Geological Society, London, Special Publications
The age and distribution of basement rocks in the Caledonide
orogen of the N Atlantic
Derek Powell, T. B. Andersen, A. A. Drake, Jr, Leo Hall and J. D. Keppie
Geological Society, London, Special Publications 1988; v. 38; p. 63-74
doi:10.1144/GSL.SP.1988.038.01.05
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© 1988 Geological Society of
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The age and distribution of basement rocks in the Caledonide
orogen of the N Atlantic
Derek Powell, T. B. Andersen, A. A. Drake, Jr, Leo Hall & J. D. Keppie
SUMMARY: Available isotopic data suggest that the Precambrian basement elements of
the northern parts of the Caledonide orogen in N America, Britain, Ireland, Greenland and
Scandinavia comprise largely Grenvillian and Svecofennian continental crust. Archaean
precursors have been identified in Greenland, Norway and Britain, but not positively in the
Appalachians. Early Caledonian (Grampian-Finnmarkian) orogenic activity gave rise, in
the Scandinavian and British-Irish sectors, to an early Palaeozoic basement which, in
Norway, is overlain by Ordovician to Silurian supracrustal sequences. In contrast, a late
Precambrian-early Cambrian (Cadomian-Avalonian and older) basement is present in
southern parts of the orogen.
The distribution of these basement elements constrains any plate-tectonic reconstructions
of the N Atlantic Caledonide orogen and suggests that the orogen was tripartite during its
early development. Further, it suggests that transcurrent movements as well as obduction
may have been important tectonic processes during Grampian-Finnmarkian tectonism.
The occurrence of Precambrian crystalline rocks
within many fragments of the Caledonide orogen,
as either autochthonous-parautochthonous massifs or components of nappe complexes, shows
that the major part of the orogen is floored by
Precambrian continental crust. Whereas in many
places isotopic data indicate the age of this
material in terms of its Precambrian history,
there are insufficient data to permit the precise
location of Precambrian age province boundaries
within the orogen (Fig. 1). Archaean materials
may be present in the Chain Lakes Massif of the
N Appalachians (Fig. 2). They also occur as
allochthonous inliers in the N Highlands of
Scotland (Fig. 3), in the southern part of the
E Greenland Caledonides (Fig. 4), the Lofoten
Islands of Norway (Fig. 5) and the Channel
Islands (Fig. 3). However, the original extent of
such rocks is masked by subsequent reworking.
A southeastern limit of Grenvillian elements
(1200-900 Ma) can be broadly identified in the
Appalachians and the British Isles, but its
extension to the E through northern Europe is
not well constrained (Fig. 1). An AvalonianCadomian basement in the southern parts of the
orogen is recognized in the Bohemian-Moldanubian Massif, southern Britain and Ireland, parts
of the Armorican Massif, Nova Scotia, southeastern Newfoundland and southeastern New
England (Fig. 1). Late Precambrian supracrustal
rocks of Avalonian aspect form much of the
Carolina slate belt in the southern Appalachians
and the Caledonian Highlands of New Brunswick, and they also occur in Morocco and Iberia
(Fig. 1) but here their status as basement is
unclear. As with the southern limit of Grenvillian
elements, the northern limit of the Av.alonianCadomian basement can only be broadly defined
on the basis of isotopic evidence (Fig. 1).
In northern E Greenland the ages of Precambrian basement elements are not well constrained,
but those in central E Greenland appear to reflect
largely Grenvillian and Svecofennian activity,
with possible relics of older material (Fig. 4). In
the Scandinavian Caledonides isotopic evidence
for Grenvillian (Sveconorwegian) and Svecofennian elements is substantial and there is some
evidence of Archaean components in northern
areas (Fig. 5). Within the southern Norwegian
Caledonides the location of the Sveconorwegian
(Dalslandian) front, which is clearly traceable
through the southwestern part of the Baltic
craton, is not well defined (Fig. 1). However,
close similarities between the Sveconorwegian
Province and rocks of the Grenville belt in
Labrador (Gower & Owen 1984) suggest its
continuation across the orogen (Fig. 6).
More positive and widespread evidence of the
age of Precambrian basement rocks and the
location of Precambrian province boundaries
within the orogen would assist in constraining
not only the positions of possible Caledonian
sutures but also the configuration of continental
crustal blocks during the late Proterozoic and the
subsequent development of the Caledonide orogen (Fig. 6).
