THE GEOCHRONOLOGY OF SCOTTISH CARBONIFEROUS VOLCA:SM Hugh AF de Souza Ph.D ...

342 Pages·2016·12.57 MB·English

THE GEOCHRONOLOGY OF
SCOTTISH CARBONIFEROUS VOLCA:SM
Hugh A. F. de Souza
Ph.D Thesis
University of Edinburgh
March 1979
0
1
DECLARATION
I declare that this thesis is entirely- my own work and has not
been submitted for a degree at any other university.
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ABSTRACT
The aims of this study were to deteimine the temporal relation-
ships, using K-Ar dating, of the complex magmatic events in the
Scottish Carboniferous Petrographic Province. Volcanic activity was
almost continuous in. Scotland between the early Carboniferous and the
early Permian; 'it reached its maximum intensity in the IJinantian. The
main problems studied were the age relationships among the major
Dinantian lava piles which are coi;piiat (cid:9) r (cid:9) uncertainties
and those between the Silesian extrusive activity and the large number
of post-Dinantian intrusions. Various, aspects of Scottish Carboniferous
geology are reviewed.
The reliability of K-Ar whole-rock basalt ages has been assessed.
Replicate age determinations on single flows agree well. The degree
of sample alteration is a broad indicator' of the extent of argon loss.
The least altered whole-rock sample dates agree with those of good
mineral geochronometers. Samples with large phenocrysts tend to have
age, variations of several percent. Hence, only size fractions of
4200,iLm were analysed. 'Crushing reduced the amount of green alteration
minerals which are liable to lose argon and contain much of the
sample atmospheric argon. The use of isochron analysis for objective
age interpretation has not been successful in practice because of argon
loss and atmospheric contamination in samples, despite precautions
taken to reduce the latter.
Over 30 dates from the Clyde Plateau Lavas, indicate that the bulk
of the lavas were erupted between 330 and 320 m.y.(mid-Dinantian or
later) contrary to earlier correlations with the early Dinantian
East Lothian lavas, previously dated at c. 345 m.y. Estimated ages of
the Burntisland and West Lothian lavas are C. 320 m.y. and 320-310 m-y.
respectively. It is argued that geological evidence indicates that
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acid and basic Border intrusions dated at c. 330 my. and the early
Asbian Glencartholrn Tuff Beds are related. Thus, in the Dinanitian, it
is proposed that an early phase of volcanism, 355-340 m.y., was
followed by uplift and erosion and then by intense volcanism from
330 to 315 rn.y.
The alkaline dolerites of Fife and the. Lothians were intruded
between 315 and 295 m.y. Following the cessation of activity in
West Lothian at C. 310 M.Y. volcanism in the eastern Midland Valley.
was sporadic and mainly intrusive. The Ayrshire Passage Group lavas
are 300-305 m.y. and were succeeded by a period of teschenite intrusion
between 300 and 290 m.y. in that area. The quartz-dolerite phase at
290-295 m-y. was followed by the emplacement of neck intrusions in
East Fife between 280 and 290 m.y. The early Permian Mauchline lavas
and the Ayrshire kylitic intrusions are 280 ± 5 m.y. The intrusion
of the N. Ayrshire and Glasgow teschenites and the Highland lamprophyres
occurred at c. 270 m.y.
A revised Carboniferous time-scale in which the Dinantian-
Silesian boundary is at 315 m.y. and with changes in other. boundaries
based on the above ages, is suggested.
It is shown that the Carboniferous magma becomes increasingly
undersaturated with time and that a similar short-term trend occurs
within each volcanic episode. The main features of Midland Valley
magmatism can be explained by the growth, maturity and decline of a
single thermal cycle, complicated by outside tectonic control.
The tectonic setting of Midland Valley volcanism to the rear of
a Hercynian destructive margin may be analogous to that of basaltic
volcanism in East Asia, developed well to the rear of the Western Pacific
arcs. Mantle upwelling related to subduction results in alkaline
magmatism in areas of tensional tectonics.
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LIST OF CONTENTS
Page
Chapter 1 (cid:9) INTRODUCTION 1
Chapter 2 (cid:9) THE CARBONIFEROUS AND PERMIAN GEOLOGY OF SCOTLAND 6
2.1 Structural Evolution of the Midland Valley 6
2.2 Stratigraphy 12
2.3 Carboniferous-Permian Volcanism in Scotland 19-
Chapter 3 (cid:9) A. (cid:9) STUDIES IN POTASSIUM-ARGON DATING 38
3.1 The Potassium-Argon Method 38
3.2 K-Ar Dates from Minerals 42
3.3 K-Ar Dating of Whole-Rock Basalts 51
3.4 Sources of Atmospheric Contamination and Blank
Measurements 63
3.5 K-Ar Isochrons (cid:9) . (cid:9) 67
.
