M I N E R A L O G I C A L MAGAZINE, JUNE 1986, VOL. 50, PP. 205 15
The Delaney Dome Formation, Connemara,
W. Ireland, and the geochemical distinction of
ortho- and para-quartzofeldspathic rocks
BERNARD ELGEY LEAKE
Department of Geology, University of Glasgow, Glasgow G12 8QQ;
also Department of Geology, University of Western Australia, Nedlands, WA 6009
AND
DHARM SINGH
Department of Geology, University of Glasgow, Glasgow G12 8QQ
ABSTRACT.A number of geochemicalplots are suggested
by means of which metamorphosed quartzofeldspathic
sedimentary rocks (meta-sandstones, meta-arkoses,
quartzites, etc.) can be distinguished from their metamorphosed igneous mineralogical analogues (metarhyolite, meta-granite, etc.). These plots are applied to an
enigmatic, partly mylonitized, quartzofeldspathic series,
the Delaney Dome Formation, which is involved in a
major thrust zone. The formation is shown to be igneous
and is probably a meta-rhyolite. The plots proposed
should have general application in identifying the precursors of both high-grade and low-grade quartzofeldspathic rocks of unknown origin.
KEYWORDS: meta-rhyolite, quartzofeldspathic rocks,
Delaney Dome Formation, Ireland.
THE Dalradian rocks of Connemara, Co. Galway,
Western Ireland (the Connemara Schists), are remarkable for lying nearly 50 km south of the main
outcrop of Dalradian rocks which extends from
Scotland to north Mayo and Donegal in Ireland.
The Connemara Schists are allochthonous, having
moved southwards on a major thrust plane, the
Mannin thrust, which has been described by Leake
et al. (1983, 1984). This thrust is only exposed
around a domal structure, the Delaney Dome, in
the southwest part of Connemara, south of Clifden
(fig. 2). Immediately above the thrust lie metagabbros which have been recrystallized to finely
lineated and foliated albite-epidote amphibolites,
the Ballyconneelyamphibolites. Above these amphibolites lie strongly foliated and partly sheared
metagabbro and quartz diorite gneiss containing
metagabbroic lumps, and above these rocks lie the
Connemara Schists (Leake, 1986).
Below the Mannin thrust is a flat-lying, monoCopyright the Mineralogical Society
tonous, mylonitic, and dynamically recrystallized
series of quartzofeldspathic rocks called the
Delaney Dome Formation (Leake et al., 1981;
Leake, 1986). These rocks, which occupy less than
25 km 2 and have a maximum elevation of only
44 m, are the only visible exposure of the foreland
on to which the Dalradian massif of Connemara
was thrust. Because of the complex nature of the
Highland Boundary Fault in Scotland and Ireland,
and the limited outcrop and complexity of the
Highland Border rocks, the nature of the Delaney
Dome Formation is of considerable importance in
any Caledonide synthesis of the British Isles.
This paper is concerned with identifying the
origin of these quartzofeldspathic rocks and using
them to develop geochemical methods which may
be applied to the identification of other metamorphosed quartzofeldspathic rocks of uncertain
origin. Thus, in both high-grade metamorphic
terranes and in sheared and mylonitized lowtemperature metamorphic sequences, rocks essentially composed of quartz and feldspar can
originate as sediments (e.g. sandstones or arkoses),
as acid volcanic rocks (e.g. rhyolites), as acid
plutonic rocks (e.g. granitoids), as high-grade
granite gneisses, migmatitic gneisses, or metasomatic rocks. Often relic textures and structures
(e.g. cross-bedding) reveal conclusively the origin of
a rock but this is not always so, and in such
instances geochemistry may be useful.
Delaney Dome Formation
This formation is made of very fine-grained
(0.05-0.1 mm) granular rocks composed of quartz
206
B.E. LEAKE AND D9 SINGH
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BALLYCONNEELu
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154
9
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DEVONIAN
GRANITES
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9
DELANEY DOME
I FORMATION
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SCHIST
9 . .~.
.[.'.'.'.'."
THRUST
GNEISS
BALLYCONNEELY
AMPHIBOLITE
METAGABBRO
CONNEMARA
.....
DIP OF
QUARTZ DIORITE
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SAMPLE
SITE
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FIG. 1. Geological sketch map showing the location of the Delaney Dome Formation in the west of Ireland9The
locations of the analysed samplesand the sample numbers are indicated9The un-numbered points can be interpolated in
the number sequence9
(60-70 ~), K feldspar (10-20 ~), plagioclase (10 ~),
muscovite plus chlorite (5 %) with accessory magnetite, epidote, and sulphides and traces of late
carbonate in a few samples9 Very rarely partially
chloritized garnet porphyroblasts occur9 In the field
the rocks are pinkish, pinkish grey or grey, usually
unbanded but possessing a strong foliation with
parallel schistosity, lensoid quartz and penetrative
shear surfaces which possess a NNW-SSE quartz
stretching lineation on the foliation. Nowhere is
any bedding or clear traces of original banding
predating the shearing visible9 Some rocks are
almost mylonites with a porcellaneous or chert-like
appearance; a few have muscovite-chlorite-rich
layers or sometimes green biotite, and occasionally
some have magnetite-rich layers or euhedral K
feldspar and plagioclase crystals in a fine-grained
quartz matrix. Some of the chlorite may be altered
biotite but probably most of the chlorite is of
primary crystallization. The pinkish layers are not
necessarily richest in K feldspar and some pink
quartz is present. Often the feldspars are completely
sericitized and saussuritized. The coarser quartz
grains are strained and possess undulose extinction
while the matrix is composed of strain-free dynamically recrystallized quartz. Thin quartz veins
(0.05-0.25 mm) parallel the foliation and contain
polycrystalline strained quartz porphyroclasts with
THE DELANEY DOME F O R M A T I O N
the chlorite, muscovite, and sometimes epidote
fabric bowing around the quartz. From probe
analyses of the rare euhedral feldspars the K
feldspar is mostly orthoclase Or80_97 (not microcline) while the plagioclase is An0_2o and has
Carlsbad and albite twinning.
It is not immediately obvious, whether, before
the thrusting, the formation was an arkose or
sandstone with recrystallized feldspar, a rhyolitic
volcanic rock or glass with feldspar phenocrysts, or
a granite or K feldspar granite gneiss such as occur
commonly in the quartz diorite gneiss suite of
Connemara. There are no exact matching rocks to
the Delaney Dome Formation known in the west of
Ireland. The exposed vertical thickness of the
formation is difficult to calculate but is probably
not more than 25 to 40 m. An undescribed borehole
(Geol. Surv. Ireland 83/8) near the centre of the
Dome has penetrated to 233 m nearly vertically
through rocks essentially similar to those at the
surface (M. D. Max and C. B. Long, pers. comm.).
