Grampian orogenesis and the development of blueschist-facies metamorphism in western Ireland

Journal of the Geological Society, London, Vol. 160, 2003, pp. 911–924. Printed in Great Britain.

911

Grampian orogenesis and the development of blueschist-facies metamorphism in

western Ireland

D. M. CHEW 1,2, J. S . DALY 1, L . M. PAGE 3,4 & M. J. KENNEDY 1

1Department of Geology, University College Dublin, Dublin 4, Ireland2Present address: Departement de Mineralogie, Universite de Geneve, Rue des Maraıchers 13, CH-1205 Geneve,

Switzerland (e-mail: [email protected])3Laboratory of Isotope Geology, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, Netherlands

4Present address: Department of Geology, Lund University, Solvegatan 13, 223 62 Lund, Sweden

Abstract: Rb–Sr and 40Ar/39Ar step-heating and in situ 40Ar/39Ar laserprobe dating of fabric-forming micas

provide new constraints on the timing of Grampian orogenesis and the associated development of blueschist-

facies metamorphism at the Laurentian margin in NW Ireland. Early (MP1) blueschist-facies assemblages were

developed in metabasites of the Dalradian Supergroup deposited near the edge of the incipient Laurentian

margin, contemporaneous with Barrovian metamorphism in the Dalradian closer to the Laurentian foreland.

The regional D2 event is associated with the formation of orogen-scale fold nappes and is constrained in the

Dalradian by S2 muscovite ages of c. 460 Ma, which are probably recording crystallization. Importantly, the

Clew Bay Complex, previously considered as an exotic terrane (correlated with the Highland Border Complex

of Scotland) and the Dalradian are in structural continuity. Muscovite from the S2 nappe fabric in the Clew

Bay Complex also yields identical c. 460 Ma ages. During D3, dextral shearing tilted the recumbent D2 nappes

into a vertical, downward-facing orientation adjacent to the Laurentian margin. D3 is constrained by S3

muscovite ages of c. 448 Ma. Synchronous deformation of the Dalradian Supergroup and the Clew Bay

Complex in the mid-Ordovician Grampian orogeny casts doubt on both the validity of Silurian microfossil

dates obtained from the Clew Bay Complex and the exotic status of both this ‘terrane’ and the correlative

Highland Border Complex of Scotland.

Keywords: Dalradian Supergroup, Clew Bay Complex, Ar/Ar, Rb– Sr, structural geology.

The NW Mayo inlier on the western extremity of the Caledo-

nides (Fig. 1a and d) preserves an excellently exposed transect

from Laurentian basement, the Annagh Gneiss Complex (Daly

1996), through presumed para-autochthonous Neoproterozoic–

Cambrian? Laurentian cover, the Dalradian Supergroup

(Winchester 1992; Fitzgerald et al. 1994) to outboard oceanic

elements, such as the Clew Bay Complex (Williams et al. 1994).

The Grampian orogeny, caused by the attempted subduction of

the Laurentian margin in the mid-Ordovician, resulted in arc–

continent collision and the development of the Dalradian nappe

pile (e.g. Van Staal et al. 1998; Dewey & Mange 1999).

However, the relationship between the Clew Bay Complex and

the Dalradian remains contentious, primarily because of the

absence of diagnostic macrofauna. Although a probable mid-

Cambrian sponge (Protospongia hicksi, Rushton & Phillips

1973) and mid-Ordovician coniform euconodonts (Harper et al.

1989) have been obtained from the Clew Bay Complex, trilete

miospores interpreted to be of Wenlock age (early Silurian)

(Williams et al. 1994, 1996) are incompatible with involvement

of the Clew Bay Complex in a mid-Ordovician Grampian

orogeny. The structural relationships between the Dalradian and

Highland Border Complex in Scotland, a presumed correlative of

the Clew Bay Complex (Harper et al. 1989), have also proved

contentious. Opinions differ on whether they have a similar

structural history and were deformed contemporaneously in the

Grampian orogeny (e.g. Johnson & Harris 1967; Tanner 1995) or

whether they were exotic to each other and juxtaposed at a later

stage (e.g. Henderson & Robertson 1982; Harte et al. 1984;

Bluck & Ingham 1997; Dempster et al. 2002).

In addition, the Dalradian of the NW Mayo inlier is unique in

that it has preserved the only evidence for blueschist-facies

metamorphism within the Dalradian Supergroup, and is the only

documented example of a blueschist in a regularly bedded

sequence within Britain and Ireland (Gray & Yardley 1979).

However, a lack of diagnostic assemblages and its occurrence

within massive epidosite pods meant that its P–T conditions and

timing relative to the regional deformation fabrics were uncer-

tain. New discoveries of blueschist-facies metabasite outcrops in

South Achill display diagnostic assemblages that can be related

to the regional structural chronology, obtained from detailed field

mapping and petrography. Rb–Sr and 40Ar/39Ar dating of fabric-

forming micas provides new constraints on the timing of

deformation and metamorphism of both the Dalradian and the

Clew Bay Complex, and on the tectonic setting of the Laurentian

margin during Grampian orogenesis.

Structure

D1 deformation of the Dalradian produced a series of low-angle

ductile shear zones (slides) (Kennedy 1969, 1980; Fig. 1c).

Associated with this high-strain event are local F1 folds, which

are in places sheath folds, with prolate pebble-stretching parallel

to the hinges. The kinematics of these late D1 high-strain zones

is difficult to establish because of the complexities of the later

D2 deformation, which is responsible for the disposition of the

major units (Kennedy 1980), but a top to the NNW sense of

movement (i.e. towards the foreland) has been suggested

(Winchester & Max 1996). D2 nappes ‘root’ in a basement core,

Fig. 1. (a) Geology of NW Mayo displaying locations of Rb–Sr muscovite and biotite and 40Ar/39Ar muscovite ages. Inset shows the location of the main

map in western Ireland. (b) Enlarged geological map of South Achill and Achill Beg displaying locations of Rb–Sr muscovite and 40Ar/39Ar muscovite

ages. (c) NNW–SSE (X–Y, located in (a)) cross-section through the NW Mayo inlier showing the P–T results of this study. (d) Location map of Ireland

within the Caledonides.

D. M. CHEW ET AL .912

the Annagh Gneiss Complex (Fig. 1c). Adjacent to, and directly

above the Annagh Gneiss Complex, the D2 nappes are upward-

facing; to the south of this ‘root zone’ recumbent D2 folds face

south (Kennedy 1980; Johnston 1995; Johnston & Phillips 1995;

Fig. 1c). SE-verging D2 nappes in NW Mayo were probably

caused by underthrusting of an unexposed crustal block at depth.

Dalradian nappes close to the Laurentian margin elsewhere in

NW Ireland (e.g. Donegal and NE Ox Mountains) have been

thrust to the SE over elements of the colliding arc and associated

basement (Alsop & Hutton 1993; Flowerdew et al. 2000). In

Scotland, the SE vergence of the D2 Tay Nappe adjacent to the

Highland Border Complex has been attributed to the under-

thrusting of an unseen landmass (Rose & Harris 2000).

