JOURNAL OF PETROLOGY
VOLUME 43
NUMBER 12
PAGES 2171–2190
2002
Mafic Magma Intraplating: Anatexis and
Hybridization in Arc Crust, Bindal Batholith,
Norway
CALVIN G. BARNES1∗, AARON S. YOSHINOBU1, TORE PRESTVIK2,
ØYSTEIN NORDGULEN3, HARALDUR R. KARLSSON1 AND
BJØRN SUNDVOLL4
1
DEPARTMENT OF GEOSCIENCES, TEXAS TECH UNIVERSITY, LUBBOCK, TX 79409-1053, USA
2
DEPARTMENT OF GEOLOGY AND MINERAL RESOURCES ENGINEERING, NORWEGIAN UNIVERSITY OF SCIENCE AND
TECHNOLOGY, N-7491, TRONDHEIM, NORWAY
3
GEOLOGICAL SURVEY OF NORWAY, N-7491, TRONDHEIM, NORWAY
4
GEOLOGICAL SURVEY OF NORWAY, MINERALOGICAL–GEOLOGICAL MUSEUM, 0562 OSLO, NORWAY
RECEIVED JULY 17, 2001; REVISED TYPESCRIPT ACCEPTED MAY 2, 2002
The dioritic Velfjord plutons (>448 Ma) were emplaced into
regional migmatitic metapelitic and metacarbonate rocks at midcrustal levels, corresponding to pressures of >700 MPa. Exhumation to >400 MPa began while the migmatites were in a
partly molten state. With increasing proximity to the plutons,
regional stromatic migmatites change to diatexite, and diatexitic
dikes are common within 500 m of the contacts. We interpret these
relationships to indicate that heat from the plutons resulted in contact
migmatization in a zone up to 1 km wide. Typical residual
mineralogy in the diatexites is plagioclase + quartz + biotite
+ garnet + sillimanite ± K-feldspar, consistent with biotite
dehydration melting. Pod- and dike-like leucosomes consist of two
types: earlier high-K (granitic) ones with mineral assemblages
identical to the migmatites and later low-K (tonalitic) ones in which
sillimanite is sparse and garnet absent. The high-K leucosome
magmas can be explained by biotite dehydration melting at 700
MPa. Within the aureole, mafic magmas were locally injected into,
and hybridized with, the diatexites and the high-K leucosome
magmas. In contrast, the low-K leucosomes are thought to result
from local, late-stage remelting of H2O-saturated diatexite. The
H2O-rich fluid was probably released from intergranular melt
trapped in the diatexites during exhumation and solidification.
Distinctive porphyritic ‘contact granites’ are common at pluton
contacts. Although the mineral assemblage of these granites is
identical to that of the diatexites, their isotopic compositions are
distinct, with Nd and 18O in the migmatites from −7·6 to −9·6
and from +10·9‰ to +13·5‰, and in the contact granites
from −5·2 to −7·5 and from +9·6‰ to +12·3‰, respectively.
Thus, the contact granites could have a source that is isotopically
distinct from, but mineralogically similar to the diatexites, or they
could result from mixing of magma similar to the high-K leucosomes
with dioritic magmas. Mass balance calculations are consistent
with the latter interpretation, with proportions of granitic to dioritic
magmas from 7:1 to 7:3. Emplacement and solidification of the
dioritic plutons provided zones of structural anisotropy along which
high-K leucosome magmas and contact granite magmas collected.
These magmas were injected by additional dioritic magma and
further hybridized. Because the solidi of the plutons were several
hundred degrees higher than that of the granitic magmas, the pluton
walls acted as long-lived, hot, rigid surfaces along which magmas
collected and migrated.
∗Corresponding author. E-mail: Cal.Barnes@ttu.edu
 Oxford University Press 2002
KEY WORDS: migmatite;
contact melting; hybridization; magma transport;
Caledonian
INTRODUCTION
The importance of crustal melting in arcs has been widely
accepted since the development of isotopic fingerprinting
JOURNAL OF PETROLOGY
VOLUME 43
methods. Controversy still exists concerning the volumetric importance of crustal melts and the mechanisms
by which crustal melts interact with mantle-derived mafic
magmas. In fact, identification of magmas that are purely
crustal in origin is commonly difficult [see Collins (1998)
and Patiño Douce (1999); and Chappell (1996) and White
et al. (1999) for contrasting views]. Patiño Douce (1999)
compared experimental data with data for a variety of
natural granitic suites and concluded that most arc granites result from some degree of hybridization between
crustal- and mantle-derived magmas.
A particularly vigorous discussion has arisen concerning
mechanisms of crustal melting and ways in which magma
is extracted, transported, and accumulated into large
plutons (e.g. Clemens & Mawer, 1992; Paterson & Fowler,
1993; Brown et al., 1995b; Sawyer, 1996, 1998; Clemens,
1998; Solar & Brown, 2001). This is at least in part due
to difficulties in the establishment of a direct relationship
between zones of crustal melting (migmatites) and granitic
plutons (e.g. Sawyer, 1996, 1998; Solar & Brown, 2001).
For example: Which processes are responsible for separation of magma from its source and removal of entrained residual solids [see review by Brown et al. (1995a)]?
Can a lithologic unit be repeatedly melted to form distinct
magmas, or does a single melting event make the source
too refractory to produce anything but ultra-high-T melts
(e.g. Beard et al., 1993)? Does crustal melting in an arc
setting depend on underplating or intraplating of mafic
magmas (Hildreth & Moorbath, 1988; Huppert & Sparks,
1988; Bergantz, 1989) or can it be related entirely to
an elevated geothermal gradient? What influence does
intrusion of mafic magmas have on the segregation,
migration, and petrology of anatectic granites?
The Bindal Batholith is a continental arc-like batholith
that probably formed near the Laurentian side of the
Caledonian orogen (Roberts, 1988). In several locations,
migmatites are intimately associated with mafic plutons,
and the inference has been made that mafic magmatism
triggered or enhanced crustal melting (e.g. Barnes et al.,
1992). We report on a suite of migmatites formed by
contact melting during emplacement of dioritic plutons,
on the granitic magmas produced, and on possible relationships with felsic magmatism in the Batholith. We
also suggest that structural anisotropy formed by emplacement of mafic plutons into migmatitic terranes enhanced the collection and transport of anatectic magmas
and focused the hybridization process.
GEOLOGIC SETTING
The Bindal Batholith (Fig. 1) is the northernmost of two
Ordovician–Silurian batholiths in north–central Norway;
the other is the Smøla–Hitra Batholith west of Trondheim
(Gautneb & Roberts, 1989). The host rocks of the Bindal
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DECEMBER 2002
Batholith are part of the Helgeland Nappe Complex
(HNC, Fig. 1) (Kollung, 1967; Myrland, 1972; Gustavson, 1978; Thorsnes & Løseth, 1991), which is the
structurally highest unit in the Uppermost Allochthon
of the Caledonian nappe stack in central Scandinavia
(Stephens et al., 1985). The HNC is composite and consists
of Late Proterozoic, medium- to high-grade pelitic, semipelitic, calc-silicate, and calcareous rocks interleaved with
Early Ordovician ophiolite complexes and their unconformable cover sequences (medium- to low-grade
sandstones, conglomerates, and calc-silicate rocks). Amalgamation of terranes within the HNC took place in
Ordovician time, before the Middle Ordovician to Early
Silurian emplacement of the Batholith. The final eastward
translation (modern coordinates) of the HNC, including
the Bindal Batholith, across lower-grade rocks of the
Upper Allochthon took place in the Late Silurian to
Devonian Scandian stage of the Caledonian orogeny.
