NORWEGIAN JOURNAL OF GEOLOGY
Ordovician and Silurian magmatism in the Upper Nappe, Uppermost Allochthon, Helgeland 121
Ordovician and Silurian magmatism in the Upper Nappe,
Uppermost Allochthon, Helgeland Nappe Complex,
north-central Norway
Calvin G. Barnes, Kristin Reid, Carol D. Frost, Melanie A. Barnes,
Charlotte M. Allen & Aaron S. Yoshinobu
Barnes, C.G., Reid, K., Frost, C.D., Barnes, M.A., Allen, C.M. & Yoshinobu, A.S.: Ordovician and Silurian magmatism in the Upper Nappe,
Uppermost Allochthon, Helgeland Nappe Complex, north-central Norway. Norwegian Journal of Geology, vol. 91, pp 121-136. Trondheim 2011.
ISSN 029-196X.
The structurally lowest parts of the Upper Nappe of the Helgeland Nappe Complex consist primarily of migmatitic quartzofeldspathic gneiss with
intercalated calc-silicate gneiss. Detrital zircons in the gneiss indicate an Ordovician depositional age. Migmatization via biotite dehydration reactions occurred at ~480 Ma at pressures of ~700 MPa; some migmatites were mobilized into cross-cutting diatexites. Diverse gabbroic to granitic
plutons and dikes of the Bindal Batholith were emplaced from 448–424 Ma. They range from calc-alkaline to alkalic and most have initial 87Sr/86Sr
<0.707 and εNd from 0 to -4. Pressures of emplacement were ca. 300 MPa. Shear zones with reverse shear sense cut 437 Ma intrusions, suggesting
that contraction accompanied magmatism.
Calvin G. Barnes, Department of Geosciences, Texas Tech University, Lubbock, Texas, 79409-1053, USA. E-mail: cal.barnes@ttu.edu. Kristin Reid
Department of Geosciences, Texas Tech University, Lubbock, Texas, 79409-1053, USA. Carol D. Frost, Department of Geology and Geophysics, University
of Wyoming, 1000 East University Avenue, Laramie, Wyoming, 82071, USA. E-mail: frost@uwyo.edu. Melanie A. Barnes, Department of Geosciences,
Texas Tech University, Lubbock, Texas, 79409-1053, USA. E-mail: melanie.barnes@ttu.edu. Charlotte M. Allen, Research School of Earth Sciences, The
Australian National University, Canberra, ACT 0200, Australia. E-mail: Charlotte.Allen@anu.edu.au. Aaron S. Yoshinobu, Department of Geosciences,
Texas Tech University, Lubbock, Texas, 79409-1053, USA. E-mail: aaron.yoshinobu@ttu.edu.
Introduction
The 478–424 Ma Bindal Batholith in north-central Norway is an excellent example of a long-lived orogenic magmatic province in which a complex combination of mantle melting, crustal melting, magma mixing, and assimilation contributed to magma compositions (Nordgulen
and Sundvoll, 1992; Nordgulen, 1993; Nordgulen et al.,
1993; Barnes et al., 1992; Birkeland et al., 1993; Barnes
et al., 2007). Because the Batholith intrudes a number of
nappe sheets in the Helgeland Nappe Complex (HNC),
an understanding of the timing of regional migmatization and its relationship to generation and coalescence of
plutonic magmas is crucial to reconstruction of the tectonic setting and thermal history of the region. Regional
migmatization and crustal melting affected parts of the
HNC from ~480 to 475 Ma (Yoshinobu et al., 2002;
Barnes et al., 2007) and granites emplaced during this
time are peraluminous to strongly peraluminous with
high initial 87Sr/86Sr values, features taken to indicate predominantly crustal origins (Nordgulen and Sundvoll,
1992; Nordgulen, 1993; Birkeland et al., 1993). Younger
magmatic activity in the Batholith (465 to 424 Ma; Nord­
gulen et al., 1993; Yoshinobu et al., 2002; Nissen et al.,
2006; Barnes et al., 2007; this paper) was much more
diverse and included crust- and mantle-derived magmas
in various forms of hybridization (Birkeland et al., 1993;
Barnes et al., 1992, 2002, 2005).
Early mapping in the HNC (Kollung and Myrland, 1970;
Myrland, 1974) recognized the presence of a number of
migmatitic rock units in central Helgeland and a later
study in the Velfjord–Tosen area (Thorsnes and Løseth­,
1991) showed that these migmatitic units belonged to
discrete nappes that were separated from one another by
non-migmatitic (lower grade) nappes. It was not clear on
the basis of this work whether regional migmatization
in different nappes was the result of a single early Ordo­
vician metamorphic event, or whether it represented
temporally and/or spatially distinct thermal events.
This paper reports a U–Pb geochronologic, thermo­
barometric, and petrologic study of a long, nearly
continu­
ous road cut near Tosenfjord that exposes
migma­titic rocks of a structurally low part of the Upper
Nappe of the HNC. The migmatites underwent partial
melting at ~480 Ma and then from 448 to 424 Ma were
repeatedly intruded by mafic to granitic dikes. Local
122 C. G. Barnes et al.
B
Explanation
Devonian - Permian basin fill
Uppermost Allochthon
Upper Allochthon
Middle Allochthon
Lower Allochthon
Parauthochthonous rocks
Proterozoic
Baltic cratonal
rocks
Sauren-Torghatten Nappe
onid
Horta Nappe
(carbonate/gneiss/migmatite)
Undifferentiated
metamorphic rocks
Normal-sense shear zone
hon
Fig. 1C
pe
rA
llo
ch t
Central Norway
Basement
Window
Up
12°00'
65°00'
200 km
14°00'
65°30'
st A
l
Lower Nappe
(pelitic schist &
migmatite/marble)
weg
ian
Trondheim
loch
Middle Nappe
~442 Ma
Andalshatten
pluton
thon
Upper Nappe
Nor
Western Gneiss
Region
Ocean
Ophiolitic rocks
Cale
d
Atlantic Ocean
Rødingsfjallet
Nappe Complex
(Uppermost
Allochthon)
<440 Ma Granitoid rocks
~465-440 Ma Granitoid rocks
~465-440 Ma Diorite & gabbro
> 465 Ma Igneous rocks
Atlantic
es
Fig. 1B
Explanation - Helgeland Nappe
Complex & Bindal Batholith
Up
p er
mo
A
NORWEGIAN JOURNAL OF GEOLOGY
14°00'
65°00'
25 km
Vikna
D
C
Velfjord
447±3
Ma
Størborja
G.A.S.P.
~ 500-700 MPa, ~800°C
2
Stromatic migmatite
TF-304, melanosome
TF-204, leucosome
Diatexitic migmatite
TF-104
Quartz-diorite pluton
TF-404
Net-veined monzonitic dike
N16.05
Biotite monzogranite
TF-82
2
447±3.7
Ma
480.3±1.9 Ma
480.5, 479.4, 465.2 Ma
480.6±3.8 Ma
447.8—1.7 Ma
436.7±3.5 Ma
~ 424-470 Ma
420 MPa2
430±7 Ma1
2
445 Ma
Tosbotn tunnel
65
N
64
64
76
Lande
G.A.S.P.