On the small island of Achill Beg, immediately south of Achill

Island (Fig. 1a and b), the Dalradian and Clew Bay Complex are

separated by the Achill Beg Fault. Detailed structural field work

has revealed that both units show the same structural history and

therefore were originally in structural continuity (Chew 2001a,

2003).

The D1 deformation event is responsible for the bulk of the

high strain seen in both the Dalradian of South Achill and the

Clew Bay Complex of South Achill Beg. On South Achill Beg,

pronounced pebble-flattening along S1 cleavage planes is visible,

and evidence for high D1 strain in the Dalradian of South Achill

is provided by the presence of tectonic slides (e.g. the Claggan

Bay Mylonite Zone of Harris 1993, 1995). F1 folds are locally

observed in the Dalradian of South Achill and the Clew Bay

Complex of South Achill Beg; in both instances they are

isoclinal, with fold hinges plunging shallowly to the east (Chew

2001a, 2003).

The early D1 high-strain fabrics are refolded by predominantly

downward-facing F2 folds, which are the most commonly ob-

served fold generation in both the Dalradian of South Achill and

the Clew Bay Complex. They are upright structures that plunge

shallowly to the east, similar to the F1 fold plunge. The F2 folds

are predominantly asymmetric, with a ‘Z’ asymmetry viewed

down plunge. Bedding is generally inverted throughout South

Achill and Achill Beg and the S2 foliation generally faces

downwards. This is consistent with the southward-younging

nature of the sequence and an overall ‘Z’ sense of asymmetry of

the eastward-plunging F2 folds (Fig. 1c; Chew 2001a, 2003).

Adjacent to the Laurentian margin, dextral shearing has

resulted in the tilting of the south-facing D2 nappes into a

downward-facing orientation (Fig. 1c). The timing of this dextral

shear episode has been regarded as either contemporaneous with

the development of the D2 nappes (Harris 1993, 1995) or as a

separate D3 event (Sanderson et al. 1980; Chew 2001a, 2003;

Chew et al. 2003). Abundant evidence of dextral shear includes

asymmetric buckle folds and extensional crenulation cleavages,

both of which affect the main S2 foliation. They are interpreted

as the reverse-slip and normal-slip crenulations of Dennis &

Secor (1990).

Metamorphism

Most of the Dalradian of the NW Mayo inlier has experienced

Barrovian metamorphism, with the highest metamorphic grade

close to the basement core in the north where the sillimanite

zone is locally reached (Max et al. 1983). There is a general

decrease in the metamorphic grade of the Dalradian through the

staurolite–kyanite and garnet zones southwards towards the

Achill Beg Fault. The lowest-grade rocks in the inlier are the low

greenschist-facies rocks of the Clew Bay Complex, south of the

Achill Beg Fault. P–T estimates for the staurolite–kyanite zone

metamorphism close to the basement core are 8 � 2 kbar and

620 � 30 8C (Yardley et al. 1987).

Petrographic studies in Achill Island and the mainland to the

east (Fig. 1a) show that the main stage of porphyroblast growth

is predominantly MP1 (Zwart 1962) in age, i.e. porphyroblasts

overgrow the S1 foliation but predate the development of the

main D2 nappe fabric. As this nappe fabric (the S2 foliation) is

commonly well developed in both Dalradian and Clew Bay

Complex lithologies, it is possible to constrain the relative timing

of porphyroblast growth in the region by using this foliation as a

time marker.

Three metamorphic subdomains can be recognized (Figs 1c

and 2; Chew 2001a). Evidence for an early high-pressure

metamorphic event is found in South Achill. Although typical

pelitic lithologies are rather undiagnostic (muscovite þ chlorite

þ albite þ quartz � garnet � epidote � chloritoid � biotite), an

early high-pressure assemblage is developed within MP1 garnets

in metabasites. This early assemblage (glaucophane þ epidote þ

Fig. 2. Metamorphic map of southern and

central Achill, showing locations of samples

illustrated in Figure 3. Although a sharp

boundary between the blueschist-facies

metamorphism and the Barrovian

metamorphism to the north is indicated, the

nature of this boundary is uncertain.

BLUESCHIST-FACIES METAMORPHISM, W IRELAND 913

Fig. 3. (a) Backscattered electron image of a metabasite from the Dalradian of South Achill (sample 102). The MP1 garnet contains abundant S1

glaucophane and epidote inclusions. The S2 foliation augening the MP1 garnet contains MS2 glaucophane porphyroblasts that are rimmed by intergrown

actinolite þ chlorite. (b) Photomicrograph of a metabasite from the Dalradian of South Achill (sample 259). S2 barroisite overgrows S2 glaucophane

porphyroblasts. (c) Photomicrograph of a metabasite from the Dalradian of Central Achill (sample 223). MP1 plagioclase contains inclusions of

hornblende þ epidote þ garnet þ titanite. A coarse-grained S2 foliation defined primarily by hornblende is visible in the top left-hand corner. (d)

Photomicrograph of a pelite from the Clew Bay Complex of South Achill Beg (sample 31), which yielded a 461 � 7 Ma Rb–Sr muscovite–apatite

isochron and a 463 � 11 Ma Rb–Sr muscovite–whole-rock isochron. Late (MP2) plagioclase overgrows an extremely fine-grained S2 foliation defined by

muscovite and chlorite. (e) Photograph of a polished rock slice of a pelite of a representative Dalradian sample selected for in situ 40Ar/39Ar laserprobe

dating of muscovite defining the S2 foliation (sample 145). This sample yielded a weighted mean of 459 � 1 Ma for nine spot fusion analyses, a

460 � 7 Ma Rb–Sr muscovite–plagioclase isochron and a 457 � 4 Ma 40Ar/39Ar step-heating plateau. The coarseness of the fabric-forming S2 muscovite

compared with the Clew Bay Complex sample, shown in (d), is evident. (f) Photograph of a polished rock slice of a pelite from the South Achill

Dalradian selected for in situ 40Ar/39Ar laserprobe dating of muscovite defining the S2 and S3 foliations (sample 79). Act, actinolite; barr, barroisite; chl,

chlorite; ep, epidote; gl, glaucophane; grt, garnet; hbl, hornblende; ms, muscovite; pl, plagioclase; ttn, titanite. The scale bar in all images represents

500 �m.


titanite þ apatite, Fig. 3a) is indicative of blueschist-facies

metamorphism, whereas later growth of calcic amphibole and

chlorite on the margins of S2 glaucophane in South Achill

suggests breakdown of the blueschist-facies assemblages to those

of greenschist facies (Fig. 3b).

Synchronous with the development of the early high-pressure

assemblages in South Achill, the Central Achill rocks were

experiencing amphibolite-facies metamorphism. Low-amphibo-

lite-facies assemblages (hornblende þ garnet þ epidote þtitanite, Fig. 3c) are preserved in MP1 plagioclases in Central

Achill. The MP1 plagioclases are augened by the main S2 fabric,

which is composed of hornblende þ biotite þ epidote þ titanite

þ quartz. In contrast to South Achill, pelitic lithologies are

characterized by the virtual absence of prograde chlorite and

chloritoid, whereas biotite is significantly more abundant.

