NORWEGIAN JOURNAL OF GEOLOGY
Geology of the Hortavær Igneous Complex 187
Geology of a magma transfer zone: the Hortavær
Igneous Complex, north-central Norway
Calvin G. Barnes, Tore Prestvik, Melanie A. W. Barnes, Elizabeth Y. Anthony &
Charlotte M. Allen
Barnes, C.G., Prestvik, T., Barnes, M.A.W., Anthony, E.Y. & Allen, C.M.: Geology of a magma transfer zone: the Hortavær Igneous Complex, northcentral Norway. Norwegian Journal of Geology, Vol. 83, pp. 187-208. Trondheim 2003. ISSN 029-196X.
The Hortavær intrusive complex consists of a wide range of plutonic rocks, from gabbro (calcic) to alkali feldspar syenite (alkalic). Emplacement at
456 ± 8 Ma (U-Pb, titanite) was primarily as a series of dikes that range from tens of cm to many meters in width. Magma mingling structures are
ubiquitous and magma mixing occurred locally. Therefore, the intrusive complex can be considered to be a zone of magma transfer and storage rather than a single pool of magma. The complex is characterized by CaO-rich rock compositions and by the presence of minerals typical of assimilation of carbonate-rich metasedimentary rocks, such as calcite, titanite, scapolite, nepheline, Ca-garnet, and idocrase. These features have been interpreted as the result of intense assimilation of host calc-silicate rocks. However, major and trace element compositions show inflections in the magmatic trend, which argues for crystal-liquid separation processes. Furthermore, some syenites are quartz bearing, which suggests a quartzofeldspathic component in the contaminant. When combined, these observations suggest that the Hortavær magmas evolved by assimilation-fractional crystallization of parental gabbroic magmas to evolved syenitic ones. Fractionation was initially dominated by separation of clinopyroxene, which was
stabilized at the expense of olivine because of assimilation of carbonate components of the host rocks. The excessive fractionation of clinopyroxene
forced melt compositions to alkali-rich, silica-undersaturated compositions, rather than SiO2-rich ones expected with olivine fractionation. Such
assimilation was possible because of the open-system behavior of the intrusive complex (magma transfer zone) such that CO2 was lost before it
could stop assimilation reactions. Development of evolved quartz syenitic magmas required a shift to assimilation of the quartzofeldspathic component of the host rocks, which occurred because carbonates are insoluble in such evolved magmas.
Calvin G. Barnes*, Department of Geosciences, Texas Tech University, Lubbock, TX 79409-1053, USA; Tore Prestvik, Department of Geology and
Mineral Resources Engineering, Norwegian University of Science and Technology, N-7491 Trondheim, Norway; Melanie A. W. Barnes, Department of
Geosciences, Texas Tech University, Lubbock, TX 79409-1053, USA; Elizabeth Y. Anthony, Department of Geological Sciences, University of Texas at El
Paso, El Paso, TX 79902, USA; Charlotte M. Allen, Research School of Earth Sciences, The Australian National University, Canberra ACT 0200, Australia; * corresponding author, e-mail: Cal.Barnes@ttu.edu.
Introduction
The Hortavær intrusive complex is exposed on small
islands and skerries northwest of the island of Leka
(Fig. 1). Its intrusive relationships and generally nontectonized nature have been interpreted to indicate that
it is of broadly Caledonian age. The intrusion has long
been recognized for its unusual mineralogy and
geochemistry: augite-rich rocks from Burøya and Vågøya were named "hortite" by Vogt (1916), a name subsequently abandoned by the IUGS. More detailed studies (Gustavson & Prestvik 1979) emphasized the complexity of the intrusive relationships and the mineralogy of the pluton. Both of these studies concluded that
the calcium-rich igneous rocks in the complex were the
result of assimilation of carbonate-rich metasedimentary rocks.
Present studies have focused on obtaining a more complete geologic map, gaining a better understanding of
intrusive relationships, and using geochemical and isotopic data to interpret the petrologic history of the
complex. We found that the intrusion consists of gabbroic, dioritic, monzodioritic, monzonitic, syenitic and
granitic magma types. In all cases except the granites
and gabbro, each rock type shows mutually-intrusive
contacts with the others, such that magma mingling is
commonplace.
We visited most of the islands and accessible skerries of
the archipelago in order to document the intrusive relations, compositional diversity, and petrologic history of
the intrusive rocks. In this contribution, the field relationships and overall chemical and mineralogical features of the pluton are presented. We conclude that the
Hortavær igneous complex represents a zone of intense
magma transfer, as well as a location in which batches
of magma were stored and fractionated. In this sense, it
is distinct from the traditional view of a magma chamber formed by one or a few pulses of magma. This
188 NORWEGIAN JOURNAL OF GEOLOGY
C. G. Barnes et al.
Fig. 1. Geologic map of Hortavær. The Hortavær igneous complex is outlined by the dashed contact. The granitic gneiss that underlies Kvingra is
interpreted as a separate intrusion, not part of the Hortavær igneous complex (see text).
NORWEGIAN JOURNAL OF GEOLOGY
Geology of the Hortavær Igneous Complex 189
Fig. 2. a. Diatexitic host-rock migmatite from Måsøya with blocks of migmatite enclosed in granitic leucosome. b. Folded, banded calc-silicate
screen from eastern Skarvflesa (southern part of central zone). Width of banded zone is approximately 5 meters. c. Garnet-rich banded melasyenite from Lågøyskjæret (northern part of western zone). d. Banded melasyenite from eastern Skarvflesa with residual clots of incompletelyreacted calc-silicate rock (adjacent to flowers on the left).
conclusion has interesting implications for the magmatic evolution of the complex, particularly its ability to
assimilate calcareous rocks.