Poles to foliations, n = 17,
average = 343/60
Lineations, n = 3
~ 500-700 MPa, ~800°C
To
s
en
fjo
rd
74
Diatexitic migmatite
TF-77B
445.1±6.4 Ma
Gabbro
TF-70
436.9±4.4 Ma
Tonalite dike
TF-82
431.9±3.5 Ma
Biotite monzogranite
TF-82
424.7±5.6 Ma
10 km
Figure 1. A. Tectonic map displaying allochthonous units within the Scandinavian Caledonides. B. Regional geologic map
of the Helgeland Nappe Complex and Bindal Batholith, located within the Uppermost Allochthon. C. Simplified geologic
map of the field area; legend as in B. Dike complex and migmatitic rocks are shown in grey and exposed along route 76 on
the northwest shore of Tosenfjord. Regional structural trends are depicted by foliations with filled triangles. SHRIMP U-Pb
zircon ages are shown in yellow; LA-ICP-MS U-Pb zircon ages are shown in white. Previously published age and depth
measurements are from: 1 = Nordgulen et al., 1993; 2 = Yoshinobu et al., 2002. D. Lower-hemisphere stereonet showing
poles to foliations and lineations from road side outcrop.
NORWEGIAN JOURNAL OF GEOLOGY
Ordovician and Silurian magmatism in the Upper Nappe, Uppermost Allochthon, Helgeland 123
deformation and development of metamorphic mineral
assemblages in these dikes hint at a more or less continuous thermal pulse from ~437–424 Ma. Apparently, this
exposure provides, in a km-long section, a significant
portion of the Caledonian thermal history of the Upper
Nappe and an important location by which to compare
other parts of the Upper Nappe and the rest of the HNC.
Geological setting
Regional setting
The Helgeland Nappe Complex is the structurally highest nappe complex of the Uppermost Allochthon (Fig.
1A; Roberts et al., 2007). In the Tosen–Velfjord–Brønnøy region, the HNC consists of four named nappes
(Thorsnes and Løseth, 1991; Yoshinobu et al., 2002)
and at least one additional nappe (Barnes et al., 2007).
From structurally lowest to highest, these nappes are
the informally named Horta nappe that is host to the
Hortavær intrusion (Barnes et al., 2007), the Sauren-­
Torghatten, Lower, Middle, and Upper Nappes (Fig.
1A, B). Basal slivers of metaperidotite (serpentinite)
and gabbro character­
ize the Sauren-Torghatten and
Middle Nappes; these slivers are unconformably overlain by cover sequences of greenschist to amphibolite facies metasedimentary and metavolcanic rocks.
The cover sequences include conglomerates and clastic metasediment­
ary rocks whose provenance ranges
from volcanic to continental­shelf (Thorsnes and Løseth, 1991; Frost et al., 2006), sug­gestive of rapidly changing sediment sources. Moreover, the Sauren-Torghatten
and Middle Nappes are evidently not overturned, as indicated by the east-­dipping contact­between the ultramafic
and meta­sedimentary rocks which was interpreted to be
an unconformable depositional contact by Thorsnes and
Løseth (1991) and Yoshinobu et al. (2002).
In contrast, the Lower and Upper Nappes contain a large
proportion of high-grade semi-pelitic schist and gneiss,
quartzofeldspathic gneiss, and marble (Gustavson, 1981;
Thorsnes, 1987; Thorsnes and Løseth, 1991; Nord­gulen
and Sundvoll, 1992). Most of the quartzofeldspathic
gneissic rocks are migmatitic, with migmatite fabrics that
range from stromatic (layered) and nebulitic (pod-like
leucosomes) to diatexitic. Diatexitic migmatites are those
in which original structure has been disrupted, generally
by magmatic flow. In the Lower Nappe, which is domi­
nated by quartzofeldspathic gneiss and marble, with
lesser calc-silicate gneiss, two migmatization events have
been recognized. The older event (>477 Ma; Yoshinobu
et al., 2002) was of regional extent and produced mildly
to strongly peraluminous alkali-calcic granites (Barnes
et al., 2002). The younger event was restricted to contact
aureoles around mafic to intermediate plutons emplaced
from 448 to 445 Ma (Barnes et al., 2002; Yoshinobu et al.,
2002). This so-called contact migmatization began when
the Lower Nappe was at ~700 MPa pressure, but ended
after exhumation to ~400 MPa (Barnes and Prestvik,
2000).
The focus of this study is on basal parts of the Upper
Nappe (Fig. 1C; Thorsnes and Løseth, 1991). The Middle­
Nappe–Upper Nappe contact is ~1.6 km west of our
study area, and is marked by the abrupt change structur­
ally upward from layered, amphibolite facies, non-­
migmatitic schists of the Middle Nappe to intercalated
migmatitic and calc-silicate gneiss of the Upper Nappe.
The latter rock types are characteristic of the Upper
Nappe along Tosenfjord from Lande to Tosbotn (Fig.
1C), with lesser amounts of quartzofeldspathic gneiss
and marble. Thus, the Upper Nappe contains rock types
nearly identical to those of the Lower nappe, although in
different proportions.
The nappes are juxtaposed along east-dipping faults that
were originally interpreted as thrusts (Thorsnes and
Løseth­, 1991). Although initial reverse motion is likely
on these faults, Yoshinobu et al. (2002) showed that final
motion on the fault separating the Lower and Middle
Nappes was in a normal sense. The Sauren-Torghatten,
Lower, Middle, and Upper nappes were intruded by the
Andalshatten pluton (Fig. 1B; Nordgulen et al., 1992).
High-precision TIMS dating of the Andalshatten pluton­
to 442 Ma (Anderson et al., 2008) places a minimum
age on terrane amalgamation. A recent study of zircon
inherit­ance in Lower Nappe plutons (Barnes et al., 2007)
indicates that terrane amalgamation was even earlier, and
was complete by ~478 Ma.
Plutonic rocks of the Bindal Batholith span an age range
of 478–424 Ma. The oldest plutons are primarily per­
aluminous to strongly peraluminous granites, have Sr,
Nd, and Pb isotopic features consistent with origins by
crustal melting (Nordgulen and Sundvoll, 1992; Birkeland et al., 1993; Marko et al., 2007), and locally conant enclaves of metasedimentary rocks
tain abund­
(Nord­gulen, 1993; Marko et al., 2008). These plutons
are referred to as “anatectic granites” (e.g., Nordgulen,
1993). Magmatism continued with emplacement of the
~466 and ~465 Ma Horta­vær and Svarthopen intrusions,
respectively (Barnes et al., 2007). Then at about 450 Ma,
a prolonged pulse of magmatism began with emplacement of the Troholmen pluton near Rødøy (Barnes et
al., 2007), followed by the Velfjord plutons (Yoshinobu
et al., 2002), the Andalshatten pluton (~442 Ma; Anderson et al., 2008), and then a broadly distributed, diverse
range of plutons from c. 440 to 424 Ma. The 450–424 Ma
magmatism encompasses alkali-calcic to calc-alkalic and
gabbroic to granitic magmas (Nordgulen and Schouenborg, 1990; Nordgulen et al., 1993; Nissen et al., 2006;
Barnes et al., 2007; this paper). The isotopic compositions of these rocks are quite variable and are broadly
consistent with mixed crustal and mantle sources (Nordgulen and Sundvoll, 1992; Birkeland et al., 1993).
124 C. G. Barnes et al.
NORWEGIAN JOURNAL OF GEOLOGY
A
A’
net-veined meta-diorite
mingled (”net veined”)
monzonite/diorite
west
N16.05
435.1 ± 2.8 Ma
stromatic
migmatite
mingled (”net veined”)
TF-204
diorite
TF-304
480.2 ± 3.6 Ma
diatexitic
migmatite
TF-77B: 443.0 ± 4.9 Ma
TF-104: 480.6 ± 3.8 Ma
A’
A’’
stromatic migmatite
mingled
dike
TF-75
N17.05
biotite leucogranite
TF-81
424.7 ± 5.6 Ma
Figure 2.