Whereas there is no international agreement
as to the division of much of Precambrian time
into eras and sub-eras the use of time-stratigraphic terms herein follows that given by
Harland et al. (1982): late Proterozoic (Pt3), 590900 Ma; middle Proterozoic (Pt2), 900-1600 Ma;
early Proterozoic (Ptl), 1600-2500 Ma.
From HARRIS,A. L. & FETTES,D. J. (eds), 1988, The Caledonian-Appalachian Orogen,
Geological Society SpeCialPublication No. 38, pp. 63-74.
63
64
D. P o w e l l et al.
M I D D L E TO L A T E P R O T E R O Z O I C A G E P R O V I N C E S
OF THE NORTH ATLANTIC CALEDONIDE & HERCYNIDE OROGENS
Exposed & inferred Grenvillian
(1200-900 Ma) province.
Exposed & inferred Avalonian Cadomian (700-550 Ma) province.
\
--~.
'...........
~
"--._..,
Caledonian deformation fronts.
"..
-.,
ssfS Grenvillian_Sveconorwegian
,.
tectonic fronts. ~
SE boundary of known Grenvillian
elements.)
0 ~ N W boundary of known Avalonian
elements. ~
~
:
j
L
~
..~...
....
..
i
! ,.. s
FIG. 1. Pre-Atlantic configuration of middle to late Proterozoic elements of the Caledonide orogen and adjacent
regions. The deformation fronts in S Britain and Europe are based on the proposals of Ziegler (1984).
Grenvillian and older basement
elements
The Appalachians
Orthogneisses and paragneisses, lithologically
comparable with the rocks of the Grenville
Province of the Laurentian Shield, occur in much
of the northwestern tract of the Appalachians
along the Blue Ridge-Green Mountain axis, in
the Indian Head-Long Ranges of Newfoundland
and in the Western Gneiss Domes of the southern
and central Appalachians (Fig. 2). U - P b and
Rb-Sr isotopic data, which suggest ages of
65
Age of basement rocks in Caledonide orogen, N Atlantic
.,/~'~
¢~/"
f
Rift facies supracrustals of mainly late Precambrian age. ~ - - ~ - ~ - ~
L~,~J el. Ionian
Isotopic a
g
e
s
.
.
age.
f;f;"" ....
/'
,~,,,.,.
?
o O-Pb, Zr.
X
...,15oo_~15~o: ~'
B,H.
- K-Ar.
,,, 6 ~
°1444,X ~ .
• 895 ° 1 1 1 5 . 1 1 4 2 ~
olo,oo~5oJ \
i
/
, ~ , y ~
e.G.D, o1028 o1140
&R. c732o655/
,i , L I ~
~ ~
~-/ /
",'/-v
° 1 1 7 4!
\r°55o
l
\ro9so
•
~e1113
~'0573!
0 6 5 0 M.G.
P.N.
;5595=612-646
--~1©619-624
5557 o600-650
Io621
0630
r M.D.
J
~50~
M.P.
L 1327.j
+
N
O1000 A.M.
j
- ?/'
S ' - ~ - ~ A594 111000 F.C.
o 810 o524
o1063
°5ss/~i~
.I
i"
/
/
==608==590 L.C.G.
~-C.S.I.
~A556 A600 P.D.
~750
.~J~Appalachian deformation front.
° 752"~
~S?
°7521
,.,~JJ
i
507 H.G.
~
7 J ~ = 5 9 2 • 5 9 9 - 6 5 3 A604 A.H.
~'o934
\ 5 5 7 5 5 5 9 6 5 6 2 6 5 6 4 2 1 " Co,H,
? /o600-620
H.H.
J
~ .x~/
,' J
• Ar-Ar.
LR
~,s94 M.C.
. , ~ . ~ -
~
,/h~'%
-
)
~ "
" "
~
?
o 1 0 6 2 5 1 0 4 5 (,~.[ ,'//I/
• 1o4o , ~ o 5 : , 1 o z o
.~7"
~
/'/.
G.M. e1047 /
• Rb-Sr, W R .
/
....
".
~ ~ /~
~, _ ~ ; -
%'/
C.L 0 9 5 5 - 1 0 4 0 01130-1200< _
//
~"
MA
/</, ,
Pb-Pb.
-
f~.#:~--~'~
pluzonic and supracrustal rocks,
and supraerustal rocks of u n c e r t a i n
Plutonic
==615
~,/~.,so-55o,gss-g,0
P R E C A M B R I A N R O C K S OF T H E APPALACHIANS.