Field setting
Host rocks of the intrusion are migmatitic gneiss,
quartzofeldspathic gneiss, and quartzite, which crop
out in the southwest and northwest, and marble, which
crops out northeast of the complex (Fig. 1). The quartzites are white, fine- to medium-grained and have
interlayers of medium-grained pelitic schist and gneiss.
The migmatites vary from layered, relatively quartzrich types to diatexitic types, herein referred to as "anatectic granite" (Fig. 1). This latter type typically forms
pods or intrusive masses that disrupt layered migmatite
(Fig. 2a) and on Fuholmen folds in layered migmatite
are surrounded by granitic matrix. Similar diatexitic
rocks underlie much of the Vega massif (~40 km NNE;
Nordgulen, 1993); anatexis has been dated at ~477 to
469 Ma (Yoshinobu et al. 2002). Metamorphic assemblages in semipelitic rocks are biotite + muscovite(?) +
sillimanite + feldspars ± cordierite ± garnet. Such
assemblages are indicative of moderate-pressure migmatization due to muscovite-dehydration melting, and
possible incipient biotite-dehydration melting (e.g.,
Spear et al. 1999).
Quartzitic and migmatitic units are intruded by dioritic and tonalitic dikes, and locally by composite dioritetonalite dikes. On the basis of geochemical data presented below, these dikes are thought to be distinct from
diorites in the Hortavær complex. The migmatitic
rocks are locally in sharp contact with the dikes, especially where the dikes were intruded parallel to foliation
in the migmatite. Elsewhere, the dikes are intruded by
leucosome material and are broken into boudins or
schollen in the migmatite.
190 NORWEGIAN JOURNAL OF GEOLOGY
C. G. Barnes et al.
Fig. 3. Photos from the central zone. a. Melasyenitic clots on diorite
on Vågøya. b. Garnet-bearing melasyenitic clots with dark, amphibole-rich reaction rinds in diorite on Vågøya. c. Dioritic pillows in
coarse-grained syenite on Vågøya. d. Synplutonic dioritic dikes in
syenite on Storfornøyta.
Metasedimentary screens and/or xenoliths throughout
the complex encompass the same lithologies as in the
host rocks. However, in spite of the relative abundance
of anatectic granite in the northwestern host rocks, it
was rarely observed, for example west of Kvåholmen
(613930, 7233420), where sheared dike-like bodies of
garnet two-mica granite are present in a syenitic host.
Screens and xenoliths of calcitic and dolomitic marble
are present throughout the intrusion and calcareous,
stretched-pebble conglomerate crops out on Lågøyskjæret.
In addition to obvious metasedimentary screens, the
complex also contains zones of rock banded on the
centimeter scale in a manner reminiscent of schlieren
banding. Banded zones range from a few square meters
in outcrop to several hundred square meters (e.g., northern Lågøyskjæret and western Kvåholmen), and some
are folded (e.g., eastern Skarvflesa; Fig. 2b). The origin
of such rocks is apparent on eastern Skarvflesa. In this
location, layered calc-silicate screens were intruded by
dioritic magma, which reacted with them (in the magmatic state) to form melanocratic syenite and monzonite. Where this process went to completion, the resultant rocks contain clinopyroxene ± amphibole + poikilitic to megacrystic garnet (Fig. 2c) + calcite ± scapolite
± wollastonite. Locally, the banded melasyenite zones
enclose less-reacted blocks of layered, calcite-rich calcsilicate rocks (Fig. 2d). Where these relationships are
observed, the banding is parallel to layering in the calcsilicates, a relationship that provides important evidence of the assimilative origin of the banded melasyenites/melamonzonites. Similar assemblages and relationships are present where melasyenitic zones are enclosed in syenitic host rocks.
The granitic and syenitic rocks that underlie Kvingra
were mapped by Gustavson & Prestvik (1979) as part of
the intrusive complex. However, this is not clear from
field relations or geochemical data. No masses of granite as large as Kvingra crop out within the rest of the
intrusive complex and Kvingra granites are compositionally distinct from granitic dikes that cut the rest of the
complex (see below). Therefore, we suggest that
Kvingra be treated as a separate intrusive body.
Intrusive relationships within the Hortavær complex
At any given outcrop, it is possible to determine the
cross-cutting sequence. However, it proved difficult to
export such sequences from one island to another, let
NORWEGIAN JOURNAL OF GEOLOGY
Geology of the Hortavær Igneous Complex 191
western syenitic part, the contact between these two
zones, and the broad, dike-like bodies of monzonite in
the eastern and west-central parts of the intrusion.
Central dioritic zone. - On Burøya and Vågøya, the
oldest intrusive rocks are (1) coarse-grained, massive to
foliated diorite and monzodiorite and (2) heterogeneous variably hybridized diorite/monzodiorite, with
probable end members of diorite and coarse-grained
syenite. The heterogeneous diorites typically consist of
cm- to dm-scale ovoid syenitic pods with indistinct
margins in a melanocratic matrix (Fig. 3a). Dark, mmto cm-wide mafic reaction (?) zones
commonly surround these syenitic pods (Figs. 3a, b). In some places,
the diorite lacks syenitic pods but contains abundant
stringers of mafic minerals similar to the reaction
zones. Titanite is typically more abundant in heterogeneous diorite than in massive diorite, and garnet that
ranges from interstitial to six cm in diameter is sparsely
present; it typically occurs in the ovoid pods (Fig. 3b).