The earliest magmatism in the Batholith consisted
of sparse, mildly to strongly peraluminous alkali–calcic
granites [e.g. tourmaline granite in Velfjord (Fig. 2a),
biotite granite near Brønnøysund (Fig. 1), and ‘anatectic
granite’ of Vega (Fig. 1)]. U–Pb (zircon) ages range from
481 to 468 Ma (Yoshinobu et al., 2001), which is coeval
with regional migmatization in the Velfjord area. Emplacement of voluminous gabbroic to granitic plutons
began at >448 Ma, with intrusion of the Velfjord and
Andalshatten plutons (Fig. 1; Nordgulen et al., 1993;
Pedersen et al., 1999). This magmatism, which was predominantly alkali–calcic to calc-alkalic, continued to
>430 Ma (Nordgulen & Schouenborg, 1990; Nordulen
et al., 1993). Pb, Sr, and Nd isotopic data (Nordgulen &
Sundvoll, 1992; Birkeland et al., 1993) require metasedimentary and meta-igneous crustal sources, as well as
a mantle component, in post-448 Ma parts of the Batholith, but pre-468 Ma plutons have isotopic compositions
consistent with crustal sources.
The Velfjord plutons consist of three large bodies
(Hillstadfjellet, Akset–Drevli, and Sausfjellet) and two
smaller ones (Svarthopen and Aunet) (Fig. 2a; Barnes et
al., 1992). The Sausfjellet pluton is dioritic whereas
the Akset–Drevli pluton consists of Fe-rich gabbro and
diorite. The Hillstadfjellet pluton was emplaced in two
stages: an early gabbroic Stage 1 and a later, more
voluminous, monzonitic Stage 2 (Fig. 2a), with compositional range from diorite to quartz monzonite. The
gabbro–diorite body west of Sørfjord (Fig. 2a) is herein
named the Svarthopen pluton [‘uralitic gabbro’ of Myrland (1972)]. This pluton is crosscut and locally mingled
with porphyritic granites that we refer to as ‘contact
granites’ in a later section. Similar granites crop out in
arcuate masses adjacent to the Svarthopen body (Fig.
2a).
The wall-rocks of all the Velfjord plutons consist of
intercalated high-grade marbles and migmatitic pelitic
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Fig. 1. Sketch map of the Bindal Batholith (after Nordgulen, 1992) and its relationships to tectonostratigraphic terranes. HNC and RNC,
Helgeland and Rödingsfjellet nappe complexes of the Uppermost Allochthon; GP, Gaupen pluton; HP, Heilhornet pluton; BNS, Bindalseid
pluton.
and quartzofeldspathic gneisses. The Sr and oxygen
isotopic compositions of the marbles suggest a Neoproterozoic age (Trønnes, 1994; Trønnes & Sundvoll,
1995). Each lithology constitutes mappable units of kilometre scale (Myrland, 1972), but variations within a unit
can encompass all three rock types. Among the migmatitic
rocks, only the pelitic gneisses show evidence of appreciable magma production during contact metamorphism, therefore our discussion will focus on them.
The high-grade metamorphic unit is structurally over-
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Fig. 2a.
lain by a package of Ordovician(?) ultramafic rocks
and non-migmatitic metasedimentary rocks (Fig. 2a).
Thorsnes & Løseth (1991) interpreted the metasedimentary rocks as a cover sequence unconformably
overlying the ultramafic rocks (e.g. at Heggefjord; Fig.
2a). A similar stratigraphic sequence is recognized in a
number of thrust sheets within the HNC (Nordgulen,
2000; Heldal, 2001).
The contact between this structurally higher ultramafic
+ cover unit and the lower migmatitic rocks was interpreted as a thrust fault by Thorsnes & Løseth (1991).
However, it places younger (Ordovician?), lower-grade,
non-migmatitic rocks over older (Neoproterozoic),
higher-grade, migmatitic rocks. This relationship, and
top-to-the-east shear sense indicators show that final
displacement on the fault was in a normal sense (Barnes
& Prestvik, 2000).
Pelitic gneisses of the lower, high-grade
unit
The pelitic gneisses vary from micaceous to quartzofeldspathic (Electronic Appendix 1, which may be downloaded from the Journal of Petrology website, at http://
www.petrology.oupjournals.org.). Probable protoliths include shales, wackes, and sparse feldspathic arenites. Most
of the unit is migmatitic, but non-migmatitic mediumgrained schists are present. These schists are typically
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calcite rich and some contain calcic amphibole. As such,
their CaO-rich bulk composition prevented partial melting. The migmatitic gneisses underwent two distinct
periods of anatexis. The first was a regional event (regional
migmatites) associated with early peraluminous magmatism in the Batholith, whereas the second was local
and related to emplacement of the Velfjord plutons
(contact migmatites).
Regional migmatites
The regional pelitic migmatites are predominantly
layered (stromatic) migmatites with millimeter- to centimeter-scale layers of alternating quartzofeldspathic
(leucosome) and mica-rich material. They typically lack
melanosomes. Less abundant vein migmatites display
discontinuous centimeter-scale leucosomes. Diatexites
(migmatites that lack internal structure) are sparse; they
are intercalated with stromatic and vein migmatites. All
types are foliated, with foliation defined by aligned biotite
and sillimanite ± staurolite ± muscovite (Table 1;
Electronic Appendix 1). They also all contain calc-silicate
pods that range from a few centimeters to several meters
in length. The various migmatite types are intercalated
with sparse marble lenses and pods of amphibolite.
Quartz veins and felsic dikes are common. The dikes
vary from garnet two-mica monzogranite and biotite
pegmatite to biotite hornblende tonalite; leucotonalite
and leucomonzogranite are most common.
Sausfjellet contact zone
Fig. 2b,c. (a) Simplified geologic map of the Velfjord plutons and
their host rocks, after Myrland (1972) and Barnes et al. (1992). East of
the Velfjord plutons, the peridotite and non-migmatitic cover sequence
overlie the plutons and migmatitic host rocks along an east-dipping,
normal-displacement fault. The areas of (b) and (c) are outlined by
boxes. (b) Geologic map of the western contact and wall-rocks of the
Sausfjellet pluton. (c) Geologic map of contact relations along the
southern Akset–Drevli pluton. All sample numbers are keyed to descriptions in Electronic Appendix 1.
Within 1 km of the Sausfjellet pluton (Fig. 2b), the pelitic
rocks are primarily diatexitic. Distinct blocks of stromatic
migmatite can be identified but are discontinuous along
strike. Calc-silicate blocks (schollen) are typically broken,
with tonalitic (‘low-K’) leucosomes filling fractures (e.g.
Fig. 3f ). In this zone, folds are locally intruded by
leucosomes parallel to axial planes. Such features are not
observed outside the aureole of the pluton.
Less than 500 m from the pluton, the migmatitic rocks
consist of well-foliated to discontinuously banded diatexite
(essentially schlieric banding defined by sillimanite ±
biotite) that is cut by isotropic diatexites. The latter units
truncate foliation in the former, have weak or no foliation,
and typically have hypidiomorphic granular texture.
These cross-cutting, isotropic diatexites commonly contain biotite-rich clots, which are thought to be remnants
of disrupted biotite-rich compositional bands. Locally,
the isotropic diatexites enclose blocks of foliated diatexite,
as well as pods of massive quartz, and calc-silicate schollen
with late-stage tonalitic leucosomes in fractures, as described above.
The pluton–diatexite contact is locally cut by dikes of
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Table 1: Summary of petrographic characteristics
General rock type
Range of
fe-no.
fe-no.