.-T.G~
//
C.B.
0
k.
i
i
km
r
r
500
7
' - " ~ o 5 6 6 / /'
o1050 e1027
f
/
/
P.M. o1051 o l 142
, o1020 o l 170 Gf.M.
O584-619
FIG. 2. Distribution of Precambrian elements of the Appalachians: AH, Antigonish Highlands; BR, Blue Ridge;
Ca.H, Caledonian Highlands; CBI, Cape Breton Island; CL, Chain Lakes Massif; COH, Cobequid Highlands;
CS, Carolina slate belt; GM, Green Mountains; LR, Long Range mountains; MD, Milford-Dedham zone; MP,
Manhattan Prong. Based in part on Schenk (1978), Williams (1978), Rankin et al. (1983), Keppie (1985) and
Drake et al. (in press).
between 1150 and 900 Ma (summarized by
Rankin et al. 1983, Drake et al., in press), confirm
the lithological correlations and may also provide
evidence for the occurrence in places (e.g. the
Manhattan Prong and the Chain Lakes Massif)
of older middle Proterozoic (Pt2) relics. Thus the
PbZ°6-pb2°6,
uz35-pb
2°7 a n d U 2 3 8 - p b 2°6 m i n i -
ages of 1500-1510 Ma, 1130-1200 Ma and
955-1040 Ma respectively from the Chain Lakes
Massif (Fig. 2) (Naylor et al. 1973) may indicate
Grenvillian reworking of older Proterozoic material, and the older ages could identify the Chain
Lakes terrane as exotic (Osberg 1978, Keppie
1985).
Gneisses and metasediments form a Precambrian basement in the Caledonian Highlands of
New Brunswick, the Cobequid Highlands and
Cape Breton Island of Nova Scotia. Isotopic data
for the former (Olszewski & Gaudette 1982) is
equivocal, but suggests a minimum age of about
mum
810 Ma. The gneisses and metasediments of the
Cobequid Highlands would appear to be older
than about 640 Ma, and the gneisses may be as
old as, or older than, about 930 Ma (Gaudette et
al. 1983). The basement gneisses and metasediments of Cape Breton Island are older than about
750 Ma (Jamieson & Craw 1983). Thus the age
of these basement elements is poorly constrained;
they may be Grenvillian, but equally they may
constitute a late Proterozoic basement to the
Avalonian-Cadomian province (see below).
The British Isles
On an early Permian continental reassembly map
of the N Atlantic area, both the northwestern
Caledonian deformation front of the N Appalachians and the Grenville front of the Laurentian
Shield should run through the British Isles
(Fig. 1). The Caledonian front probably runs just
66
D. Powell
N of the Irish mainland and thence northwards
up the W coast of the Northern Highlands of
Scotland as the Moine thrust zone (Fig. 3). In
Scotland, W of the Moine thrust zone, narrow
zones of Caledonian reworking, such as the Outer
Isles thrust and possibly the Flannan thrust
(Blundell 1984), traverse Archaean and early
Proterozoic gneisses (the Lewisian) and middle
to late Proterozoic cover sediments (the Torridonian) of the Caledonian foreland. These Precambrian basement gneisses and supracrustal rocks
otherwise show no indications of either Grenvillian or Caledonian reworking. They thus formed
a stable cratonic block during these episodes of
orogenic activity.
Orthogneisses and paragneisses of middle
Proterozoic (Grenvillian) age are present in the
Belmullet peninsula of western Ireland (van
Breemen et al. 1978), but the age of the highgrade metasediments of the NE Ox Mountain
and Lough Derg inliers is uncertain (Fig. 3) (Long
& Yardley 1979, Max & Long 1985). In the N
Highlands of Scotland, the Moine schists which
are lithologically similar to the Lough Derg rocks,
comprise an extensive group of middle
Proterozoic metasediments (Brook et al. 1976,
1977, Brewer et al. 1979) that contain allochthonous to parautochthonous inliers of early Proterozoic and Archaean basement (Tanner et al. 1970,
Rathbone et al. 1983). Lithologically, the Moine
schists do not resemble type Grenville material,
but isotopic evidence is interpreted as demonstrating regional metamorphism and plutonism
at about 1000 Ma (Brewer et al. 1979). The
regional, though sparse, development of a suite
of granitic pegmatites at 820-770 Ma (Piasecki
& van Breemen 1983, Powell et al. 1983), may
relate to later ductile crustal extension. Lithologically similar rocks to the Moine schists appear
to underlie tectonically the Dalradian Supergroup
to the SE of the Great Glen fault (Fig. 3) (Piasecki
& van Breemen 1979). Although these rocks are
older than about 750 Ma, there is no unequivocal
isotopic evidence for their sedimentary age nor
for their early metamorphism.
