David M. Chew1,†, J. Stephen Daly2, Tomas Magna3,4,5, Laurence M. Page6, Christopher L. Kirkland7,8, Martin J. Whitehouse7, and Rebecca Lam9

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Timing of ophiolite obduction in the Grampian
Orogen
Article in Geological Society of America Bulletin · October 2010
DOI: 10.1130/B30139.1
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2. ABSTRACT
This study addresses the timing and
pressure-temperature (P-T) conditions of
ophiolite obduction, one of the proposed
causes of the ca. 470 Ma Grampian orogeny of
Scotland and Ireland. This event gave rise to
the main structural and metamorphic char-
acteristics of the Grampian terrane—the type
area for Barrovian metamorphism, the cause
of which remains enigmatic despite a century
of research. Zircons from the Highland Bor-
der ophiolite, Scotland, define a 499 ± 8 Ma
U-Pb concordia age, which is interpreted as
dating magmatism. Its metamorphism is
dated by a 490 ± 4 Ma 40
Ar-39
Ar hornblende
age, and a 488 ± 1 Ma 40
Ar-39
Ar muscovite
age from a metasedimentary xenolith within
it, from which P-T estimates of 5.3 kbar and
580 °C relate to ophiolite obduction. Meta-
morphism of the Deerpark complex ophiolitic
mélange (Irish correlative of the Highland
Border ophiolite) is constrained by a 514 ±
3 Ma 40
Ar-39
Ar hornblende age, while mica
schist slivers within it yield detrital zircon
U-Pb ages consistent with Laurentian prov-
enance and Rb-Sr and 40
Ar-39
Ar muscovite
ages of ca. 482 Ma. P-T values of 3.3 kbar
and 580 °C for the mica schist constrain the
conditions of ophiolite obduction. Metamor-
phic mineral ages from the Grampian terrane
(Dalradian Supergroup) are substantially
younger (ca. 475–465 Ma) than those from the
ophiolites. If conductive heating in overthick-
ened crust was the cause of Barrovian meta-
morphism, then collisional thickening must
have started soon after ophiolite obduction at
ca. 490 Ma in order to generate the ca. 470 Ma
metamorphic peak in the Grampian terrane.
INTRODUCTION
The Grampian terrane of Scotland and NW
Ireland is the type locality for Barrovian (re-
gional) metamorphism, which is recognized in
most of the major mountain belts of the world. It
is thought to have resulted from the collision of
the Laurentian margin with an infant oceanic arc
and associated suprasubduction ophiolite (e.g.,
Dewey and Shackleton, 1984; van Staal et al.,
1998; Dewey and Mange, 1999) during the
Early–Middle Ordovician. This tectonic event
is termed the Grampian orogeny and is broadly
equivalent to the Taconic orogeny of the Appa-
lachians and eastern maritime Canada.
Abundant geochronological data demonstrate
that Barrovian metamorphism occurred over a
short time period during the Grampian orogeny.
For example, the Neoproterozoic to early Paleo-
zoic Dalradian Supergroup, a Laurentian se-
quence in Scotland and NW Ireland, underwent
polyphase deformation and metamorphism up
to upper-amphibolite-facies conditions over
10 m.y. between ca. 475 and 465 Ma (Dewey,
2005). However, models that ascribe Grampian
metamorphism and orogenesis to obduction of a
chain of suprasubduction ophiolites and associ-
ated arcs raise several issues, which can be sum-
marized as follows.
(1) The Dalradian rocks of Scotland and
NW Ireland have undergone substantial crustal
thickening, which is difficult to reconcile with
the obduction of a relatively thin slice of oceanic
lithosphere. Pressure estimates from peak Bar-
rovian metamorphic assemblages in Dalradian
rocks vary along orogenic strike, from as deep
as ~10 kbar in the core of the North Mayo Inlier
in western Ireland (Fig. 1; Yardley et al., 1987)
and the SW Scottish Highlands (Graham, 1985)
to as shallow as 2–3 kbar in the Buchan region
of NE Scotland (Beddoe-Stephens, 1990). This
suggests that the deformed Dalradian structural
pile is at least 25 km thick in places, yet, with the
exception of the Shetland archipelago 200 km
north of mainland Scotland (Fig. 1), no ophio-
litic klippe is observed to structurally overlie the
Dalradian Supergroup rocks.
(2)TheFleurdeLysSupergroupintheinternal
Humber zone of Newfoundland is a polyphase-
deformed Laurentian margin sequence that oc-
cupies a structural position analogous to that of
the Dalradian Supergroup. Large-scale Taconic
(Early–Middle Ordovician) metamorphism has
proven to be difficult to detect (Cawood et al.,
1994; Brem et al., 2007; van Staal et al., 2009),
despite abundant evidence for Taconic ophiolite
obduction and mélange formation (van Staal
et al., 2010). Additionally, Taconic metamor-
phism and deformation in the external Hum-
ber zone in Newfoundland are restricted to the
Taconic ophiolite allochthons (e.g., the Humber
Arm and Hare Bay allochthons; Cawood and
Williams, 1988; Cawood, 1989), and there is
only minimal Taconic deformation in the under-
lying autochthonous Laurentian margin shelf
carbonates and their Grenvillian basement.
(3) In Scotland and Ireland, there is a marked
time gap between the onset of ophiolite obduc-
tion and the peak metamorphism in Lauren-
For permission to copy, contact editing@geosociety.org
© 2010 Geological Society of America
1787
GSA Bulletin; November/December 2010; v. 122; no. 11/12; p. 1787–1799; doi: 10.1130/B30139.1; 8 figures; Data Repository item 2010243.
†
E-mail: chewd@tcd.ie
Timing of ophiolite obduction in the Grampian orogen
David M. Chew1,†
, J. Stephen Daly2
, Tomas Magna3,4,5
, Laurence M. Page6
, Christopher L. Kirkland7,8
, Martin J.
Whitehouse7
, and Rebecca Lam9
1
Department of Geology, School of Natural Sciences, Trinity College Dublin, Dublin 2, Ireland
2
UCD School of Geological Sciences, University College Dublin, Dublin 4, Ireland
3
Institute of Mineralogy and Geochemistry, University of Lausanne, Quartier UNIL-Dorigny, Bâtiment Anthropole, CH-1015 Lausanne,
Switzerland
4
Institute of Mineralogy, University of Münster, Corrensstrasse 24, D-48149 Münster, Germany
5
Czech Geological Survey, Klárov 3, 118 21 Prague 1, Czech Republic
6
Department of Geology, GeoBiosphere Science Centre, Sölvegatan 12, 223 62 Lund University, Sweden
7
Laboratory for Isotope Geology, Swedish Museum of Natural History, Stockholm, Box 50 007, SE-104 05 Stockholm, Sweden
8
Geological Survey of Western Australia, 100 Plain St., East Perth WA 6004, Australia
9
MicroAnalysis Facility – Inco Innovation Centre, Memorial University, St. John’s, NL A1C 5S7, Newfoundland, Canada

3. Chew et al.
1788 Geological Society of America Bulletin, November/December 2010
tian margin rocks. Peak metamorphism in the
Dalradian is constrained by Sm-Nd garnet ages
of 473–465 Ma in the type area of Barrovian
metamorphism in the Scottish Highlands (Bax-
ter et al., 2002; Oliver et al., 2000), and by U-Pb
zircon ages of 475–468 Ma from synorogenic
intrusive rocks in Connemara in western Ireland
(Friedrich et al., 1999). Constraints on the tim-
ing of ophiolite obduction include a 478 ± 8 Ma
K-Ar hornblende age from the metamorphic sole
of the Ballantrae ophiolite (Fig. 1; Bluck et al.,
1980) and K-Ar hornblende ages of ca. 479–
465 Ma from the metamorphic sole of the Shet-
land ophiolite (Fig. 1; Spray, 1988). A 498 ±
2 Ma 40
Ar-39
Ar hornblende step-heating age
from the Shetland ophiolite (Flinn et al., 1991)
is older than the 492 ± 3 Ma U-Pb zircon crys-
tallization age of the Shetland ophiolite plagio-
granite (Spray and Dunning, 1991) and may
have been affected by excess Ar. However, a
short, synchronous Grampian orogenic episode
is inconsistent with models of conductive heat
transfer in thickened crust (e.g., Dewey, 2005;
Baxter et al., 2002), and these authors have sug-
gested that the ca. 470 Ma Grampian metamor-
phic peak may have resulted from advective heat
transfer from voluminous synorogenic intrusive
rocks in the Dalradian block (Fig. 1), similar to
the original suggestion of Barrow (1893). How-
ever, much of the Dalradian block (e.g., NW
Ireland and SW Scotland; Fig. 1) is devoid of
synorogenic intrusive rocks, yet a ca. 470 Ma
orogenic peak is still identified by geochronol-
ogy (e.g., Flowerdew et al., 2000).
