Ordovician magmatism, deformation, and exhumation in the
Caledonides of central Norway: An orphan of the
Taconic orogeny?
Aaron S. Yoshinobu
Calvin G. Barnes
Department of Geosciences, Texas Tech University, Lubbock, Texas 79409, USA
Øystein Nordgulen
Norwegian Geological Survey, Leiv Eirikssons 39, 7491 Trondheim, Norway
Tore Prestvik
Department of Geology and Mineral Resources Engineering, Norwegian University of Science and Technology,
N-7491 Trondheim, Norway
Mark Fanning
Research School of Earth Sciences, Australian National University, Canberra ACT-0200, Australia
R.B. Pedersen
Geological Institute, University of Bergen, Allegaten 41, N-5007 Bergen, Norway
ABSTRACT
Magmatism, contractional deformation, and extension associated with the exhumation
of high-pressure rocks in the Scandinavian Caledonides are commonly attributed to the
Silurian-Devonian Scandian orogeny, in which eastward thrusting of allochthonous terranes over Baltica was followed by extensional collapse and exhumation. New fieldwork
and U-Pb geochronology coupled with recent pressure-temperature estimates within the
highest thrust sequence of the Caledonian orogen indicate that an earlier phase of westdirected contractional deformation was punctuated by migmatite-producing events and
voluminous magmatism ca. 477–466 Ma and ca. 447 Ma, followed by exhumation in the
Late Ordovician. Al-in-hornblende and GASP thermobarometry indicate that emplacement of a suite of 448–445 Ma plutons caused partial migmatization at pressures of 700–
800 MPa. Subsequent isothermal exhumation to pressures of 400 MPa occurred while the
host rocks were still partially molten. Rates of exhumation may have ranged from 2 to 11
mm·yr21 or greater. These data provide evidence for a previously unrecognized phase of
exhumation in the Caledonides and for aerially extensive west-vergent deformation. Deformation and magmatism associated with these events may be related to Taconic-age
orogenesis near Laurentia, where the highest nappe sequences of the Scandinavian Caledonides probably resided during early Paleozoic time.
Keywords: exhumation, Taconian orogeny, Caledonides, migmatite, SHRIMP U-Pb.
INTRODUCTION
Extension and exhumation in the Caledonian orogen of Norway are generally attributed
to postorogenic collapse of overthickened
crust following Silurian-Devonian collision of
Laurentia and Baltica (e.g., Andersen, 1998;
Braathen et al., 2000). However, the kinematics of earlier episodes of deformation, as recorded in the Middle and Upper Allochthons
(Andréasson, 1994), are generally attributed to
east-directed thrusting and nappe emplacement during the Late Cambrian to Early Ordovician Finnmarkian phase. Deformation
during the Late Silurian to Early Devonian
Scandian phase (e.g., Stephens et al., 1985)
was also east to southeast directed, so that the
tectonic evolution of central Norway is inferred to reflect repeated eastward emplacement of nappes onto Baltica. New U-Pb (zir-
con) ages and structural observations
document Ordovician crust-melting events,
west-vergent contractional deformation, and
Late Ordovician exhumation. These features
have closer affinities with the Taconic orogen
of northeastern North America than with the
Caledonian orogen as exposed in Norway.
GEOLOGIC SETTING
The Caledonides of central Norway consist
of four distinct nappes (Lower, Middle, Upper,
and Uppermost Allochthons) that were emplaced on the Baltoscandian crystalline platform during the Silurian-Devonian Scandian
orogeny along east-directed thrust faults (Stephens et al., 1985). In Norway, the Lower and
Middle Allochthons consist of Neoproterozoic
to Ordovician sedimentary and highly deformed Precambrian crystalline rocks meta-
morphosed under subamphibolite-grade conditions. The Upper Allochthon consists of
low- to high-grade volcanic, igneous, and sedimentary sequences, many clearly representing arc and ophiolite settings. Together, these
three allochthons are interpreted to have originated in part within basins along the Baltoscandian margin of the Iapetus Ocean, although portions of the ophiolitic and arc
terranes are probably exotic (Stephens et al.,
1985). Between 658N and 668N, the Uppermost Allochthon consists of two composite
nappes, the Rödingsfjället nappe complex and
the structurally higher Helgeland nappe complex (Fig. 1A). Qualitative structural restorations (e.g., Stephens et al., 1985; Roberts and
Gee, 1985) place the Uppermost Allochthon
farthest outboard of the margin of Baltica prior to Scandian thrusting. Stephens et al.
q 2002 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.