Further evidence for the possible southward
extent of Grenvillian elements is provided by the
presence of gneissic xenoliths in Carboniferous
diatremes in the Midland Valley of Scotland and
in central Ireland which lithologically do not
resemble Lewisian material and give model ages
of about 1000-1180 Ma (Fig. 3) (Strogen 1974,
Aftalion et al. 1984, Upton et al. 1984).
Greenland
The Greenland fragment of the Caledonide
orogen displays marked similarities to N Scotland
et al.
in its middle Proterozoic history. In central E
Greenland isotopic data suggest the presence of
an older basement of early Proterozoic gneisses
and plutonic rocks of Svecofennian age, together
with relics of older Archaean material (Fig. 4).
However, isotopic ages derived from intrusive
rocks and the Krummedal metasedimentary
sequence indicate deposition, regional metamorphism and plutonism between about 1250
and 950 Ma (Higgins 1976, Higgins et al. 1978,
Henriksen 1985). This, together with the subsequent history, is remarkably similar to that of
the Moine schists. As in Scotland, however,
the Grenvillian elements are dissimilar to the
type area, and together these may represent an
atypical northern arm to the Grenville orogen
(Fig. 1).
Scandinavia
In contrast with Scotland and Greenland, the
basement of the Scandinavian Caledonides appears to comprise for the most part early
Proterozoic materials together with plutonic
rocks. Isotopic data, which give ages in the 19001600 Ma range, reflects this activity, as in the
adjacent Svecofennian province of the Baltic
craton (Fig. 5) (Wilson & Nicholson 1973,
Scharer 1980, Skjernaa & Pedersen 1982). Likewise, older ages of 1750-1600 Ma in both the
southern Norwegian Caledonides and the Gothides probably relate to early Proterozoic Svecofennian precursors (Fig. 5).
Isotopic data from autochthonous and allochthonous basement gneisses and plutonic rocks
of the central and northern Norwegian area
predominantly reflect Svecofennian events (Wilson & Nicholson 1973, Griffin et al. 1978),
suggesting the continuation of the Svecofennian
Province of the adjacent craton through the
Caledonian nappes of the orogen (Fig. 5) (Pharaoh 1985). The isotopic evidence for Grenvillian
activity in these areas (Heier & Compston 1969,
Griffin et al. 1978, Reymer et al. 1980) is scant
and equivocal. However, in the southern Norwegian Caledonides middle Proterozoic ages are
widespread (Fig. 5) and, although much more
isotopic data is required, a direct continuation of
the Sveconorwegian (Dalslandian) front of the
southern Baltic craton northwestward through
the Norwegian Caledonides (Fig. 5) cannot
be dismissed (Reymer et al. 1980, Ziegler
1984), particularly in view of the close temporal and spatial similarities between the
lithotectonic regions of eastern Labrador and
the Gothides of southern Sweden (Gower &
Owen 1984).
PRECAMBRIAN ELEMENTS OF
THE BRITISH-IRISH CALEDONIDES.
.o""
r
°°•'%
i
0
KM
l
!
/o /
/" %~, ///"
100
I
e
N
(01531)
,649i~
m //
. .......}.<..'~
,/<o2o94~
/
/
/
e557o542
e 5 6 0 0563
757
o2750
o 720 o733L -:~
o7640817
~,.
e1002~
(01517)
° . . . .
<~.~.~-~
o776-~
0556
. . . .
.~;'-~o749-.-~_
-(o1330)
o1030~j%
~.,~
,~,s""~
-""~
~ "
/ e559
~
C"(e1101DllO0-1180)
o,ooo
,o,o
2
o 58r
~:q
<---~
Foreland.
3"
~
9~....., .
.... " ' ~ " C a l e d o n i a n f r o n t .
Early P r o t e r o z o i c -
B
/
..¢
.
.
.
.
.i
./
J
J
"" .......
Archaeangneisses.~_. ~
<
J
Middle to l a t e Proterozoic m e t a m o r p h i c complexes.
kf
Late Riphean-Cambrian
s u p r a c r u s t a l rocks.