(4) Structural relationships in Scotland sug-
gest that the Highland Border ophiolite (Fig. 1)
was obducted onto Dalradian rocks (Tanner,
2007) that had already experienced at least one
phase of deformation (D1). If this is the case,
then its emplacement was not accompanied
by significant internal deformation of the Dal-
radian block, since subsequent structural phases
(D2–D4) affecting the Dalradian rocks are kine-
matically incompatible with ophiolite obduction
from the southeast. Tanner (2007) thus calls into
question the role of the Highland Border ophio-
lite in the Grampian orogeny.
This study aims to quantify further the tim-
ing of ophiolite obduction and orogeny along
the Laurentian margin in Scotland and Ireland
with the aim of investigating the relationship
between them. In particular, it focuses on seg-
ments of the Laurentian margin devoid of syn-
orogenic intrusive rocks, so that the timing and
cause of regional metamorphism can be inves-
tigated in isolation from this particular heating
mechanism.
GEOLOGICAL SETTING
The principal components of the Grampian
orogeny in Scotland and Ireland are illustrated in
Figure 1. The Neoproterozoic to early Paleozoic
Dalradian Supergroup represents a basin that
was deposited during the breakup of the Rodinia
supercontinent (Dalziel and Soper, 2001); the
younger parts of the Dalradian sequence record
the transition to sedimentation on the Lauren-
tian passive continental margin (Dewey, 1969).
To the southeast, the Midland Valley terrane
(Fig. 1) is largely hidden by younger sedimen-
tary cover, but it is floored by rocks that are
believed to represent a Lower Paleozoic vol-
canic arc terrane (e.g., Dewey and Shackleton,
1984; Dewey and Mange, 1999), which formed
by subduction of Iapetus oceanic lithosphere
under an intra-oceanic arc. A subsequent sub-
duction polarity reversal is thought to have
formed a continental arc on the Laurentian
margin and injected large volumes of basic and
intermediate magma into the Dalradian rocks of
Connemara and NE Scotland (Fig. 1) during the
Grampian orogeny (e.g., Yardley and Senior,
1982; Tanner, 1990).
In Scotland, the suture between the deformed
Laurentian margin (Dalradian Supergroup) and
the colliding arc (Midland Valley terrane) is
sharply defined by the Highland Boundary fault
(Fig. 1), along which a series of Lower Paleo-
zoicdeepmarinesedimentaryrocksandisolated
occurrences of mafic and ultramafic rocks crop
out. Termed the Highland Border complex, this
belt has figured prominently in tectonic recon-
structions of the Grampian belt (e.g., Dewey,
2005), in which it is usually regarded as an ac-
cretionary complex. A recent reinterpretation
of the Highland Border complex (Tanner and
Sutherland, 2007) suggests that the majority
of the sequence is in stratigraphic continuity
with the Dalradian Supergroup, with the excep-
tion of a series of poorly exposed fault-bound
slivers of ophiolitic rocks within the fault zone,
known as the Highland Border ophiolite (Tanner
and Sutherland, 2007). Detailed reviews of the
58° N
57° N
56° N
55° N
8° W 6° W 2° W4° W
FCBL
SUF
GGF
HBF
MT
Unst
Fetlar
(continuation of
Great Glen fault)
Shetland
ophiolite
Ballantrae ophiolite
Shetland
FHCBL HBF
GGF
WBF
WBF (Walls
Boundary fault)
WBF
0 100 200 km
N
Achill Island
Connemara
Bute
SUF
Faults:
Southern Upland fault
FCBLFair Head-Clew Bay Line
HBF Highland Boundary fault
GGF Great Glen fault
MT Moine thrust
Archean / Paleoproterozoic basement
Slishwood Division
Torridonian / Colonsay Group /
Cambrian-Ordovician foreland
Moine Supergroup
Dalradian Supergroup / arc intrusives
Highland Border / Clew Bay complexes
Midland Valley terrane arc volcanics
Arran
South Mayo
Trough
Clew
Bay
Tyrone Central Inlier
Highland Border Complex
Fig. 3
Fig. 2
Figure 1. Geological map of the Caledonides of NW Ireland and Scotland. Inset shows a sim-
plified geological map of Shetland and its relationship to the British and Irish Caledonides.

4. Timing of ophiolite obduction in the Grampian orogen
Geological Society of America Bulletin, November/December 2010 1789
Highland Border ophiolite are given in Tanner
(2007) and Henderson et al. (2009).
The continuation of the Highland Boundary
fault in Ireland is referred to as the Fair Head–
Clew Bay Line (FCBL, Fig. 1), which generally
separates the Dalradian Supergroup from the
Clew Bay complex (the Irish correlative of
the Highland Border complex) and an outboard
volcanic arc terrane to the southeast. The out-
board volcanic arc terrane is represented by the
Tyrone igneous complex in the central part of
the north of Ireland, and by the Lower Ordo-
vician Lough Nafooey Group and its associated
forearc fill, and the Lower to Middle Ordovician
Murrisk Group of the South Mayo Trough in
western Ireland (Fig. 1).Additionally, unlike the
Dalradian of Scotland, the Dalradian rocks of
Connemara and the Tyrone Central Inlier (Chew
et al., 2008) also crop out to the southeast of
the Fair Head–Clew Bay Line, i.e., outboard
of the main belt (Fig. 1).
This study presents geochronological data
from Highland Border ophiolite rocks and
Dalradian Supergroup rocks on the islands of
Bute and Arran in western Scotland (Fig. 2)
and Dalradian Supergroup rocks and Clew Bay
complex rocks (Fig. 3) in western Ireland to in-
vestigate the relationship between ophiolite ob-
duction and orogeny along this segment of the
Laurentian margin. The geological setting of
each key region is described in turn, while ana-
lytical methods are described in the appendix.
LOCAL GEOLOGY AND
GEOCHRONOLOGICAL DATA
Bute
The Highland Border ophiolite (Tanner and
Sutherland, 2007; Tanner 2007) forms a dis-
continuous belt of mafic and ultramafic rocks
along the Highland Boundary fault from Bute to
Stonehaven (Fig. 1). It has a thick, locally devel-
oped “sole” of amphibolite (spilitic tholeiite) at
Scalpsie Bay on Bute and atAberfoyle (Hender-
son and Robertson, 1982). At Scalpsie Bay, the
Bute amphibolite is up to 60 m thick and occurs
along the SE extremity of the Dalradian outcrop
(Fig. 2B). The peak metamorphic assemblage
is amphibole (typically magnesio-hornblende;
sample CNH-1, Table DR11
), garnet, and tita-
nite. Epidote, albite (An5
Ab91
Or5
; Table DR1;
mineral abbreviations are after Kretz, 1983),
and chlorite are retrograde products developed
in late-stage veins. Garnet, titanite, amphibole,
and the whole rock define a 546 ± 42 Ma Sm-Nd
isochron, while the amphibole has yielded a
537 ± 11 K-Ar age (Dempster and Bluck, 1991).
Small (~50 μm diameter) euhedral zircon
grains from a sample of the Bute amphibolite
(DC 8–2-6) exhibit oscillatory growth zoning
(Fig. 4B) and are interpreted as magmatic. Ion
microprobe U-Pb analyses of five grains (Table
DR2 [see footnote 1]) yielded a concordia age
(Ludwig, 1998) of 499 ± 8 Ma (Fig. 4A), while a
sixth grain has a younger apparent age (206
Pb/238
U
age of 466 ± 11 Ma; Table DR2), which is in-
terpreted as reflecting Pb loss. Thin, U-poor
rims are also present (e.g., grain 1, Fig. 4B) but
are beyond the ~10 μm spatial resolution of the
80
0 250 m
78
74
62
85
80
72
64
83
76
80
88
70
70 54
56
64
74
72
36
88
72
45
64
45
64
75
23
60
30
0 0
26
42
58
0 0
NR47 NS47
Arran 116 (metabasite)
40
Ar-39
Ar ms: 476 ± 1 Ma
ACB-1
(amphibolite)
North Sannox
Burn
Arran 107 (psammite)
tDM: 2.07 Ga
(off map)
Arran 117 (psammite)
tDM: 1.99 Ga
Arran 119 (psammite)
tDM: 2.03 Ga (and
U-Pb detrital zircon)
Arran 120 (psammite)
tDM: 2.03 Ga
Arran 122 (psammite)
tDM: 2.71 Ga
Old Red Sandstone
HBC grits
HBC black shale
HBC lavas (pillows locally)
HBC mylonitized basic lava
Amphibolite
Dalradian
C
Arran
x
x
x
x
x
Garnet-
hornblende
schist
Scalpsie Bay
NS
585
0 100 m
68
68
59
43
72
29
54
38
53
2525
41
35
62
46
4632
34 57
73
75
29
48
35
32
NS 055
Scalpsie
Farm
46
x
x
x
Granitoid
Bute amphibolite
Semipelite xenolith
Dalradian
x
x
x
Semipelite
xenolith
DC 8–2-6 (amphibolite)
U-Pb zircon: 499 ± 8 Ma
40
Ar-39
Ar hbl: 490 ± 4 Ma
DC 8–2-8 (metased.)