Within the formation occur massive to poorly
schistose uniform amphibolites made of hornblende and highly saussuritized and sericitized
plagioclase. Chemically these metabasic rocks are
quite different from the Ballyconneely amphibolites, having much higher M g / ( M g + F e ) ratios
(typically c.0.65) compared with less than 0.60 for
most samples of Ballyconneely amphibolite (Leake,
1986). It is clear that the amphibolites in the
Delaney Dome Formation are metamorphosed
intrusive or extrusive basic igneous rocks but it is
not known which of these two possibilities is
correct. These rocks do not throw light on the
origin of the Delaney Dome Formation.
Geochemistry
Chemical analyses of twenty-nine constituents in
seventy-four samples have been completed of which
a representative fifty-nine samples are listed in
Table I* with the sample localities given in fig. 1.
The chemical analyses are by XRF using fused
beads (Harvey et al., 1973) for major elements and
pressed powders bonded with bakelite resin for
trace elements (Leake et al., 1968); FeO, HzO, and
CO2 were determined by wet chemical techniques.
All the analyses were carried out by D. Singh.
The results show a range in SiO2 from about 69
to 81 wt. ~ with three samples having rather lower
values (overall average 73.64 ~). The total SiO 2 +
A1203 is nearly always between 80 and 90 wt.
with a little over half the samples having N a 2 0 >
K 2 0 although the average N a 2 0 (3.19 ~o) is lower
* The average analysis is the mean of all seventy-four
samples.
207
than the average K 2 0 (3.45~o). The significant
contents of A1203 (9-14; average 12.01~o) and
N a 2 0 + K 2 0 (2.6-9.5; average 6.64 ~ ) immediately
rule out an origin from chert which has higher silica
and lower alumina and alkalis. Most quartzites
would also include samples with higher silica than
81 ~ but metamorphosed arkose or acid igneous
rock could equally well fit the above ranges.
Because of the predominance of SiO 2, and the
constant sum effect of chemical analyses reported
in percentages, it is inevitable that SiO 2 variations strongly negatively influence all other major
element variations. In order to obviate this negative
correlation, the major elements are considered in
terms of Niggli numbers (Niggli, 1954). In these, the
sum of the molecular proportions of A1203, CaO,
2Fe203 + FeO + MnO + MgO, and K 2 0 + Na20,
when recalculated to 100 gives the percentages ofal,
c,fm, and alk, all in molecular proportions, with si
and ti being the molecular proportions of SiO 2 and
TiO 2 reduced to the same basis as the first elements;
k = mol. prop. K20/mol. prop. (K20 + NazO) and
m9 is the mol. prop. MgO/mol. prop. (2Fe203 +
FeO + MnO + MgO).
To distinguish between quartzofeldspathic sedimentary and igneous rocks, two chemical approaches are likely to be most profitable: (1) to
establish trends of chemical variation which are
characteristically different for sedimentary and
igneous suites, one being controlled by sedimentary
processes, the other by igneous fractionation; (2) to
identify distinctively different element concentration ranges for the two suites, either graphically
or by a discriminant function. Both of these approaches have to be applied in a highly selective
manner because most of the chemical variation in
quartzofeldspathic rocks is very similar whatever
the genesis and consists largely of variations resulting from changes in quartz and feldspar. Moreover,
both igneous and sedimentary rocks have a wide
range in composition. This account does not
discuss the many chemical variations which are
common to ortho- and para-quartzofeldspathic
rocks.
Igneous and sedimentary trends of variation. Although numerous studies of igneous fractionation
have emphasized the importance of Mg/Fe change,
all variations that rely on mg fractionation are
based on ferromagnesian minerals which are only
minor constituents of sandstones, granites, or
rhyolites. Accordingly variations based on felsic
minerals will be examined first as these are potentially useful and less subject to the analytical errors
inherent in determining minor and trace constituents. Of particular value in distinguishing
sedimentary and igneous trends of variation is a
plot of Niggli al-alk against c (fig. 2).
208
B.E.
Sp. NO.
LEAKE
AND
D. S I N G H
15
37
38
39
40
41
42
43
44
45
46
47
49
50
85
sio2
TiO 2
A1203
Fe203
FeO
MnO
MgO
CaO
Na20
K20
P205
H20+
CO 2
76.92
0.25
9-74
2.05
1.02
0.05
0.91
1.64
3.21
1,81
0.04
0.78
0.02
74.34
0.31
12~01
1.40
1.28
0.04
0.50
0.38
2.23
6.52
0.03
0.25
0.I0
70.82
1.02
13.22
2.18
1.52
0.05
2,08
1.91
2.18
2,82
0.08
1.82
0.09
73.00
0.33
i[.76
2.04
1.30
0.06
0.77
1.44
4.06
2.80
0.09
1.59
0,I0
73.84
0.33
12,34
1.98
1.49
0.04
0.42
0,95
3.64
4.40
0.07
0.66
O.OO
70.78
0.33
13.48
2.36
1.83
0.05
0'.34
1.92
3.99
3.03
0.06
0.82
0.06
73.45
0.30
13.48
2.16
1.54
0.06
0.60
1.01
2.51
4.40
0.06
1.30
O.OO
75.64
0,21
10.47
1.88
0.72
0.04
0.20
0.18
2.55
5.43
75.49
0.19
10.82
2.08
0.73
0.02
0.22
0.37
2.12
6.60
0.03
0.03
0.85
0.20
0.06
0.13
75.42
0.31
11.44
2-14
0.59
0.04
0,24
0.70
2,72
6.00
0.03
0.11
0.20
76.15
0.22
11.65
1-85
0.40
0.05
0.10
0.53
2.43
6.83
0.03
0.00
0.33
73.50
0.30
12.95
2-I0
2.39
0.05
1.72
1,56
2.29
2.85
0.05
1.71
0,21
73.45
0.31
12.11
1-54
1.23
0.04
0.79
0.26
1.76
7.30
0.06
0.72
0.00
71.44
0.84
12.15
2-56
2.19
0,06
1.53
1,49
5.52
0.51
0.19
0.50
O.O0
70.87
0.05
ii.76
1-96
2.95
0.08
2,72
2.14
3.78
1,73
0.06
1.20
0.14
TOtal
98.44
100.27
99.05
99.33
100.16
99.05
100.87
98,73
98.96
99.84
100.57
101.71
99.47
98.98
99.89
667
56
4
.
.
884
74
7
.
i017
72
9
~
696
47
3
3x
739
60
4
3x
637
75
4
16
675
62
I
13
737
80
5
. .
Ba p p m
Ce
CO
.
.
.
1015
46
3
.
.
.
406
58
0
.
.
.
839
56
6
.
.
.
1247
49
2
. . .
.
261
65
i0
. .
971
75
14
.