No metabasites have been observed in the Clew Bay Complex

on South Achill Beg. Pelitic lithologies typically contain phengi-

tic muscovite þ chlorite þ albite þ quartz, with the albite

growth being MP2 in age (Fig. 3d).

The presence of newly discovered blueschist-facies assem-

blages in metabasite outcrops in South Achill means that the

textural age of the blueschist-facies metamorphism can now be

firmly established and thus related to the regional structural

chronology. In particular, it can be demonstrated that the

blueschist-facies metamorphism is contemporaneous with higher-

grade metamorphism closer to the basement core.

The tectonic evolution of this segment of the Laurentian

margin has now been further investigated by quantifying the P–T

conditions of metamorphism in the three metamorphic subdo-

mains (South Achill, Central Achill and South Achill Beg)

described above. Additionally, the timing of metamorphism has

been constrained by Rb–Sr and 40Ar/39Ar dating of fabric-

forming micas.

Analytical procedure

Electron microprobe analyses

Quantitative microprobe analyses were carried out at the EU large-scale

geochemical facility at the University of Bristol, using an automated

JEOL 8600 microprobe with an accelerating voltage of 15 kV and a beam

current of 15 nA, and a spot size as low as 1 �m was used for anhydrous

phases such as garnet. For feldspars and anhydrous phases such as

amphiboles and micas, a beam size of up to 10 �m was used to prevent

the loss of volatile elements such as Na and K. Data reduction was

carried out using ZAF corrections. Natural and synthetic standards were

used for calibration.

Rb–Sr dating

For Rb–Sr analyses, standard ion exchange methods were used for

chemical separation of elements. Samples were loaded on tantalum

filaments and were analysed on a semi-automated single-collector VG

Micromass 30 mass spectrometer at the Department of Geology,

University College Dublin. During the course of analysis, NBS SRM 987

gave 87Sr/86Sr ratios of 0.71027 � 5 (n ¼ 8, 2�) and NBS SRM 607

yielded 87Rb/86Sr ratios of 8.005 � 13 (n ¼ 7, 2�). Sr blanks averaged

1.5 ng and are not significant. Analytical uncertainties (2�) of 1.5% for87Rb/86Sr and tabulated values (Table 1) for 87Sr/86Sr ratios were used in

age calculations, which employed a value of 0.0142 Ga�1 for the 87Rb

decay constant (Steiger & Jager 1977).

40Ar/39Ar dating

40Ar/39Ar step-heating and spot fusion analyses were carried out using

the VULKAAN argon laserprobe (Wijbrans et al. 1995) at the Vrije

Universiteit in Amsterdam (Table 2). Samples were irradiated at the

HPPIF facility in the high flux research reactor at Petten, Netherlands.

Mineral separates were loaded onto Al tablets together with the flux

monitor DRA-1 sanidine (24.99 � 0.07 Ma, Wijbrans et al. 1995). Each

Al tablet contained 20 drilled holes 3 mm in diameter and 2 mm deep.

Four flux monitors were loaded per tablet, and were used to construct a J

curve with a 0.5% error (1�). Polished slices were interspersed between

the Al tablets prior to irradiation. Samples were analysed within 6 months

of irradiation to minimize the interference effects produced by radio-

active decay after irradiation. The analytical procedure has been

described in detail by Wijbrans et al. (1995) and is outlined below.

Samples were step heated using a continuous 18 W argon ion laser

(454.5–514.5 nm wavelength). Typical incremental heating steps involved

1 min of laser heating with a defocused beam to ensure uniform heating,

followed by 4 min of cleaning-up time. For spot fusion experiments,

several short laser pulses (0.1 s) excavated a pit c. 30 �m in diameter,

surrounded by a crater of melt material. The Ar released was cleaned

with Fe–V–Zr getters (250 8C), prior to analysis on a MAP-215/50 mass

spectrometer. Data reduction was carried out using in-house software,

ArArCALC V20. Blank intensities were measured every 3–5 sample

runs, and mass fractionation was corrected for by regular measurement of

shots of clean air argon.

Geothermobarometry

Electron microprobe analyses were obtained from Dalradian

garnet-bearing pelites and metabasites and from pelites within

the Clew Bay Complex to quantify the P–T conditions of MP1

metamorphism. Analytical technique is described above and data

are summarized in Figure 4. Analytical data are presented as

tables in a Supplementary Publication, which can be obtained

from the Society Library or the British Library Document Supply

Centre, Boston Spa, Wetherby, West Yorkshire LS23 7BQ, UK as

Supplementary Publication No. SUP18193 (11 pages). It is also

available online at http://www.geolsoc.org.uk/SUP18193.

Assessment of equilibrium and muscovite chemistry

Of crucial importance in all P–T studies is establishing whether

phases are in chemical equilibrium. If they are not, any P–T

information obtained may be misleading. However, in practice, it

is extremely difficult to prove if two adjacent mineral phases are

in chemical equilibrium, and in general chemical equilibrium is

assumed if two phases show textural evidence of coeval growth

in the same metamorphic event (textural equilibrium). In rela-

tively low-grade rocks such as those encountered in this study

(particularly the Clew Bay Complex rocks), even textural

equilibrium can be difficult to prove as both multiple foliations

with several generations of mineral growth and detrital phases

can be present. In particular, as geobarometry in low-grade

pelitic rocks is based on muscovite chemistry, establishing that

the muscovite generation in question has actually grown or

recrystallized during a particular deformation event is extremely

important. Such information is also required for isotopic dating

studies of fabric-forming muscovite, as described below. Chemi-

cal data are presented (Fig. 5) to demonstrate that a muscovite

that defines a particular fabric has indeed grown during that

specific event.

Analysis of three populations (S1 –S3) of phengitic muscovite

from a variety of samples from the South Achill Dalradian shows

that the early (S1) fabric is more celadonite rich and paragonite

poor than the later (S3) fabric (Fig. 5a). Similar trends have been

documented from the Scottish Dalradian (Dempster 1992). S2

muscovite overlaps both the S1 and S3 populations (Fig. 5a).

Intra-sample variability in the Si per formula unit (p.f.u.) content

of S2 muscovite in samples unaffected by later deformation


http://www.geolsoc.org.uk/SUP18193

Table1.