εHf(490 Ma) zircon:
–2.6 ± 1.2; 2.8 ± 1.0
U-Pb zircon: 490 ± 4 Ma
40
Ar-39
Ar ms: 488 ± 1 Ma
DC 4–8-1a
(granitoid)
B
Bute
OldCliff
Highland Border
complex
Dalradian
Dunoon
Innellan
TowardBUTE
ARRAN
North Glen
Sannox
(Fig. 2C)
Scalpsie
Bay (Fig. 2B)
A
Symbols in Figs. 2B, 2C
Sm-Nd depleted
mantle model age
ms: muscovite
hbl: hornblende
72 Bedding, with
younging
78 Schistosity
80 Bedding
Stretching lineation
34
tDM:
Figure 2. Geological maps and
sample localities with ages from
selected regions of the Highland
Border Complex (HBC) region
in SW Scotland (see Fig. 1).
(A) Simplified geological map
of the Highland Border region
in SW Scotland. (B) Geological
map of the Scalpsie Bay region
in Bute after Henderson and
Robertson (1982). Structural
symbols are as in A. (C) Geo-
logical map of the North Glen
Sannox region on Arran based
on the author’s own mapping
and Henderson and Robertson
(1982). Structural symbols are
as in A.
1
GSA Data Repository item 2010243, Analyti-
cal technique and Tables DR1–DR8, is available at
http://www.geosociety.org/pubs/ft2010.htm or by
request to editing@geosociety.org.

5. Chew et al.
1790 Geological Society of America Bulletin, November/December 2010
ion microprobe. Uranium concentrations are low
(10–80 ppm), and the Th/U ratios of the dated
grains range from 0.007 to 0.023 (Table DR2).
Amphibole from sample DC 8–2-6 yielded
a saddle-shaped 40
Ar-39
Ar age spectrum, indi-
cating the presence of excess radiogenic argon
(e.g., Harrison and McDougall, 1981). The four
youngest age steps are within error, comprise
43% of the total 39
Ar released, and yield a 490 ±
4 Ma plateau age (Table DR3 [see footnote 1];
Fig. 5A). These data yield an inverse isochron
age of 495 ± 1 Ma with a 40
Ar/36
Ar intercept of
180 ± 60 (Fig. 5B), which is distinct from the
accepted 40
Ar/36
Ar atmospheric value of 295.5
(Steiger and Jäger, 1977).
A 5-m-wide xenolith of garnet-muscovite
schist is intercalated within the amphibolite
at NS 0544 5852 (Fig. 2B). The metamorphic
grade is substantially higher than in the local
greenschist-facies Dalradian psammitic rocks.
THERMOCALC multi-equilibria (Holland and
Powell, 1998) yield average pressure-temperature
(P-T) values of ~5.3 kbar and 580 °C for the mica
schist xenolith (sample DC 4–8-3). These data
are consistent with a temperature of ~550 °C,
calculated using the garnet-muscovite thermom-
eter of Green and Hellman (1982). Petrographic
evidence demonstrates that the metabasites and
metasediments share the same amphibolite-facies
foliation. Thus, the calculated values provide a
P-T constraint on ophiolite obduction.
Coarse (>500 μm) muscovite from the
garnet-muscovite schist xenolith (sample DC 8–
2-8) yielded a 488 ± 1 Ma 40
Ar-39
Ar plateau age
(Table DR3; Fig. 5C). This sample also contains
small, ~50–100-μm-long zircons with “patchy”
cathodoluminescent (CL) textures (Fig. 4D),
the significance of which is discussed later.
Twenty-three concordant ion microprobe analy-
ses yielded a U-Pb concordia age of 490 ± 4 Ma
(Fig. 4C). Uranium concentrations range from
20 to 4000 ppm, and Th/U ratios are typically
below 0.1 (Table DR2). One discordant grain
with a younger apparent age (16, 206
Pb/238
U age
of 453 ± 16 Ma; Table DR2) was interpreted as
having suffered Pb loss and was excluded from
the concordia age calculation, while three others
(12, 14, 22; Table DR2) yielded older dis-
cordant ages. However, only one of these
(14) has a 207
Pb/206
Pb age that is significantly
older (1450 ± 166 Ma) than the concordia
age (Table DR2). Zircons from sample DC
8–2-8 were also analyzed by the Lu-Hf
laser ablation inductively coupled plasma–
mass spectrometry (ICP-MS) method. Fif-
teen analyses were undertaken, of which 13
were sited close to ion microprobe spots.
The εHf(490 Ma) values range from –3.51 to +5.97
(Table DR4 [see footnote 1]; Fig. 4E) and
define a bimodal distribution (Table DR4;
Fig. 4F) with peaks at –2.6 ± 1.2 and +2.8 ±
1.0 (2σ). Combined, the data yield a weighted
mean εHf(490 Ma) value of 0.6 ± 1.7 (2σ, mean
square of weighted deviates [MSWD] = 4.3).
Arran
Rocks traditionally assigned to the High-
land Border complex also crop out in North
Glen Sannox on the neighboring Isle of Arran
(Fig. 2C). These rocks constitute a 400-m-wide
sequence of pillow lavas, grits, black shales, and
cherts (Anderson and Pringle, 1944), which are
situated to the east of greenschist-facies psam-
mites and meta-arenites of the Dalradian Super-
group. Lower Paleozoic brachiopod fragments
reported from the Arran section (Anderson and
Pringle, 1944) have been lost, and no diag-
nostic fauna has subsequently been recovered.
The Dalradian and Highland Border complex in
North Glen Sannox shares the same structural
history (Johnson and Harris, 1967), suggesting
synchronous deformation during the Grampian
orogeny, and Tanner and Sutherland (2007)
regarded the Arran sequence as being in strati-
graphic continuity with the Dalradian.
In the North Glen Sannox section, the pillow
basalts are locally strongly sheared (Fig. 2C).
One mylonitized pillow basalt sample (Arran
116) contains thin seams rich in potassic white
mica (Table DR1), which may represent sheared
interpillow sediment. 40
Ar-39
Ar dating of this
white mica yielded a 476 ± 1 Ma 40
Ar-39
Ar
plateau age (Table DR3; Fig. 5D). Farther
east, pebbly grits at the eastern limit of the
metamorphic rocks yielded Sm-Nd model ages
(Table DR5 [see footnote 1]) between 1.99 and
2.71 Ga (mean = 2.18 Ga, n = 5), and detrital
zircon U-Pb concordia age spectra (sample
Arran 119; Table DR6 [see footnote 1]; Fig. 6A)
characterized by peaks at ca. 0.95–1.3 Ga, 1.4–
1.5 Ga, 1.7–1.9 Ga, and 2.5–2.9 Ga. To facilitate
comparison, all U-Pb data have been filtered us-
ing the same rejection criteria, which incorpo-
rate a filter for both (207
Pb/206
Pb)/(206
Pb/238
U) age
discordance and large age uncertainties (<20%
discordance of the centroid and a concordia age
with a 2σ uncertainty of less than 10%).
N
Dalradian Supergroup
Ordovician
Clew Bay complex
Deerpark complex
South Mayo Silurian
Corvock Granite
Achill Island
Clare Island
Clew Bay
South Mayo Trough
A B
Achill Beg fault
Leck fault
DC 8–1-24 (metased.)
40
Ar-39
Ar ms: 482 ± 2 Ma
Rb-Sr ms-plag: 483 ± 7 Ma
Kil-1 (amphibolite)
40
Ar-39
Ar hbl: 514 ± 3 Ma
DC 223 (amphibolite)
40
Ar-39
Ar hbl: 467 ± 2 Ma
40
Ar-39
Ar bt: 458 ± 2 Ma
Achill Beg
Fig. 3B
Achill
Beg
Achill Beg fault
Achill
Island
0 1 km
DC 106 (metased.)
40
Ar-39
Ar biotite: 472 ± 1 Ma
DC 27 (ultramafic)
40
Ar-39
Ar fuchsite: 464 ± 2 Ma
DC 73 (ultramafic)
40
Ar-39
Ar fuchsite: 476 ± 3 Ma
AB-69 (metased.)
40
Ar-39
Ar ms: 462 ± 1 Ma
AB-70 (metased.)