Geology; October 2002; v. 30; no. 10; p. 883–886; 2 figures; Data Repository item 2002106.
883
Figure 1. A: Simplified tectonic map of Upper and Uppermost Allochthons in central Norway.
B: Generalized geologic map displaying locations of U-Pb analyses and pressure-temperature
determinations. GASP—garnet 1 aluminosilicate 1 silica 1 plagioclase geobarometer;
CORD—reaction biotite 1 sillimanite 5 garnet 1 cordierite 1 melt (Spear et al., 1999; see text
for discussion).
(1985) and Grenne et al. (1999) suggested a
Laurentian affinity for the Uppermost Allochthon. We focus on structural relationships
and magmatism in the Helgeland nappe complex and develop a hypothesis for its precollisional tectonic setting and assembly.
Helgeland Nappe Complex and Bindal
Batholith
The southwestern Helgeland nappe complex can be divided into four nappes juxtaposed along east-dipping, west-vergent shear
zones. From structurally lowest to highest
these are the Sauren-Torghatten, Lower, Middle, and Upper nappes (Fig. 1B; Thorsnes and
Løseth, 1991). Shear zones separating the
Sauren-Torghatten from the Lower nappe, and
the Lower from the Middle nappe, show
down-to-the-east, normal displacement reactivation. The Sauren-Torghatten and Middle
nappes consist of dismembered ophiolites
(e.g., Leka ophiolite, Prestvik, 1980) unconformably overlain by metaconglomerate, pelitic schist, psammite, minor amphibolite, minor carbonate, and felsic volcanic rocks
(Thorsnes and Løseth, 1991). The contact between ultramafic rocks and metasedimentary
rocks dips eastward, indicating that both the
884
Sauren-Torghatten and Middle nappes are upright. The peak prograde metamorphic mineral
assemblage in metapelites in these two nappes
is biotite 1 muscovite 1 staurolite 1 garnet
6 sillimanite (or kyanite; Hjelmeland, 1987),
indicating amphibolite facies conditions. In
contrast, the Lower and Upper nappes consist
of stromatic and diatexitic migmatite, pelitic
schist, quartzo-feldspathic gneiss, and marble.
The entire nappe sequence is intruded by the
447 Ma granodioritic Andalshatten pluton,
which contains stoped blocks (k1 km2) of
rocks identical to those found in the Helgeland
nappe complex (Fig. 1B; Nordgulen et al.,
1993).
The Bindal batholith, including the Anhalshatten pluton, intrudes the Helgeland nappe
complex (Fig. 1; Nordgulen et al., 1993). It
spans an age range of 477–430 Ma, an interval that also encompasses at least one regional
migmatization event (discussed subsequently).
The oldest plutons (ca. 477–468 Ma; this paper) are peraluminous to strongly peraluminous and predominantly granitic; they were
emplaced during a regional migmatization
event that affected the Lower nappe. These
older granites have isotopic compositions consistent with crustal sources. From 448 to 430
Ma, a variety of plutons was emplaced; they
range from granite to gabbro and display
chemical and isotopic evidence for mixed
crust and mantle sources (Nordgulen and
Sundvoll, 1992; Nordgulen et al., 1993; Birkeland et al., 1993).
U-Pb (zircon) ages1 were determined for
five samples from the Velfjord region by thermal ionization mass spectrometry (TIMS)
methods (Table DR1; Figs. DR1–DR5; see
footnote 1) and additional samples were dated
by using sensitive high-resolution ion microprobe (SHRIMP I) at the Australian National
University (Table DR2; Fig. DR6; see footnote 1). Two samples from the Vega pluton
(Fig. 1B) were dated. This pluton carries
abundant metasedimentary inclusions and has
isotopic signatures indicative of a crustal
source (Birkeland et al., 1993). The bestestimate SHRIMP ages (Table DR2; see footnote 1) are 468.9 6 3.2 Ma and 477 6 3 Ma
(SHRIMP ages at 95% confidence interval). A
strongly peraluminous, two-mica granite west
of Velfjord was dated by TIMS methods. This
pluton is cut by the shear zone that juxtaposes
the Sauren-Torghatten and Lower nappes (Fig.
