lineated and foliated albite-epidote amphibolites, the Ballyconneely amphibolites. Above these amphi- bolites lie strongly foliated and partly sheared.
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 I
10~
ERRIS/AN NAN
ATLANTIC
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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 J'~ ~
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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 _ 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 Harvey, P. K., Taylor, D. M., Hendry, R. D., and Bancroft, 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). and eight others (1968) Chem. Geol. 5, 7 86. - - 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. Soc. Edinburgh: Earth Sci. 75, 165-71.
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. Senior, A., and Leake, B. E. (1978) J. Petrol. 19, 585625. Van de Kamp, P. C., Leake, B. E., and Senior, A. (1976) J. Geol. 84, 195-212. Wedepohl, K. H. (1969) Handbook of Geochemistry. Springer-Verlag, Berlin.
[Manuscript received 25 October 1985; revised 4 December 1985]
