(Lesser Antilles Arc), revealed from marine sediment core U1396C

185 Pages·2017·7.85 MB·English

by  Stuart Hatter

UNIVERSITY OF SOUTHAMPTON
FACULTY OF NATURAL AND ENVIRONMENTAL SCIENCE
Ocean and Earth Sciences

THE LONG-TERM EVOLUTION OF MONTSERRAT VOLCANISM
by
Stuart James Hatter

Thesis for the degree of Doctor of Philosophy
March 2018

UNIVERSITY OF SOUTHAMPTON
ABSTRACT
FACULTY OF NATURAL AND ENVIRONMENTAL SCIENCE
Ocean and Earth Sciences
Thesis for the degree of Philosophy
THE LONG-TERM EVOLUTION OF VOLCANISM ON MONTSERRAT
Stuart James Hatter
This thesis investigates the long-term geochemical trends of volcanism on Montserrat and
Guadeloupe, islands in the northern Lesser Antilles arc, through both their terrestrial and
marine (from International Ocean Discovery Program core U1396C) records, to examine the
processes driving geochemical evolution in island arcs.
Detailed mapping of the Silver Hills, the least studied volcanic centre on Montserrat, reveals
activity was dominated by andesite dome growth and collapse. New 40Ar/39Ar ages from
terrestrial samples, combined with new palaeomagnetic and biostratigraphic ages of tephra
layers from marine sediment core U1396C, reveal the previously unrecognised overlap in
volcanic activity between the Silver and Centre Hills, and Centre and Soufrière Hills.
Pb-Nd-Sr isotope and major and trace element data for the Silver Hills show that lavas fall
into two compositional groups; ‘normal’ lavas and ‘low Al O -high Fe O ’ lavas. The latter
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group formed from parental magmas which experienced up to 25% greater plagioclase
fractionation compared to the parent magmas of the normal lavas, likely as a result of lower
magmatic H O content. There is no systematic geochemical variation between lava domes of
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the Silver Hills, but there is an observed systematic shift in 143Nd/144Nd to progressively less
radiogenic values with time, which is inferred to result from mantle heterogeneity.
Tephra layers from core U1396C have been analysed for Pb-Nd-Sr isotope and major and
trace element data, which provide a record of the geochemical evolution of volcanism on
Montserrat and Guadeloupe over the past ~4.5 Ma; 2.3 and 1.7 Ma longer than their
respective terrestrial records. The main geochemical change for the Montserrat source
region occurred ~4.5–3.9 Ma, resulting from an increase in slab sediment contribution to the
mantle wedge via an aqueous fluid. Tephra layers from Guadeloupe display greater temporal
geochemical variation, driven by crustal assimilation of mid-ocean ridge basalt.
Componentry, grain size and grain morphology analyses of the thickest tephra layer in core
U1396C derived from Guadeloupe, T , reveal it was formed by deposition from fallout from
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an eruption column, followed by a co-ignimbrite ash cloud from the same eruption. Eruption
reconstructions suggest that this deposit likely formed from a VEI 6 eruption.