Chapter 3 (cid:9) B. (cid:9) REVIEW OF CARBONIFEROUS AGE DATA 74
3..6 The Carboniferous Time-Scale 74
3.7 Carboniferous-Permian Ages from.Scotland 77
Chapter 4 (cid:9) THE AGE OF THE CLYDE PLATEAU LAVAS 90
4.1 Northern Clyde Plateau 90
4.2 Kintyre (cid:9) . 95
4.3 The Cumbraes (cid:9) . 98
4.4 The Southern Clyde Plateau (cid:9) . 99
4.5 Significance of the Clyde Plateau Ages . 105
4.6 Further Dating 109
4.7 Evolution of the Clyde Plateau Lavas 112
V
Page
Chapter 5 DINANTIAN AND EARLY SILESIAN VOLCANICITY IN THE
(cid:9)
EASTERN MIDLAND VALLEY AND ALONG THE BORDERS
133
(cid:9)
5.1 Lower Oil Shale Group Volcanicity
133
(cid:9)
5.2 Upper Oil Shale Group Volcanicity- at Burntisland 138
5.3 Late Dinantian and early Silesian Volcanicity in
(cid:9)
West Lothian
139
(cid:9)
5.4 Intrusions 141
(cid:9)
5.5 Significance of the Forth Valley Group Ages 144
(cid:9)
5.6 Dinantian Volcanism in Southern Scotland 148
Chapter 6 SILESIAN AND EARLY PERMIAN VOLCANICITY IN
(cid:9)
AYRSHIRE 170
(cid:9)
6.1 The Ayrshire Passage Group Lavas 170
(cid:9)
6.2 The Mauchline Volcanic Group 173
(cid:9)
6.3 The Central Ayrshire Alkaline Dolerites 175
(cid:9)
6.4 Discussion 181
Chapter 7 THE LATE CARBONIFEROUS AND EARLY PERMIAN
(cid:9)
INTRUSIONS OF THE MIDLAND VALLEY 196
(cid:9)
7.1 The Quartz-Dolerites 196
7.2 The Alkaline Dolerites of Glasgow and North Ayrshire 201
(cid:9)
7.3 The East Fife Intrusions 207
(cid:9)
Chapter 8 THE CABONIFEROUS TIME-SCALE 220
(cid:9)
8.1 The Dinantian Time-Scale 220
(cid:9)
8.2 The Silesian Time-Scale 225
(cid:9)
Chapter 9 SCOTTISH CARBONIFEROUS VOLCANISM: A SYNTHESIS 229
(cid:9)
9.1 Southern Scotland
229
(cid:9)
9.2 Midland Valley 231
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Page
Chapter 10 THE CHEMICAL EVOLUTION OF THE CARBONIFEROUS-
PERMIAN MAGMAS (cid:9) 241
10.1 Review of Petrochemistry and Petrogenesis. (cid:9) 241
10.2 Variations in Basalt Chemistry with Time (cid:9) 246
10.3 Magmatic Evolution of the Midland Valley (cid:9) 248
Chapter 11 TECTONIC SETTING OF MIDLAND VALLEY VOLCANISM (cid:9) 253
11.1 Present Views on the Tectonic Setting of the
Midland Valley (cid:9) 253
11.2 Relationship of Midland Valley Tectonics to the
Hercynian Orogeny (cid:9) 256
11.3 Tectono-magmatic Evolution of the Midland Valley (cid:9) 264
Chapter 12 DISCUSSION OF K-AR RESULTS AND GENERAL CONCLUSIONS 272
12.1 Discussion of K-Ar Results (cid:9) 272
12.2 Conclusions (cid:9) 276
ACKNOWLEDGEMENTS (cid:9) 279
APPENDIX I (cid:9) Potassium-argon Techniques and Sample
Preparation (cid:9) 280
APPENDIX II Locations and Descriptions of Dated Samples (cid:9) 287
APPENDIX III 1. Microprobe Analysis (cid:9) 298
2. Sources of Chemical Data and its
Treatment (cid:9) 298
BIBLIOGRAPHY (cid:9) 304
(cid:9)(cid:9)(cid:9)(cid:9)
vii
LIST OF FIGURES
Page -
2.1 Outcrop of Carboniferous and Permian Rocks in the
Midland Valley 33
2.2 Outciop of Carboniferous and Permian Rocks in
Southern Scotland 34
2.3 Range and Distribution of Carboniferous and Permian
Volcanics in Central and Southern Scotland 35
3.1 Histograms of Apparent of Kintyre Basalts 80
!1y3i (cid:9)
36 A
3.2 Plot of Total 40Ar v Total (cid:9) for the Kintyre
Basalts and a Passage Group Lava 81
. (cid:9)
3.3 Plot of Blank Values against Position in Multiple-
loader (cid:9) . 82.