Rb

–S

rg

eoch

ron

olo

gy

Sam

ple

Dat

edu

nit

,lo

cali

tyan

dIr

ish

Nat

ion

alG

rid

Ref

.T

extu

ral

rela

tionsh

ipM

iner

alR

b(p

pm

)S

r(p

pm

)8

7R

b/8

6S

r8

7S

r/8

6S

r�

2�

87S

r/8

6S

r(i)

Ag

e�

2�

(Ma)

31

Cle

wB

ayC

om

ple

x,

So

uth

Ach

ill

Beg

S2

foli

atio

nm

s5

2.0

82

7.1

55

.58

0.7

62

38

1�

68

0.7

25

55

46

3�

11

31

(L7

121

92

51

)w

r6

4.0

01

05

.60

1.7

60

.73

71

59�

48

31

ap2

.50

11

47.4

90

.01

0.7

25

75

5�

46

0.7

25

71

46

1�

76

9C

lew

Bay

Com

ple

x,

So

uth

Ach

ill

Beg

S2

foli

atio

nm

s2

90

.30

17

5.0

04

.82

0.7

58

85

5�

48

0.7

26

60

46

9�

14

69

(L7

121

92

51

)w

r1

23

.52

17

0.3

92

.10

0.7

40

67

2�

48

69

ap5

.21

13

84.9

30

.01

0.7

27

33

7�

52

0.7

27

27

46

0�

78

2D

alra

dia

n,

So

uth

Ach

ill

S2

foli

atio

nm

s3

15

.22

87

.48

10

.52

0.7

96

44

4�

52

0.7

27

77

45

8�

78

2(L

69

11

95

15

)p

l2

.69

16

.73

0.4

70

.73

08

22�

52

10

7D

alra

dia

n,

So

uth

Ach

ill

S2

foli

atio

nm

s3

36

.24

58

.35

16

.89

0.8

38

92

2�

72

0.7

28

67

45

8�

71

07

(L6

971

94

76

)p

l1

.66

64

.02

0.0

80

.72

91

65�

10

81

15

Dal

rad

ian

,S

outh

Ach

ill

S2

foli

atio

nm

s2

84

.46

57

.59

14

.45

0.8

19

80

4�

48

0.7

28

80

44

2�

71

15

(F5

62

00

43

6)

(cre

nula

ted

)p

l1

.66

13

.96

0.3

50

.73

09

76�

38

14

4D

alra

dia

n,

So

uth

Ach

ill

S2

foli

atio

nm

s2

99

.70

43

.68

20

.16

0.8

65

05

1�

18

00

.732

90

46

0�

71

44

(L6

901

95

45

)p

l1

.33

29

.61

0.1

30

.73

37

50�

38

14

5D

alra

dia

n,

So

uth

Ach

ill

S2

foli

atio

nm

s2

97

.55

47

.70

18

.30

0.8

52

87

2�

54

0.7

33

22

45

9�

71

45

(L6

902

95

44

)p

l1

.45

31

.31

0.1

30

.73

40

94�

10

62

19

Dal

rad

ian

,C

entr

alA

chil

lS

2fo

liat

ion

ms

42

7.5

45

.62

25

6.3

72

.39

23

70�

14

00

.757

80

44

8�

72

19

(L6

769

97

81

)p

l3

.23

2.2

24

.24

0.7

84

82

8�

80

22

7D

alra

dia

n,

Cen

tral

Ach

ill

S2

foli

atio

nm

s4

26

.66

13

.49

97

.23

1.3

48

40

0�

60

0.7

37

20

44

1�

72

27

(F7

36

90

25

1)

wr

15

3.5

16

3.4

57

.05

0.7

81

52

5�

88

22

7b

t7

15

.35

19

.92

11

1.0

01

.40

71

40�

60

0.7

39

09

42

3�

72

62

Dal

rad

ian

,N

ort

hA

chil

lS

2fo

liat

ion

ms

43

0.3

73

4.9

93

6.4

70

.96

25

68�

68

0.7

33

38

44

1�

72

62

(F6

67

80

92

9)

pl

27

.42

15

8.3

20

.50

0.7

36

53

7�

37

27

5D

alra

dia

n,

Kin

rovar

S2

foli

atio

nm

s2

73

.30

32

2.3

82

.46

0.7

33

38

7�

42

0.7

17

63

45

0�

82

75

(F7

10

51

46

0)

pl

5.8

31

37

.30

0.1

20

.71

84

18�

44

27

5b

t4

41

.67

5.8

22

52

.73

2.2

60

58

0�

10

00

.717

67

42

9�

62

76

Dal

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typically does not exceed 0.05 formula units. However, samples

that are affected by later D3 deformation display markedly higher

intra-sample variability in the Si p.f.u. content of S2 muscovite

(as high as 0.16 formula units). This trend is evident in Figure

5b, where S2 muscovite samples unaffected by late deformation

are relatively celadonite rich and paragonite poor, and overlap

with the S1 muscovite population of Figure 5a. Conversely,

values for S2 muscovite samples where late (D3) deformation

was pervasive (Fig. 5b) overlap with the range of values

exhibited by the S3 muscovite population of Figure 5a. Only

samples that were not affected by late deformation were selected

for dating of the main (S2) fabric: it appears that deformation

and recrystallization strongly influence the muscovite chemistry.

Similarly, undeformed S2 muscovite samples from the Clew Bay

Complex were selected for isotopic dating. The S2 Clew Bay

Complex muscovite samples are celadonite rich and overlap with

the undeformed S2 Dalradian muscovite population (Fig. 5b).

Dalradian

The P–T conditions affecting the Dalradian have been assessed

using the garnet–hornblende thermometer of Graham & Powell

(1984), and the garnet–biotite thermometer calibration of

Williams & Grambling (1990), which takes into account the high

spessartine content of the Dalradian garnets from South Achill.

Barometric estimates in the Dalradian are hampered by the

low anorthite contents of plagioclase, typically 0.5–1 mol %.

Phengite pressures for the Dalradian and the Clew Bay Complex

Table 2. Ar incremental heating data

Step Laser power 36Ar(atm) 37Ar(Ca) 38Ar(Cl) 39Ar(K) 40Ar� Age (Ma) �2� 40Ar� (%) 39Ar(K) (%)

Sample 82, S2 ms; Dalradian, South Achill (L69119515)1 0.15 W 0.0003 0.0051 0.0001 0.0123 0.7646 446.3 18.5 90.6 1.12 0.20 W 0.0001 0.0014 0.0000 0.0168 1.1067 470.2 12.9 98.5 1.53 0.27 W 0.0012 0.0051 0.0000 0.2855 18.4968 464.0 2.8 98.2 26.14 0.31 W 0.0009 0.0099 0.0000 0.3253 20.8963 460.5 2.0 98.8 29.75 0.37 W 0.0001 0.0076 0.0000 0.1688 10.8523 460.7 2.3 99.7 15.46 0.50 W 0.0001 0.0000 0.0000 0.0818 5.1704 453.7 4.2 99.2 7.57 Fusion 0.0003 0.0112 0.0000 0.2042 13.0692 458.9 3.5 99.4 18.7Plateau age: 460.4 � 4.8 Ma (2�) (steps 3–7)Fusion age: 460.6 � 4.3 Ma (2�)J value: 0.004526 � 0.5% (1�)

Sample 107, S2 ms; Dalradian, South Achill (L69719476)1 0.14 W 0.0007 0.0000 0.0002 0.0538 3.4041 454.7 7.0 94.5 2.22 0.21 W 0.0011 0.0000 0.0000 0.3443 22.2850 463.6 2.2 98.6 13.93 0.25 W 0.0012 0.0129 0.0000 0.7286 46.9217 462.0 1.5 99.3 29.44 0.35 W 0.0012 0.0062 0.0000 0.5464 35.1458 461.0 1.1 99.0 22.15 Fusion 0.0010 0.0000 0.0000 0.8054 52.1130 463.4 1.2 99.5 32.5Plateau age: 462.2 � 4.3 Ma (2�) (steps 2–5)Fusion age: 462.3 � 4.1 Ma (2�)J value: 0.004526 � 0.5% (1�)