Table of Contents
Table of Contents .......................................................................................................................... i
Table of Tables ........................................................................................................................... vii
Table of Figures .......................................................................................................................... ix
Academic Thesis: Declaration Of Authorship .................................................................. xi
Acknowledgements ................................................................................................................ xiii
Chapter 1: Introduction ......................................................................................................... 1
1.1 Project rationale ........................................................................................................................ 1
1.2 Lesser Antilles arc .................................................................................................................... 1
1.2.1 Geological setting and arc structure ........................................................................ 1
1.2.2 Geochemistry of the Lesser Antilles Arc ................................................................ 3
1.3 Montserrat ................................................................................................................................... 5
1.4 Guadeloupe ................................................................................................................................. 8
1.5 International Ocean Discovery Program Expedition 340 ....................................... 10
1.5.1 Expedition overview ................................................................................................... 10
1.5.2 Site U1396 ....................................................................................................................... 10
1.5.3 Previous work on site U1396 tephra layers ....................................................... 11
1.6 Thesis overview ...................................................................................................................... 12
1.7 References ................................................................................................................................. 14
Chapter 2: The evolution of the Silver Hills volcanic centre, and revised
40Ar/39Ar geochronology of Montserrat, Lesser Antilles, with
implications for island arc volcanism ....................................................... 25
Abstract .............................................................................................................................................. 25
2.1 Introduction .............................................................................................................................. 26
2.2 Geological Setting ................................................................................................................... 27
2.3 Methods ...................................................................................................................................... 29
2.3.1 Field Work ....................................................................................................................... 29
2.3.2 40Ar/39Ar Dating ............................................................................................................ 29
2.3.3 Whole Rock Geochemistry ........................................................................................ 31
2.3.4 Core Sample Acquisition ............................................................................................ 31
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2.4 Timing of Montserrat Volcanism ......................................................................................31
2.4.1 Existing 40Ar/39Ar Dates.............................................................................................31
2.4.2 New 40Ar/39Ar Dates ....................................................................................................36
2.5 Lithofacies of the Silver Hills Deposits ...........................................................................36
2.5.1 Lava and Intensely Hydrothermally Altered Lava ...........................................40
2.5.1.1 Interpretation .......................................................................................................40
2.5.2 Massive to Diffusely Stratified, Lapilli-Tuff to Tuff-Breccia (m-dsLT-
TBr) .............................................................................................................................41
2.5.2.1 Interpretation .......................................................................................................41
2.5.3 Massive Tuff-Breccia and Breccia, with Megablocks (mTBr-BrMB) ...........41
2.5.3.1 Interpretation .......................................................................................................42
2.5.4 Massive Lapilli-Tuff, with regular Lapilli-Tuff and Tuff-Breccia Lenses
(mLTlensLT-TBr) ...........................................................................................................42
2.5.4.1 Interpretation .......................................................................................................43
2.5.5 Pumiceous, Massive to Diffusely Stratified, Lapilli-Tuff to Tuff-Breccia
(pm-dsLT-TBr) ........................................................................................................43
2.5.5.1 Interpretation .......................................................................................................43
2.6 Stratigraphy of the Silver Hills ...........................................................................................43
2.6.1 Little Bay to South Drummonds, >2 Ma ...............................................................43
2.6.2 Marguerita Ghaut and North Marguerita Bay, ~1.7–1.6 Ma ........................43
2.6.3 Yellow Hole to Old Quaw, ~1.5 Ma ........................................................................44
2.6.4 North West Bluff, ~1.5–4 Ma ....................................................................................44
2.6.5 Silver Hill, ~1.4 Ma .......................................................................................................46
2.6.6 South Marguerita Bay, ~1.3 Ma...............................................................................46
2.6.7 Little Bay, ~1.0–0.8 Ma ...............................................................................................47
2.6.8 Potato Hill, ~0.8 Ma .....................................................................................................49
2.6.9 Old Quaw Ghaut, ~0.45 Ma .......................................................................................49
2.7 Expanded Stratigraphy of the Centre Hills ...................................................................50
2.7.1 East Coast, ~1 Ma .........................................................................................................50
2.7.2 Southwest Centre Hills, ~0.45 Ma ..........................................................................51
2.7.3 Dry Waterfall and Spring Ghaut, ~0.38 Ma ........................................................51
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2.8 Provenance ................................................................................................................................ 51
2.8.1 Terrestrial Deposits ..................................................................................................... 52
2.8.2 Marine Tephra Deposits ............................................................................................. 53
2.9 Petrology .................................................................................................................................... 54
2.10 Discussion .................................................................................................................................. 55
2.10.1 Evolution of the Silver Hills Volcanic Centre ..................................................... 55
2.10.2 Comparisons with Soufrière Hills and Centre Hills ......................................... 56
2.10.3 Comparisons with Other Arc Volcanic Centres ................................................. 58
2.10.4 Revised Geochronology of Volcanism on Montserrat .................................... 60
2.11 Conclusions ............................................................................................................................... 63
2.12 References ................................................................................................................................. 63
Chapter 3: Geochemical evolution of the Silver Hills volcanic centre ................. 75
Abstract .............................................................................................................................................. 75
3.1 Introduction .............................................................................................................................. 75
3.2 Geological background ......................................................................................................... 77