17
i0
18
0
32
145
167
12
12
36
340
21
29
20
i
31
124
205
17
41
29
315
18
18
~6
0
20
i18
147
ii
23
41
227
12
17
14
0
34
68
135
12
31
38
307
14
25
~5
0
23
106
337
14
21
38
359
Ii
23
14
0
31
154
84
13
27
35
334
14
i0
17
0
16
IO1
260
17
25
49
275
16
23
19
0
47
137
294
12
30
40
278
15
33
18
0
20
I14
306
14
33
28
300
14
25
~6
0
43
160
58
16
33
37
270
16
39
17
0
18
i15
176
14
40
50
322
12
16
13
0
ll
88
306
14
24
32
181
20
29
,6
5
12
8
370
6
44
53
348
16
37
15
15
8
35
141
13
42
105
314
37.00
24.18
11.33
27~50
495.70
0~27
1,21
0.36
0.64
42.45
17.21
2.44
37.91
446.10
0.66
0.95
0.36
0.50
39.43
30.42
10.36
19.80
358.40
0.46
3.88
0.52
0.56
30.58
29.95
8.59
]1.85
406.30
0,31
1.34
0.30
0.59
40.43
18.69
5.66
35.22
410.50
0.44
1,38
0,19
0.54
40.50
19.44
10.49
29.57
360.83
0.33
1.27
0.13
0.54
43.95
21.07
5.99
28.99
406.37
0.54
1,26
0.23
0.56
42.22
15.84
1.32
40.62
517.57
0.58
1.08
0.13
0,70
41.25
16.05
3.54
40.16
453.87
O.67
0.92
0.13
0.72
41.07
14.99
4.57
39,37
459.37
0.59
0.96
0.15
0.77
42.85
11.71
3.54
41.90
457.27
0.65
1,03
0.08
0.81
39.23
31~50
8~57
20.71
376~88
0.45
1.36
0,42
0,44
41.63
19.63
1.63
37.12
428~44
0.73
0.92
0.35
0,53
34.98
29.49
7.80
27,73
348.96
0.06
3,09
0.38
0,51
31,42
36,36
i0.43
21.69
322.31
0,23
1.71
0.51
0 .37
102
133
134
135
136
137
138
139
140
141
143
144
145
146
147
Sio 2
TiO 2
AI203
Fe203
FeO
MDO
MgO
CaO
Na20
K20
P205
H20+
CO 2
76.62
0.27
11.52
0.26
3~06
0.04
0.59
1.62
5.O1
0.50
0,04
0.53
0-07
73.97
0.27
12.60
2.21
1.25
0.03
1.29
i.I0
2~85
2.53
0.05
1.56
0.19
74.87
0.48
10.88
2.05
2.49
0.05
1.33
0.66
5.61
0.34
0.09
0,97
0.07
74.34
0.26
12.53
2.45
0.79
0.06
0.58
2,34
3.68
1.68
0.07
1,18
0.05
75.38
0.23
10.34
i~79
1.53
0.05
0.65
0.59
4.94
3.61
0,03
0.85
0,06
77.30
0.17
I0,12
2.01
0~94
0.05
0.46
0.30
2.12
5.29
0.02
0,62
O.13
71.43
0.40
12.00
2.06
2.57
0.05
2.39
0.93
1.83
4.75
0.07
1.64
0.09
74.43
0.24
11.57
1.87
1,26
0.03
0.27
0.41
3.34
4.87
0.04
0,60
0.21
74.04
0.38
12.09
0.49
2.63
0.04
0.93
0~86
3~43
3,37
0,08
1.20
0,18
73.66
0.21
11.92
1.37
1,63
0.04
0.55
0.55
4,34
3.19
0.03
0.77
0.15
75.41
0.25
Ii.48
2.04
i.ii
0.04
0,55
0.I6
3.10
3.56
0.04
0.77
0.24
73.53
0.24
11.83
[~87
1.32
0~05
0.69
0.34
1.56
6.75
0.04
1.28
0.13
79.66
0,24
9.74
1.42
0.50
0.03
0.23
0+29
1.71
6,16
0,04
0.34
0.19
71.40
0.48
12.81
2.34
2.05
0.06
1.22
2.61
3.08
2.95
0.i0
1.82
0.17
75.54
0.23
I1.65
2.12
0.84
0.04
0.66
0.45
2.98
4.60
0.03
0.53
0.14
Total
100.13
99.90
99.89
100.01
i00,05
99.53
100.21
99.14
98172
98.41
98.87
99.93
100.55
100.09
99.81
187
63
3
36
0
14
23
1022
64
5
57
4
17
35
83
73
5
34
2
iS
34
730
56
2
63
12
15
31
1072
41
3
54
4
16
22
1133
45
I
83
0
14
23
773
77
7
12
6
15
32
727
40
3
,34
8
16
IS
643
59
2
9
8
15
30
919
50
1
15
0
16
19
711
60
2
21
3
16
32
674
43
2
21
5
16
26
1781
40
0
62
7
ii
17
986
67
4
154
2
16
35
720
46
1
2
9
8
142
14
46
30
332
1
21
99
157
14
33
51
261
1
iQ
i0
90
4
76
59
527
0
22
26
186
18
27
36
208
O
25
99
124
1
35
43
280
0
30
100
71
13
23
43
194
6
26
103
113
12
18
52
229
0
25
81
67
14
28
35
257
2
17
81
109
ii
28
32
211
0
19
59
95
14
32
41
259
0
14
106
71
16
35
38
221
0
24
126
38
14
29
92
240
1
17
105
62
13
29
22
256
7
25
135
184
ii
29
55
250
2
22
83
76
14
43
30
263
42.16
26.29
6.69
24.85
419.99
0.37
1.15
0.42
0,61
34.88
30.50
3.85
30.77
407.29
0.04
1.96
0.35
0.43
41.25
18.82
14.01
25.92
415.30
0.23
i~O9
0.26
0.74
34.99
20~65
3.63
40.73
432.88
0.32
1.99
0.27
0.51
40.57
20.30
2.19
36,94
525.86
0.62
0.87
0.23
0.66
35.12
36.87
4.95
23.86
354.77
0.63
1.49
0.49
0.42
41.41
17.39
2.67
38.53
452.02
0.49
I.I0
0.14
0.57
42.32
23.48
5.47
28.73
439.71
0.31
1.70
0~35
0.14
41.15
18.83
3.45
36.57
431.50
0.33
0.93
0.26
0.43
43.48
21.10
I.I0
34.32
484.62
0.44
1.21
0.25
0.62
41.05
20,84
3,14
35*97
432.91
0~70
i~06
0.29
0.56
42.62
13.58
2.31
41.49
591.48
0.70
1.34
0.19
0.72
38.64
27.09
14.31
19.95
365.44
0.48
1.85
0.34
0.51
41.73
19.94
2.93
35.39
459.12
0.50
1.05
0.30
0.69
~N~
Ni
Pb
Rb
Sr
Th
Y
Zn
Zr
i0
15
18
0
ii
31
89
13
31
70
377
1281
38
2
.
