Longman, Christopher David (1980) Age and affinity of granitic detritus in Lower Palaeozoic ...

223 Pages·2012·7.44 MB·English

Longman, Christopher David (1980) Age and affinity of granitic detritus
in Lower Palaeozoic conglomerates, S.W. Scotland: implications for
Caledonian evolution.

PhD thesis

http://theses.gla.ac.uk/3510/

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Glasgow Theses Service
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AGE AND AFFINITY OF GRANITIC DETRITUS IN
LOWER PALAE040IC CONGLOMERATES, S. W.
SCOTLAND IMPLICATIONS FOR CALEDONIAN
:
EVOLUTION
Christopher David Longman
A thesis for the degree Doctor
submitted of
Philosophy the University Glasgow
of at of
December 1980
CONTENTS
Page
1. INTRODUCTION
1.1 Prologue 1
1.2 Previous research and project
development 3
1.3 Stratigraphy 5
8
1.4 Conglomerate composition and sampling
12
1.5 Depositional environment
17
1.6 Tectonic
setting
19
1.7 Sample collection and preparation
2. GEOCHRONOLOGY
21
2.1 Analytical
methods
21
2.1.1 Rb-Sr
geochronology
23
2.1.2 Isotope dilution
25
2.1.3 Mass
spectrometry
27
2.1.4 Analytical
precision
30
2.1.5 Data
evaluation
32
2.1.6 U-Pb
geochronology
data
2.2 Geochronological
34
2.2.1 Kirkland
conglomerate
34
2.2.2 Benan
conglomerate
36
2.2.3 Kilranny
conglomerate
36
2.2.4 Tormitchell
conglomerate
36
2.2.5 Craigskelly
conglomerate
37
2.2.6 Corsewall
conglomerate
framework 38
2.3 Summary
of geochronological
3. GEOCHEMISTRY & PETROGRAPHY
59
3.1 Analytical techniques
59
3.1.1 XRF
60
3.1.2 'Wet' chemistry
60
3.1.3 Microprobe analyses
61
3.1.4 CIPW Norm
61
3.1.5 XTLFRAC
62
Analytical
3.1.6 precision
3.2 petrography
88
3.2.1 Granites
102
3.2.2 Xenoliths
Page
3.3 Problems 103
of classification
granite
3.4 Major 117
element chemistry
3.5 Trace 136
element chemistry
3.6 Geothermometry 144
3.7 Summary 146
4. CALEDONIAN EVOLUTION OF SOUTHERN SCOTLAND
147
4.1 Previous
plate-tectonic models
4.2 Evidence for fore-arc 154
a regime
160
4.3 Island development
arc
164
4.4 Midland Valley as a marginal basin
for the
4.5 Plate-tectonic model
166
Scotland
development of southern
169
4.6 Implications of the model
4.6.1 Agreement/disagreement
with
169
models
previous
170
4.6.2 Dalradian sedimentation
170
4.6.3 Timing of events
to
4.6.4 Relationship other episodes
174
intrusion
of
granite
177
4.7 Correlation east and west
5. ORDOVICIAN TIME SCALE
178
5.1 Present applications
research
178
time
5.2 Published scales
180
data
5.3 Implications of present
to
5.3.1 Pluton emplacement erosion
180
timespan
for
5.3.2 Maximum age estimates
181
horizons
stratigraphic
time
Comparisons
5.3.3 with previous
181
scales
APPENDICES
184
descriptions
I Thin
section
the
II Ordovician and
conglomerates
189
Valley
the Midland
of
evolution
: constraints
III Ordovician
conglomerates
191
the time
on scale
198
REFERENCES
LIST OF TABLES
Page
1A Conglomerate
constituents
11
2A Replicate
analyses
29
2B U-Pb
data
sample
33
2C Rb-Sr isochron (Benan
data
conglomerate) 53
2D Rb-Sr isochron (Corsewall,
data
Craigskelly,
Kilranny,
Kirkland
Tormitchell
and conglomerates) 54
2E Rb-Sr
analytical data 55-58
3A
Detection limits
63
3B
Rb Sr
and values 64
3C Chemical
67-80
rock analyses
3D Norm for
and mode analysed clasts 81-87
3E Granite
135
environmental characters