TF-82
431.9 ± 3.5 Ma
biotite tonalite
with stromatic migmatite
enclaves
stromatic
migmatite
east
Figure 2. Composite photograph of the northern side of the road cut with a sketch showing the various rock units and their
crosscutting relationships. Dated units are labeled and the specific locations of dated samples are shown, where known.
Shear zones are shown with red lines.
Table 1: Table 1. Summary of rock units and ages.
Rock unit
Rock types
Age (Ma)*
migmatitic gneiss
stromatic (banded) gneiss with tonalitic leucosomes
480.3 ± 1.9
calc-silicate gneiss
blocks in migmatite: diopside, garnet, plagioclase, calcite
diatexitic (disrupted) migmatite
massive–weakly banded garnet biotite tonalite migmatite
gneissic metamorphic rocks
pegmatitic leucogranite dike
eastern pluton
480.6 ± 3.8
-----
augite biotite hornblende qtz diorite/qtz monzodiorite
447.8 ± 1.7
gabbroic blocks in leucogranite
hornblende pyroxene gabbro
436.9 ± 4.4
net-veined monzonitic dikes
monzonite (enclaves) cut by leucomonzonite veins
436.7 ± 3.5
homogeneous & mingled dikes
homogeneous monzodiorite
-----
composite tonalite (host)--qtz diorite (enclaves)
-----
composite melagranite (host)--qtz diorite (enclaves)
-----
gabbroic to granitic dikes
granitic & tonalitic dikes
muscovite-biotite granodiorite/tonalite
biotite monzogranite
*Ages reported for migmatites are interpreted to date partial melting. All other ages interpreted to date magma emplacement.
431.9 ± 3.5
~424
NORWEGIAN JOURNAL OF GEOLOGY
Ordovician and Silurian magmatism in the Upper Nappe, Uppermost Allochthon, Helgeland 125
Local setting
The study area is located along highway 76 west of
Tosenfjord and 1.4 km northeast of Lande (Fig. 1C). The
main focus of research is a ~500-meter-long set of double­
road cuts into high-grade metamorphic rocks crosscut by mafic, felsic, and composite dikes. The western
end of the road cut has the following UTM coordinates:
zone 33, 397370 easting, 7239419 northing (WGS84). The
sequence of crosscutting relationships involves three distinct metamorphic units and a number of cross-­cutting
intrusive units diagrammatically illustrated in Figure
2 and summarized in Table 1. Photographs in Figure 3
show some of these relationships in greater detail and
additional detail is presented in Appendix 1. The various rock units are described below in order of decreasing
age, as determined by U–Pb (zircon) dating and inferred
from crosscutting relationships.
Migmatitic and calc-silicate gneiss. The oldest rocks in
the road cut exposures are isoclinally folded semi-pelitic
gneiss, migmatitic gneiss, and associated calc-silicate
gneiss. These migmatites are banded (stromatic), with
distinct peraluminous, tonalitic leucosomes and biotite­rich, garnet- and sillimanite-bearing melanosomes (Fig.
2; Appendix 1). A sample of the stromatic migmatite was
collected for geochronology and was split into a leucosome fraction (TF-204) and a melanosome fraction (TF304). Migmatitic rocks exposed from the base of the
Upper Nappe to Tosbotn are broadly similar to those in
the road cut, but in some cases also contain staurolite
and cordierite. Calc-silicate rocks are intercalated with
the migmatites and are present as boudins or schollen in
stromatic migmatite. They typically consist of variable
proportions of diopside, garnet, plagioclase, and calcite.
In the central part of the road cut, stromatic migmatite
is crosscut by medium-grained, foliated diatexitic migmatite (Figs. 2, 3A). The diatexite is peraluminous, tonalitic to quartz dioritic in composition and contains biotite,
garnet, sillimanite, and muscovite. Garnet is similar in
composition to that in the stromatic migmatite (Appendix 1). The diatexite encloses blocks (m- to dm-scale) of
stromatic migmatite and calc-silicate gneiss (Fig. 2) and
is therefore interpreted to be intrusive. Two samples for
U–Pb dating were collected (TF-104 and TF77B; Fig. 2).
Figure 3. Outcrop photographs. A. Stromatic migmatite
intruded by diatexite. Hammer handle is 38 cm long. B.
Blocks of foliated, banded diorite (e.g., TF-70) intruded
by leucogranite. Pen is 13 cm long. C. Net-veined diorite–
monzonite intrusion cutting stromatic migmatite, showing
the location of dated sample N16.05. Pen is 13 cm long. D.
Circa 435 Ma mingled dike. Hammer is 59 cm long.
diatexite
A
stromatic
migmatite
B
C
D
sample N16.05
Figu
126 C. G. Barnes et al.
Structural development within the migmatitic and calcsilicate gneiss is dominated by a moderate to steeply eastdipping, axial planar metamorphic foliation and outcrop-scale isoclinal folds. Gneissic banding and foliations
are penetrative within the migmatitic rocks (Fig. 3). Discreet, cm- to m-scale shear bands occur throughout the
road cut. In many cases, these shear bands are localized
on leucosomes, indicating melt-present deformation.
Three outcrop-scale shear bands approximately 3–10 cm
thick are shown on Figure 2 and exhibit top-to-the west
sense of displacement along east-dipping planes. However, asymmetric kinematic indicators were not widely
observed within the outcrop; a few normal-sense (downto-the east), cm-scale shear bands were observed. A limited number of lineations defined by recrystallized aggregates of quartz, feldspar, and biotite plunge moderately
to the east-northeast along northwest-trending foliation
planes (Fig. 1D), consistent with west-directed thrusting
(e.g., Thorsnes and Løseth, 1991; Yoshinobu et al., 2002).
Pegmatitic leucogranite dikes. Near the western end of
the road cut, white, pegmatitic leucogranitic dikes crop
out. Grain size increases from medium-grained cores to
very coarse-grained margins and the proportion of alkali
feldspar increases toward the margins. Biotite reaches 4
cm in length and is oriented perpendicular to dike margins (Fig. A1). These dikes are interpreted to be accumulations of leucosome magma from adjacent migmatites,
but have not been dated.
Pluton east of main road cut. Approximately 100 m east of
the section studied in detail, a small, medium- to coarsegrained pluton of augite biotite hornblende quartz diorite/quartz monzodiorite intrudes the stromatic migmatite. It contains rounded mafic magmatic enclaves and
xenoliths of the host rocks. A sample of this pluton (TF404) was collected for U–Pb dating.
Gabbroic to granitic dikes. The remaining intrusive rocks
in the road cut range from gabbro to granite. They display an array of grain size, intrusive shapes (Fig. 2), mineral assemblages, chemical composition, deformation,
and degree of metamorphic fabric development (see
Appendix 1 for detailed descriptions and photographs).
Many of the dikes are mingled and some show evidence
of physical and chemical hybridization. U–Pb dating
(see below) and field relationships show that they were
emplaced from 437 to 424 Ma, and most were emplaced
between 437 and 432 Ma.
Green, fine-to coarse-grained mafic rocks crop out near
the west-central part of the road cut where they form
blocks and boudins in white leucogranite (Fig. 3B).
The blocks range from coarse-grained and massive to
medium-grained, foliated, and banded (Fig. 3B); they
contain diopsidic pyroxene, poikilitic hornblende, biotite, and plagioclase (Appendix 1). In the field, these
mafic blocks were interpreted to represent mafic intrusions emplaced prior to migmatization; however U-Pb
NORWEGIAN JOURNAL OF GEOLOGY
dating of a sample of banded diorite (TF-70) indicates
that this is not the case (see below), at least for some of
the mafic rocks.