40
Ar-39
Ar ms: 462 ± 1 Ma
0 10 km
F
L
60 90 69 70 71 72 73
96
95
94
93
92
DC-82 (psammite)
U-Pb detrital zircon
AB-4 (psammite)
U-Pb detrital zirconDC 8–1-24 (metased.) εNd(490) = –15.1
(and U-Pb detrital zircon)
DC 8–1-25 (metased.) εNd(490) = –7.5
DC 8–1-26 (metadol.) εNd(490) = 6.3
DP 11 (metadol.) εNd(490) = 5.8
DP 9 (metadol. and metased.)
PT = ~550 °C, 3.3 kbar
Figure 3. Geological maps and sample localities with ages from the Clew Bay region in western Ireland (see Fig. 1). (A) Simplified geological
map of the Clew Bay region in western Ireland. (B) Geological map of southern Achill Island and Achill Beg. Legend is as in A.

6. Timing of ophiolite obduction in the Grampian orogen
Geological Society of America Bulletin, November/December 2010 1791
30 μm
27
25
22
19
17
23
28
24
18
530 510 490 470 450
0.03
0.04
0.05
0.06
0.07
11.4 11.8 12.2 12.6 13.0 13.4 13.8 14.2
238
U/
206
Pb
DC 8–2-6
Concordia age = 499 ± 8 Ma
MSWD = 7.4
750
650
550
450
14
22
12
16
0.03
0.05
0.07
0.09
0.11
11 13 157 9
238
U/
206
Pb
207
Pb
206
Pb
207
Pb
206
Pb
DC 8–2-8
Concordia age = 490 ± 4 Ma
MSWD = 0.20
A C
30 μm
1 2
4
B
E
F
1
2
4
16
15
20
24
29
26
21
32
8
3
A
B
–10
–6
–2
2
6
10
DC 8–2-8 Lu-Hf data
Weighted mean = 0.6 ± 1.7 (2σ)
MSWD= 4.3, error bars are 2σ
0
1
2
3
4
5
–10 –6 –2 2 6 10
Number
εHf (490 Ma)
-2.6 ± 1.2
(2σ)
2.8 ± 1.0
(2σ)
D
εHf
(490 Ma)
6
Figure 4. Tera-Wasserburg concordia diagrams, scanning electron micrograph cathodoluminescence (SEM-CL) images, and Lu-Hf iso-
topic analyses of zircon from the Highland Border ophiolite on Bute. Spot numbers are those used in the corresponding data table. (A) Tera-
Wasserburg concordia diagrams, sample DC 8–2-6. (B) SEM-CL images, sample DC 8–2-6. (C) Tera-Wasserburg concordia diagram,
sample DC 8–2-8. (D) SEM-CL images, sample DC 8–2-8. (E) Lu-Hf isotopic analyses of zircon, sample DC 8–2-8. (F) εHf(490 Ma) probability
distribution diagram for sample DC 8–2-8. MSWD—mean square of weighted deviates.

7. Chew et al.
1792 Geological Society of America Bulletin, November/December 2010
A
B
HG
C
D
E
F
IJ
0 10 20 30 40 50 60 70 80 90 100
300
400
500
600
490 ± 4 Ma
DC 8–2-6 hornblende
Cumulative %39
Ar released
Cumulative %39
Ar released
Cumulative %39
Ar released
Cumulative %39
Ar released
Cumulative %39
Ar released
Cumulative %39
Ar released
Cumulative %39
Ar released
Cumulative %39
Ar released
Cumulative %39
Ar released
Cumulative %39
Ar released
Cumulative %39
Ar released
Cumulative %39
Ar released
0 10 20 30 40 50 60 70 80 90 100
300
400
500
600
0 10 20 30 40 50 60 70 80 90 100
300
400
500
600
DC 223 hornblende
0 10 20 30 40 50 60 70 80 90 100
300
400
500
600
DC 27 fuchsite
0 10 20 30 40 50 60 70 80 90 100
300
340
380
420
460
500
0 10 20 30 40 50 60 70 80 90 100
300
340
380
420
460
500
DC 8–2-8 muscovite
0 10 20 30 40 50 60 70 80 90 100
300
340
380
420
460
500
Arran 116 muscovite
0 10 20 30 40 50 60 70 80 90 100
300
400
500
600
0 10 20 30 40 50 60 70 80 90 100
300
340
380
420
460
500
DC 8–1-24 muscovite
0 10 20 30 40 50 60 70 80 90 100
300
340
380
420
460
500
AB-69 muscovite
0 10 20 30 40 50 60 70 80 90 100
300
340
380
420
460
500
0 10 20 30 40 50 60 70 80 90 100
300
340
380
420
460
500
DC 106 muscovite
Isochron age = 510 ± 1 Ma
40
Ar/36
Ar intercept = 330 ± 30
A
B
H G
C
D
E
F
DC 8–2-6 hornblende
476 ± 1 Ma
Kil-1 hornblende
Kil-1 hornblende
DC 8–1-24 muscovite
483 ± 7 Ma
AB-70 muscovite
DC 73 fuchsite
DC 223 biotite
Isochron age: 496 ± 1 Ma
40
Ar/36
Ar intercept: 180 ± 60
(isochron uses
steps C-F)
(isochron uses
steps F-J)
488 ± 1 Ma
514 ± 3 Ma
482 ± 2 Ma
musc
plagioclase
462 ± 1 Ma
476 ± 3 Ma464 ± 2 Ma
462 ± 1 Ma
472 ± 1 Ma
467 ± 2 Ma
458 ± 2 Ma
0.005
0.004
0.003
0.002
0.001
0
0.010 0.02 0.03
39
Ar/40
Ar
87
Rb/86
Sr
36
Ar
40
Ar
0.003
0.002
0.001
0
0.01 0.020 0.03
39
Ar/40
Ar
36
Ar
40
Ar
87
Sr
86
Sr
0.95
0.85
0.80
0.75
0.70
0.90
0 5 20 25 3010 15
Age
(Ma)
Age
(Ma)
Age
(Ma)
Age
(Ma)
Age
(Ma)
Age
(Ma)
Age
(Ma)
Age
(Ma)
Age
(Ma)
Age
(Ma)
Age
(Ma)
Age
(Ma)
A
D
G
J
M
B
E
H
K
N
C
F
I
L
O
Figure 5. Mineral geochronology from the Highland Border ophiolite (A–C), Highland Border complex/Trossachs Group
(D), Deerpark complex ophiolitic mélange (E–H), Clew Bay complex (I–J), and the Dalradian Supergroup (K–O).

8. Timing of ophiolite obduction in the Grampian orogen
Geological Society of America Bulletin, November/December 2010 1793
Geology of the Clew Bay Region
Western Ireland (and in particular the Clew
Bay region) provides one of the most complete
sections through the Grampian orogen, and all
of the major components of the orogen are well
exposed (Fig. 3), including Laurentian cover
(Dalradian Supergroup), accretionary complex
rocks (Clew Bay complex), a dismembered
suprasubduction ophiolite (Deerpark complex),
and arc volcanic rocks and forearc basin (Lough
Nafooey arc and South Mayo Trough). The
Deerpark complex is best exposed on the south
side of Clew Bay, and it consists of an ophio-
litic mélange of amphibolite-facies basic and
ultrabasic rocks and associated metasediments
(Ryan et al., 1983). It lies to the south of green-
schist-facies turbiditic metasedimentary rocks
of the Clew Bay complex, which have yielded a
Middle Cambrian sponge (Protospongia hicksi;
Rushton and Phillips, 1973) and a long-ranging
(Early–Middle Ordovician) fauna of coni-
form euconodonts (Harper et al., 1989). These
metasedimentary rocks are in structural continu-
ity with Dalradian Supergroup rocks to the north
(Chew, 2003).
The Dalradian rocks immediately north of
the Clew Bay complex on southern Achill Is-
land (Fig. 3) have undergone blueschist-facies
metamorphism (Gray and Yardley, 1979), in
contrast to Dalradian rocks farther north, which
experienced Barrovian metamorphic conditions
locally up to sillimanite grade (Max et al.,
1983). The blueschist-facies assemblages devel-
oped at P-T conditions of 10.5 ± 1.5 kbar and
460 ± 45 °C contemporaneously with the
Barrovian metamorphic assemblages (Chew
et al., 2003). Peak MP3 metamorphism in
the Dalradian of NW Ireland has been dated
at ca. 460 Ma by the Sm-Nd garnet method
(Flowerdew et al., 2000). The 40
Ar-39
Ar and Rb-Sr
ages of fabric-forming minerals (principally
hornblende, biotite, and muscovite) between
470 and 455 Ma are consistent with crystalliza-
tion during the Grampian orogeny at ca. 470 Ma
(Flowerdew et al., 2000; Chew et al., 2003; Daly
and Flowerdew, 2005) and subsequent cooling.