Many of these early dioritic rocks are pyroxene- ±
amphibole-rich and contain magmatic calcite and scapolite; they are the "hortite" of Vogt (1916). Locally, the
dioritic dikes are appinitic, with equant amphibole
phenocrysts and interstitial feldspars. (Note that the
term "amphibole" is used for convenience; the range of
calcic amphibole in the complex is discussed below).
Younger rocks in the central zone are predominantly
dikes. These encompass coarse- to medium-grained
syenite, composite and pillowed (Fig. 3c) dikes with
fine-grained diorite in syenitic host, and synplutonic
dioritic dikes in both dioritic and syenitic (Fig. 3d)
hosts. It is noteworthy that diorite in the composite
dikes contains enclaves of more mafic diorite, and that
some composite dikes contain hybrid monzonite.
Fig. 4. Photos from the western zone. a. Monzonitic synplutonic dikes
in syenite from Grøningen. b. A composite dike containing a swarm
of dioritic enclaves in a syenitic host cuts medium-grained monzonite, Kvåholmen. c. Garnet rosettes in a mafic syenitic host on Ørnholmen. Width of field of view is approximately 25 cm.
alone across the entire complex. The common exception to this is that granitic dikes (fine-grained to pegmatitic) were the last to be emplaced. Even this generalization is uncertain, because on southern Langdraget
granitic dikes contain dioritic enclaves, which suggests
the presence of mafic magmas throughout the history
of the intrusive complex. Therefore, we present intrusive relations (prior to the granitic dikes) for four distinct areas in the complex: the central dioritic part, the
Foliation is defined by alignment of pyroxene, plagioclase, and, where present, medium- to fine-grained,
angular to rounded mafic microgranular dioritic enclaves. Where present, compositional banding is parallel
to foliation. In the heterogeneous diorite/monzodiorite, felsic zones may be randomly oriented or flattened
to form a foliation with approximately the same orientation as that in the foliated diorite.
An island at the northwestern end of Burøya (Fig. 1) is
underlain by coarse-grained biotite amphibole olivine
augite gabbro that contains veins and pods of pegmatoidal olivine gabbro with glassy amphibole and feldspar. No contacts between this gabbro and adjacent
dioritic rocks are exposed. However, in view of the
location of the olivine gabbro in the central part of the
pluton, we interpret it to be part of the Hortavær intrusive complex.
Western syenite zone. - In this region, the oldest intrusive rocks are typically coarse-grained syenite. The lar-
192 NORWEGIAN JOURNAL OF GEOLOGY
C. G. Barnes et al.
Fig. 5. Photos from the sheeted zone. a. Alternating intrusive sheets of
diorite and syenite on Kleppan, view to the north. b. Thin sheets of
diorite intruding syenite northeast of Andersøya, view to the northeast. c. Banded and mingled diorite, monzonite, and syenite, northeastern Ørnholmen, view to the south. d. Dioritic dike cutting a composite diorite/syenite dike on Kleppan.
gest exposures of banded screens are present in this
zone (e.g., Kvåholmen, Lågøyskjæret, Sylskjæret). On
the basis of mineral assemblage, they range from
medium- to coarse-grained garnet melasyenite (Fig. 2c)
to nepheline- and scapolite-bearing amphibole pyroxene diorite. The largest of the melasyenite units are
shown in Figure 1.
It is rare to find an outcrop of syenite in this zone that
does not contain fine- to medium-grained enclaves
and/or synplutonic dikes (Figs. 4a). The enclaves and
dikes vary in composition from syenitic through monzonitic to dioritic. In some cases, the enclaves and synplutonic dikes also contain enclaves of more melanocratic rock (Fig. 4b). Some enclaves are blocky and
appear to result from brittle disruption of synplutonic
dikes (e.g., Barnes et al. 1986), others are fusiform to
ovoid (10 to 30 cm long) and still others have bulbous
pillow-like shapes. For many examples of such mingling relationships, see Didier & Barbarin (1991). Enclave abundances can reach as much as 70%, for example in outcrops on the Båsen island group (Fig. 1).
The sheeted zone: the contact between central and western zones. - The transition from central diorite zone to
western syenite zone is at least 500 m wide (Fig. 1). It is
characterized by coarse-grained syenitic host rock with
screens of diorite; all intruded by sub-parallel swarms
of fine- to medium-grained dioritic dikes (Fig. 5a, b)
and composite dikes of fine- to medium-grained diorite and syenite/monzonite. The composite dikes range
from one meter to tens of meters in width and generally consist of fine- to medium-grained dioritic enclaves in coarse-grained syenite. They also vary in degree
of disruption from uniform, straight-walled dikes (Fig.
5a) to broken and hybridized (Fig. 5c). Mafic enclaves
range in size from one cm to many meters and can be
angular, tabular, ovoid, or bulbous. Some composite
dikes display hybrid zones whose color index is intermediate between the enclaves and the host syenite.
These hybrid zones commonly contain mafic enclaves
whose color index is somewhat lower than mafic enclaves enclosed in syenite. Two additional points are of
note. First, composite dikes are not necessarily the
youngest dikes in the sheeted zone. Figure 5d shows a
massive dioritic dike cutting a composite dike. Second,
the sheeted zone is not sharply bounded, particularly
on the western side. Instead, it is gradational, with a
westward decrease in the number of dioritic dikes and
an increase in the amount of disruption of these dikes.