Sample 137, S1 ms; Dalradian, North Achill (F56200436)1 0.14 W 0.0004 0.0019 0.0000 0.0084 0.4329 377.1 36.9 79.4 0.82 0.22 W 0.0004 0.0000 0.0003 0.2164 13.3969 445.6 2.6 99.2 20.33 0.30 W 0.0002 0.0000 0.0002 0.3011 18.5166 443.1 2.4 99.7 28.34 0.40 W 0.0002 0.0000 0.0000 0.3230 20.2108 449.8 3.3 99.7 30.35 0.60 W 0.0002 0.0129 0.0000 0.1612 10.0218 447.4 3.1 99.5 15.16 Fusion 0.0002 0.0238 0.0000 0.0553 3.3688 439.6 7.5 98.3 5.2Plateau age: 445.9 � 4.8 Ma (2�) (steps 2–5)Fusion age: 445.6 � 4.2 Ma (2�)J value: 0.004526 � 0.5% (1�)




(Fig. 4) are estimated using the Si p.f.u. content of phengite

(Massonne & Schreyer 1987), but must be taken as minima as

K-feldspar is absent. Further P–T information for MP1 meta-

morphism in the Dalradian is obtained from the average P–T

method of thermocalc (Powell & Holland 1994) using the 26

September 1997 version of the thermodynamic dataset (Holland

& Powell 1998). Further details are available in table 10 of the

Supplementary Publication. Average P–T estimates define a band

in P–T space (Fig. 4) depending on XH2O in a H2O–CO2

mixture, as there is no independent control on fluid composition.

Clew Bay Complex

Geothermobarometry in the Clew Bay Complex was undertaken

using local equilibria of chlorite and phengite pairs (Vidal &

Parra 2000) using the tweequ program (Berman 1991). This

approach employs several chlorite and potassic white mica

end-members, which circumvents the problems traditionally

associated with obtaining P–T estimates in high-variance

assemblages such as the Clew Bay Complex pelites. The

thermodynamic data and solid solution models used in the P–T

calculations have been given by Vidal et al. (2001) for the

chlorite end-members and Parra et al. (2002) for the phengite

end-members. Multi-equilibrium P–T intersections obtained

using tweequ define a band in P–T space (Fig. 4) assuming a

range of values for the chlorite Fe3þ/Fe2þ ratio of between zero

and 0.1. The uncertainty in the Fe3þ content of potassic white

mica is less significant as iron-bearing end-members were not

used in the P–T calculations; changing the Fe3þ content has little

effect on the calculated activities of the other potassic white

mica end-members and hence on the P–T estimates (Vidal &

Parra 2000).

Results and error propagation

One of the chief sources of difficulty in quantitative thermo-

barometry is the assessment of uncertainties. In many cases

uncertainties (at the 2� level) of �0.5 kbar and �30 8C are

quoted in the literature, whereas rigorous error assessment

suggests uncertainties (at the 2� level) as high as 2–3 kbar and

c. 100 8C (e.g. Hodges & McKenna 1987; Worley & Powell

14

12

10

8

6

4

2

0300200 400 500 600

P (

kbar

)

T (°C)

223TC

2273.22

Si p.f.u.

223grt-hbl

227 grt-bt

227 TC

1063.40 Si p.f.u.

106grt-bt

102 TC107 TC

32 3.33 Si p.f.u.

Central Achill(525 ± 45 ºC,6.5 ± 1.5 kbar)

CBC, SouthAchill Beg(c. 10 kbar,325 - 400 °C)

South Achill(460 ± 45 ºC,

10.5 ± 1.5 kbar)

1.00.1

1.0

0.1

1.0

1.0

0.2

0.2

0

0

0.10.1

32 TWQ

106grtbthblSi p.f.u.

TC

TWQ

Sample numberGarnetBiotiteHornblendeSi per formulaunit of phengiteThermocalc av.PT calculationTWEEQU PTintersection

Fig. 4. Summary of P–T data for South and Central Achill and South

Achill Beg. Numbers in italics adjacent to South Achill data correspond

to XH2 O values for binary H2O–CO2 fluids used for different

thermocalc average P–T calculations. Numbers in italics adjacent to

the Clew Bay Complex P–T data from South Achill Beg correspond to

the chlorite Fe3þ/Fe2þ ratio used for different tweequ P–T intersections.

The highest temperatures are obtained using a chlorite Fe3þ/Fe2 ratio of

zero, whereas significantly lower temperatures are obtained using a

chlorite Fe3þ/Fe2 ratio of 0.1. However, a peak metamorphic temperature

of c. 325–400 8C is thought likely (see text).

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35 3.40

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35 3.40

S2 South Achill, no late deformationS2 CBC, South Achill Beg

S2 South Achill, pervasive D3 deformation

(b)

S2 South AchillS1 South Achill

S3 South Achill

(a)

Si p.f.u.

Si p.f.u.

NaNa + K

NaNa + K

Fig. 5. Graphs illustrating the variation in phengite composition

according to structural age. (a) Composition of three generations of

phengite from several samples from the South Achill Dalradian (S1 –S3).

The youngest generation (S3) of phengite present is significantly more

paragonite rich and celadonite poor than the phengite defining the older

fabrics. (b) Composition of S2 phengite from several samples from the

South Achill Dalradian and the Clew Bay Complex of South Achill Beg.

Samples of S2 phengite that have experienced pervasive late (D3)

deformation are differentiated from those S2 phengite samples that are

undeformed. S2 phengite that has experienced D3 deformation is more

paragonite rich and celadonite poor than undeformed S2 phengite.


2000). P–T estimates for the three metamorphic subdomains

identified from petrography are described below along with

conservative error estimates.

P–T estimates of the conditions of MP1 Barrovian metamorph-

ism (low amphibolite facies) in the Dalradian of central Achill

are 525 � 45 8C and 6.5 � 1.5 kbar (1�) (Figs 1c, 2 and 4).

Temperature estimates were calculated using the garnet–biotite

thermometer of Williams & Grambling (1990). Those workers

estimated 1� uncertainties of c. 45 8C for spessartine-rich garnet

samples using the error assessment procedure outlined by Powell

(1985), similar to the 1� uncertainties of the thermocalc

average P–T data. Barometric estimates and their uncertainties

are derived from the thermocalc average P–T data. P–T

estimates of the contemporaneous MP1 blueschist-facies meta-

morphism in South Achill are 460 � 45 8C and 10.5 � 1.5 kbar

(1�) (Figs 1c, 2 and 4).