Fine- to medium-grained net-veined monzonitic dikes
cut diatexite and stromatic migmatite (Fig. 3C). The
dikes consist primarily of biotite hornblende augite monzonite, which occurs as enclaves surrounded by thin
veins of hornblende ± augite monzonite/quartz monzonite. The enclave-dike margins are commonly interdigitated. Where the net-veined dikes are in contact with
migmatite, 8–10 cm-wide zones of “hybrid” quartzbearing hornblende monzonite are present. A sample of enclave material from the largest net-veined dike
(N16.05) was collected for U–Pb dating (Figs. 2, 3C).
One of the net-veined dikes is deformed by a throughgoing, top-to-the-west shear zone (Fig. 2).
Homogeneous and mingled dikes. The net-veined dikes
are crosscut by dikes of fine-grained, dark gray, homogeneous hornblende monzodiorite and by fine- to mediumgrained, pillowed, composite dikes. There are two types
of mingled dike: pale gray, biotite tonalite with enclaves
of dark gray, hornblende biotite quartz diorite, and pale
gray, porphyritic, biotite quartz diorite with enclaves of
hornblende biotite melagranite (Fig. 3D). Deformation
of these dikes is variable, from undeformed to strongly
deformed and mylonitic.
Granitic and tonalitic dikes. The stromatic migmatite,
diatexite, and homogeneous and net-veined monzonitic dikes are crosscut by dikes of white, fine- to coarsegrained, biotite monzogranite and pale gray muscovitebearing biotite granodiorite/tonalite. The largest example of the granodiorite/tonalite is exposed near the eastern end of the road cut and reaches at least 15 meters in
width (Fig. 2); it was sampled for U–Pb dating (sample
TF-82). The granodiorite/tonalite is cut by a pale gray,
medium-grained biotite monzogranitic dike in a splayed
array with both curved and angular contacts (Fig. 2).
Two samples of this dike were dated (TF-81 and N17.05).
Geochemistry
Analytical methods.
Analytical methods are presented in Appendix 2. In
brief, mineral compositions were determined by electron microprobe at the University of Wyoming, major
element rock compositions were measured by inductively-coupled plasma optical emission spectrometry at
Texas Tech University, rare earth and other trace element
concentrations were determined by inductively-­coupled
plasma mass spectrometry (ICP-MS) at Washington
State University, and isotope ratios by isotope dilution
thermal ionization mass spectrometry at the University of Wyoming. U–Pb ages of zircon were determined
by Sensitive High Resolution Ion Micro Probe-Reverse
NORWEGIAN JOURNAL OF GEOLOGY
Ordovician and Silurian magmatism in the Upper Nappe, Uppermost Allochthon, Helgeland 127
Geometry (SHRIMP-RG; Stanford University) and laser
ablation (LA) ICP-MS at the Research School of Earth
Sciences, Australia National University. The geochronologic data are given in Appendix Tables 1 (LA-ICP-MS)
and 2 (SHRIMP-RG).
probability plot at ~420 and ~460 Ma. These zircons have
a relatively large range of U contents (Appendix Table 1),
but there is no correlation between age and abundance
of U. Using zircons in the 420–460 Ma range, an age of
436.7 ± 3.5 Ma is obtained (MSWD = 1.20).
U–Pb dating
Tonalitic dike. SHRIMP-RG analysis of sixteen zircons
from the biotite tonalite (TF-82) yielded dates from 425
to 448 Ma (Fig. 4H). This population yielded an average
age of 431.9 ± 3.5 Ma.
Stromatic migmatite melanosome. Zircons from sample
TF-304 were dated by LA-ICP-MS. Two crystals gave
concordant ages at 717 and 773 Ma (Appendix Table 1).
The remaining concordant zircon ages range from 467
to 498 Ma (Fig. 4A). An average of all ages in this range
yields 480.2 ± 3.6 Ma (MSWD = 2.43). Exclusion of the
three youngest and four oldest zircons results in a statistically identical age of 480.3 ± 1.9 Ma (MSWD = 0.73;
Fig. 4A).
Stromatic migmatite leucosome. The leucosome fraction
(sample TF-204) yielded few zircons. Two zircons yielded
concordant ages of 480.5 and 479.4 Ma with a third at
465.2 Ma. Five older concordant ages were obtained at
974, 1014, 1060, 1407, and 1735 Ma.
Diatexitic migmatite. Two samples of diatexitic migmatite were dated, one (TF-104) by LA-ICP-MS (Appendix Table 1) and the other (TF-77B) by SHRIMP-RG
(Appendix Table 2). Sample TF-104 yielded 13 concordant ages in the range 472–492 Ma (Fig. 4B) and others at 649, 796, 1021, 1401, and 1412 Ma. Within the age
range 470–495 Ma, an average age of 480.6 ± 3.8 Ma was
obtained. SHRIMP-RG data for sample TF-77B resulted
in five approximately concordant detrital zircon ages of
602, 772, 1280, 1348, and 1565 Ma, along with two discordant ages of 463, and 473 Ma and a group of younger
zircons whose average age is 445.1 ± 6.4 Ma (Fig. 4C &
D). We call attention to the discrepancy in these ages
compared to sample TF-104 at the end of this section.
Pluton east of main road cut. The quartz dioritic intrusion
sampled east of the main study area (TF-404) yielded
20 concordant dates by LA-ICP-MS (Fig. 4E) that average 447.3 ± 2.8 Ma. If the five youngest and four oldest
dates are excluded, an average of 447.8 ± 1.7 Ma (MSWD
= 0.27) is obtained. No inherited zircons were observed.
Banded gabbro. SHRIMP-RG dating of the banded gabbro sample TF-70 yielded a range of concordant ages
from 420 to 458 Ma (Fig. 4F). With the possible exception of the single date at ~458 Ma, no distinct break in
the zircon age data is apparent, therefore all of the data
were used to obtain an age of 436.9 ± 4.4 Ma. As with
quartz diorite sample TF-404, no inherited zircons were
observed.
Net-veined monzonitic dike. Enclave material from a
net-veined dike that cuts diatexite (N16.05) yielded zircons with LA-ICP-MS dates that span a range from ~415
to 479 Ma (Fig. 4G), with changes in the slope of the
Late biotite monzogranite. Laser ablation ICP-MS analysis of zircons from sample N17.05, the biotite monzogranite that cuts the biotite tonalite dike (TF-82), showed
a large variation in U content, from < 130 to >26,000
ppm. Moreover, within the normal limits used to define
concordance, many zircons were classified as discordant. Figure 4I shows the age spectrum obtained when
slightly less stringent limits were placed on concordancy.
In this case, two zircons have ages at 424.0 ± 1.2 Ma, two
at ~444 Ma, and a cluster of 8 analyses between 458 and
470 Ma. There is no correlation between age and U content. Six zircons from a second sample of this body (TF81) were analyzed by SHRIMP-RG. The average age for
all six analyses is 425.2 ± 9.7 Ma and if two extreme samples (441 and 404 Ma) are rejected, the remaining four
crystals yield an age of 424.7 ± 5.6 Ma. We interpret the
SHRIMP and LA-ICP-MS data to indicate an age of ~424
Ma for this monzogranite, with inheritance at ~444 Ma
as well as older ages. The small number of circa 424 Ma
zircons is consistent with the monzogranite having crystallized from a low-T magma in which most zirconium
in the sample was sequestered in inherited zircon. This
conclusion is supported by zircon-saturation temperatures (Watson and Harrison, 1983) of 654–691 °C.