NORWEGIAN JOURNAL OF GEOLOGY
Geology of the Hortavær Igneous Complex 193
Fig. 6. Structural data. a. Poles to planes of magmatic foliation, orientation of host-rock screens, and sheet-like intrusions in the sheeted zone.
Lower hemisphere equal-area plot. b. Rose diagram of dike orientations. See text for discussion.
The strike of the sheeted zone changes along its length
(Fig. 1) from about 010 and moderate westward dip in
the Kleppan group to about 060 and moderate northwesterly dip further north (Fig. 6a). This change in orientation is also reflected in the attitudes of the dioritic
and composite dikes elsewhere in the complex, as
shown in a rose diagram of dike orientations (Fig. 6b).
Maxima from 010 to 050 are reflective of dikes from the
northern half of the complex, whereas the 155 maximum is reflective of the southern half. The E-W maximum represents orientations of late-stage syenitic,
composite, and granitic dikes. This group is underrepresented because many late-stage dikes with this orientation were not measured.
Dike-like monzonitic bodies. - Dike-like zones of monzonitic rock crop out in at least two parts of the complex. The largest of these is a 3 km-long, 500 m-wide
body east of Vågøya. A similar body may underlie Småfornøyta (Fig. 1), but contact relations are not exposed.
Finally, in the contact zone between the syenitic and
dioritic zones (east of Ørnholmen; Fig. 1), a 100 mwide dike cuts dioritic host rocks.
Typical cross-cutting relations in the monzonitic zones
are well exposed on southern Langdraget (Fig. 1) where
monzonite cuts the diorite, but "pods" or "pillows" of
fine-grained diorite are present in adjacent coarse-grained monzonite and the grain size of the diorite coarsens away from the contact. As elsewhere, these mutu-
ally intrusive contact relations are taken to indicate
intrusion of the monzonitic unit while the diorite was
still in a magmatic state.
Late-stage dikes. - Late stage syenitic, granitic, and dioritic dikes are present throughout the intrusion. The
syenitic dikes are generally coarse- or very coarse-grained, but white, fine-grained dikes are also present. The
granitic dikes are commonly fine-grained and have
accessory fluorite. The dioritic dikes are equigranular
and fine-grained. Although the late stage dikes display a
range of orientations, they most commonly have
approximately E-W strike, whereas dike swarms in the
contact zone between western and central parts of the
intrusion strike N-S (in the south) or ENE (in the
north; Fig. 6).
Orientation of magmatic foliation and metasedimentary
screens.
Figure 6 also shows the orientation of magmatic foliation. Foliation in the east-central and southwestern
parts of the intrusion is oriented NNE, whereas foliation in the western and northwestern parts of the intrusion is typically oriented ENE. Qualitative observations
indicate that foliation in metasedimentary screens is
generally parallel to magmatic foliation. In many cases
the long dimensions of these screens share this orientation.
194 NORWEGIAN JOURNAL OF GEOLOGY
C. G. Barnes et al.
Lithology and Petrography
Detailed petrographic descriptions of many of the
rocks types were presented by Vogt (1916) and Gustavson & Prestvik (1979). Rather than repeat their work,
we provide descriptions of samples that contain as-yet
undescribed assemblages and summaries of rock types
described previously. Mineral compositions are summarized in Tables 2, and 3.
Olivine gabbro
Fig. 7. Concordia plot of results of laser-ablation ICP-MS dating of
titanite, plotted using the program ISOPLOT of Ludwig (2002). The
upper intercept represents common Pb composition, the lower intercept the crystallization age of the titanite. Note intercept ages are
inherently less precise than those derived from concordant minerals
which is why zircon (when available) is the mineral of choice for
dating intrusive ages.
These rocks crop out on one island at the north end of
Burøya (Fig. 1). They are medium- to coarse-grained
amphibole olivine gabbro with subophitic texture. Olivine (Fo66 to Fo55) is rounded and commonly enclosed
by augite, amphibole, or plagioclase. The augite is poikilitic, pale pink, and color zoned, with Mg#
(=Mg/(Mg+Fetot) from 0.83 to 0.73. Brown to olive
pargasitic amphibole is poikilitic, reaches at least 1 cm
in diameter, and has Mg# of ~0.65. Plagioclase shows
normal zoning from An77 to An45. Accessory minerals
are pyrrhotite, pyrite, calcite, and biotite.
Geochronology
Diorite and monzodiorite
Gustavson & Prestvik (1979) reported an Rb–Sr isochron age for the Hortavær complex of 471 ± 5 Ma.
Further attempts at isochron dating using either the
Rb–Sr or the Sm–Nd isotope data (data presented in
Barnes et al., in review), failed to obtain consistent
results. This is largely a function of the isotopic variability in the complex (op. cit.). Titanite from sample
91.32H, a biotite amphibole alkali feldspar syenite from
Flatskjæret was dated by laser ablation ICP-MS at Australia National University. Nineteen 54µ-diameter spots
were used to construct a substitute concordia plot
(Figure 7). The data (Table 1) define a chord with an
upper intercept approximating the composition of
common Pb (207Pb/206Pb=0.691±0.07) and a lower
intercept corresponding to an age of 455.7 ± 8.4 Ma,
with an MSWD of 1.05. The goodness of fit of the
chord suggests that what was measured is a mixture of a
single common Pb component, and radiogenic Pb that
has grown since crystallization of the syenite.
