requires revision: no significant geochemical or morphological differences 174
were found to distinguish between the French Hill Formation and the Te Oka 175
Formation, no boundary between the two was found in transects 176
Ash deposits can inform on early eruption cycle dynamics: in conjunction 177
with Shankle (2015), a model was created to integrate an early explosive 178
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eruption stage, supported by intermittent ash layers with picrite basalt 179
origins, into the already existing shallow magma chamber model 180
Future study should focus on pre eruptive magmatic processes revealed 181
through thin section crystal alteration, in addition the field area should be 182
revisited to find more ash deposits overlain by lava flows to further the 183
model created here 184
ACKNOWLEDGMENTS 185
A massive thank you to Frontiers Abroad and the University of Canterbury 186
for the opportunity to conduct this research. I would like to acknowledge the help 187
and guidance given by Dr. Sam Hampton, Mr. Max Borella, Dr. Darren Gravley, Dr. 188
Travis Horton, Ms. Elisabeth Bertolett, and Mr. Rob Spiers. I would also like to thank 189
my fellow FA students for inspiring new questions and especially Maddie for her 190
collaboration. Lastly, thank you to Zoe for supplementing me with technology while 191
in New Zealand. 192
FIGURES 193
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Figure 1 195
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Figure 2 197
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Figure 3 199
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Figure 4 201
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Figure 5203
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Figure 6 205
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Figure 7 207
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Figure 8 210
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Figure 9 212
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Figure 10 214
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Figure 11 216
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Figure 1219
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Figure 13222
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FIGURE CAPTIONS 223
Figure 1. Highlighted study area on a map of Banks Peninsula taken and outlined on 224
Google Earth 225
226
Figure 2. Haylock’s Bay stratigraphic geochemical plot. Horizontal dashed lines 227
represent batch dividers, vertical solid lines represent related flows within single 228
batches, and vertical dashed lines represent unrelated flows between two batches. 229
Geochemical data trends shown for MgO, SiO2, Al2O5, K2O, P2O5, FeO, V, Zr, and Sr. 230
231
Figure 3. Damon’s Bay stratigraphic geochemical plot. Horizontal dashed lines 232
represent batch dividers, vertical solid lines represent related flows within single 233
batches, and vertical dashed lines represent unrelated flows between two batches. 234
Geochemical data trends shown for MgO, SiO2, Al2O5, K2O, P2O5, FeO, V, Zr, and Sr. 235
236
Figure 4. Flea Bay South stratigraphic geochemical plot. Horizontal dashed lines 237
represent batch dividers, vertical solid lines represent related flows within single 238
batches, and vertical dashed lines represent unrelated flows between two batches. 239
Geochemical data trends shown for MgO, SiO2, Al2O5, K2O, P2O5, FeO, V, Zr, and Sr. 240
241
Figure 5. Geochemical plots mapped spatially onto the study area. Red lines indicate 242
transects and arrows within the red lines indicate stratigraphic up. 243
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Figure 6. Beckham’s (2015) petrographic guide created using Viccaro et al. (2010) 245
as a model. 246
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Figure 7. Batch 1 from the Haylock’s Bay transect, resorbed rims and melt inclusions 248
at the beginning of the batch indicating a magma recharge event with stages of rapid 249
crystal regrowth. 250
251
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Figure 8. Batch 2 from the Damon’s Bay transect, large phenocrysts with varying 252
textural indicators begin this batch, steadily decreasing in size and percentage as 253
well as plagioclase microlites in the matrix becoming more aligned. 254
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Figure 9. Batches 1 and 10 from the Flea Bay South transect, batch 1 evolving from a 256
highly altered, crystal rich picrite basalt to an almost aphyric mugearite, batch 10 257
evolving from a porphyritic flow with clear resorbed rims to an aphyric, aligned 258
flow. 259
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Figure 10. Map area taken from Sewell et al. (1992) showing the four collected 261
transects as red lines. 262
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Figure 11. Spider diagram showing picrite basalt flows from Damon’s Bay, Flea Bay 264
South, Flea Bay Northeast, and Haylock’s Bay transects. 265
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Figure 12. Goldrick and Shankle (2015) eruptive model that take contemporaneous 267
explosive eruptions and lava flows into account as explained by the captions below: 268
1) A capped vent traps volatiles within the vent and the attached shallow magma 269
chamber, crystal cumulates sit at the bottom of the chamber, leftover from the last 270
magma batch 271
2) A new injection of magma into the chamber begins to mix the crystals, partially 272
melting and recrystallizing them, as well as increasing the pressure within the 273
chamber and conduit as volatile bubbles grow and coalesce 274
3) The cap fails at a critical pressure resulting in an explosive eruption driven by the 275
rapid expansion of volatiles, depositing layers of ash – immediately followed by 276
continued explosive activity, erupting picritic basalt containing the sieved and 277
resorbed minerals phenocrysts 278
4) The magma injection is over and explosive eruptions become less frequent, 279
magma within the chamber begins to fractionate new crystals which sink to the 280
bottom without the mixing the injection had provided 281
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5) Lava erupted becomes more evolved and aphyric over time, from picrite basalt to 282
hawaiite to mugearite, and fractionated crystals build up on the bottom of the 283
magma chamber 284
6) In the final stages the evolved, aphyric lava is erupted effusively and the conduit 285
closes signifying the end of the injected lava batch and returning to stage 1 286
287
Figure 13. Picrite basalt overlying a thick ash deposit in Haylock’s Bay 288
289
References 290
Beckham, E. (2015). Magmatic evolution of The Akaroa Volcanic Complex, Eastern 291
Banks Peninsula. University of Canterbury. 292
Hampton, S. J., & Cole, J. W. (2009). Lyttelton Volcano, Banks Peninsula, New 293
Zealand: Primary Volcanic Landforms and Eruptive Centre Identification. 294
Geomorphology, 104(3-4), 284-298. 295
Johnson, J. (2012). Insights into the magmatic evolution of Akaroa Volcano from the 296
geochemistry of volcanic deposits in Okains Bay, New Zealand. University of 297
Canterbury. 298
Patel, S. (2013). On the Subsurface Evolution of Batch Lavas at Stony Bay, Banks 299
Peninsula, New Zealand: A Study in Plagioclase Phenocryst Development. 300
University of Canterbury. 301
Sewell, R.J. and Weaver, S.D. (1990). Geology of the Akaroa West Area. New Zealand 302
Geological Survey, Department of Scientific and Industrial Research. 14-19. 303
Sewell et al. (1992) Geological Map of New Zealand: Banks Peninsula. 1:50 000. New 304
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Shankle, M. (2015). Ash Homogeneity Informing Eruption Dynamics on Banks 306
Peninsula. University of Canterbury. 307
Timm, C. Hoernle, K. Van Den Bogaard, P., Bindeman, I., & Weaver, S. (2009) 308
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Viccaro, M., Giacomoni, P. P., Ferlito, C., & Cristofolini, R. (2010). Dynamics of magma 312
supply at Mt. Etna volcano (Southern Italy) as revealed by textural and 313
compositional features of plagioclase phenocrysts. Lithos, 116(1), 77-91. 314