.
Niggli Values
al
fm
c
alk
si
k
ti
mg
w
Sp.No.
Ba p p m
Ce
0
0
14
23
Niggli Values
fl
m
c
alk
si
k
ti
mg
w
39.16
20.96
i0.01
29~86
411.99
0.06
1.17
0~24
0.07
THE DELANEY
DOME FORMATION
209
Table I (cont.}
Sp.NO.
163
164
165
166
167
168
169
170
4492
4493
4494
4495
4496
4497
4498
Sio2
Ti02
A1203
Fe203
FeO
MnO
MgO
CaO
Na20
K20
P205
H20+
CO 2
75.32
0"33
11.91
1.90
1.31
0,04
i.I0
0,98
3.47
1.84
0.07
1.74
0.16
73,45
0.24
11.59
i~87
1.60
0.04
0.88
0.23
1.59
6.92
0.04
1.26
0.00
76.17
0-54
11,32
1.61
2.40
0.05
0.88
1.32
2.90
1.83
0.i0
1.98
0.15
71.24
0.57
12.60
2.18
2.78
0.05
1,38
3.16
2.95
1.93
0,ii
2.26
0.13
68.98
0.62
13,15
3~13
2.44
0.06
i~95
2*89
2.91
1.88
0.I0
2.64
0.21
73.28
0.31
12.78
2.38
0.93
0.04
0.78
1,56
2.91
3.22
0.08
2.32
0.51
70.28
0.60
12.69
3.01
2.18
0.05
1.34
3.54
2.66
1.49
0.12
1.47
0,00
75.96
0.30
12.46
1.97
1.30
0.05
0.91
1.03
3.63
1.76
0,07
1,81
0,00
65.26
0.85
14.O8
2.74
3.45
0.07
1.76
2.68
4.55
1,86
0.19
0.97
0.21
70.94
0.28
ii.64
0.00
6.~9
0.04
0.82
2.75
2.78
2.04
0.07
1.44
0.09
76.24
0.31
11.41
i.~7
1,23
0.03
0.72
0.95
4.50
2.07
0.08
1.19
0.07
75.06
0.37
11.73
1,59
0.72
0,04
0,13
0.42
1.79
7.69
0,04
0.04
0.05
74.05
0.39
12.11
I~80
i~85
0.04
0.91
1.23
4.96
0.67
0.09
0.52
0,II
71.38
0.34
12,21
2~24
1.61
0.07
1.16
2.25
2.84
3.08
0.06
1.77
0.I0
71.31
0.68
11.28
2.19
2.77
0.07
1,29
1.51
3.72
2.87
0.13
1.77
0.02
TOtal
100.17
99.17
101.25
101,32
100.96
100.82
99.43
101.25
98,67
99.18
100.47
99.67
99.73
99.11
99.59
co
cr
cu
Ga
461
41
2
15
O
16
681
54
4
45
5
16
23
616
44
5
29
4
13
17
395
54
12
208
5
18
23
626
57
0
4
3
15
30
zn
Zr
23
38
268
1
20
126
44
12
25
43
265
5
20
54
138
7
16
49
215
534
66
i0
25
i~
17
29
.
8
20
46
156
ii
42
147
361
748
40
3
34
0
16
14
0
21
69
174
568
43
t2
36
9
19
14
.
17
23
131
213
Ii
20
65
254
937
52
1
18
5
14
21
Ni
Pb
Rb
Sr
458
62
8
31
ii
16
29
.
i0
22
91
190
12
26
49
231
2
27
96
205
12
19
37
210
0
26
60
97
13
27
31
213
t257
52
3
62
17
14
24
14
2
25
148
50
12
24
32
153
144
51
5
121
23
ii
53
13
4
23
21
77
Ii
31
33
271
1088
65
8
I0
5
17
29
16
0
18
97
195
i0
48
48
300
114
30
il
43
22
18
20
14
2
16
63
77
15
26
59
252
39.97
23.74
1.44
34.86
432,13
0,74
1~06
0.32
0.51
40.20
27.30
8.52
23.98
458.99
0.29
2.45
0.29
0.38
35,51
28.73
16.19
19.57
340.70
0.30
2.05
0.34
0,41
4499
4500
4501
sio 2
TIO 2
Al203
Fe203
FeO
MnO
M90
CaO
N~20
K20
P205
H20+
CO 2
73.82
0.36
12.15
1.93
2,37
0.05
1.07
0.59
5.60
1.13
0-05
1.47
0.05
74.28
0.24
11.75
2.03
1,20
0.04
0.79
0.08
2.63
5.31
0.05
1.54
0.40
Total
100.64
202
54
3
i0
17
16
38
18
0
9
29
~
ppm
~;
20
0
32
115
165
12
20
44
217
14
18
84
265
12
23
57
240
401
54
4
0
i
16
23
.