Precambrian (to Cambrian ?) gneisses &
j
650
2565
550
s u p r a c r u s t a l rocks.
Solway line (Iapetus suture).
FIG. 3. Basement elements in the British-Irish Caledonides: A, Anglesey; LD, Lough Derg inlier; OX, Ox
Mountains; R, Rosslare. The symbols for isotopic data are as in Fig. 2 except for the following: o, Rb-Sr mineral
age. The ages in parentheses are upper intercepts on condordia derived from Caledonian granites.
68
D. Powell et al.
PRECAMBRIAN ELEMENTS
OF THE EAST GREENLAND CALEDONIDES.
300 KM
[
.;3
t
.
oe1000-1250
e1700-1980
1
o950
1060
~. ~Al125
_~o950
oe1840-2020--------
1315
O2300-2520
1650
Al182
1490
--- n 6 1 6 e 1 1 6 2
0e990-11(
el 162
=,1890 [ ] 2 2 9 0
Precambrian shield.
Precambrian to Palaeozoic
platform sediments.
/
Upper R i p h e a n to S i l u r i a n
sediments.
Precambrian-Caledonian
metamorphic complexes.
...~'~" Caledonian front.
-----"
D e f o r m a t i o n f r o n t of N.Greenland
Fold Belt.
FIG. 4. Precambrian to lower Palaeozoic elements of the E Greenland Caledonides.
Avalonian-C adomian elements
The late Proterozoic history of parts of the eastern
Appalachians, NW Africa, Iberia, southern
Britain and northern Europe differs markedly
from that of the areas described above in as much
as late Precambrian supracrustal sequences have
in many places undergone late Proterozoic to
early Cambrian deformation, metamorphism
and/or plutonism between 800 and 540 Ma
(Fig. 1).
Gneisses that may constitute a basement to
these supracrustal sequences are present in
Anglesey and the Welsh Borderland, Rosslare,
the Channel Islands and Brittany, the Caledonian
and Cobequid Highlands, and Cape Breton
Island (Figs 2 and 3). The gneisses of Anglesey,
however, give ages of 593 and 562 Ma and the
associated granite has an age of 603 Ma (Beckinsale & Thorpe 1979), but similar gneisses in the
Rosslare Complex of SE Ireland are certainly
older than 538 Ma and may be as old as 2400-
Age of basement rocks in Caledonide orogen, N Atlantic
PRECAMBRIAN
ELEMENTS
I
OF THE
AND
69
<'FINNMARKIAN"
SCANDINAVIAN C ~ N I D E S .
Autochthonous 1
Allochthonous Proterozoic(& Archaean ?) basement.
L a t e P r o t e r o z o i c to l a t e C a m b r i a n s u p r a c r u s t a l rocks & o p h i o l i t e s plus
older a l l o c h t h o n o u s b a s e m e n t a f f e c t e d by F i n n m a r k i a n O r o g e n y .
e668
Late Proterozoic to late Cambrian supracrustal
_ •8,25
r o c k s plus o l d e r b a s e m e n t n o t a f f e c t e d
by F i n n m a r k i a n Orogeny.
K-At
Isotopic ages.
oU-Pb.
• Rb-Sr, W.R.
• 16~t~
4-''"
• 1703 I L
i1815--~,
e1175~2
e1550
...:
...-
:
:
~e"
t..:'
•..
:":"
t
i
-'~,~' e1'635
~1 /
......l
el 707
e1565
>~
el
el 820
_e1625
1800-1900
i
e1725
?~.
/
e1800
• o932-1816 •
• 1 1 5 0 ~ '1~
• 105E~
• 1035"~
• 1253 " ~
L:IO
0tel
• 1004 d
11064~i
i1750, ,
o946
~
Sveconorwegian
basement.
01810
• 1 1 0 0 j ~ i L. . . . . i 1 6 1 6 - •1600
/ - - e961 - - - ~
•928~
0932
Baltic Shield.
-J/
i1062
Svecofennian
basement.
.......
II106 0930
1062
FIG. 5. Basement elements of the Scandinavian Caledonides. Isotopic data from Wilson & Nicholson (1973),
Griffin et al. (1978), Reymer et al. (1980), Scharer (1980), Pharaoh et al. (1982), Skjernaa & Pedersen (1982) and
Harvey (1983). Note that details of the geology are not given for Sweden.
7O
D . P o w e l l e t al.