Essential minerals
plag An%
biotite
garnet
n.d.
0·60–0·52
0·89
Accessory minerals
Textures
Regional (stromatic and vein) migmatites
plag, biotite, sillimanite, staurolite, garnet, quartz, musc
Lepidoblastic; micas, sillimanite, and staurolite define
ilm, zirc, Ksp, tour,
pyrr
foliation
Diatexitic migmatites >50 m from plutons
plag, quartz, biotite, garnet, sillimanite
37–12
0·51
0·89–0·79
st, ilm, tour, zrn/mnz
plag, quartz, biotite, garnet, sillimanite ± cordierite
45–23
0·50–0·48
0·80
ilm, zrn/mnz, st
± kyanite ± K-feldspar
87–52 (rare)
0·43–0·32
35–22
0·61–0·55
0·88
musc, ilm, zrn
62–41
n.d.
n.a.
ap, sill, musc, ilm, zrn
plag, Ksp, quartz, garnet, biotite
34–32
0·59
0·88
ap, ilm, zrn or mnz
[sample N17.91 contains opx with fe-no. 0·58]
46–27
0·62–0·54
0·84
Foliated, locally with biotite-rich clots and schlieren
Diatexitic migmatites <50 m from plutons
Weak foliation or massive; hypidiomorphic granular
High-K leucosomes
plag, Ksp, quartz, sillimanite, garnet, biotite
Weak foliation or massive; hypidiomorphic granular
Low-K leucosomes
plag, quartz, biotite
Massive; hypidiomorphic granular
Contact granites
Weak foliation or massive; hypidiomorphic granular
ap, ilm, zrn or mnz,
spinel
Mineral abbreviations: plag, plagioclase; musc, muscovite; Ksp, K-feldspar; ilm, ilmenite; zrn, zircon; mnz, monazite; ap,
apatite; tour, tourmaline; pyrr, pyrrhotite; st, staurolite. Biotite adjacent to garnet is typically more magnesian than biotite
isolated from garnet. n.d., not determined; n.a., not analysed.
feldspar-phyric garnet-bearing biotite quartz monzonite
to granite (Fig. 3d). As will be shown below, such rocks
are characteristic of these contacts. Therefore, for the
sake of simplicity and despite their range of modal
variation, the dikes are referred to as ‘contact granites’
(Table 1; Electronic Appendix 1). Diatexite dikes intrude
the southwestern margin of the Sausfjellet pluton. These
dikes have limited lateral extent and they commonly
brecciate the host gabbro and diorite. The contact migmatites, contact granites, and pluton are cut by dikes of
medium-grained, equigranular leucocratic monzogranite
and tonalite (Electronic Appendix 1).
of <500 m. Furthermore, at its southern contact, the
Akset–Drevli pluton is separated from migmatites by a
band of contact granite of 100–200 m width (Fig. 2c)
identical to the contact granite adjacent to the Sausfjellet
pluton. Mafic dikes intruded the contact diatexites while
the diatexites were still molten. This resulted in formation
of mafic pillows in, and magma mingling with, leucosome
magma-rich zones of the diatexites (Fig. 3h). Hybridization during mingling formed garnet-bearing mafic
enclaves in variably granitic to diatexitic dikes.
Hillstadfjellet contact zone
Akset–Drevli contact zone
A similar change from stromatic to diatexitic migmatites
can be seen along the southern margin of the Akset–Drevli
pluton, although the transition occurs over a distance
The contact between the Hillstadfjellet pluton and migmatites is exposed along Sørfjord and Heggefjord (Fig.
2a). Leucosome-rich diatexite with blocks of stromatic
migmatite and calc-silicate is typically in contact with
the pluton. Diatexite dikes intrude the margins of the
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pluton and hybridization between the diatexite and the
pluton locally produced hornblende-rich garnet-bearing
diorite.
Medium- to fine-grained leucosomes are generally
leucogranite (‘high-K leucosomes’); but sparse leucotonalite (low-K leucosomes; Table 1 and Electronic
Appendix 1) cuts the high-K leucosomes. Stringers of
leucogranite are connected to larger pods of leucosome
(Fig. 3a), which are elongate parallel to foliation and
interfinger with the diatexite along foliation planes. Some
leucosomes are intimately folded with the host diatexite
(Fig. 3b) but the same leucosomes contain folia and
disrupted fragments of the host migmatite. Along Heggefjord, contact granite dikes intrude, and are locally
hybridized with, Stage 1 diorites (location N40.91, Fig.
2a), whereas along Sørfjord, contact granite is sparse, is
typically elongate in the plane of foliation, and is pinched
out along foliation-parallel shear zones.
The diatexites, leucosomes, and contact granite were
intruded by fine- to coarse-grained dioritic dikes (Fig.
3c). Along strike these dikes are broken into angular to
cuspate mafic enclaves; some dikes have cuspate margins
against contact granite. Such features are identical to
mingled synplutonic dikes that are common in many
granitic plutons (see Barnes et al., 1986) and indicate
that mafic magmas were emplaced into the migmatitic
margins of the plutons while the migmatites were partly
molten.
Late-stage, near-solidus deformation formed a strong
foliation parallel to the intrusive contact. The intensity
of the foliation is greater in the contact migmatites than
in the pluton; the migmatites locally have S–C fabrics
with down-to-the-east sense of shear.
Ages of migmatization
The proximity of contact granites to the Velfjord plutons
suggests that the granites resulted from contact anatexis
associated with pluton emplacement (Barnes et al., 1992).
This idea was tested with U–Pb (SHRIMP) dating of
zircon from contact granite sample N19.91 collected
adjacent to the eastern contact of the Akset–Drevli pluton
(Fig. 2a). Details have been presented by Yoshinobu et
al. (2001). Zircons from sample N19.91 yield a bimodal
age distribution, with a cluster of five ages with mean
467·8 ± 4·9 Ma and a cluster of six ages with mean
447·1 ± 3·7 Ma (ages at 95% confidence interval). The
467·8 Ma age is within the age range of the older
anatectic granites of the Bindal Batholith (see above). It
is interpreted to represent the age of regional migmatization in the Velfjord area. The 447·1 Ma age is
identical to that of the adjacent pluton (448 Ma; Pedersen
et al., 1999) and is consistent with formation of the granitic
body by contact anatexis.
GEOCHEMISTRY
Elemental data
Analytical methods are described in the Appendix.
Sample locations are shown in Fig. 2. Major and trace
element compositions of granitic rocks, migmatites, and
host rocks are given in Electronic Appendix 2, which
may be downloaded from the Journal of Petrology website,
at http://www.petrology.oupjournals.org.
Figure 4 gives a normative classification (Barker, 1979)
of the leucosomes, contact granites, and diatexitic dikes;
it illustrates the range of rock types among the contact
granite group, from granite to tonalite. Other geochemical distinctions between the various migmatites,
non-migmatitic metasedimentary rocks, and granitic
rocks are illustrated in Fig. 5, which shows variations of
K2O, CaO, Mg/(Mg + Fe), and normative corundum
as a function of SiO2 content. The tie-lines in Fig. 5a
and b connect compositions of leucosomes and adjacent
diatexites. The single analyzed diatexite–high-K leucosome pair was sampled in the southern contact zone of
the Akset–Drevli pluton.
Compositional differences between the two types of
leucosome (low vs high K) and the contact granites are
shown in Fig. 5. The low-K leucosomes have K2O
contents <2·4 wt %, CaO contents from 2·5 to 4·6 wt %
and Mg/(Mg + Fe) >0·43. In contrast, the high-K
leucosomes have K2O from 2·5 to 5·8 wt %, CaO
contents <1·5 wt % and Mg/(Mg + Fe) between 0·3
and 0·4. All of the leucosomes have SiO2 contents
>68 wt %, with the high-K leucosomes between 73 and
80 wt % SiO2.