These rocks show a wide range of grain size, texture,
and degree of homogeneity. Detailed descriptions are
presented in Vogt (1916) and Gustavson & Prestvik
(1979). They range from fine- to coarse-grained and
their mafic assemblage varies from pyroxene ± amphibole, to biotite + pyroxene + amphibole, to biotite +
amphibole. Clinopyroxene is either pale violet or pale
green; its Mg# varies within the group from 0.65 to
0.25. In some samples clinopyroxene occurs as granular
aggregates surrounded by amphibole. The Al2O3 content of clinopyroxene is rather high compared to typical medium- to low-pressure pyroxene (Al reaches 0.4
atoms per formula unit). Amphibole habit and color
vary widely; it is chemically classified as ferropargasite.
It occurs as 1 to 3 mm diameter ragged prisms with
reddish brown to olive pleochroism, as equant, deep
olive to deep green, subhedral prisms, and as yellowgreen to olive, subhedral to anhedral grains. Amphibole
Mg# varies from 0.47 to 0.24. Biotite occurs as sparse
flakes. Zoned plagioclase has cores of An53-24 to rims of
An28-20. K-feldspar (microcline) is poikilitic to interstitial. Accessory minerals are titanite, apatite, calcite,
minor magnetite and pyrite, and, in some samples, scapolite. Late-stage epidote locally replaces plagioclase.
This result is, within analytical uncertainty, similar to
U-Pb (zircon) ages of mafic plutons of the Velfjord
massif, e.g., 448 ± 2 Ma for the Akset-Drevli pluton and
447 ±2 for the Hillstadfjellet pluton (Yoshinobu et al,
2002) and the Andalshatten pluton (447 ± 7 Ma; Nordgulen et al., 1993). All of these plutons contain volumetrically significant mafic components. The possibility
exists, therefore, that the Hortavær complex is a slightly
older part of this pulse of mafic magmatism in the Bindal Batholith.
Appinitic diorites grade into amphibole pegmatoids.
The typical appinitic diorite has mafic phases in subequal proportions: clinopyroxene forms pale green to
pale olive granular to prismatic crystals that reach 1mm
in length and amphibole (ferropargasite with Mg# 0.33
to 0.24) forms equant, subhedral prisms 1 to 2 mm
Geology of the Hortavær Igneous Complex 195
NORWEGIAN JOURNAL OF GEOLOGY
Table 1. U-Pb-Th isotope composition of Titanite from 91.32H
Anal
No notes
1 excl.
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
610
238U/206Pb
11.227
10.473
11.263
11.742
12.221
12.068
12.207
10.974
12.108
11.892
11.690
12.473
11.067
12.119
12.539
11.913
10.978
10.887
11.213
11.197
4.0005
+1σ
207Pb/206Pb
0.161
0.138
0.175
0.152
0.159
0.150
0.157
0.144
0.154
0.154
0.149
0.151
0.140
0.148
0.155
0.148
0.147
0.136
0.140
0.144
0.17640
0.20212
0.16812
0.14440
0.12270
0.13471
0.14781
0.19466
0.12660
0.14873
0.16647
0.11429
0.18807
0.12146
0.11765
0.14211
0.19114
0.19340
0.17484
0.17207
0.909779
Pbtot
+1σ
U
(ppm)
atomic
(ppm)
0.00286
0.00312
0.00339
0.00209
0.00185
0.00199
0.00196
0.00324
0.00185
0.00188
0.00259
0.00195
0.00306
0.00175
0.00191
0.00217
0.00297
0.00258
0.00250
0.00286
11.22
6.41
8.19
10.00
8.91
7.35
6.61
7.37
10.49
8.02
6.74
9.85
8.70
16.00
10.68
9.18
8.59
14.84
13.29
7.92
423.0
44.5
25.7
38.5
46.1
44.6
35.3
33.5
32.0
48.5
40.7
30.6
53.0
36.3
69.2
56.7
43.4
35.5
54.8
51.4
34.6
471.7
Th/U
6.635
5.608
4.696
5.561
5.247
5.486
4.795
5.186
5.917
4.680
5.470
4.765
5.752
6.750
4.918
5.408
5.802
7.205
6.916
5.558
1.01874
207Pb/235U
+1σ
2.166
2.661
2.058
1.696
1.384
1.539
1.669
2.446
1.442
1.724
1.963
1.263
2.343
1.382
1.294
1.645
2.401
2.449
2.150
2.119
31.356
0.046
0.053
0.043
0.032
0.027
0.029
0.030
0.051
0.027
0.031
0.039
0.026
0.047
0.026
0.026
0.032
0.048
0.044
0.040
0.044
208Pb/232Th
0.02710
0.02975
0.02911
0.02655
0.02559
0.02582
0.02693
0.02929
0.02559
0.02703
0.02735
0.02538
0.02844
0.02496
0.02521
0.02650
0.02844
0.02727
0.02708
0.02775
0.528333
+1σ
0.00034
0.00036
0.00037
0.00034
0.00031
0.00033
0.00033
0.00039
0.00031
0.00033
0.00035
0.00030
0.00032
0.00029
0.00030
0.00032
0.00036
0.00031
0.00029
0.00034
The ANU’s LA-ICPMS facility comprises a Lambda-Physik LPX120i excimer laser and Agilent 7500S quadrupo ICP. A 54 µ beam was used to
ablate 20 spots in 18 grains for 60 seconds to a depth of about 25 µ: 20 s of background, 10s of laser-on stabilization time, and 30 seconds of
laser-on "peak" time. A mass sweep took 0.382 s. Zr and P were measured to check for inclusions, and none was identified. In only one case did
uncertainty in 206Pb/238U exceed twice that expected for counting statistics, and no. 1 was excluded from the intercept calculation. NIST 610
glass was used as the standard with Pb isotope composition determined by Woodhead and Hergt (2000). For the titanite, isotope counts were
background subtracted, ratioed to another isotope, and then on a mass-sweep by mass-sweep (depth) basis, a fractionation factor determined
from the average of 11 ablations of the 610 glass was applied to the corresponding mass sweep in the unknown. Uncertainties quoted here
include uncertainty in the measured average of the standard (1% for 206Pb/238U and 0.5% for 207Pb/235U). Concentration data assume
CaO=26wt%. Fish Canyon Tuff (USGS-FC3) titanite was used as a monitoring standard and twenty analyses yielded a lower intercept age of
27.8 Ma+2.2 Ma (MSWD=2.0). The fine details of emplacement age of the tuff are in debate but probably the best emplacement age is from
U-Pb single crystal TIMS work on zircon which yielded an age of 28.48 + 0.06 Ma (Schmitz et al., 2003).