A summary of the metamorphic conditions and
metamorphic cooling age constraints from the
Irish Dalradian is given in Chew (2009).
Isotopic Data and P-T Constraints from the
Deerpark Complex
Sample DC 8–1-24 is a small block of garnet-
muscovite-albite schist intercalated within the
Deerpark complex ophiolitic mélange. It exhibits
a strong foliation defined by coarse (>500 μm)
muscovite, which has yielded a 40
Ar-39
Ar pla-
teau age of 482 ± 2 Ma (Table DR3; Fig. 5G)
and a Rb-Sr age of 483 ± 7 Ma (Table DR7
[see footnote 1]; Fig. 5H). Sample DC 8–1-24
yielded a Nd model age of 2.2 Ga (Table DR5)
and U-Pb detrital zircon age spectra with peaks
at ca. 0.95–1.3 Ga, 1.4–1.5 Ga, and 1.7–1.9 Ga,
with a broad peak from 2.4 to 3.0 Ga and a
minor peak at 3.4 Ga (Table DR6; Fig. 6B).
With the exception of the grains of 3 Ga and
older, the overall pattern indicates a Laurentian
affinity similar to sample Arran 119 (Fig. 6A).
THERMOCALC multi-equilibria yield average
P-T values of ~3.3 kbar and 580 °C (Fig. 7A;
Table DR8 [see footnote 1]) for a mica schist
intercalated on a centimetric scale within a
metabasite block (sample DP-9, Table DR1),
consistent with a temperature estimate of
~600 °C (Fig. 7A) using the garnet-muscovite
thermometer of Green and Hellman (1982).
Petrographic evidence demonstrates that the
metabasites and metasediments share the same
amphibolite-facies foliation, and thus these data
are argued to provide a P-T constraint on ophio-
lite obduction.
Amphibolite-facies metabasites within the
Deerpark complex have a mid-ocean-ridge
basalt (MORB)–like trace-element chemistry,
but pronounced Nb anomalies and initial Nd
isotopic compositions (εNd[490] = +6; Table DR5)
are consistent with a juvenile subduction-
related origin (Chew et al., 2007). Hornblende
defining a strong lineation in one of these meta-
basite blocks (sample Kil-1) yielded a 40
Ar-39
Ar
plateau age of 514 ± 3 Ma (Fig. 5E), with an
inverse isochron that has a 36
Ar/40
Ar intercept
marginally outside error of the atmospheric
value (330 ± 30; Fig. 5F). This indicates that
the age is likely meaningful and not affected
by the presence of excess radiogenic argon.
Isotopic Data from the Dalradian and Clew
Bay Complex
Metamorphic mineral ages have been ob-
tained from the Dalradian of Achill Island
and from the Clew Bay complex on Achill
Beg (Figs. 3A and 3B). Sample DC 223 is an
amphibolite-facies pretectonic metadolerite
DC 8–1-24, n = 136
0.000
0.001
0.002
0.003
0.004
300
500
700
900
1100
1300
1500
1700
1900
2100
2300
2500
2700
2900
3100
3300
3500
3700
3900
Age (Ma)
Probability
0
10
20
30
40
Frequency
DC 82, n = 168
0.000
0.001
0.001
0.002
0.002
0.003
300
500
700
900
1100
1300
1500
1700
1900
2100
2300
2500
2700
2900
3100
3300
3500
3700
3900
Age (Ma)
Probability
0
5
10
15
20
25
Frequency
AB-4, n = 156
0.000
0.001
0.001
0.002
0.002
0.003300
500
700
900
1100
1300
1500
1700
1900
2100
2300
2500
2700
2900
3100
3300
3500
3700
3900
Age (Ma)
Probability
0
2
4
6
8
10
12
14
16
Frequency
Arran 119, n = 215
0.000
0.001
0.002
0.003
0.004
0.005
0.006
300
500
700
900
1100
1300
1500
1700
1900
2100
2300
2500
2700
2900
3100
3300
3500
3700
3900
Age (Ma)
Probability
0
10
20
30
40
50
60
70
80
Frequency
A
C D
B
Figure 6. U-Pb zircon proba-
bility density distribution dia-
grams for samples from (A) the
Highland Border complex/
Trossachs Group on Arran,
(B) the Deerpark complex ophio-
litic mélange, (C) the Clew Bay
complex, and (D) the Dalradian
Supergroup.

9. Chew et al.
1794 Geological Society of America Bulletin, November/December 2010
intrusion within the Dalradian Supergroup.
It yielded a 40
Ar-39
Ar hornblende plateau age
of 467 ± 2 Ma (Fig. 5N) and a biotite cooling
age of 458 ± 2 Ma (Fig. 5O). All other mineral
age data from the Dalradian come from farther
south in rocks that have experienced blueschist-
facies metamorphism (Fig. 3B). Samples DC 27
and DC 73 are fuchsite-muscovite schists from
the same locality. The fuchsite is incorporated
within the main schistosity, and it grew during
Grampian metamorphism, probably nucleat-
ing on detrital chromite (Chew, 2001). Sample
DC 27 yielded a 40
Ar-39
Ar fuchsite plateau age
of 464 ± 2 Ma (Fig. 5K), while sample DC 73
yielded a 40
Ar-39
Ar fuchsite plateau age of
476 ± 3 Ma (Fig. 5L). Sample DC 106 is
a garnet-biotite schist interbedded with the
fuchsite-bearing schists. It yielded a 40
Ar-39
Ar
biotite plateau age of 471 ± 2 Ma (Fig. 5M).
South of the Achill Beg fault (Fig. 3B), two
samples from low-grade metasedimentary rocks
of the Clew Bay complex yielded identical
muscovite 40
Ar-39
Ar plateau ages of 462 ± 1 Ma
(samples AB-69 and AB-70, Figs. 5J and 5K).
Detrital zircon U-Pb age spectra from the
Clew Bay complex (sample AB-4, L72139214)
and the Dalradian rocks 5 km to the NW (sample
DC 82) are similar (Figs. 6C and 6D; Table DR6)
and are characterized by a broad peak from 0.95
to 1.9 Ga, with larger peaks at ca. 1.1, 1.35,
and 1.8 Ga. Older detritus is restricted to the
2.5–2.9 Ga age range (Figs. 6C and 6D;
Table DR6).
INTERPRETATION
Geochronology Data from the Dalradian,
Clew Bay Complex, and Highland
Border Complex
The geochronological data presented here
demonstrate that a clear time difference
exists between the mineral cooling ages from
the Dalradian (combined with the low-grade
metasedimentary sedimentary rocks of the Clew
Bay complex and the Highland Border com-
plex) and the high-grade metamorphic rocks
(amphibolites and metasedimentary rocks) of
the Highland Border ophiolite and its Irish cor-
relative, the Deerpark complex. All cooling age
data presented here from Dalradian and Clew
Bay complex metamorphic rocks from western
Ireland are consistent with rapid cooling from
a ca. 475–470 Ma Grampian metamorphic
peak (Fig. 7B). Since these two units are also
in structural continuity (Chew, 2003) and are
characterized by virtually identical U-Pb detrital
zircon signatures (Figs. 6C and 6D), it is argued
here that the distinction between the Dalradian
and Clew Bay Complex rocks is unnecessary,
and that the low-grade metasedimentary rocks of
the Clew Bay complex should simply be regarded
as the youngest (i.e., Lower Paleozoic) portion
of the Dalradian Supergroup in Ireland. This is
consistent with the reinterpretation of Tanner and
Sutherland (2007) of the low-grade metasedi-
mentary rocks of the Highland Border complex,
which occur structurally below the Highland
Border ophiolite. They term these rocks the Tros-
sachs Group, which includes the sequence previ-
ously assigned to the Highland Border complex
in North Glen Sannox on Arran. The 40
Ar-39
Ar
white mica age from the Glen Sannox sequence
of 476 ± 1 Ma is also consistent with cooling
from a ca. 475–470 Ma Grampian metamorphic
peak. The geochronological data presented thus
imply that the Dalradian and Clew Bay Complex/
Trossachs Group were deformed synchronously
in the Grampian event.
Geochronology and P-T Data from the
Highland Border Ophiolite and the
Deerpark Complex
The geochronological data from the Highland
Border ophiolite and its correlative sequence in
western Ireland, the Deerpark complex, are more
difficult to interpret. The small oscillatory-zoned
zircons from the Bute amphibolite (Fig. 4B) are
interpreted as magmatic in origin, and therefore
the U-Pb concordia age of 499 ± 8 Ma (Fig. 4A)
is interpreted as dating igneous crystallization of
this segment of the Highland Border ophiolite.