0
20
73
i19
15
26
39
203
34.96
32.94
13.97
18.14
311~15
0.30
2.10
0.40
0.54
42.11
20.86
9.34
27.69
409.67
0,41
1,30
0.31
0.70
35.81
29.14
18.16
16.90
336.48
0.27
2.16
0.33
0.55
43.16
23.07
6.49
27.28
446.43
0.24
1.33
0.35
0.58
34.10
31.11
11.80
23.00
268.14
0.21
2.63
0.35
0.42
33.82
31.96
14.53
19.70
349.70
0.33
1.04
0.19
0.00
40.06
20,01
6.06
33.86
454.24
0,23
1.39
0.32
0.55
43.22
12.46
2.8[
41~51
489~23
0.74
1.74
0.i0
0.67
39.76
23.72
7.34
29.17
412.56
O.08
1.63
0.32
0.47
37.70
24.94
12.63
24.72
373.99
0.42
1.34
0.36
0.56
33.89
30.07
8.26
27.77
384.22
0.34
2,61
0,33
0.42
4502
4503
4504
4505
4506
4507
4508
4509
4510
4511
4512
Average
72.85
0.35
11.77
1.92
1.96
0.04
1.09
0.18
3.12
5.22
0.05
1.61
0.85
71.63
0,39
13,12
1.99
2.26
0.05
1.43
0.53
4.17
4.16
0.05
1.42
0.14
65.35
0.71
14.17
2.47
3.57
0.05
2.69
3.55
3.44
1.63
0.13
2,34
0,07
69.39
0.62
13.12
2.31
3.02
0.06
1.35
1.06
3.75
3.21
0.II
2.02
0.09
67.95
0.61
13.90
2.41
3.43
0.06
1.93
1.32
4.65
1.48
0.12
2.25
0.03
71.12
0.55
12,57
2.09
2.75
0.06
1,87
1.29
3.72
2.12
0.Ii
1.20
D.05
70.05
0.59
12.66
2,73
2.27
0.06
1.44
3.19
1.78
2~48
0.II
2.20
0.08
70,99
0.56
12.48
2.25
2.65
0.05
1.03
1.33
3.29
3~78
0.ii
1.01
0.07
73.89
0.22
12.21
2.32
1.03
0.05
0.71
1.60
1.25
4.84
0.04
i.i0
0.80
78.50
0.18
10.77
2.00
0.69
0.03
0.[8
0.38
3~O6
4.32
0,03
0,00
0,14
78.33
0.23
10.62
2.28
0.70
0,03
0.14
0.23
3.84
3.07
0.05
0.ii
0.07
77.69
0.28
11.04
1.57
1.25
0.05
0.36
1.06
3.55
2.55
0.04
0.38
0,03
73.64
0.37
12.01
2.02
1.68
0.05
0,91
L.28
3.19
3.45
0.07
l.ll
0.13
100.34
I01.01
101,43
i00.17
i00.ii
100.14
99.50
99.64
99.60
100.06
100.28
99.70
99.80
99.91
696
55
2
s
19
16
23
13
576
64
8
8
8
13
26
15
628
50
7
9
7
20
i5
20
619
70
i0
61
16
18
36
14
688
60
12
36
8
16
38
13
327
75
15
38
50
16
47
ii
710
48
ii
68
12
15
30
13
631
99
13
34
64
10
34
57
898
61
ii
36
14
20
28
57
596
70
9
13
7
18
36
14
1088
40
4
39
lO
10
19
9
612
58
4
9
17
9
29
9
568
46
6
35
ii
12
26
15
717
56
5
46
8
15
26
18
43
115
43
13
27
94
236
1
71
47
13
54
53
371
1
75
91
15
58
56
450
32
80
311
14
31
69
284
22
85
92
12
25
58
284
19
44
ii0
9
28
64
310
41
58
138
i0
26
58
259
25
137
908
15
26
194
79
24
99
123
14
23
56
73
23
149
I01
14
36
59
0
22
83
83
ii
22
36
150
12
97
51
7
37
38
244
22
58
124
13
33
35
252
12
31
49
260
41.58
22.27
0.51
35.64
445.96
0.57
1.08
0.32
0.60
38.13
25.88
1.06
34,93
400.44
0.52
1.45
0.35
0.47
37.69
26.90
2.77
32.64
349,15
0.40
1.43
0.39
0.44
32.90
34.88
14.99
17.24
257.45
0.24
2.10
0,45
0.38
37.13
30.13
5.45
27.29
333.17
0.36
2.24
0.32
0,41
36.22
33.42
6.25
24.11
300.44
0.17
2.03
0.38
0,39
36.30
32,63
6.77
24.30
348.48
0.27
2.03
0.42
0.41
36.78
30.06
16.85
16.81
345.31
0.48
2.19
0.35
0.52
37.10
27.46
7.19
28.25
358.07
0.43
2.12
0.28
0.43
42.64
21.72
10.16
25.48
437.86
0.72
0.98
0.29
0.67
42.81
15-89
2.75
38.59
529.43
0.48
0.91
0.i[
0.72
41.84
17.18
2.72
38.26
523.67
0.48
1.16
0.i0
0.75
42.05
17,86
7.34
32.75
502.07
0.32
1,12
0.19
0.53
39.33
23.67
7.67
29.37
408,10
0.42
1.53
0.32
0.52
.
.
.
Niggli Values
~mI
41.85
24.83
6.26
27.06
449.08
0.26
1,48
0,39
c
alk
si
i
mg
w
0.57
Sp.No,
~
Co
cr
cu
Ga
La
Nb
Ni
Ph
Rb
$r
Th
Y
Zn
Zr
PPm
72
14
64
61
462
o
~
~
.
.
.8
.
.
.
.
.
.
.
.
.
.
.
22
89
151
Niggli Values
~ml
alk
si
k
ti
mg
w
37.74
26,51
3.33
32.42
389,10
0.12
1.43
0.32
0.42
210
B. E. LEAKE AND D. SINGH
50
a
~\
BUSHVELD COMPLEX
JO~ , \
Muscovite
pAn100
j~
J[~
/
I ~ ~- \
~^ ~
//m~"~'"
30
, S AERGAARD
Biotite
J ....
/linen~
oi alk I
-
'' ~.n#~ u" ~ ' ~ - ~ . S." CALIFORNIA
Ir
BATHOLITH
~/
~ ~, / / _ ~ GALWAY GRANITE
2O
/
~". \
~k"\
ou r
~-""-/
al-alk
/
/
'~n
L
~
j
tp
FIELD OF
CALIFORNIAN
F,ELDOFALPINE
/'SEDIMENTS AND
,,~.,%/A~..~/"
~4ETASEDIMENTS
* ~
~/,,/r~,,,,,~....
FIELD OF
"'*..
CONNEMARA
/ ~ ' ~ ~ C / ~
ETASEDIMENTS
Biotite J
~%..../
CALC,TE
.
......
DOLOMITE
Or
I
20
Olv An0
Opx
m
CALCITE
I
I
40 C~X 60
9
80
Niggli
100
c
A
Or k"
OIv
DOL"O~E
'-
t
~
CAR
~h
20
n
40 CPX 60
,
"I
80
10
Niggli c
Opx
To An 100
C
25
.//~so
9
20
9 9
9
9
.)~An 40
ANORTHITE
~o
d
40
o / / /
9
Muscovite
Y
FERENTIATION
TRENO
15
0
,9
_/7
~o et~ ~/~n~
9
e":"//'eTM 9
al-alk
io
RIVER SANDS /
lnSO
40
Muscovite
10
50 ~.~ FIELDOF BIG
~~nAn100
9
30
9
"
t P ~ ' - ~ . / ~ : ~:~:l
al-alk
~,~_ AVERAGE
~zJ- COMPOSITION
-;,~
~
/
~
~
jFIELD OF DELANEY
DOME FORMATION
F!~LD OF
/ " ~ ~ , i
!~,/~/~ HORNBLENDE
/
r
9
/ b
9" . , : x~
:'..~r
9
o~
ely
Opx
~
Biotite
-
10
f
~
' ~ ~ " k \ qu/"OX~ LIMESTONE
::
Dee
ee~eeC 9
5-
,
10
t
,
,
15
20
25
Niggli c
ALBITE
OIv
Opx
20
!~-~"~:I _
<-~/~c~
\
:~.::~;~I.DgLOMnE ~_'9,p~.\
40
Niggli
60
80
100
c
FIG. 2. Niggli al atk against c plots. (a) Igneous rocks. Fields of four igneous suites outlined; also the feldspar variation
line An~10o in which K feldspar and albite, An0, plot at zero. (b) Sediments. Fields of variation of four typical
sedimentary suites (see text) are shown and also the positions of important sedimentaryrock types and the feldspar line.