Summary of U–Pb ages. We interpret the LA-ICP-MS
ages determined for samples of the stromatic and diatexitic migmatites to indicate that crustal melting occurred
in this part of the Upper Nappe at ~480–481 Ma. The
discrepancy between the ca. 480 Ma age determined for
diatexite sample TF-104 by LA-ICP-MS and the ~445 Ma
age determined for TF-77B by SHRIMP is problematic.
Some of the most intricate and difficult intrusive relationships exposed in this outcrop are leucocratic veins
that appear to emanate from stromatic or diatexitic migmatite and then mingle with or cut various dike rocks.
In the field, these relationships were interpreted to indicate that the migmatites were partly molten at the time
of later dike emplacement. This observation was particularly important with regard to the net-veined monzodiorite that intrudes the diatexite (e.g., N16.05; Fig.
3C), because some rocks at the margin of this intrusion
were interpreted to be hybrids of the net-veined dike and
migmatite. However, the 436.7 ± 3.5 Ma age of sample
N16.05 is interpreted to be an intrusive age because the
dated zircons were separated from the mafic part of the
dike. Because of this result, the ca. 445 Ma date obtained
from diatexite sample TF-77B may be interpreted as (1)
128 C. G. Barnes et al.
NORWEGIAN JOURNAL OF GEOLOGY
500
495
490
495
490
480.3 ± 1.9
0.73 (n=15)
485
485
date (Ma)
date (Ma)
500
TF-304 stromatic migmatite
melanosome
480
480
475
475
470
465
460
A
-3
207
470
entire range:
480.2 ± 3.6
2.43 (n=21)
206
-2
-1
Pb/ Pb
0
stdev
1
465
460
2
3
TF-77B diatexite
0.066
0.058
480
0.054
440
420
470
13.0
13.4
13.8
14.2
238U/ 206 Pb
14.6
-3
460
-2
-1
0
stdev
1
2
3
TF-77B diatexite
445.1 ± 6.4
3.4 (n=13)
440
420
C
0.050
12.6
15.4 400
15.0
D
TF-404 quartz diorite
entire range
447.3 ± 2.8 Ma
1.24 (n=20)
465
460
455
date (Ma)
460
B
480
date (Ma)
0.062
TF-104
diatexitic migmatite
480.6 ± 3.8
1.17 (n=13)
450
445
440
435
447.8 ± 1.7 Ma
0.27 (n=11)
430
E
425
420
-3
Figure 4
-2
-1
0
stdev
mis-sampling in the field, (2) evidence that the diatexite
remained in a partly molten state for more than 40 m.y.,
(3) resetting of part, but not all of the zircons in the diatexite at ca. 447 Ma, (4) evidence that the migmatite was
remelted during magma intrusion at ~437 Ma, permitting hybridization, or (5) contamination of the migmatite by younger leucocratic magmas. At present, we discount the first possibility because the chemical composition of TF-77B is similar to other migmatitic rocks
(see below), and discount the second possibility because
there is no evidence for similar long-term high temperatures elsewhere in the HNC. Resetting at ca. 447 Ma is
1
2
3
possible if the middle crust was sufficiently heated by
magma emplacement at that time (e.g., Velfjord plutons).
However, the nearest large plutons of this age crop out 10
km from the field area. The latter two explanations are
quite possible, particularly in view of the intricate veins
and dikes that were emplaced from 437–424 Ma. Further detailed geochronologic study of the Upper Nappe is
needed to constrain better its thermal history.
After migmatization, the next identifiable igneous
event in the area is emplacement of the quartz dioritic pluton (TF-404) at 447.8 Ma. This is the same age
NORWEGIAN JOURNAL OF GEOLOGY
Ordovician and Silurian magmatism in the Upper Nappe, Uppermost Allochthon, Helgeland 129
470
480
TF-70 banded diorite
436.9 ± 4.4 Ma
460
2.4 (n=22)
460
440
450
430
440
420
400
430
420
F
460
TF-82 tonalitic dike
431.9 ± 3.5 Ma
450 0.89 (n=16)
440
410
G
-3
-2
-1
0
stdev
1
2
3
480
470
460
date (Ma)
date (Ma)
436.7 ± 3.5 Ma
1.20 (n=19)
date (Ma)
date (Ma)
450
410
N16.05
net-veined monzodiorite dike
470
450
440
430
430
420
410
H
420
410
N17.05 granitic dikes
424.0 ± 1.2
I
-3
-2
-1
0
stdev
1
2
3
Figure 4. Results of U–Pb dating of zircon. A. Probability plot of results for stromatic migmatite melanosome TF-304.
Uncertainties are 1 σ. The shaded area represents analyses selected to interpret the age of migmatization. B. Probability
plot for diatexitic migmatite TF-104. C. Concordia plot for diatexite sample TF-77B. D. Age distribution of concordant
zircons younger than 480 Ma in TF-77B. See text for discussion. E. Probability plot for quartz diorite pluton sample
TF-404. The shaded area represents analyses selected to interpret the age of emplacement. F. Age distribution of zircons
from banded diorite sample TF-70. G. Probability plot of net-veined monzodiorite sample N16.05. The shaded area
represents analyses selected to interpret the age of emplacement.Figure
H. Age 4
distribution of zircons from tonalitic dike TF-82.
I. Probability plot for granitic dike N17.05. The shaded area represents analyses selected to interpret the age of emplacement. Data in panels A, B, E, G, and I were determined by LA-ICPMS, for panels C, D, F, and H by SHRIMP-RG. See
text and Appendix 1 for details.
as emplacement of voluminous dioritic to granitic plutons of the Velfjord area to the west (Fig. 1; Yoshinobu
et al., 2002; Anderson et al., 2008). In the study area,
there evidently was a magmatic lull until about 437 Ma.
From ~437 to ~424 Ma, the Upper Nappe was intruded
by mafic (gabbroic and dioritic), tonalitic, monzonitic,
and granitic magmas, many of which were mingled. This
time range is very similar to the ages of the large felsic
plutons that characterize the eastern part of the Bindal
Batholith (Nordgulen et al., 1993; Nissen et al., 2006).
Major and trace element compositions
Migmatites. The migmatite samples vary in SiO2 content
from ~53 to 73% (Appendix Table 3; Fig. 5). Two samples
of diatexite have SiO2 contents near 55%, whereas a bulk
sample of the stromatic migmatite and two analyzed leucosomes range from 65 to 73% SiO2 (Fig. 5). All of the
analyzed migmatites are strongly peraluminous, with A/
CNK > 1.1 (Fig. 5A). The melanosome has the highest
A/CNK value (~2.1), which is indicative of its high biotite content. The migmatite samples have quite uniform