3.3 Methods ...................................................................................................................................... 78
3.3.1 Sample acquisition ....................................................................................................... 78
3.3.2 Whole rock geochemistry .......................................................................................... 79
3.4 Results ......................................................................................................................................... 81
3.4.1 Major and trace element data .................................................................................. 81
3.4.2 Isotope data .................................................................................................................... 83
3.5 Geochemical variation between Silver Hills domes .................................................. 84
3.6 Discussion .................................................................................................................................. 85
3.6.1 Crustal assimilation ..................................................................................................... 85
3.6.2 Petrogenesis of Silver Hills lavas ............................................................................ 88
3.6.2.1 Role of enclave fractionation .......................................................................... 88
3.6.2.2 Formation of ‘low Al2O3-high Fe2O3’ lavas ................................................ 88
3.6.3 Examining the role of magma source components in changing
143Nd/144Nd .............................................................................................................. 92
3.7 Conclusions ............................................................................................................................... 95
3.8 References ................................................................................................................................. 96
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Chapter 4: Geochemical evolution of volcanism on Montserrat and
Guadeloupe (Lesser Antilles Arc), revealed from marine sediment
core U1396C .................................................................................................... 103
Abstract ........................................................................................................................................... 103
4.1 Introduction ........................................................................................................................... 103
4.2 Geological Setting ................................................................................................................ 105
4.2.1 Lesser Antilles ............................................................................................................. 105
4.2.2 Montserrat .................................................................................................................... 106
4.2.3 Guadeloupe .................................................................................................................. 107
4.3 Methods ................................................................................................................................... 107
4.3.1 Sample acquisition .................................................................................................... 107
4.3.2 Componentry analysis ............................................................................................. 107
4.3.3 Geochemistry............................................................................................................... 108
4.4 Results ...................................................................................................................................... 109
4.4.1 Core componentry ..................................................................................................... 109
4.4.2 Mode of deposition .................................................................................................... 110
4.4.3 Tephra Layer Provenance ...................................................................................... 111
4.4.4 Temporal Geochemical Variation ........................................................................ 113
4.4.4.1 Pb-Nd-Sr isotopes ............................................................................................ 113
4.4.4.2 Trace elements .................................................................................................. 116
4.4.4.3 Major elements ................................................................................................. 116
4.5 Discussion ............................................................................................................................... 119
4.5.1 Crustal assimilation .................................................................................................. 119
4.5.2 Changes in source component contributions ................................................. 125
4.6 Conclusions ............................................................................................................................ 127
4.7 References .............................................................................................................................. 128
Chapter 5: Discovery of a large 2.4 Ma Plinian eruption on Basse-Terre,
Guadeloupe, from the marine sediment record ................................. 137
Abstract ........................................................................................................................................... 137
5.1 Introduction ........................................................................................................................... 137
5.2 Methods ................................................................................................................................... 138
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5.2.1 Geochemistry .............................................................................................................. 138
5.2.2 Age Assignment .......................................................................................................... 139
5.2.3 Grain Characteristics Analysis .............................................................................. 139
5.2.4 Componentry analysis ............................................................................................. 140
5.3 Sediment Core Observations ........................................................................................... 140
5.3.1 Tephra Provenance ................................................................................................... 140
5.3.2 Tephra Age ................................................................................................................... 141
5.3.3 Number of Eruptions ................................................................................................ 141
5.4 Mode of Deposition ............................................................................................................. 142
5.4.1 Primary fallout versus flow deposit ................................................................... 142
5.4.2 Depositional processes ........................................................................................... 143
5.4.3 Eruption column ........................................................................................................ 144
5.5 Comparison with the Subaerial Record ...................................................................... 145
5.6 Eruption Reconstruction .................................................................................................. 145
5.7 References .............................................................................................................................. 148
Chapter 6: Conclusions and future work .................................................................... 157
6.1 Conclusions ............................................................................................................................ 157
6.2 Future Work .......................................................................................................................... 159
6.3 References .............................................................................................................................. 162
Appendices .............................................................................................................................. 165
Appendix A ........................................................................................................................................ 165
Appendix B ........................................................................................................................................ 166
Appendix C ......................................................................................................................................... 166
Appendix D ........................................................................................................................................ 166
Appendix E ......................................................................................................................................... 167

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