1B; Table DR1; see footnote 1). Two dated
fractions result in a discordia plot (Fig. DR1;
see footnote 1) with a lower intercept of 470.4
6 3.6 Ma and an upper intercept of 1308 6
140 Ma. The lower intercept is taken to represent an igneous age that is similar to the age
of the Vega pluton.
South of Velfjord, three large dioritic plutons yielded nearly concordant TIMS U-Pb
ages of 448–445 Ma (Fig. 1B; Table DR1;
Figs. DR2–DR4; see footnote 1). They intrude
migmatitic gneiss and carbonate rocks of the
Lower nappe (Fig. 1B), but do not intrude the
Middle nappe. Barnes and Prestvik (2000)
demonstrated that heat released from these dioritic plutons resulted in remelting of the host
migmatites to form diatexitic contact migmatites and associated small, peraluminous granitic bodies. Two of these granitic bodies were
dated, one by conventional methods and one
by SHRIMP. The former yielded a 207Pb/206Pb
age of 477 6 9.3 Ma (Fig. DR5; see footnote
1). This age, although consistent with the early peraluminous magmatic event, is not consistent with an origin by remelting proposed
by Barnes et al. (2002). SHRIMP analyses of
zircons from the second body (Fig. DR6; Table DR2; see footnote 1) show a cluster of six
ages, the mean value of which is 447.1 6 3.7
Ma, and a cluster of five ages at 467.8 6 4.9
Ma. The younger age cluster is identical to
1 GSA Data Repository item 2002106, U/Pb
SHRIMP and TIMS data tables, concordia plots, histograms, and Al-in-hornblende data table, is available
on request from Documents Secretary, GSA, P.O.
Box 9140, Boulder, CO 80301-9140, USA, editing@
geosociety.org, or at www.geosociety.org/pubs/
ft2002.htm.
GEOLOGY, October 2002
Figure 2. Major tectonic elements in Ordovician evolution of Helgeland nappe complex and
Uppermost Allochthon. A: Proposed location of continental slivers proximal to Laurentia (L)
and formation of suprasubduction-zone Leka ophiolite above east-dipping subducting slab.
B: Collision of continental fragments, preservation of ophiolitic rocks, and generation of
early peraluminous plutons (e.g., Vega). C: Continued amalgamation of Uppermost Allochthon with Laurentia and loading of lower nappes prior to 448 Ma during Taconic orogenesis.
Loading of nappe pile is shown schematically in pressure-temperature diagram. D: Exhumation of Helgeland nappe complex ca. 447 Ma, followed closely by initiation of west-dipping subduction and eventual closure of Iapetus during Scandian phase of Caledonide orogeny. Schematic pressure-temperature diagram illustrates known conditions for prograde
metamorphism and exhumation based on this study and Barnes and Prestvik (2000); K—
kyanite, S—sillimanite, A—andalusite, B—Baltica. See text for details.
that of the adjacent pluton (448 Ma) and is
consistent with formation of the granitic body
by contact anatexis. We interpret the older
cluster to represent zircon crystallization at the
end of the thermal event responsible for the
older peraluminous granites and interpret the
age range 477–468 Ma as a period of regional
migmatization. According to this interpretation, the 477 Ma age (Fig. DR5; see footnote
1) results from zircon that grew during the
older migmatite event.
Thermobarometric Constraints in the
Helgeland Nappe Complex
Al-in-hornblende pressures of crystallization for the Velfjord plutons range from 700
to 800 MPa based on the Anderson and Smith
(1995) calibrations and are consistent with
pressure-temperature estimates from enclosing
pelitic aureole rocks (Barnes and Prestvik,
2000). Estimates (see footnote 1) from the
crosscutting Andalshatten pluton, which contains the necessary mineral assemblage for Alin-hornblende barometry, yield pressures of