1600 Ma (Max & Long 1985). Analyses of the
orthogneisses of the Malvern Complex and the
Rushton schists of the Welsh Borders (Fig. 3)
yield isotopic ages of 680Ma and 670Ma
respectively (Thorpe et al. 1984). In contrast,
isotopic data from gneisses of the Channel Islands
and Brittany give evidence for metamorphic
activity at 1200-900 Ma (Leutwein 1968) and
2600-2300 Ma (Leutwein et al. 1973). However,
plutonic activity in these areas, in part overlapping late Proterozoic (Cadomian) tectonism, is
bracketed between 650 and 550 Ma (Bishop et al.
1975).
The age of basement gneisses in the Caledonian
and Cobequid Highlands and Cape Breton Island
in Canada (Fig. 2) is not well constrained, but
they are possibly older than about 810 Ma, about
640 Ma and 750 Ma respectively (Olszewski &
Gaudette 1982, Gaudette et al. 1983). These
gneisses are overlain by metasediments and
~-"
metavolcanic rocks intruded by plutonic rocks
with intrusive ages of 526 and 530 Ma (Caledonian Highlands), 575 and 642 Ma (Cobequid
Highlands), 604 and 653 Ma (Antigonish Highlands), 574 Ma (Cape Breton Island) and 621580 Ma (SE Newfoundland) (Benson 1974, Dallmeyer et al. 1981, Krogh & Papezik 1983,
Wanless, personal communication). In many
places these Precambrian rocks are overlain by
Cambrian strata.
Plutonism in the Milford-Dedham zone of SE
New England at 630-550 Ma is isotopically well
constrained (Zartman & Naylor 1984), and synto post-tectonic events in the Carolina slate belt
are dated at 650-564 Ma (Rankin et al. 1983).
These, together with similar events recorded in
Nova Scotia, Newfoundland, the British Isles
and Brittany, suggest broad contemporaneity of
orogenic and plutonic (Cadomain-Avalonian)
activity along the southeasten tract of the Cale-
Grenvillian deformation front
~
Subduction-obduction zone
L..'
~ J
.....:.!?<......... Ridge-transform system
..~
Transcurrent systems
--
~
/~:
/ \ 7 ( "-I
\
ualedonian thrusts
\ I " 1 \1
:!i~,- .ii:: ~
Grenvillian supracrustal rocks
[~C!i:ti!..]! ~
I"-/Zy!
\ ~ I ~'
\,~/-t/
/,~\ ~'_/
.......... Continental margin
Archaean-early Proterozoic
basement
Grenvillian basement
Passive margin
i "b!.~:i:"
I ~ i : ili?] ~
~11~7.~
! ? .-
Active margin
J~l~dl~i"t
'!
~::ii '~
\
'~ti~i:
-=
'~'.....
ii,-,I
""
~/,~/'~r~
VI
II, III~!
/.-,; /\'L\%',
r'.-',D-i~G'c.
(" '_.._.J~\ '/ t,
......
.-
.
/]
...-
.,
,/
.... .(,~
::::
...
..: .-,
, ~
-' Y~7""
;:'
.,/.
:.,/ ... '
_.
~
.......
%\
\
,
.
FIG. 6. Hypothetical configuration of the N Atlantic Caledonides at the onset of the Grampian-Finnmarkian
orogeny: GH, Grampian Highlands; MV, Midland Valley; NWIa and NWIb, alternative positions for
northwestern Ireland; NH, Northern Highlands of Scotland; OH, Outer Hebrides. For further explanation see
text.
Age of basement rocks in Caledonide orogen, N Atlantic
donides (Fig. 1). However, the age and distribution of older Precambrian basement materials in
this zone is not clear.
The Grampian-Finnmarkian
basement
There is substantial isotopic and some biostratigraphic evidence of pervasive polyphasal deformation, metamorphism and plutonism in the late
Cambrian-early Ordovician interval (520480 Ma) in parts of the N British-Irish Caledonides (the Grampian orogeny), throughout the
upper Altochthon of the orogen in Norway and
Sweden (the Finnmarkian orogeny), and perhaps
in N Germany and Poland (Lambert & McKerrow 1976, Sturt 1984, Ziegler 1984). The
Penobscotian deformation in the northern Appalachians is a correlative of this GrampianFinnmarkian orogeny. In the Appalachians it
involved the initial emplacement of ophiolite
between 508 and 490 Ma, and is recognized in
the Burlington Peninsula of Newfoundland
through to southern Quebec, along the southern
side of the Chain Lakes Massif and in N central
Maine (Keppie et al. 1983, Keppie 1985). It is
uncertain, however, whether the Penobscot event
represents an early phase of the Taconic orogeny
or was distinct.