values of FeO*/(FeO*+MgO) of ~0.74 (Fig. 5B). Such
uniformity is not characteristic of magmatic suites (e.g.,
130 C. G. Barnes et al.
A
mel
A/CNK
2.0
1.5
leu
dtx
1.0
vnA
enAz
hostB
enA enB
0.5
leu
432
SiO2
B
dia
vnA
0
dia
ic
lc
-ca
li
a
alk
enAz
enB
enB
SiO2
50
55
60
65
D
70
75
80
vnA
enB
enAz
enA
hostB
dia
enAz
500
enB
str
enA
SiO2
65
leu
432
enA
1000
60
str
2000
0.6
55
leu
1500
hostB
50
hostB
leu
432
0.7
alic
-alk
calc cic
cal
ppm Sr
FeO*/(FeO*+MgO)
str
leu
dia104
mel
dia
-10
45
2500
magnesian
mel
0.4
45
5
ferroan
0.8
0.5
lic
alka
-5
A=net-veined
B=pillowed dike
0
C
10
str
dtx
0.9
15
migmatite
448 Ma
437-432 Ma
~424 Ma
Na2O+K2O-CaO
2.5
NORWEGIAN JOURNAL OF GEOLOGY
70
75
80
432
leu
leu
dia
mel
0
0
1
2
CaO
3
4
5
6
7
8
9
Figure 5. Representative geochemical variation diagrams, with samples grouped according to approximate age, where
known. Black triangles with dashed tie lines connect coexisting enclave (enA, enAz) and host vein (vnA) compositions in
the 437 Ma net-veined dike. Black triangle labeled 432 is dated sample TF-82, the tonalite/granodioritic body at the eastern end of the road cut. Orange triangles connected by dashed tie lines represent a mingled (pillowed) dike with quartzdioritic/tonalitic host (vnB) and melagranitic enclaves (B). Abbreviations associated with migmatitic samples are: str
= stromatic migmatite; dtx = diatexite; mel = melanosome; leu = leucosome. A. weight percent SiO2 vs. A/CNK, where
A/CNK is molar Al2O3/CaO + Na2O + K2O. B. All igneous samples plot in the magnesian field of Frost et al. (2001).
C. Samples of igneous rocks range from calc-alkalic to mildly alkalic according to the modified alkali-lime index (Frost et
al., 2001). D. Plot of CaO versus ppm Sr showing the wide range of Sr contents, particularly among the most mafic samples. Note the lack of correlation of Sr content with CaO in the mafic and intermediate samples.
Figure 5.
Frost et al., 2001). Instead it is suggestive of uniform
compositions of ferromagnesian phases in melanosomes,
leucosomes, and diatexites.
Among the migmatites, the melanosome has the highest rare earth element (REE) abundances (Fig. 6A) and in
particular the highest abundances of heavy REE. Diatexite
sample TF-104 has a similar pattern to the melanosomes;
however, the pattern of diatexite TF-77B is distinct from
the melanosome pattern in having much lower heavy REE
abundances. Leucosomes are characterized by heavy REE
concentrations, ~10x chondrites (Fig. 6A). One leucosome
has lower light REE abundances than the other samples
and a positive Eu anomaly, all of which suggest that this
sample contains cumulate plagioclase.
448 Ma pluton. Two samples of the ~448 Ma quartz dioritic to quartz monzodioritic pluton east of the main road
cut are metaluminous, magnesian, and span the boundary between calc-alkalic and alkali-calcic (Fig. 5). They
have moderate REE slopes (normalized La/Lu = 33) and
slight negative Eu anomalies (Fig. 6B).
Younger intrusive rocks. Field relationships and U–Pb
ages suggest that all of the post-migmatite intrusive rocks
in the main road cut have ages from 437–424 Ma, and
most range from ~437 to 432 Ma. Therefore, we have
not attempted to subdivide these units into narrower age
groups and instead discuss them as a single suite. Samples with SiO2 contents < 60 wt% are metaluminous,
whereas those with higher SiO2 contents lie on or just
Ordovician and Silurian magmatism in the Upper Nappe, Uppermost Allochthon, Helgeland 131
NORWEGIAN JOURNAL OF GEOLOGY
Figure 6. Chondritenormalized REE diagrams. A. Migmatitic
rocks and related (?)
comb-layered dike. B.
Samples from the 448
Ma pluton east of the
main road cut and
monzodioritic sample
TF-79 (~437 Ma). C.
Mingled dikes. D. Evolved dikes ≤ 435 Ma.
1000
1000
A
stromatic migmatite
diatexite (TF-104/TF-77B)
TF
-7
B
leucosome/melanosome pair
3
comb-layered dike (TF-71)
100
100
leu
co
so
me
10
10
TF-17
sample/chondrites
TF-404
}448 Ma
TF-79 (~435 Ma)
1
1000
La Ce Pr Nd
Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
1000
C
mingled dikes
(437-432 Ma)
La Ce Pr Nd
D
Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
tonalitic to granitic rocks
100
enclave
felsic component
hybrid (TF-66)
100
10
1
10
TF-74
TF-67 (~435 Ma)
TF-64
TF-82 (431 Ma)
TF-84
TF-81 (424 Ma)
TF-72 (
424 Ma)
1
La Ce Pr Nd
Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
1
La Ce Pr Nd
Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Figure 6
above the metaluminous–peraluminous boundary (Fig.
5A). All samples plot in the magnesian field of Frost et al.
(2001), although two samples associated with the 437 Ma
net-veined unit plot near the magnesian–ferroan boundary (Fig. 5B). In the modified alkali-lime index diagram
(Fig. 5C), compositions vary from calc-alkalic to alkalic.
The single sample that plots in the calcic field is a 437 Ma
banded gabbro that is probably an augite cumulate. This
suite of dikes is characterized by relatively high Sr contents (Fig. 5D). Most dikes with less than 3% CaO have
>1000 ppm Sr and in this compositional range, no correlation exists between the Sr and CaO contents. In contrast, the older 448 Ma quartz diorite has lower Sr contents than the younger dikes (~750 ppm; Fig. 5D) and the
~424 Ma granitic dike has <500 ppm Sr.
Figure 6C shows REE patterns for mingled dikes and
related rocks. All show similar patterns, with slight negative Eu anomalies. In two of the enclave-host pairs, the
net-veined dike (TF 74) and the pillowed dike (TF-84),
the REE abundances of the host are higher than in the
enclaves. In contrast, for the mingling zone at the west
end of the exposure (samples TF-64 & 66; see Appendix
1), the REE patterns of the enclave and host cross, with
higher light REE and lower heavy REE in the host rock
(Fig. 6C). The hybrid rock associated with TF-64 (sample
TF-66; Fig. 6C) has REE abundances that are generally
intermediate between the enclave and host.
Figure 6D shows REE patterns for 437–424 Ma felsic
dikes. Samples TF-67 and TF-72 have patterns similar
132 C. G. Barnes et al.
to those of the felsic parts of mingled dikes TF-74 and
TF-84 (Fig. 6C), whereas sample TF-81 has lower heavy
REE, similar to the felsic part of mingled dike TF-64
(Fig. 6C). The pattern for the dated granodiorite (TF-82)
shows low REE abundances and a positive Eu anomaly
which are typical for plagioclase accumulation.
NORWEGIAN JOURNAL OF GEOLOGY
2
Nd (t)
0
A
Nd and Sr isotope data
All three samples of migmatitic rocks analyzed for Nd
and Sr isotopic compositions (Appendix Table 4; Fig.
7) have low initial εNd values (-7 to -8) and high initial 87Sr/86Sr values (0.714–0.718). The diatexitic migmatite sample has distinctly lower initial 87Sr/86Sr than the
stromatic migmatite and the leucosome. In view of the
concentrations of Sr in all migmatite samples (332–622
ppm) this variation is probably related to isotopic heterogeneity in the sedimentary protolith rather than differences in the growth of radiogenic Sr after migmatization.
The 448 Ma quartz dioritic pluton east of the main road
cut has initial εNd and 87Sr/86Sr values of -4.8 and 0.71043,
respectively (Appendix Table 4). These isotope ratios are
quite distinct from those of the 437–432 Ma dike rocks,
which have εNd from -0.2 to -2.4 and initial 87Sr/86Sr
between 0.706 and 0.708. This rather narrow range of isotopic values encompasses rocks from 50 to 65 wt% SiO2
and 1.2–8% MgO. Within this group, there is no correlation between initial 87Sr/86Sr and SiO2 or 1/Sr, although
the two samples with the highest 1/Sr values also have the
highest initial 87Sr/86Sr. In contrast, a weak negative correlation exists between εNd and 1/Nd (and SiO2 content).