36 A
3.4 Plot of (cid:9) against Baking Times 83
33..55 40Ar/ 6Ar v 40 Isochron Diagram for Samples
containing Initial Argon with a Ratio less than
Atmospheric 84
. (cid:9)
3.6 (cid:9) Isotope Ratio Plot illustrating how Appparently
Significant Isochrons may be Obtained 85
3.7 The Current Carboniferous Time-Scale 86
3.8 Summary of Scottish Carboniferous Age Determinations 87
4.1 Outcrop of the Northern Clyde Plateau Lavas and
Location of Dated Samples 116
4.2 Outcrop of Carboniferous Lavas and Sediments in
Southern Kintyre and Location of Dated Samples 117
4.3 Isotope Ratio Plot of Kintyre Basalts 118
4.4 Isotope Ratio Plot for MV226(cid:9) - 119
4.5 Outcrop of the Southern Clyde Plateau Lavas and
Location of Dated Samples .120
4.6 (cid:9) Histogram of Dates from the Clyde Plateau Lavas (cid:9) 121
5.1 (cid:9) Outcrop of Carboniferous Lavas and Intrusions around
the Firth of Forth and Location of Dated Samples (cid:9) 155
5.2 (cid:9) Rb-Sr Isochron Diagram for the Traprain Law Phonolite (cid:9) 156
5.3 (cid:9) Histogram of Dates of Intrusions from around the
Firth of Forth (cid:9) . (cid:9) 157
- (cid:9) viii
Page
5.4 Alkali-silica Diagram for Lavas and Intrusions from
Fife and the Lothians 158
5.5 Outcrop of CarboniferousVolcanics along the. Borders
and Location of Dated Samples
159
5.6 Histogram of Dates of Lavas and Intrusions from the
South of Scotland 160
0 A
5.7 4 rad v 40K Diagram for the Southern Scotland
Whole-rock Samples (cid:9) . 161
5.8 Isotope Ratio Plot for the. Southern Scotland
Whole-rock Samples 162
6.1 Outcrop of Lavas and Intrusions in North and Central
Ayrshire and Location of Dated Samples 186
6.2 Histogram of Dates from the Passage Group Lavas 187
6.3 Isotope Ratio Plot for the Passage Group Lava's 188
6.4 Sketch Map showing the Distribution of Intrusions in
South-Central Ayrshire and Location of Dated Samples 189
6.5 Histogram of dates of Cantral Ayrshire Lavas and
Intrusions 190
6.6 Differentiation Index plotted againDegree of Silica-
saturation for the Silesian Lavas and Intrusions of
Ayrshire (cid:9) . (cid:9) 191
.
7.1 Distribution of Quartz-Dolerites and Tholeiites in
Central-Scotland and Location of Dated Samples 212
7.2 Isotope Ratio Plot for Minerals from the Quartz-Dolerites. 213
.
7.3 Differentiation Index plotted against Degree of Silica-
Saturation for the East Fife Intrusions (cid:9) 214
.
7.4 P25 against Tb 2 for the East Fife Intrusions 215
8.1 The Revised Carboniferous Time-Scale 228
9.1 Possible Extent of Early Visean Lavas in the Midland
Valley (cid:9) . 240
10.1 Age against Degree of Silica-saturation for
.
Carboniferous-Permian Alkaline Basic Rocks 251
10.2 Age against Degree of Silica-saturation for all
Carboniferous-Permian Basic Rocks (cid:9) . 252
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Page
11.1 Histogram of Ages of Carboniferous Rocks from
Western and Central Europe (cid:9) 268
11.2 Divisions of Hercynian Europe (cid:9) 269
11.3 Plate Configurations in the Dinantian (cid:9) 269
11.4 Schematic Diagram illustrating a Possible Plate
Tectonic Model for Western Europe in the Dinantian (cid:9) 270
11.5 Strike-slip Faults between Newfoundland and Britain
in the Upper Palaeozoic (cid:9) 271