The contact granites display a wider SiO2 range than
any other granite group (Fig. 5) and all but two of them
have SiO2 contents <69 wt %. The two high-SiO2
exceptions consistently plot in or near the field of highK leucosomes. The contact granites show decreasing
CaO with increasing SiO2 and variable K2O contents
(Fig. 5). Although characterized by considerable scatter,
it is apparent that the contact granite trend is not collinear
with the compositional trends of the migmatites (diatexites
and bulk stromatic migmatites).
All of the pelitic migmatites have SiO2 <65 wt %. On
average, the stromatic migmatites display higher bulk
SiO2 than the diatexites, from 62 to 65 wt %. Most
samples are low in CaO (<1·5%; Fig. 5b); these samples
show negative correlation between CaO and Al2O3 (not
shown). The few high-CaO, low-Al2O3 migmatites are
similar in composition to non-migmatitic rocks from the
area, except that the latter have higher Mg/(Mg + Fe)
(Fig. 5c).
The migmatites are conspicuous in their high normative corundum (Cor) contents relative to the granitic
rocks and non-migmatitic metapelites (Fig. 5d). The
migmatites plot on a steep trend nearly perpendicular to
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Fig. 4. An–Ab–Or normative classification (Barker, 1979). The gray shaded field is the compositional range of low-K leucosomes; the diagonal
ruled field is the range of high-K leucosomes. N17 is sample N17.91, a garnet opx tonalite; T is the average composition of the 481 Ma
tourmaline two-mica granite in northwestern Velfjord (Fig. 2a).
the trend of the contact granites. Stromatic migmatites
are typically less Cor rich than the diatexites. Diatexitic
dikes have high Cor values and plot with the diatexites
rather than the granites. If, as field relations suggest, the
contact diatexites formed by contact melting of regional
stromatic migmatites, then their overall higher normative
Cor suggests that melt was lost from the diatexites, leaving
them with refractory compositions.
Isotopic data
Samples for oxygen, Sr, and Nd isotopic analysis were
chosen to be representative of the mineral assemblages
of each group and to encompass their ranges of major
element compositions. Analytical methods are presented
in the Appendix. One of the plotted contact granite
analyses is taken from Nordgulen & Sundvoll (1992) and
Birkeland et al. (1993).
Among the leucosomes and migmatitic rocks, Nd(448
Ma) ranges from −5·3 to −9·6 (Table 2); however, all
but one sample have values more negative than −7·6
(Fig. 6a). The contact granites range from −5·2 to −7·5.
Although there is minor overlap between the contact
granites and the migmatites, the contact granites have
generally higher Nd values. For the purposes of comparison, the range of Nd for the Velfjord plutons is +0·3
to −3·8 (average −2·0; Fig. 6a; C. G. Barnes, T. Prestvik
& B. Sundvoll, unpublished data, 1994) and Nd values
in metapelites from the structurally higher non-migmatitic
terrane average −11·3.
Oxygen isotope ratios for whole rocks and quartz
separates are given in Table 2. For samples in which
Fig. 3. (a) Fine-grained, banded, K2O-rich biotite muscovite leucogranite leucosome along the NW contact of the Hillstadfjellet pluton. The
leucosome interfingers with adjacent diatexite; biotite bands in the leucosome are probably part of the diatexite. Hammer head is 13 cm long.
(b) High-K leucosome interfingered and folded with diatexite along the NW contact of the Hillstadfjellet pluton. (c) ‘Synplutonic’ dioritic dikes
in contact migmatite along the northwestern Hillstadfjellet contact. Dikes show bulbous, pinch-and-swell shapes and grade into linear enclave
swarms (left side of photograph). Hammer handle is 52·5 cm long. (d) Block of contact granite enclosing a fusiform mafic enclave; location N83.
(e) Garnet sillimanite biotite diatexite (on left) brecciating medium-grained diorite of the NW Hillstadfjellet pluton. (f ) Calc-silicate schollen in
diatexitic migmatite, NW Hillstadfjellet pluton. (Note leucosome collected in low-strain zones adjacent to the rigid calc-silicate blocks.) Coin is
22 mm in diameter. (g) Pocket of K2O-rich, garnet-bearing muscovite biotite leucogranite leucosome south of the Akset–Drevli pluton adjacent
to the contact granite zone. The leucosome pocket formed amid broken, massive, calc-silicate blocks (left side of photograph) and diatexitic
migmatite (right side of photograph). Field of view is >30 cm. (h) Sketch of a mingled dike that cuts migmatite south of the Akset–Drevli pluton;
location N155. Mafic pillows and hybrid zones are present in a matrix of diatexite and leucosome. The contacts between diatexite and leucosome
are gradational over >2 cm.
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Fig. 5. Variation of K2O (a), CaO (b), Mg/(Mg + Fe) (c), and normative corundum (d) as a function of SiO2. Tie-lines connect coexisting
diatexite (or stromatic migmatite) and leucosome compositions. The gray shaded field is the compositional range of low-K leucosomes; the
diagonal ruled field is the range of high-K leucosomes. N17 is sample N17.91, a garnet opx tonalite. T represents the average of five samples
from the 481 Ma tourmaline two-mica granite west of Velfjord (Fig. 2a; Nordgulen, 1992).
both were measured, quartz is >1‰ higher than wholerock values, which is typical of quartz–whole-rock fractionation at high T (e.g. Taylor & Sheppard, 1986).
Whole-rock 18O values for stromatic migmatites and
diatexites range from +10·9‰ to +13·5‰ (Table 2;
Fig. 6b). This range of values is nearly identical to that
of the leucosomes. The contact granites have 18O values
from +9·6‰ to +12·3‰, which are slightly lower
than in the migmatites. In contrast, the oxygen isotopic
compositions of the Velfjord plutons range from +5·9‰
to +9·8‰ and average +7·5‰.
Initial Sr isotope ratios (calculated for 448 Ma) are
available only for the Velfjord plutons and the contact
granites. The former range from 0·7057 to 0·7070
(Nordgulen & Sundvoll, 1992; C. G. Barnes, T. Prestvik
& B. Sundvoll, unpublished data, 1994) and the latter
from 0·7123 to 0·7223 (Fig. 7; Table 2). The contact
granite values are within the range of metasedimentary
rocks from the region reported by Nordgulen & Sundvoll
(1992). However, Nordgulen & Sundvoll (1992) did not
analyze samples of the high-grade migmatites exposed
adjacent to the Velfjord plutons.