(c) Delaney Dome Formation, showing variation close to the feldspar line. (d) Igneous and sedimentary fields. Arrows
on the feldspar line show the direction of normal igneous fractionation. Stippled area is the fieldof igneous rocks (which
extends to negative al-alk values) excluding carbonatites. Mixtures of various common sediments indicated and the
Delaney Dome Formation (from c).
On this plot albite (AND),K feldspar, olivine, and
orthopyroxene all plot at the origin while anorthite
Anlo o plots at al-alk 50, c.50. Consequently a line
joining An0 to An100 shows the total variation
produced by varying compositions and modal
proportions of K feldspar and plagioclase, two of
the major felsic minerals. Consequently, it is predictable that most igneous differentiation series will
cluster around and roughly parallel the feldspar
line and this is well shown by four representative
igneous suites, the Bushveld Complex (from ultrabasic to granitic), Skaergaard (gabbro to granophyre), the South California batholith (gabbro to
granite), and the Galway Granites (granodiorite to
alkali granite). Details of these plots and of other
igneous series are given in Leake (1969). Of the
major igneous minerals, only Ca clinopyroxene
(augite, etc.) plots significantly away from the
general feldspar line; biotite and hornblende are
only slightly off the main trend. Changes in the
THE DELANEY DOME F O R M A T I O N
proportions of quartz have no influence on this
plot. As nearly all differentiating igneous series
increase in K feldspar, quartz, and albitic plagioclase with decline of anorthitic plagioclase, typical
igneous variation is well represented by the plagioclase line with variation towards the origin as
shown in fig. 2. This figure also depicts the field of
igneous rocks (excluding carbonatites) as defined
by the positions of the principal igneous minerals,
olivine, pyroxenes, feldspars, hornblende, and
biotite.
On this plot sedimentary variations have quite
different trends to those of igneous rocks. Clays,
being rich in al-alk and generally poor in Ca, plot
on one side of the igneous field and trend while
dolomites and limestones plot on the other side.
Typical quartz arenites, arkoses, and greywackes,
being dominated by quartz and feldspar, overlap
the centre of the igneous field. But mixtures of
(1) clays or shales with quartz arenites, arkoses,
or greywackes, (2) clays or shales with dolomites or limestones, or (3) quartz arenites with
dolomites or limestones, all characteristically give
trends nearly at right angles to the igneous trend
as shown by 4 representative sedimentary series;
about 250 alpine sediments and metasediments
compiled by Niggli (references in Leake, 1969),
over 250 Connemara metasediments (Leake et
al., 1975; Senior and Leake, 1978; Leake, 1980),
26 Californian arkoses (Van de Kamp et al.,
1976), and 43 representative sand analyses from
the world's largest rivers, kindly provided by P. E.
Potter. Thus sedimentary variation is essentially
from high al-alk with low e towards lower alalk and increasing c for the three major sedimentary
suites of shales, siliciclastic sandstones, and
carbonates.
The Delaney Dome Formation clearly shows a
distinct igneous trend consistent only with an
igneous origin (fig. 2c).
Although emphasis has been placed on circumventing the influence of silica changes on other
elements, silica variation can in itself be useful,
as major variation in silica occurs in most sedimentary and igneous rocks. Plots which involve
silica or Niggli si against ratios of other oxides or
molecular amounts, or plots against trace elements,
have potential in the present problem. Thus si
against mg plots give a strong negative correlation because silica generally increases in igneous
differentiation as Mg/Fe falls, this being well
recognized in numerous igneous suites. In sedimentary rocks a wide scatter of si and rnO values occurs
giving a pattern quite unlike that characteristic of
igneous suites. In particular very high si values
occur in siliciclastic sandstone and arkoses, frequently exceeding 500 or 1000 and reaching up to
211
values in excess of 10 000. Coupled with high values
is a wide scatter. Fig. 3 displays these points (note
two scale changes on the si axis). It should be noted
that of the 43 analyses of big river sands available,
only 13 have si < 500, whereas only 8 of the
Delaney Dome analyses have si > 500 and none
exceed 700. In igneous suites the range of si and mg
is relatively limited, whereas sediments show a wide
scatter, as there is no necessary correlation between
Mg/Fe (which is largely controlled by the clay
minerals, detrital Fe oxides and ferromagnesian
minerals) and si, which is largely in quartz and
feldspars. Only sedimentary suites involving dolomite show consistent negative correlations o f s i and
rag, and necessarily, because dolomite is silica-free,
such suites either have much lower values of si than
the present rocks or higher Ca and Mg, or both
characteristics.
The Delaney Dome rocks clearly match best to
the igneous trend of increasing si with fall o f m g and
also display similar amounts of scatter to that
shown by typical igneous suites of which the S.
California batholith is quite representative (fig. 3).
Low levels of MgO, as in the present rocks, must
undoubtedly contribute to the scatter in any plot
relying on rag, especially when MgO is determined
by XRF analysis.
From the aI-aIk against c diagram it can be
deduced that with igneous differentiation al-alk
declines and therefore there should be a positive
correlation between mg and al-alk, a plot which (as
with the si vs. mg plot) combines a feature based
largely on the mafic minerals with one based largely
on the salic minerals. Fig. 3 shows that the present
samples exhibit an overall positive correlation of
rno and al-alk which is only consistent with igneous
fractionation; sedimentary rocks either show no
correlation or tend to have a negative correlation as
instanced by the Connemara metasediments and
Californian arkoses.
The scatter in fig. 3 emphasizes the need to study
a considerable number of samples; a small number
could give a misleading impression. Particularly
important in all studies of variation trends is the
need to collect samples covering as full a range of
variation as possible; merely collecting typical
'average' samples could result in clusters of points
on the geochemical plots in which variation trends
are not apparent. Arithmetic averages might be
closely similar for igneous and sedimentary suites.
Thus in sampling for geochemical work of this kind
unusual samples need to be included as well as
typical ones. Because Niggli al-alk, c, and m9 are
all independent of the amount of SiO 2 in the
analysis, varying proportions of quartz have little
influence on the trends and thus quartz arenites,
which can contain over 9 0 ~ SiO2, have one major
B. E. L E A K E A N D D. S I N G H
212
+BIG RIVER SANDS
600
9
9
~~
2000~
1000
500
9 CALIFORNIAN ARKOSES
AND SANDS
1O.0O0
~ AVERAGE
9
9
\"a
9
9
9
9
§247 \\§
.+
§247~ §
500 ~
9
9
400
"~"\*
Niggli
"9
si
I~
9
\
30o
\\
Niggli400
9
Si
9 9
k
FIELD OF
S, CALIFORNIA /
200
"
\
300
e\
\
BATHOLITH
\
\
!/I:.>.i), 9
FIELD O F / " 4 ~ (
ALPINE
SEDIMENTS& ~.