4A Summary
for the
of plate-tectonic models north
the Iapetus Ocean 153
margin of
4B Sequence
173
of events
5A Ordovician time 183
scales
iv
LIST OF FIGURES
Page
1/1 Regional location
map
2
1/2 Stratigraphic
scheme
7
1/3 Geological
Girvan
map of showing sample
localities
10
1/4 Facies
relations
16
2/1 Nicolaysen
diagram
23
242 Rb-Sr isochrons
for
889 & 399
samples 39
2/3
198 & 1197
40
of
2/4
196 & 192
41
of
2/5
188 & 187 42
2/6
186 & 176 43
2/7 P?
101 44
h
(cid:30)
2/8 Concordia
diagram for 101 45
sample
2/9
Rb-Sr isochrons for Benan WR & 996 46
samples
2/1.0
993 & 988 47
of
2/11
985 & 982 48
2/12
591 & 587 49
2/13
580 & 579 50
2/14 284 & Kilranny WR 51
87Sr/86Sr0
2/15 Frequency vs. age an d vs, age 52
3/1 H & CO2 determination 65
graphical
20
3/2 Microprobe for 101 &187 89
analysis points samples
3/3 Microprobe for 176 & 182 90
analysis points samples
3/4 Feldspar for 182 & 187 91
analyses plot samples
3/5 Feldspar for 176 92
analyses plot sample
3/6 Feldspar for 101 93
analyses plot sample
3/7 Thin for 101 96
section sketch sample
0/8
Thin for 187 97
section sketch sample
3/9 Amphibole for 187 98
analyses plot sample
3/10 Granophyric intergrowth by 197 99
shown sample
3/11 Granophyric/graphic texture from 284 100
sample
3/12 Thin for 179 102
section sketch xenolith
1'iul, C regc. r'U111g i. LgUre5 3/1J LU 5/, 50 1U/
.
3/13 Streckeisen
108
modal granite classifiacation
3/14 An-Ab-Or diagram (Heitanen & Streckeisen) 109
3/15 An-Ab-Or diagram (O'Connor, Barker & Glikson) 110
V
Page
3/16 ANOR
classification 111
granite
3/17 Ca-Na-K diagram
112
3/18 K20/Na20 (Fe+Mg+Mn)
113
vs.
3/19 R1-R2 114
major cation granite classification
3/20 Normative data on a Streckeisen diagram 115
plotted
3/21 Modal data Streckeisen diagram 116
on a
plotted
3/22 AFM ternary 123
plot
3/23 Frequency DI 124
of and normative plagioclase
3/24 K20+Na20 CaO S102 125
and vs.
126
3/25 Log (CaO/(Na20+K20)) Si02
vs.
127
3/26 AA SiO
vs.
2
128
(Normative) Si02
3/27 Corundum or Diopside vs.
129
3/28 Mgo P205
vs.
130
(Winkler
3/29 Q-Ab-Or & Breibart)
131
(Winkler
3/30 An-Ab-Or & Breibart)
132
Zr
3/31 Molecular Al/(Ca Na)
+ vs
133
3/32 Rb (annotated data)
Sr age
vs. with
134
3/33 Rb Zr Rb Nb
vs. and vs.
139
3/34 K/Rb
DI
vs.
140
3/35 Ternary Ba-Rb-Sr diagram
141
3/36 Log Ba Rb
vs
142
3/37 Log Sr Rb
vs.
143
3/38 Log Ce Rb
vs.
4/1 Plate-tectonic models for the northern margin
152
150-
the Iapetus Ocean
of
156
4/2 Anatomy
of a volcanic
157
fore-arc basin
4/3 Development of a residual
Valley during
4/4 Evolution of the Midland part
168
the Ordovician
of
176
4/5 Northward subduction related granites
VI
ACKNOWLEDGEMENTS
The is for facilities
author
grateful provided
at the Scottish Universities Research and Reactor Centre by
Professor H. Wilson in the Department Geology,
and of
Glasgow University by Professor B. E. Leake. Technical
assistance by Dr. M. Aftalion. Mr. J. Hutchinson,
was provided
Mr. J. Jocelyn, Mr. D. Skinner, Mr. W. Neilson Mrs. A.
and
MacDougall for the is indebted.
author
which
The in the benefitted
computations used study
from discussions from, Drs. C. M.
greatly with, and advice
Farrow and M, R. Giles and Messrs. J. H. Hutchinson and B. E.
Keeling.
Various the by
aspects of study were aided
discussion
and communication with numerous geologists