Estimating peak metamorphic temperature in low-greenschist-

facies assemblages (phengitic muscovite þ chlorite þ albite þquartz) such as those of the Clew Bay Complex is extremely

difficult. Peak temperatures were initially crudely estimated at

250–350 8C based on thermal alteration values of amorphous

organic matter (Chew 2001a). More recent estimates utilizing

local equilibria of chlorite and phengite pairs (Vidal & Parra

2000) yield P–T conditions of c. 300–450 8C and 10 kbar for the

Clew Bay Complex (Figs 1c and 4) depending on the value used

for the chlorite Fe3þ/Fe2þ ratio, with the highest temperatures

produced (c. 450 8C) corresponding to chlorite containing no

ferric iron. However, a temperature range of c. 325–400 8C is

thought more likely by utilizing a procedure that estimates the

amount of Fe3þ in chlorite by minimizing the difference between

(AlIV � AlVI)/2 and the number of vacancies (Le Hebel et al.

2002). Rigorous error assessment in tweequ is complicated by

the fact that uncertainties in the thermodynamic data and

activity–composition models are not reported.

The uncertainties involved in the absolute P–T estimates arise

chiefly as a result of uncertainties in experimental calibrations

and in activity–composition models. However, by using com-

parative thermobarometry (applying a single thermobarometer to

different samples to calculate differences in P–T conditions), it

is possible to eliminate the systematic error associated with these

uncertainties (e.g. Hodges & McKenna 1987; Worley & Powell

2000). Thus differences in pressure and temperature may be

calculated much more precisely than absolute values, and

uncertainties may be a reduced by a factor of 5–10 (Worley &

Powell 2000). Thus it is likely that the c. 65 8C temperature

difference between South Achill and Central Achill obtained

using the garnet–biotite thermometer of Williams & Grambling

(1990) is statistically significant. Likewise, using a similar

approach, analysis of S1 inclusions within MP1 garnet in South

Achill suggests that the rocks were colder prior to the MP1

S1: 457 ± 6 MaS2: 455 ± 11 Ma

Age(Ma)

500

475

425

400

450

Sample 16(Dalradian, South Achill) (a)

S2: 459 ± 1 MaAge(Ma)

500

475

425

400

450

Sample 145(Dalradian, South Achill)

S3: 448 ± 3 Ma

S2: 451 ± 2 Ma

Age(Ma)

500

475

425

400

450

Sample 79(Dalradian, South Achill) (b)

200 100806040400

600

550

500

450

Age(Ma)

Cumulative % Ar39

Sample 137(Dalradian, North Achill)

446 ± 5 Ma

200 100806040400

600

550

500

450

Age(Ma)

Cumulative % Ar39


460 ± 5 Ma

200 100806040400

600

550

500

450

Age(Ma)

Cumulative % Ar39


462 ± 4 Ma

(c)(d)

(g)

(e) (f)

200 100806040400

600

550

500

450

Age(Ma)

Cumulative % Ar39


463 ± 4 Ma

200 100806040400

600

550

500

450

Age(Ma)

Cumulative % Ar39


457 ± 4 Ma

(h)

Fig. 6. Summary of 40Ar/39Ar laserprobe

incremental heating (a–c) and in situ spot

fusion ages (d–h). Error bars (for

individual spot fusion ages), error boxes

(for plateau steps) and errors in sample

ages (i.e. plateau ages and weighted average

of spot fusion ages) are expressed as 2�.

The weighted averages of the spot fusion

analyses were calculated using isoplot and

use the 2� error associated with each

analysis.


blueschist-facies metamorphism. S1 phengite inclusions in garnet

cores yield temperatures c. 50 8C lower than the corresponding

S2 phengite–garnet rim pairs using the garnet–phengite barom-

eter of Green & Hellman (1982) for pelitic systems. This

temperature rise from D1 to D2 is consistent with the breakdown

of the early S2 blueschist-facies assemblages that is observed

petrographically (Fig. 3b), with S2 glaucophane being rimmed by

barroisite and actinolite.

Implications for the evolution of the Laurentian marginduring Grampian orogenesis

Significantly, the Clew Bay Complex has a low geothermal

gradient similar to that of South Achill. The metamorphic break

between the Clew Bay Complex and the blueschist-facies

metamorphism in the Dalradian of South Achill is sharply

defined by the Achill Beg Fault (Figs 1a and 2), but the nature of

the boundary between the blueschist-facies metamorphism and

the Barrovian metamorphism of the Dalradian to the north (Fig.

2) is uncertain. It is possible that the boundary is defined by an

early (i.e. ductile) tectonic discontinuity, such as the Claggan

Bay Mylonite Zone of Harris (1993, 1995) or the Ashleam Bay

Slide (Chew 2001a). Alternatively, it may be due to late faulting

or may even be gradational in nature.

It is suggested that the original boundary between the

blueschist-facies metamorphism and the Barrovian metamorph-

ism is a very late D1 or early D2 thrust that runs through

Ashleam Bay (Fig. 2). Here there is evidence of stratigraphic

excision, as the uppermost Appin Group and the lower portions

of the Argyll Group (including the Port Askaig tillite equivalent)

appear to be absent (Chew 2001a). However, it should be

emphasized that the majority of the intervening ground between

these two metamorphic subdomains (from Claggan Bay in South

Achill to the northern part of Central Achill, Fig. 2) yields little

in the way of diagnostic metamorphic assemblages.

As blueschist-facies metamorphism has not been observed

elsewhere within the Dalradian Supergroup, the question arises

of why it is found in South Achill. The Dalradian of South

Achill is likely to represent extremely distal Dalradian sedimen-

tation (Chew 2001b). With the onset of subduction during

Grampian orogenesis, it is likely that distal elements of the

Laurentian margin would have experienced high-pressure–low-

temperature metamorphism as a result of their proximity to the

subduction-zone environment (e.g. Fig. 7). The synchronous

(MP1) development of Barrovian metamorphism in the Dalradian

to the north is best explained by its position away from the

subduction zone, closer to the foreland. It is proposed that with

the onset of foreland-directed thrusting in late D1 times, the

blueschist-bearing rocks are removed from the cold subduction

zone setting and are incorporated into the development of the

main D2 nappe. Once incorporated into the nappe pile, the S1

and early S2 blueschist-facies assemblages are broken down to

the greenschist-facies assemblages.

Geochronology of fabric-forming micas

To date, there has been no published mineral geochronology

from the Clew Bay Complex, and the only metamorphic mineral

age data from the Highland Border Complex are from the Bute

Amphibolite (Dempster & Bluck 1991), thought to be the

metamorphic sole of a tectonically emplaced ophiolite

(Henderson & Robertson 1982). In particular, there has never

been a systematic attempt to date correlatable fabrics across the

Dalradian–Highland Border Complex and Clew Bay Complex

contact in either Scotland or Ireland. Most of the isotopic ages

presented in this paper were obtained from S2 muscovite defining

the main nappe fabric in both the Dalradian and the Clew Bay

Complex, as the S2 foliation is the dominant foliation in the NW

Mayo inlier. The majority of the dated samples are from the

Dalradian (e.g. Fig. 3e), as the foliation-forming muscovite in

Clew Bay Complex pelites is extremely fine grained (e.g. Fig.

3d), which makes conventional mineral separation or in situ

laserprobe dating technically challenging.