200
~.
\
I0o
~
~ ~
____
~- -
F,ELDDE
100
9
2 "
~
METASE0,ME.+S~_
'~
F,ELDOF'
CALIFORNIAN
~
~
CONNEMARA
/METASEDIMENTS /
o'.1
0b
0%mg &
30
0'5
&
I
0'z
0.2
\
/
/, /
25
%
/~'*
al-alk
15
/
-9
9 /
ioi
//
9
/
~
9
el
9
9 .
.
ee
9
/
/
/
/
/
//
FIELD OF
CALIFORNIAN
\
15 §
§
F,0LOOEALP,OE',/++
\
/
/. 9
9
9
~ (9
g'%
0.7
"'.:."
:,\ \ ~ . . - . . § . . . .
§
"~. "~. i
\',,
\',
~§
+
\\
§
"\
I0
- . . ~
/
5
0-6
~ \ \
N - - \
ARKOSES/
',/\§
al-alk
0-5
\\.
2C
*
9 ~,/'. o
\
\
\
i
t
0-4
rng
*
\,\
FIELD OF
//
S. CALIFORNIA
/
BA TH O LI TH ""---....~e/.
20
\
I
03
/
\
(~ AVERAGE
25
I
1
0.1
3C 9
~ ~ ~
- ~ ~ ~,~.
9
FIELD OF CONNEMARA
METASEDIMENTS
5
;
§
+
"~ \
/
i
i
i
4,
AVERAGE
~
0.15
~f
FIELD OF S.
CAL!FORNIA ~
BATHOLIT H ~
0.10
P2OsI
9 9
/
". ~
,~_..~'"
/ ~ t
d~
t
d.2
~
9
L
015
~ .I
9 oe
9
9 ~. .
,,,o,o~5/"
[
9
99
~
~
j
/
~
~
§
[
FIELD OF
9
0-10
. . .
~.
~
9 |
P205
""
w,~
005
§
§
§
+..
0-4
0-5
.r
i
&
mg
d,
ds
d~
0!7
0.1
0-2
03
mg
0.6
0-7
FIG. 3. Plots of Niggli rn9 against si, al-alk, and P205 wt. ~ for the Delaney Dome Formation and the S. California
batholith (left hand side) and typical sedimentary series (right hand side). The big river sands are shown by crosses
and because of the wide scatter, which also extends outside the range of the plot, no field is defined for these
analyses. The solid spots are Californian arkoses representative of variation in arkoses; the Alpine and Connemara fields
cover a wide range of sediments and sedimentary rocks (references in text).
variable removed. Plots of A1203 wt. ~ minus
N a 2 0 q- K z O wt. ~o and of C a D wt. ~o, or even of
the molecular proportions of these values, diminish
as SiO 2 wt. ~o increases.
Other plots that are useful in the solution o f this
problem include m y against P 2 O s (fig. 3) and Zr in
which acid igneous rocks show fall in m g with fall in
P 2 0 5 and Zr as apatite and zircon crystallization
depletes the m a g m a of P and Zr. In contrast,
neither of these elements vary systematically with
m 9 in metasediments, as neither apafite nor zircon
are related to the Fe- and Mg-bearing minerals in
THE DELANEY D O M E F O R M A T I O N
sediments, being determined neither by the salic
nor femic minerals.
Although Cr and Ni variation with m9 is well
established as a reliable indicator of igneous fractionation in basic magmas, these elements are much
less useful in acid igneous rocks because Cr and Ni
have very low abundances in such rocks and m# has
considerable experimental scatter. Typically Cr
and Ni values in sediments are higher than in
granites and rhyolites. Thus typical quartz arenites
and arkoses have Ni 20-40 ppm (Wedepohl, 1969;
Van de Kamp et al., 1976) whereas rhyolites
average less than 10 ppm and granites 7-15 ppm
(Wedepohl, 1969). The present samples average
only 3 ppm Ni and although possessing higher Cr
values (av. 46 ppm) are still low for a sediment.
Ni-m 9 and Cr-m9 plots (not reproduced) do not
conflict with an igneous origin.
Although plots which involve al-alk are subject
to the criticism that alkalis are among the most
mobile of the common elements in rocks, plots
which involve mg, si, Zr, and P are all independent
of alkali variation. Accordingly if such plots give an
ortho or para indication consistent with that given
by al-alk plots, as in the present instance, then there
seems no reason to regard the al-alk parameter as
unreliable. If the al-alk derived deductions conflict
with those derived from other evidence then a dose
examination of the nature of the al-alk variation is
called for. The commonest form of alkali metasomatism in metamorphic rocks involves feldspathization and such metasomatism should not be
difficult to detect because the ratios Na/Ca, Na/K,
or K/Ca should progressively change with feldspathization towards those ratios found in the stable
feldspar (i.e. the porphyroblasts) because the control on chemical composition with respect to the
mobile elements is the composition of the stable
porphyroblastic feldspar. If no change in rock
composition occurs with respect to Na/Ca, Na/K,
or K/Ca, depending whether the feldspar is K
feldspar or plagioclase, then the feldspathization
is probably isochemical recrystallization. Because
each feldspar composition, whether plagioclase or
K feldspar, has a characteristic al-alk value, the
al-alk values of feldspathized alkali metasomatized
rocks should cluster around this characteristic
value rather than be scattered along an igneous or
sedimentary trend thus enabling the recognition of
alkali movement connected with feldspathization.
Other types of alkali movement may be more
difficult to detect but can be suspected ifal alkplots
consistently deviate from the trend expected using
the evidence of other elements or ratios. Indeed an
alkali metasomatized rock can be expected to
display neither a sedimentary nor an igneous al-alk
trend but a metasomatic trend of variation ofal-alk
213
with little change in c (unless Ca has also moved).
Such a vertical variation on the al-alk against c plot
has been found in alkali-metasomatized material
(Leake, 1970).
Averaoe composition. Table II shows that the
average composition of the Detaney Dome Formation major oxides matches fairly closely that of
typical rhyolite or calc alkali rhyolite, the main
differences being the rather higher Fe, Mg, and Ca
and lower A1 and K of the Delaney Dome rocks.
This is consistent with a slightly less differentiated
magma than those which form the basis of the
averages cited. The Delaney Dome rocks equally
well match with typical granites, being again rather
richer in Fe and Mg and lower in A1 and K than the
average granites listed. There is also, however,
rather more Si and less P in the Delaney Dome
Formation. Little more can be deduced by comparing averages which encompass considerable variations in the rocks averaged but it is notable that
most analyses ofrhyolites consulted show distinctly
lower P than granites of otherwise similar major
element chemistry (see fig. 3). This may result
from intratelluric crystallization of apatite within
plutons in which F, CI, and OH are available to
crystallize apatite [the higher H 2 0 of the rhyolites
compared with the granite in Table II is almost
certainly a function of post-magmatic addition of
water to the rhyolites].