670 MPa (Table DR3; see footnote 1).
Contact migmatite host rocks adjacent to
the Velfjord plutons provide evidence for exhumation from mid-crustal depths while the
leucosomes were still partially molten. Near
the pluton contacts, early kyanite is overprinted by multiple generations of sillimanite. At
near-solidus conditions, cordierite (commonly
included in poikilitic alkali feldspar) was a
stable magmatic phase (Barnes and Prestvik,
2000). This sequence of aluminosilicate stability, combined with the Al-in-hornblende
and GASP thermobarometry, indicates synmagmatic decompression and a clockwise
pressure-temperature path, as shown in Figure
2 (Barnes and Prestvik, 2000). The pressuretemperature path consequently indicates that
the migmatites remained partially molten durGEOLOGY, October 2002
ing exhumation to pressures of 400 MPa. New
Al-in-hornblende pressure estimates from the
Tosbotn pluton (Fig. 1B; Table DR3; see footnote 1) indicate that by 435 Ma, the Helgeland
nappe complex was at pressures of ;400
MPa.
Timing of Deformation and Rates of
Exhumation
The timing of juxtaposition of the nappes
in the Helgeland nappe complex is bracketed
by crosscutting relationships between the
nappes and the plutons within the batholith.
The oldest regionally deformed pluton in the
Velfjord region is the 470 Ma two-mica granite, which is cut by the shear zone between
the Sauren-Torghatten and Lower nappes (Fig.
1B). Therefore, contraction associated with
northwest-vergent imbrication and recorded as
the shear zone that cuts the two-mica granite
and extension, recorded as exhumation of the
contact migmatites, were complete by the time
the crosscutting Andalshatten pluton solidified
at 447 Ma. Exhumation of the Velfjord plutons from 700 to 400 MPa most likely occurred at 448–447 Ma, because the contact
migmatites were still partially molten.
Rates of exhumation can be evaluated if the
temperature interval over which the contact
granites and plutons crystallized and the duration of crystallization can be defined. Locally abundant retrograde muscovite in the
contact granites and migmatitic leucosomes
suggests final crystallization at an H2Osaturated solidus. At ;700 MPa, solidus temperatures are in the 640–680 8C range (e.g.,
Johannes and Holtz, 1996). Maximum temperatures for the contact granites were ;800
8C, on the basis of phase relationships and zircon saturation temperatures (Barnes and Prestvik, 2000). Because melting and formation of
the contact granites and related leucosomes
occurred at 700 MPa between 447 6 2 Ma
and 448 6 2 Ma and crystallization occurred
at 400 MPa in the same time interval, the uncertainties in the U-Pb age data can be used
to bracket the duration of cooling through the
solidus. A maximum duration for crystallization of 5 m.y. yields a time-averaged exhumation rate of 2.2 mm·yr21. Given the overlap
in uncertainties of the U-Pb ages, a minimum
duration of 106 yr (or less) yields a timeaveraged rate of 11 mm·yr21. If durations of
cooling from ;800 8C to ;660 8C are much
less than 106 yr, then the exhumation rates
were even faster.
DISCUSSION
Crosscutting relationships and U-Pb zircon
ages indicate that juxtaposition of nappes
within the Helgeland nappe complex, pluton
emplacement, and exhumation must have followed closely in time during the Late Ordovician. This structural and magmatic evolution
cannot be due to an early phase of the Scandian orogeny for the following reasons. (1)
Structural vergence within the Helgeland
nappe complex is predominantly top to the
west (Fig. 1B); (2) Scandian, extension-related
exhumation is Silurian or younger in age (Andersen, 1998; Braathen et al., 2000); and (3)
existing lithologic correlations suggest that
rocks of the Uppermost Allochthon are exotic
with respect to the paleo–Baltican margin
(Roberts et al., 2001). Instead we suggest that
the Ordovician tectonic evolution of the Helgeland nappe complex is more consistent with
Taconic-style deformation as documented in
the northeastern United States and Canada
(Stanley and Ratcliffe, 1985).