The youngest sample analyzed for isotope ratios (424
Ma leucogranite TF81) has isotopic compositions nearly
identical to the 448 Ma quartz diorite (Fig. 7).
In the preceding field descriptions, we noted field evidence that suggested significant interaction between
migmatitic rocks and younger dikes. One specific case
was the possible interaction of magma of the net-veined
dike with adjacent diatexite (Fig. 2). The isotopic compositions of the enclave and the vein material from this
dike are nearly identical, and show no evidence of a mixing line with the adjacent migmatite (field A on Fig. 7).
In a broader sense, isotopic compositions of the majority of the 437–432 Ma dike rocks are difficult to explain
as hybrids of a mafic end member and migmatitic rocks
(or magmas). This is because such hybridization should
result in a decrease in the Sr content of the igneous rocks,
with corresponding negative correlation of Sr and initial
87
Sr/86Sr with SiO2. Neither correlation is observed.
Pressure and temperature estimates
Migmatitic rocks.
We attempted to use Fe-Mg exchange equilibria between
garnet and biotite to estimate temperature (Reid, 2004).
migmatite
448 Ma
437-432 Ma
<425 Ma
Velfjord
plutons
enB
-2
hostB
-4
424 Ma
448 Ma
-6
-8
0.705
dia
0.710
0.715
87
leu
str
0.720
86
initial Sr/ Sr
Figure 7. εNd (t) plotted versus initial 87Sr/86Sr. Symbols as
in Figure 5. The unpatterned field (A) is the range of compositions in the net-veined dike (TF-74A, 74B, N16.05).
Samples from pillowed dike (B) are connected by a tie line.
The horizontally-hachured field is the range of compositions of the Velfjord plutons (Barnes et al., 2002).
However, temperatures estimated on this basis result in
values that are below the wet solidus (ca. 640°C), which
Figure 7.
indicates that biotite compositions were reset during
cooling. Temperature was also estimated on the basis of
Ti-in-zircon thermometry (Watson et al., 2006) with
activities of SiO2 and TiO2 set to unity. A decrease in
the activity of TiO2 to 0.9 would increase estimated T by
10–20°C. No pressure correction was applied (see discus­
sion in Fu et al., 2008). Temperature was calculated for
each zircon that was used in the U–Pb age determin­
ations. Nineteen zircons separated from melanosome of
the stromatic migmatite yielded an average temperature
of 811 ± 24 °C and 11 zircons from the diatexite gave an
average of 793 ± 30 °C.
Some migmatitic samples contain the assemblage garnet
+ sillimanite + plagioclase + quartz, which permits the
use of the GASP thermobarometer. Figure 8 shows the
results of GASP calculations for five samples of the Upper
Nappe: two from the Tosen road cut, two from Upper
Nappe rocks sampled along Tosenfjord, and one collected ~20 km to the north in Størborja (Fig. 1C). If it is
assumed that the Ti-in-zircon temperatures determined
for the ~480 Ma zircons are representative of the T of
melting, then intersection of this temperature range with
the GASP equilibrium curves should provide an estimate
Ordovician and Silurian magmatism in the Upper Nappe, Uppermost Allochthon, Helgeland 133
NORWEGIAN JOURNAL OF GEOLOGY
of the pressure of melting. A further constraint on pressure is the observation that cordierite is present in many
of the migmatite samples from the Upper Nappe, which
restricts melting conditions to be at P lower than the
biotite-sillimanite-garnet-cordierite-liquid equilibrium
(Fig. 8). We conclude that partial melting of the Upper
Nappe migmatites occurred between 500 and 700 MPa
and ~800°C. These conditions are appropriate for biotite-­
dehydration melting (e.g., Patiño Douce and Johnston,
1991; Vielzeuf and Montel, 1994; Montel and Vielzeuf,
1997).
Younger intrusive rocks.
Pressure-temperature estimates for intrusive rocks were
made on the basis of Al-in-hornblende thermobaro­
metry (Anderson and Smith, 1995). All of the analyzed
amphibole has calculated Fe3+/(Fe3+ + Fe2+) > 0.20, which
Schmidt (1992) considered acceptable for use with hornblende barometry. Anderson and Smith (1995) suggested
that amphibole outside the Fetot/(Fetot + Mg) range of
0.40-0.65 should not be used for hornblende baro­metry.
Two samples (TF-73, TF-17) contain amphibole that meet
these criteria and contain the equilibrium assemb­lage
of plagioclase, K-feldspar, quartz, hornblende, biotite,
and Fe-Ti oxide (Hammarstrom and Zen, 1986; Hollister et al., 1987). For these calculations, rim compositions
of amphibole and plagioclase were used with the pressure calibration of Anderson and Smith (1995) and the
edenite–richterite T calibration of Holland and Blundy
(1994).
Sample TF-17 is from the 447 Ma quartz dioritic pluton at
the east end of the outcrop. Hornblende-plagioclase pairs
yielded a P estimate of 329 ± 44 MPa and a T estimate of
712 ± 19°C. Monzonite sample TF-73 is thought to be a
hybrid between the monzonitic host and enclaves of the
net-veined dike dated at 437 Ma (dated sample N16.05;
Fig. 4G). The estimated pressure was 365 ± 37 MPa and
the T estimate was 723 ± 12°C. This temperature estimate
is consistent with the Ti-in-zircon temperature calculated
from magmatic zircons from sample N16.05 of 747 ± 77
C. The peach-colored box in Figure 8 indicates the most
likely P and T conditions of hornblende, plagioclase, and
zircon equilibrium among these younger intrusive rocks.
Discussion and Conclusions
Timing and conditions of magmatism
Bt Grt
As K
01
VF2
L
99A
0.9
As
rd
L
Bt G
rt
xC
rd
L
C
Grt As L
Crd Kfs
b
sA
G
0.7
0.8 = Fe/(Fe + Mg) of
garnet in assemblage:
Grt+Bt+As
Grt+Bt+Crd
Grt+Crd+As
L
Cr
rt
Ab
Ms
V
Kfs
As
600
d
Bt
0.6
il
tS
Gr
rd
lC
Sp
Kf
200
fs L
tK
Gr d Bt
Cr
0.8
andalusite
500
rt
G
sillimanite
400
Bt
Op
kyanite
Opx As
Grt Crd
5
1
VF2
VF1
0.9
Opx As L
0.6
0.7
fs
Grt K
Bt As
800
600
Ti in zircon T
Ab
fs L
As L
8
Ms
0.
Ms Ab V
leucosome T
NaKFMASH
V
Bt Grt
dV
Opx Cr
700
T (oC)
800
Figure 8.
900
Al-in-hornblende P (A & S)
P (MPa)
1000
A
Figure 8. P-T estimates. P-T grid for fluid-absent partial
melting of pelitic compositions, after Spear et al. (1999).
Colored curves are equilibrium conditions for five Upper
Nappe samples with the GASP (garnet-sillimanite-silicaplagioclase) assemblage calculated using TWQ (Berman,
1988, 1990). The gray box indicates estimated conditions
of migmatization determined on the basis of Ti-in-zircon
thermometry of ~480 Ma zircons and the results of GASP
calculations. The pink-shaded box indicates the results of
Al-in-hornblende barometry and hornblende-plagioclase
thermometry on 448 and 437 Ma intrusive rocks. Phase
abbreviations are: Ms, muscovite; ab, albite; As, aluminosilicate; Kfs, K-feldspar; Bt, biotite; Grt, garnet; Opx, orthopyroxene; Crd, cordierite; Spl, spinel; Sil, sillimanite; L,
silicate melt; V, aqueous fluid.