Although individual siliciclastic sandstone or
arkose analyses closely similar to the average
composition of the Delaney Dome rocks no doubt
exist, Table 1I shows that, on the whole, quartz
arenites and arkoses are richer in Si and Ca and
poorer in A1, Na, and K (i.e. such sediments have
relatively more quartz and less alkali feldspar).
Nevertheless sole reliance on average compositions
is unsatisfactory because both igneous and sedimentary rocks show great diversity, and trends of
variation related to the formative processes are
likely to be more reliable.
Conclusions and discussion
The Delaney Dome Formation is a deformed low
grade metamorphosed quartzofeldspathic igneous
rock, originally a rhyolite or granite. Although
geochemically it has not been possible to distinguish between these two alternatives, the extremely
fine grain size ( < 0.1 ram), the total lack of quartz
porphyroclasts and the rare euhedral orthoclase
and albitic plagioclase crystals strongly suggests a
sheared rhyolite or ignimbrite, perhaps originally
rich in volcanic glass. The Carlsbad-albite twinning in the plagioclase suggests an igneous feldspar
because metamorphic plagioclase does not normally
contain Carlsbad and albite twinning. The lack of
214
B.E. LEAKE AND D. SINGH
TABLE
II:
AVERAGE
CHEMICAL
ANALYSES
OF Q U A R T Z O F E L D S P A T H I C
1
2
3
4
5
Sio2
73.64
72.82
73.66
71.30
72.08
TiO 2
0.37
0.28
0.22
0.31
0.37
12.01
13.27
13.45
14.32
13.86
AI203
6
ROCKS
7
8
77.1
78.66
80.15
0.3
0.25
0.42
8.7
4.78
6.43
Fe203
2.02
1.48
1.25
1.21
0.86
1.5
1.08
1.29
FeO
1.68
i.ii
0.75
1.64
1.67
0.7
0.30
i.i0
MnO
0.05
0.06
0.03
0.05
0.06
0.2
0.01
0.05
MgO
0.91
0.39
0.32
0.71
0.52
0.5
0.17
0.85
CaO
1.28
1.14
1.13
1.84
1.33
2.7
5.52
3.32
Na20
3.19
3.55
2.99
3.68
3,08
1.5
0.45
1.19
K20
3.45
4.30
5.35
4.07
5.46
2.8
1.32
1.20
P205
0.07
0.07
0.07
0.12
0.18
0.i
0.08
0.06
0.78
0.64
0.53
0.9
1.33
0.92
3.0
5.04
2.10
H20+
i.ii
i.i0
CO 2
0.13
0.08
0.05
i.
Average
of 74 analyses
of the D e l a n e y Dome Formation.
2.
Average
of 670 typical
rhyolites
3.
Average
calc alkali
4.
Average
of 2485 granites
5.
Average
calc alkali
6.
Average
arkose
rhyolite
(Le Maitre,
granite
(Pettijohn,
(Le Maitre,
(Nockolds,
1954)
1976).
[or 22].
1976).
(Nockolds,
1954)
[of 72]~
1963).
7.
Average
of 253 sandstones
8.
Average
of 43 Big river sands
(Pettijohn,
(Potter,
1963).
pers.comm.).
microcline in euhedral K feldspar crystals also al., 1983; 1984). This, combined with the lack of
suggests a non-metamorphic origin. Chemically the pre-thrusting metamorphism of the rhyolites (aslow P205 values agree particularly well with a suming the igneous nature of the euhedral feldspars
rhyolite.
is correct), suggests that the volcanic rocks were not
A number of plots have been shown to be of great pre-thrust antiquity, perhaps lower to
useful in distinguishing quartzofeldspathic sedi- middle Ordovician, Cambrian, or late Precambrian
ments and igneous rocks, in particular a l - a l k vs. in age. Geographically the nearest rhyolites and
c; a l - a l k vs. mg; and m g vs. si, whereas other plots
felsic lavas and tufts of suitable age occur in the
such as rng vs. P205 and si vs. Zr may also be of South Mayo trough, e.g. lower Ordovician of
assistance.
Tourmakeady, west of Lough Mask, although the
The Delaney Dome Formation is the foreland on particular rocks at Tourmakeady are associated
to which the Dalradian rocks were thrust. The with limestones which are unlikely to occur under
formation is likely to be fairly thick for otherwise, 9 the Delaney Dome. However, there are reasons for
instead of mylonitizingthe rocks under the Mannin supposing that the Dalradian block of Connemara
thrust, it is likely that movement would have may have been strike faulted into its present
continued on an underlying more ductile horizon position in the lower Silurian (Bluck and Leake, in
such as a shale or carbonate; there can be few less prep.) and if so, the Delaney Dome Formation is
suitable rocks for a thrust zone than an unbedded unlikely to correlate directly with any rocks in the
acid igneous rock. Thus a substantial thickness of South Mayo trough.
rhyolite is probably present which must predate
460_+ 20 Ma, the Rb-Sr age of the Mannin thrust Acknowledgements. B.E.L. thanks the University of
and also the K-Ar age of lineated hornblendes in Western Australia for research facilities during study
the overlying Ballyconneely amphibolite (Leake et leave from the University of Glasgow.
THE DELANEY DOME FORMATION
REFERENCES
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F. (1973) X-Ray Spectrometry, 2, 33-44.
Leake, B. E. (1969) Indian Mineral. 10, 89 104.
- - ( 1 9 7 0 ) J. Geophys. Res. 75, 349 56.
- - ( 1 9 8 0 ) Am. Mineral. 65, 26 36.
- - ( 1 9 8 6 ) J. Geol. Soc. London, 143 (in press).
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- - T a n n e r , P. W. G., and Senior, A. (1975) J. Petrol. 16,
237 -77.
(1981) The Geology of Connemara (map).
University of Glasgow.
----Singh,
D., and Halliday, A. (1983) Nature, 305,
210 13.
----Macintyre,
R. M., and Elias, E. (1984) Trans. R.
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215
Le Maitre, R. W. (1976) J. Petrol. 17, 589 637.
Niggli, P. (1954) Rocks and mineral deposits. Freeman, San
Francisco.
Nockolds, S. R. (1954) Bull. Geol. Soc. Am. 65, 100732.
Pettijohn, F. J. (1963) U.S. Geol. Surv. Prof. Paper, 440 5.
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Van de Kamp, P. C., Leake, B. E., and Senior, A. (1976)
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[Manuscript received 25 October 1985;
revised 4 December 1985]