including Profs. P. Brown K. Condie Drs. M. Aftalion,
and and
G. Brown, C. Carter, H. Gabrielse, A. Halliday, J. Legget, D.
Powell, A. Saunders, D. Strong J. Whitacker. Also the
and
in the Glasgow Geology Dept.
stafffand research students
to
contributed much useful, and much not so useful
discussion.
for
My thanks to Henry Williams a vain
also go
to interest in the delights of
me graptolithology
attempt
in
for leaving few feet in which to work,
and me a square
homage to the of muck and
plethora
a room otherwise paying
!.
surrounding palaeontologists
paperwork
from the Natural Environment
Financial
support
Research Council appreciated.
was greatly
Drs. B. J. Bluck and 0. van Breemen, who supervised
help
for the and
this thanked substantial
study, are
to
afforded me.
patience
Finally I like to thank my parents and
would
interest.
brother for their support and
vii
The herein is the
material presented result
independent
of by the
research author
between September 1977 December
undertaken and
1980 the Scottish Universities Research
at
(East
Reactor Centre Kilbride) the
and and
Department Geology, Glasgow University.
of
Any
published or unpublished results of other
full
have been
workers given acknowledgement
in the text.
Christopher D. Longman
viii
SUMMARY
Granitic
clasts of hitherto
unknown
affinity are
found in Ordovician
Silurian
and
conglomerates S. W.
of
Scotland.
The host
conglomerates form
part of a sequence
envisaged to have
accumulated the
on
southern
margin of
a
continent under Iapetus Ocean
which crust being
was
consumed
in
a northward dipping
subduction zone. Palaeocurrent
and
clast
size evidence suggests a provenance to the immediate
north. The
age and chemistry of the
granitic clasts was
thus important
in
elucidating the type,
extent and evolution
of
magmatic the
activity north of the
subduction zone, crust
into
the
which
plutons were emplaced
and regional chemical
trends in
rocks of comparable age.
The
data
age obtained in from
show a spread age
c. 580 Mä to 450 Ma, and the chemistry indicates that the
granitoids originated in
a compressional
calc-alkaline
environment. This
evidence coupled with other clast types
present suggests the island founded
existence of an arc on
continental crust in the region now occupied by the Midland
Valley during the late Precambrian to Ordovician. Both to
the
north and south of this arc there is evidence, from the
Ballantrae
the Highland Border
complex and series, of
marginal basin oceanic crust having existed. This leads to
the for the the
proposition of a new model evolution of
Caledonides involving
southern volcanic/island arc and
marginal basin development above a northward dipping
The basins, to
subduction zone. closure of marginal prior
final Iapetus Ocean in
closure, results arc-continent
tectonic in the Southern
collisions manifest as effects
Highlands.
Magmatism in the be
volcanic arc may correlated
to the by this
with progressively younger events north
through the
model effects of subduction enhanced melting
increasing depths.
at
The isotopic for the
ages clasts also provide
additional data for the Lower Palaeozoic time
points scale
due to the
proximity of radiometric and stratigraphic ages,
broad
and show agreement Rb-Sr K-Ar
with pre-existing and
time
scales.
ix