Although influenced slightly by the presence
of excess argon, the 40
Ar-39
Ar amphibole age of
490 ± 4 Ma is interpreted as constraining meta-
morphism of these rocks. Although these data
apparently contradict the Early Cambrian age for
the Bute amphibolite implied by a 546 ± 42 Ma
Sm-Nd isochron (garnet-titanite-amphibole–
14
12
10
8
6
4
2
300200 400 500 600
P(kbar)
T (°°C)
A B
Dalradian (Central Achill)
(~6.5 kbar, 525 °C)
Clew Bay complex
(Achill Beg)
(~10 kbar, 375 °°C)
Dalradian (South Achill)
(~10.5 kbar, 460 °°C)
Deerpark complex
(~3.3 kbar, 580 °°C)
1.00.1
DP9 TC
DP9
grt-ms
DP9 Sample number
TC Thermocalc average
P-T calculation
grt-ms garnet-muscovite
thermobarometry
ca. 475 Ma, onset of
Grampian thermal pulse
on the Laurentian margin
ca. 465–455 Ma, main
time bracket of Grampian
mineral cooling ages
ca. 514 Ma, Ar-Ar
hbl. age from the
Deerpark
complex
ca. 482 Ma, mica ages
from the Deerpark complex
ca. 490 Ma, mineral ages
from the Bute ophiolite
Mineral ages from the Dalradian
Supergroup and Clew Bay complex
(= Laurentian margin)
Mineral ages from the Deerpark
complex and Highland Border
ophiolite
0.00
0.02
0.04
0.06
0.08
0.10
400
410
420
430
440
450
460
470
480
490
500
510
520
Age (Ma)
Probability
0
1
2
3
4
5
6
7
8
Frequency
Figure 7. (A) Summary of pressure-temperature (P-T) data from the Deerpark complex ophiolitic mélange, the Clew Bay complex, and the
Dalradian Supergroup. The blueschist-facies assemblages (south Achill Dalradian) developed contemporaneous with the Barrovian meta-
morphic assemblages (central Achill Dalradian). Numbers in italics adjacent to Deerpark complex data correspond to XH2O values for binary
H2
O-CO2
fluids used for different THERMOCALC average P-T calculations. (B) Schematic temporal reconstruction of the tectonic evolution
of the Laurentian margin in NW Ireland and Scotland based on the geochronological data presented in this study and Chew et al. (2003).

10. Timing of ophiolite obduction in the Grampian orogen
Geological Society of America Bulletin, November/December 2010 1795
whole rock) and a 537 ± 11 K-Ar amphibole age
(Dempster and Bluck, 1991), it should be noted
that these Early Cambrian ages are not well
constrained. The precision on the Sm-Nd analy-
sis is poor, and hence an Early Cambrian age
rests solely on one K-Ar amphibole age, which
could be influenced by excess radiogenic argon,
as is evident in our 40
Ar-39
Ar data.
The geochronological data from the metasedi-
mentary xenolith intercalated within the Bute
amphibolite, which yielded a 488 ± 1 Ma
40
Ar-39
Ar muscovite age (Fig. 5C) and a U-Pb
concordia age of 490 ± 4 Ma (Fig. 4C), imply
that a ca. 490 Ma metamorphic event has
affected the Bute amphibolite. The remarkably
homogeneous nature of the zircon age popula-
tion would suggest that the xenolith has a vol-
caniclastic provenance with a predominant input
from a 490 Ma source. The CL textures of the
zircons are complex, but we argue that they too
imply a magmatic origin. The CL textures of
the zircons show oscillatory-zoned outer rims
that overgrow complex-textured interiors. Al-
though some of the “patchy” CL textures in the
interiors are reminiscent of reequilibration of
radiation-damaged (metamict) zircon domains in
the presence of a hydrothermal fluid (cf. Geisler
et al., 2003) or reequilibration of crystalline (non-
metamict) zircon by dissolution-reprecipitation
(cf. Tomaschek et al., 2003), it should be noted
that the higher CL response overgrowths display
oscillatory zoning (e.g., grains 22, 25, and 27;
Fig. 4D). The textures suggest magmatic inter-
actions in almost all cases, with the final phase
usually not only zoned, but often idiomorphic.
Both core and overgrowth domains appear to
yield identical ages within analytical uncertainty,
and there is no correlation between uranium con-
centration (which varies from 20 to 4000 ppm)
and 206
Pb/238
U age (Table DR2).
Moreover, the cores and overgrowths repre-
sent discrete Hf isotopic populations. The Hf
isotopic analyses define a bimodal distribution
(Fig. 4F). One population comprising mainly
complex-textured cores (grains B, 16, 32;
Table DR4) has a weighted mean εHf(490 Ma)
value of +2.8 ± 1.0 (2σ). These domains usually
have low U contents, consistent with crystalli-
zation from a mafic magmatic component. Al-
ternatively, the low U contents could be due to
replacement of higher U-Th-Y zircon by lower
U-Th-Y zircon (e.g., Gagnevin et al., 2010).
A second population with a weighted mean
εHf(490 Ma) value of –2.6 ± 1.2 (2σ) is generally
made up of oscillatory-zoned domains (grains 1,
3, 15, 21, 24;Table DR4).This can be interpreted
as a relatively later magmatic component domi-
nated by crustal melt. This crustally influenced
component has the highest U contents, while U
and Th are generally positively correlated, con-
sistent with a magmatic rather than hydrother-
mal origin, considering the relative solubilities
of the two elements in aqueous fluids. Thus,
the sample is interpreted as a volcaniclastic
rock with a predominant input from a 490 Ma
magmatic source. Metamorphism occurred
shortly afterward, based on the 488 ± 1 Ma
40
Ar-39
Ar muscovite age from the same sample
and the 490 ± 4 Ma 40
Ar-39
Ar amphibole age
from the enclosing amphibolite.
The detrital zircon U-Pb data suggest that
the garnet-grade mica-schist blocks within the
Deerpark complex ophiolitic mélange probably
represent distal Laurentian margin material in-
corporated into the mélange during obduction.
Muscovite from one of these has yielded a
Rb-Sr age of 483 ± 7 Ma and a 40
Ar-39
Ar age of
482 ± 2 Ma (Figs. 5G, 5H, and 7B). Metamor-
phism and deformation affecting an amphibolite
body within the Deerpark complex are dated at
514 ± 3 Ma by a 40
Ar-39
Ar age of hornblende
defining a strong tectonic lineation (Figs. 5F and
7B). The discrepancy between the white mica
and amphibole mineral cooling ages is puzzling.
It is likely that the amphibole age is meaning-
ful and not affected by the presence of excess
radiogenic Ar due to a near-atmospheric inter-
cept on the inverse isochron (Fig. 5F), while the
white mica Rb-Sr and 40
Ar-39
Ar ages are inter-
nally consistent. It is suggested that the amphi-
bole age records the onset of subduction-related
deformation and metamorphism at ca. 510 Ma.
Although this age is older than any documented
magmatic rocks in the Deerpark complex and
Highland Border ophiolites, similar ages have
been recorded from detrital zircon grains from
the South Mayo Trough, which are thought
to have been derived from the Deerpark com-
plex (see figs. 5b, 6b, 7b, and 8b in McConnell
et al., 2009). A ca. 510 Ma age also temporally
overlaps with the Lushs Bight oceanic tract
in the Notre Dame subzone in north-central
Newfoundland. The Lushs Bight oceanic tract
has been interpreted as a fragment of supra-
subduction-zone oceanic crust formed dur-
ing the initiation of subduction at ca. 510 Ma
(van Staal et al., 2007). The P-T values of
~3.3 kbar and 580 °C are argued to constrain
the conditions of ophiolite obduction. These
P-T data clearly demonstrate that the Deer-
park complex underwent metamorphism under
high-T, low-P conditions (176 °C/kbar). This
is in contrast to the Dalradian Supergroup on
the Laurentian margin, which underwent
blueschist-facies metamorphism at ~10.5 ±
1.5 kbar and 460 ± 45 °C (Fig. 7A; Chew et al.,
2003), which corresponds to high-P, low-T con-
ditions of 44 °C/kbar. Exhumation of the Deer-
park complex at ca. 482 Ma is inferred from
the white mica cooling ages. This event tempo-
rally overlaps with the appearance of ophiolitic
detritus, such as chrome spinel, in the forearc
basin to the Grampian orogeny (the South Mayo
Trough) in Early Arenig (ca. 480 Ma) times
(Dewey and Mange, 1999).
TECTONIC MODEL—INTEGRATING
OPHIOLITE OBDUCTION AND
BARROVIAN METAMORPHISM
Most constraints on the timing of peak Gram-
pian metamorphism in the Dalradian are re-
stricted to regions characterized by the presence
of abundant synorogenic intrusions, such as in
NE Scotland and Connemara (Fig. 1). These
intrusions are thought to represent a continen-
tal (Andean-type) arc on the Laurentian margin
that formed after a subduction polarity rever-
sal during the Grampian orogeny, and they are
characterized by the intrusion of large volumes
of calk-alkaline basic and intermediate magma-
tism (Yardley and Senior, 1982; Tanner, 1990).