We interpret the U-Pb data for the migmatites to indicate that partial melting of the Upper Nappe occurred
at ~480 Ma. Melting occurred at ca. 500–700 MPa pressure (ca. 20–28 km depths) and ~800°C (Fig. 8), conditions consistent with biotite-dehydration melting (e.g.,
Vielzeuf & Holloway, 1988; Patino Douce & Johnston,
1991; Vielzeuf & Montel, 1994; Montel & Vielzeuf, 1997).
Under these conditions, enough melt was produced to
allow for mobilization of melt-rich parts of the migmatite to form cross-cutting diatexites. It is noteworthy that
the diatexitic rocks are chemically more refractory than
the stromatic migmatites (i.e., diatexites have lower SiO2
and higher TiO2, CaO, total Fe). This chemical difference
indicates that although the diatexites may have initi­
ally had larger proportions of melt than the stromatic
migma­tites, melt was lost during or after emplacement.
There is no field or geochemical evidence that migmati­
zation was accompanied by mafic magmatism, at least at
the level of exposure. This leaves the source of heat for
ca. 480 Ma crustal melting in the Upper Nappe, and elsewhere in the HNC, an unanswered question.
The next magmatic event recorded in the study area was
emplacement of the small quartz dioritic pluton at 448
Ma. The age of this pluton is identical to larger plutons
that intrude the underlying nappes of the HNC, specifically the Velfjord plutons ~10 km to the west and northwest (Fig. 1B, C; Barnes et al., 1992, 2004). The quartz
diorite is distinct from these plutons in having signifi­
cantly higher initial 87Sr/86Sr values (Fig. 7) and also in
its calc-alkalic (versus alkali-calcic to alkalic) compositions at comparable SiO2 contents. These distinctions
134 C. G. Barnes et al.
NORWEGIAN JOURNAL OF GEOLOGY
suggest that the quartz dioritic magma either had a distinct source or underwent significant crustal contamination compared to the Velfjord plutons.
et al., 2007). Evidently, middle Ordovician metamorphism
and local crustal melting occurred throughout the HNC in
response to nappe amalgamation (Barnes et al., 2007).
The various dikes with ages from 437–432 Ma are noteworthy because of the wide range of bulk compositions,
from gabbroic to granitic. The abundance of mingled
dikes among the 437–432 Ma dikes shows that mafic
and felsic magmas coexisted in this time range. Overall,
these rocks are metaluminous to weakly peraluminous
and have low initial 87Sr/86Sr compared to the migmatites
and 448 Ma quartz diorite (Figs. 5, 7). There is no correlation between the chemical composition (e.g., SiO2 content) and isotope ratios and among the rocks with SiO2
>71 wt %, i.e. rock types that range from tonalite to granite (2.5 to 7.0 wt % K2O), with no correlation of SiO2 with
K2O. Nevertheless, most samples have small negative Eu
anomalies (Fig. 6) and mafic–intermediate rocks have
high (>1000 ppm) Sr contents (Fig. 5). These observations suggest that the various 437–432 Ma dikes are not
related by differentiation of a single, evolving magma,
but instead arise from a deep-seated source region in
which isotopic compositions vary within a narrow range
but major element concentrations can vary significantly.
Specifically the combination of low initial 87Sr/86Sr, high
Sr contents and lack of correlation between Sr and differentiation among intermediate and mafic rocks, small,
uniform Eu anomalies, and low alumina saturation index
is suggestive of magma generation or differentiation
under conditions where plagioclase fractionation was
minor, consistent with lower crustal conditions.
Although anatexis in the Upper Nappe occurred at ~700
MPa pressure, by the 448 Ma emplacement of the quartz
dioritic pluton the Upper Nappe rocks were at a pressure
of ~300 MPa. Similar amounts of exhumation have been
documented in the Velfjord region (Barnes and Prestvik, 2000) and at Vega (Marko et al., 2007; Oalmann,
2010). In Velfjord, exhumation was interpreted to occur
during and after emplacement of ca. 448–445 Ma plutons (Barnes and Prestvik, 2000; Yoshinobu et al., 2002),
whereas exhumation preceded the 448 Ma magmatism at
Tosenfjord. The age of exhumation at Vega is uncertain,
but may have been as old as 475 Ma (Marko et al. 2007).
Evidently, although high-grade metamorphism was
coeval across the HNC, exhumation was diachronous.
In contrast, the young 424 Ma granitic dike has much
higher initial 87Sr/86Sr and lower εNd than the 437–432
Ma dikes, suggesting a distinct source for this youngest
magma batch. The low HREE of this granite (Fig. 6D)
indicates residual garnet in the source, which is in contrast to nearly all of the older dikes. It is noteworthy that
although the 424 Ma granite displays a garnet REE signature, it is only slightly peraluminous, which rules out an
origin by partial melting of typical metapelitic rocks in
the HNC (e.g., Nordgulen and Sundvoll, 1992; Barnes et
al.2002).
Several of the ca. 437–432 Ma dikes are foliated, including mingled dikes in which mafic enclaves underwent
sub-solidus deformation (Appendix 1), and the ~437 Ma
net-veined rocks are cut by a shear zone with reversesense (top-to-west) displacement. In contrast, the 424
Ma granitic dikes show no evidence of sub-solidus deformation. Evidently, magma emplacement at ca. 435 Ma
was accompanied by local penetrative deformation, at
least some of which was contractional.
Relationship to regional geology
The age of migmatization of the Upper Nappe at~ 480
Ma is essentially identical to that of migmatization in the
Horta and Lower Nappes (Yoshinobu et al., 2002; Barnes
The work of Nordgulen and colleagues (Nordgulen,
1993; Nordgulen et al., 1993; Yoshinobu et al., 2002;
Barnes et al., 2007) have shown that the earliest plutons
in the Bindal Batholith (478–475 Ma) are restricted to
the western HNC and that plutons from 467–444 Ma
crop out primarily in the central and western parts of the
nappe complex. In contrast, plutonic rocks in the 440–
430 Ma age range crop out across the HNC, from the
westernmost exposures (e.g., Sklinna pluton; Barnes et
al., 2007) to the easternmost ones (Nissen et al., 2006).
This change from early, relatively focused magmatic
activity to later, widespread magmatism in the HNC may
be related to changes in tectonic environment (contraction versus extension), dip of a subducting slab (older
steep dip to younger shallow dip), or to the development
of a slab window in early Silurian time and initiation
of the Scandian collision. The possibilities can only be
resolved with further detailed petrological and geochronological study of the HNC.
Acknowledgments. - We thank Øystein Nordgulen for bringing this
exceptional exposure to our attention and for his geologic insights on
HNC geology. S. Swapp provided able assistance with the microprobe
analyses. We also thank H. Schiellerup and R. Larsen for their thoughtful reviews. This research was supported by NSF grant EAR9814280 and
by the Norwegian Geological Survey.
Electronic Appendix Tables
Table 1. U-Pb age data determined by LA-ICP-MS
Table 2. U-Pb age data determined by SHRIMP-RG.
Table 3. Representative major and trace element analyses
Table 4. Nd and Sr isotope data.
Table 5. Representative garnet analyses.
Table 6. Representative hornblende analyses.
Table 7. Representative biotite analyses.
Table 8. Representative plagioclase analyses.
Table 9. Clinopyroxene analyses.
NORWEGIAN JOURNAL OF GEOLOGY
Ordovician and Silurian magmatism in the Upper Nappe, Uppermost Allochthon, Helgeland 135
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