All ages are shown in Figure 1a and b, and 40Ar/39Ar data are

displayed graphically in Figure 6. Raw data are given in Tables

1–3. Rb–Sr ages were in general calculated using coexisting

plagioclase to constrain the 87Sr/86Sr initial ratio, with the

exception of samples 31, 69 and 227, which used the whole-rock

sample. Samples 31 and 69 also used coexisting S2 apatite to

further constrain the 87Sr/86Sr initial ratio.

South Achill and the Clew Bay Complex crystallizationages

Muscovites defining the main S2 nappe fabric in South Achill

yield c. 460 Ma ages by all three methods (Rb–Sr, 40Ar/39Ar

step-heating and in situ laserprobe dating, Figs 1b and 6, Tables

1–3). Likewise, Rb–Sr dating of the main S2 nappe fabric of the

Clew Bay Complex in South Achill Beg yields similar c. 460 Ma

ages (Fig. 1b, Table 1) using both apatite and the whole-rock

sample to constrain the 87Sr/86Sr initial ratio. The peak tempera-

ture of the low-greenschist-facies Clew Bay Complex does not

exceed 450 8C, and is likely to be c. 325–400 8C (Fig. 4). Hence

the Rb–Sr ages for S2 muscovite in the Clew Bay Complex are

Fig. 7. Schematic reconstruction of the Laurentian margin of western Ireland at c. 470 Ma. Incipient development of blueschist-facies metamorphism

(asterisk) in distal Dalradian sediments and the Clew Bay Complex accretionary wedge. Onset of foreland-directed thrusting and ophiolite obduction.


likely to be crystallization ages. A closure temperature of c.

500 8C for the Rb–Sr muscovite system is commonly quoted in

the literature (e.g. Cliff 1985), although it is likely to be lower

for fine-grained muscovite samples such as those from the Clew

Bay Complex. Using the Dodson (1973) formulation for closure

temperature and the empirical diffusion parameters of Jenkin

(1997), a closure temperature of c. 350 8C is obtained for the

Clew Bay Complex muscovite samples (assuming a grain

diameter of 75 �m and a cooling rate of 25 8C Ma�1), but it

should be emphasized that Sr diffusion coefficients in micas are

currently very poorly constrained.

As the South Achill Dalradian and the Clew Bay Complex

share the same structural history (Chew 2001a, 2003), the

identical c. 460 Ma Rb–Sr muscovite ages for the S2 nappe

fabric in South Achill may also be recording crystallization.

Further evidence that these ages may be recording crystallization

is obtained from the rather low peak temperature estimates

obtained from the South Achill Dalradian (460 � 45 8C, Fig. 4).

The marked similarity with the S2 muscovite 40Ar/39Ar step-

heating data (Fig. 6) would suggest that the Ar/Ar system is

recording crystallization as well, despite growing c. 50–100 8C

above its closure temperature (c. 350–400 8C, e.g. Wijbrans &

McDougall 1988). Although the simplest explanation for the

similarity in Rb–Sr and Ar/Ar ages would be that of rapid uplift

and cooling, it has been suggested (e.g. Villa 1998) that in the

absence of later deformation and fluid circulation, isotopic

closure can occur at significantly higher temperatures (c. 100–

200 8C) than those conventionally assumed.

In situ 40Ar/39Ar laserprobe ages of earlier S1 muscovite in

South Achill yield a weighted mean age of 457 � 6 Ma, with S2

muscovite from the same sample yielding a weighted mean age

of 455 � 11 Ma (sample 16, Fig. 6a). In situ dating of S3

crenulations (Fig. 3f) related to dextral shearing yield a weighted

mean age of 448 � 3 Ma. This age is interpreted as a crystal-

lization age based on the low-greenschist-facies assemblage

(phengitic muscovite þ chlorite þ albite þ quartz) observed in

the S3 crenulation seams. The older S2 muscovite seams (Fig. 3f)

yield a weighted mean age of 451 � 2 Ma (sample 79, Fig. 6b).

It appears that 40Ar/39Ar system is only reliably recording the

youngest deformation fabrics present. In addition, sample 115

(442 � 7 Ma, Fig. 1b, Table 1) is the only Rb–Sr S2 muscovite

age from South Achill to deviate significantly from the c.

460 Ma mean and it is affected by late deformation, which is

pervasive at the southern tip of the island, and continues to

intensify towards the Achill Beg Fault.

Cooling ages from further north

Rb–Sr S2 muscovite and biotite dates from the central and

northern portions of Achill Island and the mainland, within the

Table 3. Ar spot fusion data

Spot Laser power 36Ar(atm) 37Ar(Ca) 38Ar(Cl) 39Ar(K) 40Ar� Age (Ma) �2� 40Ar� (%) 39Ar(K) (%)

Sample 16, S1 and S2 ms; Dalradian, South Achill (L68929561)1 (S1) Fusion 0.0789 0.1962 0.0000 2.7870 199.6289 463.1 1.2 89.5 34.52 (S1) Fusion 0.0016 0.0320 0.0000 0.8867 62.6202 457.4 2.5 99.2 11.03 (S1) Fusion 0.0022 0.0303 0.0000 0.7043 49.4344 454.9 1.6 98.7 8.76 (S1) Fusion 0.0010 0.0308 0.0000 0.8643 60.2106 451.9 1.7 99.5 10.77 (S1) Fusion 0.0010 0.0190 0.0000 0.7633 53.4692 454.1 1.7 99.4 9.54 (S2) Fusion 0.0030 0.0626 0.0000 1.3182 92.4487 454.6 1.3 99.1 16.35 (S2) Fusion 0.0016 0.1900 0.0002 0.7543 53.1331 456.4 1.9 99.2 9.3J value: 0.004086 � 0.5% (1�)

Sample 79, S2 and S3 ms; Dalradian, South Achill (L69089511)2 (S2) Fusion 0.0002 0.0000 0.0001 0.1262 9.0407 453.4 3.4 99.2 7.14 (S2) Fusion 0.0002 0.0266 0.0000 0.0320 2.2206 441.3 10.1 97.3 1.86 (S2) Fusion 0.0000 0.0299 0.0002 0.0209 1.5135 457.8 14.5 99.4 1.210 (S2) Fusion 0.0010 0.0208 0.0000 0.5635 40.1131 450.7 1.3 99.3 31.711 (S2) Fusion 0.0004 0.0308 0.0004 0.1893 13.5803 453.8 3.5 99.1 10.71 (S3) Fusion 0.0001 0.0000 0.0000 0.1492 10.5535 448.2 2.8 99.7 8.43 (S3) Fusion 0.0000 0.0000 0.0001 0.0972 6.8772 448.2 3.5 99.8 5.55 (S3) Fusion 0.0001 0.0345 0.0001 0.1325 9.3329 446.6 3.6 99.8 7.57 (S3) Fusion 0.0013 0.0511 0.0002 0.0683 4.7528 441.7 5.5 92.4 3.88 (S3) Fusion 0.0002 0.0371 0.0000 0.0891 6.2796 446.9 4.8 99.0 5.09 (S3) Fusion 0.0003 0.0178 0.0000 0.3099 22.0392 450.4 1.9 99.6 17.4J value: 0.003987 � 0.5% (1�)