DISCUSSION
Models for development of the contact migmatites and
the origins of the contact granites must explain the
following:
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ANATEXIS AND HYBRIDIZATION IN A CALEDONIAN ARC, NORWAY
Table 2: Nd and oxygen isotopic compositions
Sample
Sm
(ppm)
Nd
147
(ppm)
144
Sm/
±
Nd
143
Nd/
144
Nd
±
143
Nd/144Nd Nd
448 Ma
448 Ma
T(DM)
18O
18O
(Ga)
wr
qtz
Granitic dikes intruding the Velfjord plutons
75.12
11·4
N27.91
12·8
10·1
N10.97
11·2
N64.99
12·8
Contact granites
90.26
12·3
91.07A
11·4
13·1
N17.91
13·762
77·855
0·10762
0·00027
0·512120
0·000005
0·511804
−5·2
1·23
9·6
N19.91
17·672
98·655
0·10906
0·00027
0·512114
0·000005
0·511794
−5·4
1·26
12·0
12·4
11·4
13·3
N21.91
10·5
N20.97
12·7
N33.97
14·4
N34.97
11·4
N53.99B
11·3
N80.99
10·2
Stromatic migmatites
N28.97
12·1
N92.99A
12·915
66·271
0·11865
0·00028
0·512018
0·000007
0·511670
−7·8
1·52
12·1
N122.99
13·339
69·869
0·11623
0·00028
0·511974
0·000007
0·511633
−8·5
1·55
12·9
N144.00
10·736
57·561
0·11355
0·00028
0·511948
0·000007
0·511615
−8·9
1·55
10·9
82·374
0·11309
0·00028
0·512006
0·000007
0·511674
−7·7
1·46
13·5
Diatexitic migmatites
N14.97A
15·301
N14.97B
14·8
N26.97
N36.97
14·0
11·342
60·389
0·11435
0·00028
0·511989
0·000007
0·511653
−8·1
1·50
13·449
70·228
0·11059
0·00029
0·512013
0·000007
0·511689
−7·4
1·50
N102.99
N165.00
10·9
12·6
11·7
Diatexitic dike
N91.99
11·3
High-K leucosomes
N14.97C
N35.97
15·4
4·204
21·737
0·11775
0·00028
0·511926
0·000007
0·511580
−9·6
1·64
12·2
N69.99
11·1
N118.99A
11·9
Low-K leucosomes
5·617
30·285
0·11292
0·00028
0·512002
0·000007
0·511671
−7·8
1·46
14·2
N92.99B
6·424
32·965
0·11864
0·00025
0·512028
0·000007
0·511680
−7·6
1·50
11·9
N161.00
13·768
83·566
0·10031
0·00025
0·512092
0·000007
0·511798
−5·3
1·19
12·7
N15.97
N24.97
12·9
Non-migmatitic schist, from aureole of Hillstadfjellet pluton
N90.99D
13·8
Pelitic schists of the overlying nappe
N136.00
4·753
23·756
0·12181
0·00028
0·511840
0·000007
0·511483
−11·5
1·84
12·8
N138.00
5·783
30·739
0·11454
0·00029
0·511835
0·000007
0·511499
−11·1
1·72
13·0
wr, whole-rock composition 18O values in per mil (V-SMOW).
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Fig. 6. Isotopic compositions. Variation in (a) Nd (calculated 448 Ma)
and (b) whole-rock 18O as a function of SiO2 content of the rock.
Values are per mil (V-SMOW). N17 is sample N17.91, a garnet opx
tonalite. The circled crosses are analyses of pelitic schists from the
nappe that overlies the Velfjord plutons and their migmatitic host
rocks (samples N136.00 and N138.00; Table 2). The remaining cross
represents a non-migmatitic calcareous schist from the eastern aureole
of the Hillstadfjellet pluton.
(1) close to the Velfjord plutons, the migmatites become predominantly diatexitic and isotropic, cross-cutting diatexites are present;
(2) the contact granites are proximal to the Velfjord
plutons, locally intrude and hybridize with them, yet are
more intensely penetratively deformed than the adjacent
diorites;
(3) the mineral assemblages in the diatexites, high-K
leucosomes, and contact granites are similar (garnet,
NUMBER 12
DECEMBER 2002
Fig. 7. Comparison of Velfjord contact granites with granitic rocks of
the Bindal Batholith. (a) Plot of Sr content (ppm) vs SiO2 shows that
the bulk of Bindal granitic rocks are Sr rich. In contrast, the >447
Ma contact granites are similar to older (466–481 Ma) Bindal anatectic
granites (e.g. Vega, Fig. 1) and tourmaline granites. Relatively low Sr
contents also characterize the Andalshatten, Gaupen, and Heilhornet
plutons (Fig. 1), which are coeval with the Velfjord plutons and contact
migmatization. (b) Initial 87Sr/86Sr (448 Ma) plotted against 1/Sr shows
the high initial 87Sr/86Sr of the contact granites, Bindal anatectic and
tourmaline granites compared with the Velfjord diorites and the bulk
of Bindal granitic rocks. Contact granite samples labeled with an ‘S’
were collected adjacent to the Svarthopen pluton (Fig. 2a). The
Andalshatten and Gaupen plutons have relatively high initial 87Sr/86Sr
compared with the Velfjord diorites, which suggests hybridization with
anatectic magmas near the level of emplacement. Data from Barnes et
al. (1992), Nordgulen (1992), Nordgulen & Sundvoll (1992), and this
paper.
biotite, plagioclase, quartz ± alkali feldspar ± sillimanite), whereas the low-K leucosomes typically lack
garnet and alkali feldspar;
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ANATEXIS AND HYBRIDIZATION IN A CALEDONIAN ARC, NORWAY
(4) the compositional trend of the contact granites is
distinct from that of the migmatites;
(5) the most siliceous contact granites are compositionally similar to the high-K leucosomes;
(6) the contact granites have a U–Pb age identical to
that of the Velfjord plutons;
(7) contact granites are coeval with the high-K
leucosomes, but are isotopically distinct from them;
(8) low-K leucosomes are the youngest leucosome type.
Our first consideration is whether development of
diatexitic migmatites is related to emplacement of the
Velfjord dioritic magmas. This close association was
originally interpreted to result from contact melting
caused by heat from the plutonic magmas (Barnes et al.,
1992). The alternative is that the pluton-ward transition
from stromatic to diatexitic migmatites and the presence
of isotropic, cross-cutting diatexites adjacent to the plutons is fortuitous. We find the latter explanation unlikely
in view of the absence of voluminous diatexites distal
from mafic plutons.
Our second consideration is the origin of the leucosomes and the contact granites; specifically, whether their
source was the local diatexitic migmatites. A local origin
for the high-K leucosomes is consistent with their mineral
assemblage and the broad overlap of oxygen and Nd
isotope compositions; however, the data do not rule out
melt migration within the migmatite by vertical or lateral
flow. This is particularly true in view of the evidence for
decompression while the migmatites were still partly
molten (Barnes & Prestvik, 2000). A local origin for the
low-K leucosomes is likewise consistent with the isotopic
data and the prevalence of such leucosomes in boudin
necks and filling fractures in schollen blocks. If such is
the case, then an origin of high-K and low-K leucosomes
from the same source must be explained.
Finally, the isotope data show that the contact granites
cannot be simple partial melts of the local migmatites.
Thus, the contact granites could represent magmas from
a deeper, isotopically distinct source, or they could represent the effects of local partial melting combined with
hybridization (magma mixing). In this regard, it is noteworthy that feldspar-phyric plutonic rocks are common
in the Bindal Batholith (Nordgulen, 1992). These plutons
are characterized by K-feldspar phenocrysts, by Sr contents that reach 800 ppm at 67% SiO2, and by initial
87
Sr/86Sr <0·710 (Fig. 7; Nordgulen & Sundvoll, 1992).
In contrast, the Velfjord area contact granites contain
K-feldspar and plagioclase phenocrysts (Electronic Appendix 1), commonly carry accessory garnet and sillimanite, have Sr contents no higher than 320 ppm, and
have initial 87Sr/86Sr between 0·7123 to 0·7223 (Fig. 7).
These features, combined with the gradation between
coarse leucosomes and contact granites, suggest a local
origin for the contact granites. In the following sections,
we discuss the origin of the leucosomes and of the most
siliceous contact granites. We then consider possible
petrogenetic links between these high-SiO2 rocks and the
range of contact granite compositions. This is followed
by a discussion of possible implications for transfer of
anatectic magmas, magma mixing in a migmatitic realm,
and the significance of such activity in the Bindal Batholith.