across; it is deep olive to deep green. Plagioclase (An53
to An26) and perthitic K-feldspar are poikilitic, with
inclusions of clinopyroxene ± amphibole. Some plagioclase was prismatic, primary crystals were originally
~5mm across; they now consist of ~1mm-diameter
subgrains. Accessory phases are titanite (euhedral, to 1
mm long), elongate apatite, and minor magnetite and
pyrite. In pegmatoidal varieties, elongate amphibole
reaches 15 cm in length.
Nepheline-bearing monzodiorite is medium- to finegrained and hypidiomorphic-granular. It is texturally
similar to monzodiorite described above, with granular
to poikilitic plagioclase, poikilitic K-feldspar, and as
much as 10% nepheline, which forms cores of some
plagioclase crystals and is also interstitial to poikilitic.
Where plagioclase rims nepheline, amphibole and
pyroxene are inclusions at the crystal boundaries. The
nepheline typically has ratios of Na/(Na+K+Ca) of 0.82
to 0.76. Accessory minerals are scapolite, calcite, stubby
to prismatic apatite, titanite, biotite, pyrrhotite, and
pyrite, and in one sample, idocrase.
Monzonite
Monzonites in the Hortavær complex are medium- to
fine-grained, have hypidiomorphic granular texture,
and contain pyroxene, biotite, and amphibole. Clinopyroxene is pale green and partly replaced by pale brown
to olive ferropargasite; biotite occurs as yellow-brown
flakes. These samples have variable proportions of plagioclase and K-feldspar. Furthermore, some samples
contain andesine (An36-33) and others oligoclase (An2520), and a single sample (93.45H) contains nepheline.
Accessory phases are fine apatite, pyrite, and zircon (?).
Late, fine-grained epidote replaces some plagioclase.
196 NORWEGIAN JOURNAL OF GEOLOGY
C. G. Barnes et al.
Table 2. Summary of plagioclase, scapolite, and nepheline compositions.
rock mg#
olivine gabbro
91.49H
0.73
91.51H
0.74
93.49H
0.67
diorite/monzodiorite
93.44H
0.33
93.46H
0.34
93.52H
0.49
93.56H
0.39
93.62H
0.39
93.63H
0.44
93.72H
0.40
monzonite/syenite
93.45H
0.078
93.57H
0.370
93.76H
0.156
93.02H
0.100
melasyenite/melamonzonite
91.06H
0.084
91.20H
0.180
93.06H
n.a.
plagioclase
Ca/(Ca+Na)
max
min
scapolite
Ca/Ca+Na)
max
min
nepheline
Na/(Na+K+Ca)
max
min
76.6
76.2
76.8
53.0
64.1
45.0
----------
----------
----------
----------
23.7
36.0
31.1
40.5
52.8
47.8
32.7
20.4
32.8
23.6
24.4
26.3
27.9
----
63.2
74.8
------------61.5
52.4
72.4
----------------
------81.3
---------79.6
------76.0
---------65.0
---25.0
8.8
5.5
---19.7
5.3
1.5
-------------
-------------
81.5
----------
79.9
----------
28.0
29.3
36.7
21.8
17.1
27.8
----------
----------
----------
----------
Syenite
The syenitic rocks are typically coarse-grained, but
range from fine-grained to very coarse-grained. The
felsic assemblage varies, from plagioclase (oligoclase)
syenite to plagioclase-free alkali feldspar syenite, to
alkali feldspar quartz syenite. In all samples, the original alkali feldspar has undergone intense exsolution
and consequent recrystallization. A few elongate, relict
crystals (to several millimeters long; now microcline)
are preserved in a matrix of ragged grains that range
from < 1 mm to 2-3 mm in diameter. Granular, patchy,
and lamellar exsolution are ubiquitous; it is common to
observe bead-like albite surrounding blocky microcline. As a result of this extensive recrystallization, the
nature of the primary alkali feldspar cannot be determined petrographically. Clinopyroxene and amphibole
are the principal mafic minerals and dark brown biotite
is present in some samples. Stubby hedenbergitic clinopyroxene (Mg# from 0.21 to 0.05) is the most common
mafic phase. It is typically green, with weak pleochroism, but varies from yellow green to green to dark
green. In some syenites, clinopyroxene is rimmed and
partly replaced by ferropargasitic amphibole (Mg#
from 0.10 to 0.05), with typical pleochroism from bluegreen to dark green. Still other syenites lack clinopyroxene and contain ferropargasite and brown biotite.
Accessory minerals are titanite, zircon, calcite, and apatite. Quartz is either absent or present in abundances >
5% (quartz syenite).