SHRIMP data indicate at least two separate
Ordovician zircon-crystallizing events. The
first occurred in the ca. 477–468 Ma interval
and culminated in generation of widespread
migmatites and emplacement of the voluminous, peraluminous, crust-derived granites
(e.g., Vega pluton; Fig. 1). The second, which
began at 448–447 Ma, involved emplacement
of isotopically complex plutons with mixed
crust and mantle components, such as the Velfjord and Andalshatten plutons, and associated
contact migmatization. We postulate that migmatization and emplacement of the peraluminous, older part of the Bindal batholith occurred during collision and amalgamation of
continental fragments outboard of Laurentia
above an east-dipping (present geography)
subducting slab (Fig. 2). West-vergent structural asymmetry (this paper; Roberts et al.,
2001), probable continental-shelf assemblages, faunal correlations (Robinson et al., 2001),
and timing relationships are consistent with a
paleogeographic setting near Laurentia in the
Late Ordovician. Amalgamation of the Helgeland nappe complex with probable Lauren885
tian lithosphere occurred between ca. 468 and
448 Ma, prior to emplacement of the ca. 448–
445 Ma Velfjord plutons (Fig. 2C). Nevertheless, synmagmatic folds in the plutons and
their aureoles, and axial-planar leucosomes in
447 Ma contact migmatites, indicate that contraction was also synchronous with solidification of the plutons (Barnes et al., 2002).
Following collision, subduction polarity
switched but magmatism continued above a
west-dipping subduction zone (Fig. 2D). Extension and exhumation (Fig. 2D) were contemporaneous with, or followed closely after,
contraction and pluton construction at 447
Ma, because contractional and extensional
shear zones appear to be cut by the 447 Ma
Andalshatten pluton.
Although disagreement exists over the timing and style of Taconic deformation and magmatism (e.g., Karabinos, 2001), the lithology,
structural style, and ages of magmatism in the
Helgeland nappe complex are similar to those
in Newfoundland (e.g., Notre Dame arc,
Whalen et al., 1997; Dashwoods block, Waldron and van Staal, 2001) and in the New England Appalachians (e.g., Bronson Hill arc,
Robinson et al., 2001). However, magmatism
in the time-equivalent North American Taconic arcs noted here is generally metaluminous,
intermediate in composition, and has isotopic
signatures indicative of a mantle component
(e.g., Whalen et al., 1997), in contrast to the
diverse suite of 448 Ma and younger plutons
of the Bindal batholith.
Our working hypothesis is that the Helgeland nappe complex represents orphaned allochthonous, along-strike equivalents of these
peri-Laurentian arc sequences (e.g., van Staal
et al., 1998) that were stranded on Baltica during final breakup of Pangea. The geologic
evolution of the Helgeland nappe complex, including west-directed thrusting and crustally
derived magmatism followed by or synchronous with exhumation, may be characteristic
of the northern portion of the Taconic orogen.
Future geochronologic and field and petrologic studies in East Greenland should provide
further insight into the nature of the northern
extent of the Taconic orogen.
CONCLUSIONS
1. The nappes of the Helgeland nappe complex were imbricated along east-dipping,
west-vergent shear zones after 477 Ma and
prior to 447 Ma.
2. SHRIMP U-Pb data indicate at least two
zircon crystallizing events centered around
477–468 Ma and 448–445 Ma; both events
produced voluminous crustal magmas.
3. Construction of large, 448–445 Ma Bindal
plutons at ;800–700 MPa was coincident with
formation of proximal (contact) migmatites.
4. Exhumation of the Bindal plutons and
886
migmatites while melt was present occurred
ca. 448–445 Ma.
5. Estimates of the rates of exhumation
range from 2 to 11 mm·yr21; the actual rates
may have been much faster. Together, these
data represent new evidence for Ordovician
orogenic events in the Uppermost Allochthon
and establish further links between Taconic
orogenesis in North America and the tectonic
evolution of exotic rocks in the central Norwegian Caledonides.
ACKNOWLEDGMENTS
We thank M. Barnes, G. Dumond, K. Reid, P.
Beebe, K. Beebe, and T. Foster for field assistance
and Petra Aune and Håkon Aune for delightful hospitality in Velfjord. C. van Staal, D. Roberts, and B.
van der Pluijm provided constructive and thorough
reviews that are greatly appreciated. Research was
supported by National Science Foundation grant
EAR-9814280 and the Texas Tech University Research Enhancement Fund. Logistical support from
the Norwegian Geological Survey is gratefully
acknowledged.
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Manuscript accepted June 19, 2002
Printed in USA
GEOLOGY, October 2002