Origin of the leucosomes
Recent experimental studies on partial melting of pelitic
and semipelitic rocks [summarized by Patiño Douce
(1999)] permit direct comparison of observed leucosome
compositions with experimentally produced glasses. Any
such comparison must consider the possibility that leucosomes do not represent melt compositions but instead
are partial cumulates (e.g. Brown et al., 1995b; Sawyer,
1998) or contain appreciable amounts of residual minerals. Figure 8 is a plot of molar K, (Na + Ca), and (Fe
+ Mg + Ti) for the granites and migmatites, along with
compositions of constituent minerals. The compositional
ranges of experimentally produced glasses are shown for
biotite dehydration melting of metawacke, muscovite
dehydration melting of biotite muscovite schist, and H2Osaturated melting of metawacke. (See the caption of Fig.
8 for data sources.)
Three of the high-K leucosomes and the most siliceous
(i.e. low Fe + Mg + Ti) members of the contact granites
(seven samples) plot in or near the fields for dehydration
melting of metawacke or biotite muscovite schist (Fig.
8). The experiments show that such melts are in equilibrium with the assemblage quartz + K-feldspar +
plagioclase + biotite ± sillimanite ± garnet; the assemblage present in the high-K leucosomes and contact
granites. At a pressure of 700 MPa, the melting temperatures necessary to form these residual minerals are
in the 750–800°C range (Barnes & Prestvik, 2000).
Previously, it was noted that the contact diatexites are
more refractory than the regional stromatic migmatites
and this relationship is apparent when average compositions are plotted (Fig. 9a). The refractory nature of
the contact diatexites is consistent with loss of melt similar
in composition to the high-K leucosomes. This idea was
tested with linear least-squares mass balance calculations
(Table 3), which show that 22–24% melting of an average
regional stromatic migmatite (Table 3) produces a magma
of SiO2-rich high-K leucosome or contact granite type,
leaving a residue similar to the average diatexite composition (Fig. 9a). The sum of squares of residuals (r2)
is >0·5. The goodness of fit is improved if 2–4% feldspars
are fractionated from the assemblage (r2 >0·2).
Four of the low-K leucosome compositions plot to
the left of the field for H2O-saturated partial melts of
metawacke (Fig. 8), and two other samples plot at much
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Fig. 8. Compositions of analyzed samples in terms of (Na + Ca), (Fe + Mg + Ti), and K, cation proportions (after Solar & Brown, 2001).
Fields represent compositions of experimentally produced glasses from dehydration melting of metawacke (diagonal ruled; Vielzeuf & Montel,
1994; Patiño Douce & Beard, 1996; Montel & Vielzeuf, 1997; Patiño Douce & McCarthy, 1998), dehydration melting of muscovite schist
(vertical ruled; Patiño Douce & Harris, 1998; Pickering & Johnston, 1998) and H2O-saturated melting of metawacke (shaded; Conrad et al.,
1988; Patiño Douce & McCarthy, 1998). Tie-lines connect compositions of adjacent migmatite and leucosome. N17 is sample N17.91, a garnet
opx tonalite; T is the average composition of the 481 Ma tourmaline two-mica granite in northwestern Velfjord (Fig. 2a).
higher proportions of (Fe + Mg + Ti). The lack of
overlap with the field of experimentally produced, H2Osaturated, tonalitic glasses (Fig. 8) is due to the slightly
higher (Fe + Mg + Ti) contents and somewhat lower
K contents in the leucosomes.
One explanation for such compositions is accumulation
of plagioclase ± ferromagnesian minerals (cumulates or
residual phases) in a leucosome magma. Furthermore, the
simplest explanation is to call on such accumulation from
magmas of high-K leucosome type. However, plagioclase
in the low-K leucosomes is more calcic (An62–41) than
plagioclase in the high-K leucosomes (An35–22), is distinctively oscillatory zoned, and has a characteristic blocky
habit (Electronic Appendix 1). Furthermore, some high-K
leucosomes contain K-feldspar phenocrysts; therefore, Kfeldspar should be a cumulus phase in at least some of the
low-K leucosomes. We conclude that even if the low-K
leucosomes are cumulates, they are not cumulates from
magmas of high-K leucosome type, but from a magma
capable of precipitating calcic plagioclase.
Two types of leucosome from a single
source?
The isotopic evidence is consistent with a local origin for
both the high-K and the low-K leucosomes. Furthermore,
field relations indicate that both leucosome types formed
during the contact melting event, and that the high-K
leucosomes formed before the low-K ones. If true, this
relationship requires early, higher-T, dehydration melting
to form the high-K leucosomes (750–800°C; Barnes &
Prestvik, 2000), followed by later, lower-T, H2O-saturated
melting to form the low-K leucosomes (as low as 675°C;
Conrad et al., 1988). This sequence is unusual in that
one would expect H2O to be consumed at the beginning
of the melting process.
A resolution to this contradiction can be found in the
nature of the migmatitic source rocks. In spite of their
residual chemical compositions, most of the contact diatexites retained significant melt fractions, either because
melt could not completely escape or because melt migrated into the diatexite (Sawyer, 1998). In either case, the
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BARNES et al.
ANATEXIS AND HYBRIDIZATION IN A CALEDONIAN ARC, NORWAY
Fig. 9. Models for melting and mixing. (a) At high-pressure conditions, stromatic migmatite compositions melt to yield high-K leucosome–siliceous
contact granite compositions. These rocks represent >23% partial melting. The residue of melting is modeled as a refractory diatexite
composition. (b) High-K leucosome magmas mix with dioritic magmas similar to those of the Velfjord plutons. The hybrid magma also
accumulates variable proportions of plagioclase, K-feldspar, garnet, and biotite. (c) As the diatexites solidify at >400 MPa, H2O is liberated
and promotes local H2O-saturated melting to form the low-K leucosomes.
contact diatexites represent refractory, yet melt-bearing
compositions. Because H2O was concentrated in the
melts during biotite dehydration melting, the trapped
melts would exsolve H2O-rich fluid during decompression
and crystallization (Spear et al., 1999). The common
retrograde development of muscovite in many of the
diatexites attests to this exsolution. We suggest that transport and local collection of exsolved H2O-rich fluid
resulted in local, H2O-saturated remelting of the refractory contact diatexites to produce low-K tonalitic
leucosomes (Fig. 9c). This explanation is consistent with
the higher Mg/(Mg + Fe) values and CaO contents of
the low-K leucosomes, because they formed by melting
of a somewhat more refractory source. A refractory
source is also consistent with the relatively small volume
of these leucosomes.
Compositional variation of the contact
granites
The contact granites are puzzling because they are isotopically distinct from the local migmatites but show
chemical, structural, textural, and geographic evidence
for a local origin (see above). In Fig. 8, the contact
granites define a broad trend that extends away from
mica-dehydration minimum melt compositions toward
the (Na + Ca)–(Fe + Mg + Ti) join. The trend cannot
result solely from variable separation of residual minerals
because many compositional plots lack linearity between
the contact granites and the diatexitic migmatites (Figs
5, 6 and 8). This does not preclude the presence of
residual phases in the contact granites, but indicates
that separation of such phases is not the only cause of
compositional variation.
The contact granites have a phenocryst assemblage
(plagioclase, alkali feldspar, biotite, garnet ± quartz) that
is stable in a narrow T range of 750–800°C (Barnes &
Prestvik, 2000), yet they span a large compositional range.
This suggests that the compositional variations must result
from a process other than variable degrees of partial
melting.