Geochronological constraints include U-Pb
zircon ages of 475–468 Ma from synorogenic
intrusive rocks in Connemara in western Ireland
(Friedrich et al., 1999) and by Sm-Nd garnet
ages of 473–465 Ma in the type area of Barro-
vian metamorphism (Glen Clova) in the Scottish
Highlands (Baxter et al., 2002).
Barrow (1893) noted that metamorphic iso-
grads (in what was to become the Barrovian
type locality) increased toward the synoro-
genic granites, and he postulated that these pro-
vided the heat source. As a rapid, synchronous
Grampian orogenic episode would appear to
be inconsistent with models of conductive heat
transfer in overthickened crust (see England and
Thompson, 1984; Jamieson et al., 1998), various
authors (Dewey, 2005; Baxter et al., 2002) have
suggested that the ca. 470 Ma Grampian meta-
morphic peak may have at least partly resulted
from advective heat transfer from the volumi-
nous synorogenic intrusive rocks in the Dalra-
dian block, similar to the original suggestion
of Barrow (1893). This hypothesis is supported
by thermal modeling of Sr diffusion profiles in
apatite from the Barrovian zones of NE Scot-
land, which demonstrates that the thermal peak
was brief and lasted only a few hundred thou-
sand years, which is one or two orders of mag-
nitude shorter than the time scales predicted by
conductive relaxation of overthickened crust
(Ague and Baxter, 2007). However, although
this model may be appropriate for much of NE
Scotland and Connemara, most of the Dalradian
block is devoid of synorogenic intrusive rocks,
and a ca. 470 Ma orogenic peak is still detected
in such rocks by geochronological studies in
NW Ireland (e.g., Flowerdew et al., 2000).
Given that advective heat transfer from large

11. Chew et al.
1796 Geological Society of America Bulletin, November/December 2010
volumes of intrusive rocks does not appear to
be a viable option in NW Ireland, the cause of a
short, synchronous Grampian orogenic episode
in such areas remains enigmatic. Additionally,
Barrovian metamorphism within the Dalradian
structural pile cannot be explained by obduction
of a hot ophiolite slab. For example, our one-
dimensional heat conduction modeling would
predict that obduction of a 900 °C, 10-km-thick
slab of oceanic lithosphere onto a cold (200 °C)
Laurentian margin would produce temperatures
greater than 400 °C only in the upper 2 km of the
Laurentian margin footwall. This is consistent
with the thin Taconic metamorphic soles in the
external Humber zone in Newfoundland asso-
ciated with the Taconic ophiolite allochthons
(e.g., the Humber Arm and Hare Bay alloch-
thons; Cawood and Williams, 1988; Cawood,
1989). Additionally, Taconic deformation in the
underlying autochthonous Laurentian margin
shelf carbonates and their Grenvillian basement
is minimal.
This study demonstrates that in SW Scotland,
generation of oceanic crust was under way by
500 Ma, and that high-grade metamorphism
associated with ophiolite obduction took place
at 490 Ma. In the Clew Bay region in western
Ireland, high-grade metamorphism was under
way by 510 Ma, and exhumation of the ophio-
lite and the associated intercalated Lauren-
tian margin rocks took place at ca. 482 Ma. A
subduction-related magmatic arc founded on
ophiolitic basement was active in both regions
by ca. 490 Ma. These early stages in the devel-
opment of the Grampian orogeny are illustrated
in Figures 8A and 8B. As much of this infor-
mation is derived from fault-bounded lenses
and mélange units within the Highland Bound-
ary and Clew Bay fault zones, which makes it
very difficult to establish the original relation-
ship between units, the first stages of this model
draw on previously published tectonic models
for the early stages of the Taconic orogeny in
Newfoundland (van Staal et al., 2007). Subduc-
tion and the onset of obduction is inferred at ca.
510 Ma (Fig. 8A), analogous to the Lushs Bight
oceanic tract in Newfoundland. In the Grampian
orogen, this event is represented by obduction of
the Deerpark complex ophiolitic mélange, per-
haps initiated by subduction lock-up adjacent
to outboard peri-Laurentian microcontinental
blocks. Several such microcontinental blocks
have been identified or proposed, including
the Slishwood Division and the Tyrone Cen-
tral Inlier (Chew et al., 2008; Flowerdew et al.,
2009) and could even include Connemara. By
analogy with Newfoundland, separation of nar-
row microcontinents possibly happened as a
result of an inboard ridge jump during the for-
mation of the Iapetus Ocean. Early obduction
substantially outboard of the Laurentian margin
can explain why the Laurentian passive margin in
NW Scotland (the Cambrian to Ordovician
Ardvreck and Durness Groups) was not termi-
nated until at least Late Arenig–Early Llanvirn
times (ca. 470–465 Ma) (Huselbee and Thomas,
1998). Additional evidence that the Grampian
orogenic belt was originally far removed from
the Laurentian passive margin (Fig. 8A) in-
cludes major differences in the detrital zircon
signature between the Cambrian-Ordovician
passive margin and its temporal equivalents in
the Dalradian Supergroup (Cawood et al., 2007)
and the absence of any Grampian detritus in the
Laurentian passive margin (Bluck, 2007). Early
outboard obduction onto attenuated Laurentian
microcontinental block(s) also explains the asso-
ciation of ophiolitic mélange and serpentinized
subcontinental lithospheric mantle in the High-
land Boundary fault zone. These serpentinites
are believed to have been produced by exhuma-
tion of serpentinized subcontinental lithospheric
mantle within the extending, distal portions of
the Laurentian margin during the opening of
the Iapetus Ocean (Chew, 2001; Tanner, 2007;
Henderson et al., 2009), which were then incor-
porated into the ophiolitic mélange during the
onset of collision.
Clogging of the subduction zone by the
microcontinental block(s) forced subduction
to step back toward the Laurentian margin
(Fig. 8B) and development of a subduction-
related magmatic arc founded on ophiolitic
basement by 490 Ma. The arc locally assimi-
lated old continental crust in both SW Scot-
land (based on the zircon Hf isotopic data of
this study) and in the Clew Bay region (Chew
et al., 2007), although the early stages of arc
magmatism in this region (the Lough Nafooey
arc) were predominantly juvenile. This assimi-
lated material was likely either distal sediment
derived from the Laurentian margin or the Lau-
rentian microcontinental block(s) described
previously (Fig. 8B). In Bute, volcaniclastic
sediment derived from the arc was immediately
subducted and rapidly exhumed as obduction
onto the Laurentian margin commenced. The
mineral cooling ages from this study suggest
that the main phase of ophiolite exhumation
occurred from 490 to 480 Ma, which overlaps
with the appearance of ophiolitic detritus in
the Grampian forearc basin (the South Mayo
Trough) in Early Arenig (ca. 480 Ma) times
(Dewey and Mange, 1999). The regions of
high-pressure metamorphic assemblages on the
Laurentian margin in western Ireland (Chew
et al., 2003) and the SW Highlands of Scotland
(Graham, 1985) preserve mineral cooling ages
as old as 475 Ma (Fig. 7). This high-pressure
metamorphism can be readily explained by
subduction of the leading edge of the Lauren-
tian plate under the Grampian suprasubduc-
tion ophiolite–arc system. The high-pressure
rocks were then transferred to the overriding
plate within the subduction channel and were
thrust toward the Laurentian margin (Figs. 8B
and 8C). Final exhumation of the high-pressure
rocks (Fig. 8C) is thought to have occurred by
oceanward extensional collapse of the collid-
ing arc during subduction polarity reversal fol-
lowing collision (Clift et al., 2004). A similar
tectonic scenario for the development of high-
pressure assemblages in subducted continental
margin sediments has been developed for the
Semail ophiolite in Oman, where eclogite-
facies metamorphism of the subducted Arabian
continental margin at ca. 79 Ma significantly
postdates formation of the Semail oceanic crust
and its near contemporaneous metamorphic
sole at ca. 95 Ma (Warren et al., 2005).
Although it is beyond the scope of this study
to evaluate thoroughly the thermal cause of
Barrovian metamorphism, conductive heat
transfer in overthickened crust (cf. Richardson
and Powell, 1976), possibly accompanied by
viscous heating (cf. Burg and Gerya, 2005),
was probably important, particularly in re-
gions devoid of syntectonic intrusive rocks.
Although the geochronological data set for
such regions within the Dalradian Supergroup
is limited, it appears that they still yield a
470 Ma thermal pulse. Collisional thickening,
probably initiated as long ago as 490 Ma (cf.