Sample 145, S2 ms; Dalradian, South Achill (L69029544)1 (S2) Fusion 0.0003 0.0226 0.0000 0.1506 11.0506 456.1 2.8 99.3 5.32 (S2) Fusion 0.0009 0.0220 0.0000 0.6316 46.7845 459.8 1.7 99.4 22.43 (S2) Fusion 0.0006 0.0127 0.0000 0.3509 26.0835 461.2 2.8 99.3 12.44 (S2) Fusion 0.0005 0.0197 0.0002 0.0954 7.0994 461.8 4.5 97.9 3.45 (S2) Fusion 0.0003 0.0041 0.0000 0.0567 4.1341 453.5 6.7 98.2 2.06 (S2) Fusion 0.0010 0.0358 0.0000 0.6250 46.0855 458.0 1.7 99.4 22.27 (S2) Fusion 0.0005 0.0531 0.0000 0.3175 23.3966 457.8 2.1 99.4 11.38 (S2) Fusion 0.0009 0.0314 0.0001 0.3608 26.6725 459.0 1.7 99.0 12.89 (S2) Fusion 0.0005 0.0117 0.0002 0.2326 17.2598 460.5 2.6 99.2 8.3Normal isochron: 459.0 � 18.0 Ma (2�) (all spots)Inverse isochron: 456.6 � 5.3 Ma (2�) (all spots)J value: 0.003919 � 0.5% (1�)


recumbent nappes, yield c. 440–450 Ma and c. 420–430 Ma

ages, respectively (Fig. 1a, Table 1), and sample 137 from west

Achill yields a 446 � 5 Ma 40Ar/39Ar step-heating plateau for S1

muscovite (Figs 1a and 6f). As a peak temperature of 525 �45 8C has been estimated for central Achill and the temperature

rises towards the north (Yardley et al. 1987), these ages are

interpreted as recording cooling.

Comparison with other Grampian age data from Ireland

Comparing the development of the c. 460 Ma D2 nappes in the

southern portion of the NW Mayo inlier with the timing of

Grampian orogenesis elsewhere in Ireland reveals a diachroneity

of c. 5–10 Ma. In Connemara, late D3 (i.e. synchronous with the

last stages of nappe development) quartz-diorite gneisses yield

U–Pb zircon ages of 467 � 2 Ma (Friedrich et al. 1999). Pre-

tectonic dolerite dykes in the Annagh Gneiss Complex on the

north Mayo coast (Fig. 1a) display a composite S1 –S2 Grampian

foliation, and hornblende from this fabric has yielded a

473 � 3 Ma 40Ar/39Ar step-heating plateau (Flowerdew 2000).40Ar/39Ar hornblende and Rb–Sr muscovite dating of the main

S3 nappe fabric in the Ox Mountains Dalradian yields cooling

ages as old as c. 470 Ma (Flowerdew et al. 2000).

Although a difference of c. 5–10 Ma is only just resolvable

with the current data, it may be significant. However, the main

nappe fabric in both north Mayo (S2) and the Ox Mountains (S3)

precedes the metamorphic peak, whereas the main nappe fabric

(S2) in the southern portion of the NW Mayo inlier postdates the

main phase of porphyroblast growth. It is unclear whether it is

the metamorphic peak or the main phase of nappe development

that is diachronous through the NW Mayo and Ox Mountains

inliers. The validity of correlating fabrics throughout the various

inliers of the Irish Dalradian is also uncertain.

Comparisons with blueschist development in theAppalachian orogen

Blueschists have been documented from a similar tectonic setting

in correlative rocks in the Appalachian orogen (Van Staal et al.

1998). Blueschist-facies assemblages are developed in rocks that

are structurally overlain by ophiolitic rocks of the Baie Verte

Oceanic Tract in Newfoundland (Jamieson 1977) and are also

developed underneath correlative ophiolitic rocks in Quebec

(Trzcienski 1976) and New England (Laird et al. 1993). Obduc-

tion of the Baie Verte Oceanic Tract onto the Laurentian margin

took place in the Early Ordovician (Van Staal et al. 1998, and

references therein). The oceanic components of the Clew Bay

Complex (Fig. 7) and Highland Border Complex are widely

regarded to be represent the continuation of the Baie Verte

Oceanic Tract into the Caledonides (e.g. Van Staal et al. 1998;

Dewey & Mange 1999).

Conclusions

Blueschist-facies assemblages were developed at the leading

edge of the subducting Laurentian plate and were thrust in late

D1 times towards the foreland where inboard elements of the

Dalradian were undergoing contemporaneous MP1 Barrovian

metamorphism (Fig. 7). Continued crustal thickening, probably

associated with the underthrusting of a southern landmass, led to

the formation of D2 crustal-scale nappes. At the Laurentian

margin, white mica defining the S2 nappe fabric in the Dalradian

yields c. 460 Ma Rb–Sr and 40Ar/39Ar crystallization ages.

Closer to the core of Laurentian basement, S2 muscovite and

biotite yield Rb–Sr cooling ages of c. 445 Ma and c. 425 Ma,

respectively. South of the Achill Beg Fault, the outboard Clew

Bay Complex exhibits a structural chronology identical to that

seen in the Dalradian, with white mica yielding c. 460 Ma Rb–

Sr white mica crystallization ages. The geochronology confirms

the structural observation that the Dalradian and the Clew Bay

Complex have shared the same deformation history. Contempora-

neous deformation of the Dalradian and the Clew Bay Complex

in the mid-Ordovician Grampian orogeny suggests that the

Silurian microfossil data from the Clew Bay Complex (Williams

et al. 1994, 1996) need to be reassessed, as does the exotic status

for the Highland Border Complex in Scotland. Later D3 dextral

shearing resulted in the tilting of the recumbent, south-facing D2

Dalradian nappes into a downward-facing orientation. In situ Ar/

Ar laserprobe dating shows that this event is also pre-Silurian

and occurred at 448 Ma, similar to the c. 460–440 Ma age

estimates (Dempster 1985) for the formation of the analogous

Highland Border Downbend in Scotland.

D.M.C. gratefully acknowledges a Forbairt Basic Research Grant and a

University College Dublin (UCD) Research Doctoral Scholarship. Access

and financial support to use the EU Large Scale Geochemical Facility at

the Department of Earth Sciences at the University of Bristol was

provided by the European Community–Access to Research Infrastructure

Action of the Improving Human Potential Programme. We are grateful to

M. Murphy for skilled laboratory assistance at UCD. O. Vidal is thanked

for helpful advice and discussions regarding the phengite–chlorite

equilibria. G. Droop and an anonymous reviewer are thanked for their

constructive and careful reviews of this paper, and R. Strachan is thanked

for patient scientific editing.

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Received 29 January 2003; revised typescript accepted 10 July 2003.

Scientific editing by Rob Strachan


Grampian orogenesis and the development of blueschist-facies metamorphism in western Ireland

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