The elemental and isotopic compositions of the contact
granites can be modeled by variable hybridization of
mafic magmas with local, high-K leucosome-like granitic
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JOURNAL OF PETROLOGY
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Table 3: Representative results of mass balance
calculations
Mix
Partial melting of average stromatic migmatite with possible
no.
fractionation
1
Residual diatexite
79%
High-K granitic melt (90.26)
21%
r2
2
0·50
Residual diatexite
56%
Contact granite (N17.91)
44%
r2
3
1·6
Residual diatexite
80%
High-K granitic melt (N53.99B)
Fractionated plag (An31)
24%
−4%
r2
4
0·3
Residual diatexite
83%
High-K granitic melt (90.26)
23%
Fractionated plag (An39)
−2%
Fractionated Kspar (Or88)
−4%
r2
0·2
Mixing high-K and dioritic magmas
± crystal accumulation
5
Simple mixing to form contact granite N17.91
High-K granitic melt (N20.97)
72%
Monzodioritic magma
28%
r2
6
7
1·4
Mixing ± accumulation of phenocrysts
High-K granitic melt (N53.99B)
71%
Monzodioritic magma
11%
Plag (An43)
8%
Kspar (Or88)
9%
r2
0·2
70%
Dioritic magma
7%
Plag (An43)
9%
Kspar (Or88)
8%
Biotite
1·5%
Garnet
3%
Apatite
0·5%
r2
0·1
DECEMBER 2002
member can crudely fit the data (r2 from 1·4 to 2·3).
These simple mixing models assume that the mixing endmembers and products were melts; the models fail to
account for accumulated phenocryst and residual crystals.
Mass balance calculations that couple magma mixing
with crystal accumulation yield mass percentages of felsic
melt, mafic melt, and accumulated crystals in the range:
>70% felsic melt, 7–9% mafic melt, >17% accumulated
feldspars, >3% accumulated garnet, and <1% accumulated apatite and ilmenite. The r2 values for these
models range from 0·03 to 0·05.
Isotopic mass balance (simple mixing) for oxygen and
Nd suggests that contact granites can result from mixing
proportions of >85% magma of high-K leucosome type
and >15% mafic magma. The discrepancy between
major element and isotopic models could result from the
fact that some of the cumulate feldspars and garnet
carry isotopic signatures of the granitic end-member.
Therefore, we interpret the contact granites to be hybrids
of mafic magmas and local, high-K granitic melts, modified by accumulation of phenocrysts ± residual phases.
A consequence of this interpretation is that hybridization need not occur in a single, well-defined locality, but can occur wherever sufficient mafic magma is
available for mixing. At Velfjord, hybridization occurred
in mafic dikes that cut the migmatites (Fig. 3h) and in
contact granite zones adjacent to the Velfjord plutons
(Fig. 3c). Because hybridization was local, non-uniform
elemental and isotopic trends would be expected. Homogeneous hybridization would require convective mixing
of the contact granite magmas.
Migration of the contact granite magmas
Mixing ± accumulation of phenocrysts and residue
High-K granitic melt (90.26)
NUMBER 12
magmas, combined with accumulation of phenocrysts
and residual minerals. A simple mixing scenario was
tested with major element mass balance calculations
(Table 3). If leucocratic contact granitic magma is mixed
with ‘average’ Akset–Drevli or Sausfjellet pluton compositions (Fig. 9b), then mixtures of 20–30% mafic end-
The contact granites are characterized by small proportions of residual material and by relatively low-T
mineral assemblages. As noted above, these factors suggest that the location of the contact granites immediately
adjacent to the Velfjord plutons is not a function of raised
T and consequent greater degrees of melting. This is in
contrast to the increase in proportion of leucosomes and
increased mobility of the diatexites, which clearly are
related to contact melting effects. Instead, we suggest
that the contact granites occur at the boundary between
diatexite and mafic plutons because of rheological gradients imposed by juxtaposition of the mechanically
strong, pyroxene ± melt-bearing plutons and weak,
quartz + feldspar + melt-bearing diatexites.
Because biotite dehydration melting results in a weak
positive volume change (Rushmer, 2001), disruption of
the migmatite crystal framework is critical to development
of significant accumulations of magma. Collection of
leucosomes and contact granite intrusions occurred via
magma migration to regions of lower differential stress
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ANATEXIS AND HYBRIDIZATION IN A CALEDONIAN ARC, NORWAY
and anisotropic effects associated with solidification of
the large, mafic plutons. At the outcrop scale, leucosomes
collected in strain shadows such as boudin hinges (Collins
& Sawyer, 1998) and adjacent to resistant blocks (Fig.
3f and g). During transport of the larger contact granite
bodies, the mafic Velfjord plutons probably behaved as
resistant blocks (‘mega-schollen’) because they reached
their solidus at much higher temperatures than the surrounding migmatites. [The pluton solidi are as high as
>900°C for the Sausfjellet pluton on the basis of pyroxene closure temperatures and as low as 750°C for the
Hillstadfjellet pluton on the basis of hornblende–plagioclase thermometry (Barnes & Prestvik, 2000). The contact
granite wet solidus is >650°C.]
Contact granite magma may have migrated to strain
shadows protected by the resistant mafic plutons (e.g.
southwestern margin of the Akset–Drevli pluton, Fig. 2a
and c). However, because contact granites also occur
within regions that are perpendicular to the maximum
principal shortening direction (e.g. southeastern margin of
the Akset–Drevli pluton, Fig. 2a), other mechanisms must
have been active. We suggest that the temperature and
rheological gradient across the pluton–diatexite boundary
provided an anisotropy into which the contact granite
magmas were focused as dikes (Fig. 10). Such anisotropic
effects could allow magmas to migrate in dikes that are
not in the optimum orientation relative to the maximum
principal shortening and extension directions (i.e. parallel
to the z-axis and perpendicular to the x-axis of the finite
strain ellipsoid; Lucas & St.-Onge, 1995). High-K leucosome magmas and related hybrids could migrate into
these regions and collect to form pockets of granitic
magma. Further contraction of the plutons during solidification and cooling could promote or maintain a strain
field that was optimal for localization of magmas along
the pluton–diatexite boundary. Collection of the contact
granite magma at pluton margins allowed for longer
cooling, acted as traps for additional mafic input and
subsequent hybridization (Fig. 10), and permitted growth
and accumulation of feldspar phenocrysts.
If the plutons acted as mega-schollen, then their rigid
walls may have provided conduits for contact granite
magmas to migrate upward, away from the zone of
melting (Fig. 10). This form of magma transport would
provide a pre-heated channel for magma flow and a
long-lived zone into which anatectic magmas could migrate. We suggest that the large contact granite bodies
adjacent to the Svarthopen pluton (Fig. 2a) were emplaced by such a process and that they collected along
the margins of a larger mafic body at slightly deeper
structural levels.
Diatexites as open magmatic systems
The relationship between migmatites and granitic plutons
has been widely discussed [e.g. see reviews by Brown
(1994), Brown et al. (1995b) and Clemens (1998)]. Stromatic migmatites have clear mechanisms and evidence
of melt transfer, such as veins and concentrations of
leucosome material (e.g. Brown et al., 1995b). In contrast,
diatexites commonly lack field evidence for melt transfer
and loss. On the basis of elemental concentrations, Sawyer
(1998), Milord et al. (2001) and Solar & Brown (2001)
have shown that some diatexites are refractory relative
to their protolith and therefore must have lost a melt
component.