Melasyenite and melamonzonite (banded zones)
These rocks are remarkably heterogeneous at outcrop,
hand sample, and thin section scales. Grain size is
medium to very coarse. Orange, color-mottled garnet
varies from equant (Fig. 2c), to interstitial, to rosettelike (Fig 4c). Garnet compositions (mole fractions) are
in the range grossular (0.773-0.700), pyrope (<0.005),
almandine (0.001-0.097), spessartine (<0.013), andradite (0.248-0.148). Clinopyroxene is yellow-to-green
pleochroic (Mg# 0.40 to 0.05); some samples also contain large (to 3 mm in diameter) grains with olive cores
(Mg# 0.44 to 0.40). Ferropargasitic amphibole (Mg#
0.25 to 0.10) ranges from nearly opaque deep green to
pleochroic bluish-green to yellow. K-feldspar (microcline) is prismatic, with original crystals ≥ 3 mm long.
Both granular and lamellar exsolution has occurred. In
addition, K-feldspar commonly surrounds garnet, although many clinopyroxene-garnet contacts are in textural equilibrium. Where present, plagioclase ranges
from An37 to An17 and is riddled with fine inclusions of
epidote. Common accessory minerals are interstitial
0.020
0.055
----0.014
0.007
0.027
0.050
0.019
0.009
---
0.25-0.33
0.48-0.64
----0.40-0.44
0.36-0.40
0.05-0.15
0.10-0.24
0.13-0.21
0.10-0.13
---
0.019
0.011
--0.58-0.65
--0.061
0.011
0.019
0.37-0.41
0.44-0.51
0.57-0.77
---
0.033
0.016
---
s.d.
0.73-0.83
0.78-0.83
n.d.
range
* clinopyroxene core compositions. **clinopyroxene rim compositions
gabbro
91.49H
0.677
0.787
91.51H
0.712
0.801
93.49H
0.672
n.d.
diorite
93.46H
0.339
0.391
93.62H
0.387
0.464
monzodiorite
93.56H
0.391
--93.63H
0.439
0.622
93.57H
0.370
0.532
nepheline diorite
93.52H
0.487
0.667
93.58H
0.316
--nepheline monzodiorite
93.44H
0.291
0.300
93.72H
0.404
0.515
monzonite
93.02H
0.100
--93.45H
0.078
0.115
melasyenite/melamonzonite
89.02H*
0.199
0.421
89.02H**
0.199
0.370
91.06H
0.066
0.070
91.20H
0.180
0.154
93.06H
n.d.
0.177
93.78H
0.098
0.109
garnet-bearing biotite granite dike
93.84H
0.320
---
bulk rockclinopyroxene
ave.
Mg#
Mg#
Table 3. Representative compositions of mafic minerals.
---
-------------
-----
-----
-----
-----
-----
0.61
0.67
0.58
olivine
ave.
Mg#
---
-------------
-----
-----
-----
-----
-----
0.63-0.60
0.69-0.65
0.62-0.55
range
---
-------------
---
-----
---
-----
-----
0.15
0.01
0.027
s.d.
---
--------0.105
0.060
0.089
0.092
0.243
0.386
0.424
0.305
0.395
0.419
0.355
0.255
0.290
0.651
n.d.
n.d.
amphibole
ave.
Mg#
---
--------0.005
0.004
0.013
0.009
0.005
0.007
0.025
0.026
0.006
0.013
0.016
0.003
0.018
0.009
-----
s.d.
0.052
0.632
--0.713
0.787
0.772
0.718
-----
-----
-----
-------
-----
-------
0.718
0.021
--0.079
0.045
0.075
0.064
-----
-----
-----
-------
-----
-------
0.028
0.337
--0.199
0.161
0.142
0.209
-----
-----
-----
-------
-----
-------
grossular almandine andradite
garnet (mole fractions)
NORWEGIAN JOURNAL OF GEOLOGY
Geology of the Hortavær Igneous Complex 197
198 NORWEGIAN JOURNAL OF GEOLOGY
C. G. Barnes et al.
Monzogranite is most common, but a few alkali-feldspar
granite and one biotite trondhjemite were collected.
Most of the granitic dikes are leucocratic, with biotite
(Mg# 0.11 to 0.07) as the common mafic mineral. However, a few samples also contain blue-green amphibole
and a few contain hedenbergitic clinopyroxene. Accessory minerals are apatite, Fe-Ti oxides, tourmaline, fluorite, calcite, pyrite, and rare titanite and muscovite.
Granitic dikes in the host rocks northwest of the intrusive complex consist of two-mica trondhjemite, biotite
tonalite, muscovite alkali-feldspar granite, and garnet
two-mica granite. These dikes are typically hypidiomorphic granular and show little evidence of deformation. Biotite in the garnet-bearing sample has much
higher Mg# (0.31) than biotite in granitic dikes within
the intrusive complex. Garnet in this sample is almandine-rich, with average composition (mole fractions) of
grossular, 0.052; pyrope, 0.107; almandine, 0.718;
spessartine, 0.095; andradite, 0.028.
The Kvingra granite ranges from sub-mylonitic to
intensely mylonitized (Gustavson & Prestvik, 1979).
Some samples preserve relict K-feldspar phenocrysts.
Biotite was probably the varietal phase; it is replaced by
chlorite. Accessory minerals are epidote, fluorite, Fe-Ti
oxides, pyrite, calcite, and muscovite.