Chew et al., 2007), immediately followed the
start of ophiolite obduction. Because there is
limited evidence for obduction of a thick slab
of oceanic lithosphere, it is inferred that the
deformed Laurentian margin structural pile
was composed mainly of Dalradian nappes.
Further dating of peak metamorphic minerals
with high closure temperatures (e.g., Sm-Nd or
Lu-Hf dating of garnet) is required in several
key regions of the Dalradian belt (e.g., NW
Ireland, SW Scottish Highlands, Shetland) to
determine if a short, synchronous Grampian
orogenic episode developed along the entire
strike length of the orogen.
CONCLUSIONS
This study presents new constraints on the
timing and the P-T conditions of ophiolite ob-
duction in the Highland Border ophiolite in SW
Scotland and its correlative in western Ireland,
the Deerpark complex.
Magmatic zircons from the Bute amphibo-
lite define a 499 ± 8 Ma U-Pb concordia age,
interpreted as dating the crystallization of its
igneous protolith and therefore the formation of
this part of the Highland Border ophiolite. The

12. Timing of ophiolite obduction in the Grampian orogen
Geological Society of America Bulletin, November/December 2010 1797
?
amountofseparation
uncertainduetomovementon
GreatGlenfaultandMoinethrust
Hebrideanterrane
HebrideanterranefarremovedfromGrampianterrane:
1)NoGrampiandeformationordetritus
2)Passivemargincontinuesto465Ma
3)Detritalzirconsignatureofpassivemarginvery
differenttotemporalequivalentsintheDalradian
passive-margin
paleoshorelineabsentinIrishsectionsspreadingcenteroutboardmicrocontinent(examplesincludetheSlishwoodDivision
andtheTyroneCentralInlier)
(arcabsentinScotland)
siteofeventualdevelopmentofblueschist-facies
metamorphismindistalDalradiansediments
inferredophioliticmélange
SouthMayoTrough
forearcbasin
HProckstransferredtohanging
wallandthrustovermargin
TrossachsGroup,
HBCandCBC
ophioliticmélanges
extensionaldetachmentdevelops
later,exhumesHProcks
Lough
Nafooey
Arc
High-pressuremetamorphism
Arcvolcanics/magmaplumbingsystem
Fore-arcbasinsediments
Ophioliticmélange
Oceaniccrust
Serpentinizedsubcontinentallithosphericmantle
Mantle
Accretionarycomplex(~HBC/CBC)
Cambro-Ord.passivemargin(shallowmarine/distal)
Rift-relatedbasicvolcanisim
DalradianSupergroup
Sub-GrampianGroupbasement
Torridoniansandstone
Laurentiancrystallinebasement
Key
orogenpropagates
towardsforeland
zonelaterexploitedbyHBFandFHCBL
C480Maobductionontocontinentalmargin
B490Maintra-oceanicarc
A510Maearly(intra-oceanic)obductionontooutboardmicrocontinent
earlyobductioneventonto
outboardmicrocontinent?
natureofcrystallinebasementuncertain
Figure8.SchematicmodelofthetectonicevolutionoftheLaurentianmargininScotlandandIrelandat(A)510Ma,(B)490Ma,and
(C)480Ma.HBC—HighlandBordercomplex,CBC—ClewBaycomplex,HBF—HighlandBoundaryfault,FHCBL—FairHead–Clew
BayLine.

13. Chew et al.
1798 Geological Society of America Bulletin, November/December 2010
same body has yielded a 490 ± 4 Ma 40
Ar-39
Ar
hornblende age, which is interpreted as the
age of metamorphism. A homogeneous zircon
age population from a mica schist intercalated
within the ophiolite at the same locality de-
fines a U-Pb concordia age of 490 ± 4 Ma. This
date is interpreted to suggest a volcaniclastic
origin for this rock, possibly originating from
a subduction-related magmatic arc founded on
ophiolitic basement. A late magmatic compo-
nent in these zircons is defined by oscillatory-
zoned domains and idiomorphically zoned rims.
They yield a weighted mean εHf(490 Ma) value of
–2.6 ± 1.2 (2σ), which may reflect contami-
nation of the magmas by old continental crust,
probably assimilated Laurentian margin mate-
rial (cf. Chew et al., 2007). Muscovite from the
same rock yielded a 488 ± 1 Ma 40
Ar-39
Ar age,
which is interpreted as the age of metamor-
phism. THERMOCALC multi-equilibria yield
average P-T values of ~5.3 kbar and 580 °C
for the mica schist. Since the amphibolite
and metasedimentary xenolith share the same
amphibolite-facies foliation, these data are ar-
gued to provide a P-T constraint on ophiolite
obduction. Pebbly grits within the Highland
Border complex rocks in North Glen Sannox
on the Isle of Arran have yielded detrital zircon
age spectra characterized by peaks at ca. 0.95–
1.3 Ga, 1.4–1.5 Ga, 1.7–1.9 Ga, and 2.5–2.9 Ga
and support correlation of the Arran Highland
Border complex rocks with the Dalradian Tros-
sachs Group. The 40
Ar-39
Ar dating of white
mica from mylonitized pillow basalts in North
Glen Sannox has yielded a 476 ± 1 Ma 40
Ar-39
Ar
age, which is consistent with involvement in a
ca. 475–465 Ma Grampian orogeny.
Early metamorphism and deformation (pos-
sibly produced by intra-oceanic thrusting over
an outboard Laurentian microcontinent) within
an amphibolite body from the Deerpark com-
plex ophiolitic mélange in western Ireland are
constrained at 514 ± 3 Ma by 40
Ar-39
Ar dating
of hornblende defining its lineation. Mica schist
blocks are intercalated within the Deerpark
complex mélange and yield detrital zircon spec-
tra similar to the upper portions of the Dalradian
Supergroup. Muscovite from one of the schist
blocks yielded a Rb-Sr age of 483 ± 7 Ma and
a 40
Ar-39
Ar age of 482 ± 1 Ma. P-T values of
~3.3 kbar and 580 °C for the mica schist ar-
guably provide a P-T constraint on ophiolite
obduction, since petrographic evidence demon-
strates that the metabasites have experienced the
same metamorphic event.
The 40
Ar-39
Ar age mineral age data from the
Dalradian and Clew Bay complex are consis-
tent with rapid cooling from a ca. 475–470 Ma
Grampian metamorphic peak. There is there-
fore a pronounced time gap between mineral
ages in the Dalradian (along with the low-grade
metasedimentary rocks of the Clew Bay com-
plex and the Highland Border complex) and
the higher-grade metamorphic rocks of the
Highland Border ophiolite and the Deerpark
complex. The boundary between these units is
also defined by a marked difference in P-T con-
ditions: high-T, low-P metamorphic conditions
in the Deerpark ophiolitic mélange compared
to the high-P, low-T (blueschist-facies) meta-
morphic conditions in the subducted Laurentian
margin sediments of the Dalradian Supergroup.
These results demonstrate that the Highland
Border and Deerpark complex ophiolites expe-
rienced metamorphism and deformation at least
15 m.y. before the Grampian orogeny. It is en-
visaged that subduction of the leading edge of
the Laurentian plate, probably initiated as long
ago as 490 Ma, was contemporaneous with
the start of ophiolite obduction. This produced
high-pressure metamorphism on the Laurentian
margin. The high-pressure metamorphic rocks
were then transferred to the hanging-wall plate
and thrust back onto the margin, and exhumed
shortly afterward by extensional collapse, pre-
serving mica cooling ages as old as ca. 475 Ma
close to the Laurentian margin. Away from the
Laurentian margin, collisional thickening cre-
ated the thick Dalradian nappe stack and asso-
ciated Barrovian metamorphism, with possibly
minimal involvement of obducted oceanic litho-
sphere. If conductive heat transfer in overthick-
ened crust was the major heating mechanism,
then collisional thickening may have initiated
shortly after the start of ophiolite obduction at
ca. 490 Ma in order to generate the ca. 470 Ma
peak metamorphism in the Dalradian Supergroup.
ACKNOWLEDGMENTS
Richard Spikings and Michael Murphy are
thanked for technical assistance with the 40
Ar-39
Ar
and isotope dilution–thermal ionization mass spec-
trometry (ID-TIMS) analyses, respectively. Catherine
Ginibre and Hans Harryson are thanked for techni-
cal assistance with the electron microprobe analyses.
The NordSIMS facility is operated under an agree-
ment between the research councils of Denmark,
Norway, and Sweden, the Geological Survey of Fin-
land, and the Swedish Museum of Natural History.
This is NordSIMS contribution 247. Cees van Staal,
Rob Strachan, and Associate Editor Brian McConnell
are thanked for insightful comments, which improved
the manuscript. Geoff Tanner, John Dewey, Bill
Henderson, and Bill Church are thanked for many
stimulating discussions on the geology of the High-
land Border and Clew Bay regions.
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