This relationship is apparent in the Velfjord contact
diatexites. In fact, the most refractory compositions are
generally those of the mobile contact diatexites (i.e. dikelike and isotropic cross-cutting diatexites). This implies
either that the melt component migrated to adjacent
diatexites by permeable flow or migrated away from the
local zone of migmatization. The former explanation is
unlikely because few, if any, diatexites have compositions
that indicate addition of a melt component. The latter
explanation is consistent with collection of anatectic
magmas adjacent to the dioritic plutons.
The contact diatexites were open systems not only with
regard to melt loss, but also with regard to addition of
mafic magmas. The presence of mingled, hybrid zones
within the diatexites and along the margins of plutons
(e.g. Hillstadfjellet and Svarthopen), and the isotopic
evidence for mixing in the contact granites, suggests that
dioritic magmas were able to invade and mix with the
contact diatexitic mush. Such hybrid zones can be viewed
as small-scale analogs of the lower-crustal MASH zones of
Hildreth & Moorbath (1988) in which wholesale injection,
melting, and hybridization of the crust occur.
RELATIONSHIP TO OTHER
GRANITES OF THE BINDAL
BATHOLITH
Figure 7a is a comparison of Sr contents in the contact
granites with other granitic rocks in the Bindal Batholith.
The contact granites belong to a group of plutons (e.g.
the Gaupen, Andalshatten, Heilhornet, and Bindalseidet
plutons, and the Alsten Massif, Fig. 1) whose Sr contents
are low relative to the rest of the Batholith (Nordgulen,
1992). Dated plutons in this group have U–Pb ages
[444 Ma (Nordgulen et al., 1993). Low Sr contents also
characterize the older (481–468 Ma) anatectic Vega
pluton and the tourmaline granite west of Velfjord (Fig.
1). In contrast, the few dated plutons in the ‘high-Sr’
field have U–Pb ages Ζ443 Ma (Nordgulen et al., 1993).
Many of the low-Sr plutons are also distinct in their
higher initial 87Sr/86Sr compared with the rest of the
Batholith (Fig. 7b) and at least one (the Gaupen pluton)
has contact granites in its aureole.
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DECEMBER 2002
Fig. 10. Schematic illustration of processes involved in development of the contact granites. (a) Injection of mafic dikes into migmatites in the
aureole causes pillowing of the mafic magma and hybridization with the migmatite. (b) Hybrid magmas and magmas of high-K leucosome type
collect along the stiff contact with a dioritic pluton. (c) Additional injection of mafic magmas into the magma collection zones, with subsequent
disruption and hybridization. Differentiation of the contact granite thus proceeds by hybridization and crystal accumulation. This evolved, hybrid
magma can then feed overlying granitic magma chambers.
The presence of anatectic granites, the lower Sr contents, and higher initial 87Sr/86Sr among the older plutons
in the Batholith suggest that crustal anatexis of pelites
played a prominent role early in the development of the
Batholith. This suggests that (1) pelitic crustal source(s) or
mixing end-members of the lower-Sr plutons underwent
anatexis before 444 Ma, but were not important after
that time, or that (2) the pelitic source rocks were geographically restricted to certain parts of the Batholith.
The latter possibility could result from pre- or synBatholith tectonic imbrication of source terranes. In either
case, additional thermobarometric, geochronologic, and
isotopic data are necessary for a thorough understanding
of the positions of crustal magma sources and the contributions of advected (magmatic) heat, regional metamorphism, and tectonic imbrication before or during
formation of the Batholith.
CONCLUSIONS
Mid-crustal melting in the Helgeland Nappe Complex
is at least locally related to emplacement of mafic (dioritic)
magmas. Following regional high-grade metamorphism
from 481 to 468 Ma, emplacement of mafic plutons at
448–447 Ma caused mica dehydration melting of pelitic
and semipelitic rocks. This contact melting event resulted
in a transition from regional stromatic migmatites to
contact diatexitic migmatites with proximity to the plutons. Leucosomes produced during this event were granitic; they tended to collect adjacent to the plutons. Dioritic
dikes emplaced into the partly molten contact diatexites
and related leucosomes hybridized with them to form
‘contact granite’ magmas. Crystallization of the migmatites resulted in release of H2O-rich fluid, which caused
local, volumetrically minor, H2O-saturated remelting to
produce tonalitic leucosomes.
The higher solidi of the dioritic plutons compared with
the migmatitic and contact granite magmas meant that
the plutons acted as mega-schollen in their migmatitic
host. As a consequence, the plutonic contacts locally
acted as anisotropic boundaries where contact granite
magmas collected. Once in place, the contact granites
trapped additional influxes of mafic magma, and hybridized with them. The hot, rigid, vertically extensive
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BARNES et al.
ANATEXIS AND HYBRIDIZATION IN A CALEDONIAN ARC, NORWAY
plutonic contacts also accommodated upward transport
of the granitic magmas.
Thus, emplacement of mafic plutons into mid-crustal
rocks can result in additional melting of those rocks, can
provide a locus for melt collection and upward transport,
and can provide sites of hybridization. Such crustal
‘intraplating’ may provide a mechanism to explain the
hybrid nature of many ‘anatectic’ granites (e.g. Elburg
& Nicholls, 1995; Collins, 1998).
ACKNOWLEDGEMENTS
We thank M. Barnes for her considerable assistance in
the field and laboratory, and S. Swapp, G. J. DeHaas,
and I. Vokes for help in the laboratory. Thorough reviews
by M. Brown, T. Rushmer, and K. Skjerlie helped us
clarify our arguments; however, they are not implicated
in our conclusions. Partial support for this work came
from NSF grant EAR9814280.
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APPENDIX: ANALYTICAL METHODS
Major elements and Rb, Sr, Zr, Y, Nb, Ba, Cr, Cu, and
Zn were determined by X-ray fluorescence (XRF) at the
Department of Geology and Mineral Resources Engineering, Norwegian University of Science and Technology. Loss on ignition (LOI) was first determined by
the weight loss of adsorbed water by heating 2–3 g sample
powder in a porcelain cup at 120°C overnight (M1).
Subsequently, the weight loss was determined by heating
the samples in the same porcelain cup to 900°C in a
furnace for 5 h (M2). The reported LOI values are M2
– M1.
XRF analyses were made using a Philips PW1480
XRF system. Major elements were determined on glass
beads, which were made by fusing 0·5000 g ignited rock
powder with 5·0000 g LiBO2–LiB4O7 (66:34).
International standards were used for calibration. The
accuracy, the deviation of the ‘true’ value, as determined
on the international standards BCR-2 and GSP-2, is in
general better than 3% with the exception of MnO
(10%). Precision for the major elements is typically better
than 1·5% and better than 4% for the trace element.
Detection limits for trace elements are 2 ppm for Zn,
Cu, and Co; 3 ppm for Zr, Y, Sr, Rb, Ni, and Th; 4
ppm for Pb; 5 ppm for Cr and V; and 10 ppm for Ba.
Oxygen was liberated from silicates using the BrF5
method of Clayton & Mayeda (1963) and converted to
CO2 by passage over a hot graphite rod. Oxygen isotopic
ratios were obtained on a VG SIRA-12 dual-inlet mass
spectrometer. All values are relative to V-SMOW. Silicate
analyses are precise to ±0·2‰. The average and standard
deviation obtained for NBS-28 is 9·45 ± 0·2‰ (standard
error is ±0·02‰).
The Nd isotope ratios were determined by isotope
dilution methods at the Mineralogical–Geological Museum, University of Oslo, and using a VG Isotope VG354
TIMS instrument. The analytical procedure was identical
to that reported by Mearns (1986). During the analytical
work the JM-reference standard gave a value of 143Nd/
144
Nd = 0·51111 ± 0·00005. Errors quoted are 2
standard errors.
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