No time equivalent of the Grampian-Finnmarkian Orogeny has yet been recognized in the
E Greenland Caledonides.
A post-Finnmarkian pre-Scandian unconformity is recognized in many places in nappes of the
upper Allochthon of the Norwegian Caledonides
(Brekke et al. 1984 and personal communication,
Sturt 1984, Sturt et al. 1984, Ramsay et al. 1985).
Although such Ordovician and younger cover
rocks are absent in the Scottish sector, in places
rapid uplift occurred between 460 and 440 Ma
(Dempster 1984). In Ireland Grampian deformation was essentially complete by about 460 Ma
(Powell & Phillips 1984). Thus, through these
areas
the Grampian-Finnmarkian orogen
formed a basement to later Palaeozoic sedimentation and could have provided a source for
sedimentary detritus.
Isotopic evidence for Grampian orogenic activity in Scotland is limited to the Dalradian zone of
the Grampian Highlands and Shetland, and
possibly the Ballantrae ophiolite complex (Bradbury et al. 1976, Flinn & Pringle 1976, Bluck et
al. 1980). There is no evidence for tectonometamorphic activity in the Northern Highlands
in the 555-480 Ma interval (Powell & Phillips
1984). Indeed, peak Caledonian metamorphic
conditions were achieved at about 470-450 Ma
(van Breemen et al. 1974, Brewer et al. 1979).
71
Despite arguments to the contrary (van Breemen
et al. 1979, Coward 1983), there is no evidence
for diachroneity of activity or structural continuity across the Great Glen other than differences
in the isotopic ages of events; no progressive
changes in age within the two zones are apparent.
It is therefore possible to consider that the
northwestern and southeastern Highlands of
Scotland behaved as separate entities during
much of the evolution of the Grampian-Caledonian orogeny (Powell & Phillips 1984). The
boundary between these zones may lie along the
Ossian steep belt-Loch Awe syncline-Ox Mountains line rather than the Great Glen fault zone.
Conclusions
A hypothetical plate-tectonic configuration that
might explain the distribution of Grenvillian
elements in Laurentia and Baltica and the
location of the tectonically passive and active
margins to Iapetus during the Grampian-Finnmarkian orogeny is illustrated in Fig. 6. The
reconstruction is based on regarding the 'Grenvillian' rocks and events of N Scotland and
Greenland as relating to an atypical northern
arm of the Grenville-Gothides belt, on the
occurrence of Grenville-type rocks in western
Ireland (van Breemen et al. 1978) and on the
match of the Grenville front with its counterpart
in Baltica. The alternative positions for Baltica
arise from (a) the extension of the Grenville Front
through the Rockall plateau (supported by Piper
(1976)) and (b) the match of rocks in eastern
Labrador with those of southern Sweden (Gower
& Owen 1984).
Justification for considering a transcurrent
model for the Grampian orogeny in the BritishIrish sector (ef. Dewey & Shackleton 1984) lies in
the known large transcurrent movements associated with the Highland Boundary fault zone
(Bluck 1983), the possibility that the 'fountain of
nappes' structure of the Dalradian (Thomas 1979)
represents a deeper crustal analogue of a major
'flower structure', and the differences between
the late Precambrian and/or early Palaeozoic
history of the N Highland block, the Dalradian
zone and the Highland Border (Powell & Phillips
1984). The obliquity of major structures in the
Dalradian zone not only to the Highland Boundary fault but also to the trend of the Ossian steep
belt (Thomas 1979) is also of note.
ACKNOWLEDGMENTS: DP acknowledges the financial
assistance of the Natural Environment and Research
Council (Grant GR3/3998). During the preparation of
this paper the untimely death of Leo Hall occurred. The
authors wish to acknowledge his contribution, his help
and his enthusiasm; he will be sadly missed.
D. Powell et al.
72
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D. POWELL, Department of Geology, Royal Holloway & Bedford New College, University
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T. B. ANDERSEN,Geologisk Institutt Avd. A, Universitetet i Bergen, 5000 Bergen, Norway.
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J. D. KEPPIE, Department of Mines and Energy, PO Box 1087, Halifax, Nova Scotia,
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