Geochemistry
Analytical methods
Fig. 8. Geochemical classification according to Frost et al. (2001). The
proposed magmatic trend is shown by the dashed gray line. Samples
labeled with and ‘N’ contain nepheline. a. The FeO/(FeO+MgO)
ratio is calculated using measured FeO contents, therefore not all
samples are plotted. The proposed magmatic trend crosses the boundary between magnesian and ferroan suites. b. The proposed magmatic trend begins in the calcic field and crosses boundaries into the
alkalic field. The compositions of granitic dikes in the Hortavær complex then cross the boundary from alkaline to alkali-calcic. Note the
dissimilar compositions of granitic dikes from the Hortavær complex
and anatectic granites from the western host rocks.
calcite (as much as 6% of the rock), titanite, apatite,
and epidote; some samples contain wollastonite, pyrrhotite (?) or pyrite, and zircon (?) as inclusions in Kfeldspar, and magnetite.
Granitic rocks
Granitic dikes in the intrusive complex are generally
medium- to fine-grained and hypidiomorphic granular.
Mineral analyses were made on a JEOL-JXA733 Superprobe at the University of Wyoming using natural and
synthetic standards and ZAF corrections. Analytical
conditions were 15kV accelerating voltage and 10 na
beam current. Analytical spots were ~1µm diameter
except for plagioclase, nepheline, and scapolite (10 µm).
Major oxides and Rb, Sr, Zr, Y, Ba, V, Cu, and Zn were
analyzed by XRF at the Institute for Geology and Mineral Resources Engineering at the Norwegian University
of Science and Technology or by ICP-AES at Texas Tech
University. The rare earth elements (REE), Cs, Ta, Th,
Hf, and U were analyzed by INAA in three different
labs, as noted in Table 4.
Classification
According to the classification scheme of Frost et al.
(2001), the gabbroic and dioritic rocks range from calcic to alkaline and from magnesian to ferroan (Fig. 8),
with monzonitic and syenitic rocks virtually entirely
alkaline and ferroan. This transition involves enrichment in total Fe and the alkalis within the dioritic
group of rocks to nearly 12% Fe (as FeO) and 8% total
NORWEGIAN JOURNAL OF GEOLOGY
Geology of the Hortavær Igneous Complex 199
Fig. 9. Major element and Sr variation. The shaded field represents the compositional range of clinopyroxene. Samples labeled with and ‘N’
contain nepheline. a. Na2O + K2O versus CaO. Dioritic dikes that intrude host rock migmatites and anatectic granites plot in a trend parallel
to, but offset from the proposed magmatic trend of the Hortavær complex. b. CaO versus Mg/(Mg+Fet) showing locations of dioritic samples.
The field outlined with a dashed line is the compositional range of dioritic and gabbroic rocks from the central zone. The field outlined with a
solid line encompasses diorite compositions from the sheeted zone. c. CaO versus K2O showing the curvilinear magmatic trend and particularly
the decrease in K2O among the most evolved syenites. d. Inset of Na2O + K2O versus CaO for syenitic and granitic compositions. Compositional
fields for cumulate syenites and for quartz-bearing syenites are shown. e. CaO versus TiO2 showing curvilinear magmatic trend. f. Sr versus
CaO showing the curvilinear magmatic trend. Sr-rich samples labeled with a ‘d’ are dikes.
Fig. 10. Chondrite-normalized REE plots. a. Gabbroic and dioritic rocks that are part of the proposed magmatic trend. Values of Mg/(Mg+Fe) are listed after the sample number. b. Diorite and calciumrich diorite ("hortite"; shown with shaded symbols). Values of Mg/(Mg+Fe) listed after sample numbers. c. Monzonitic rocks (unfilled symbols) and melasyenite and melamonzonite (filled symbols). d.
REE patterns for syenites with Sr > 500 ppm. e. REE patterns of syenites with Sr < 500 ppm. f. REE patterns of syenites interpreted to be alkali-feldspar cumulates. Note change in scale. g. REE patterns of
granitic dikes that intrude the intrusive complex. h. REE patterns of granitic rocks from Kvingra and dikes of residue-poor, peraluminous, anatectic granite (diatexite) from the western host rocks. The
shaded field shows that range of the latter samples except for probable feldspar cumulate, sample 91.24H.
200 NORWEGIAN JOURNAL OF GEOLOGY
C. G. Barnes et al.
NORWEGIAN JOURNAL OF GEOLOGY
alkalis (Na2O + K2O). Most granitic dike compositions
plot in the ferroan field and range from alkaline to
alkali-calcic (Fig. 8).
Compositional variation
Figures 8 and 9 exemplify the wide compositional range
of the complex. When granitic dikes are included, SiO2
contents range from 43% to 77%, CaO contents from
24% to 0.4%, and total alkalis from 13.8% to 0.8%.
Figure 8 permits identification of a number of compositional groups; data from the intrusive complex, the
granitic rocks from Kvingra, and granitic and dioritic
dike samples from the western host rocks are plotted.
In Figure 9a, a linear relationship can be defined among
samples of olivine gabbro, diorite and monzodiorite,
monzonite, and syenite. Melasyenitic samples plot to
the right of this line, as do many samples from the dioritic zone. Granitic dikes in the intrusive complex and
Kvingra granites plot to the left of the line; they generally have low CaO contents and total alkalis somewhat
less than the syenites. In contrast, diatexitic granite
samples from the western host rocks have much lower
alkali contents. Although not apparent in this plot, a
compositional gap exists between the dioritic and syenitic rocks. This gap is filled by data from eight monzonitic samples. However, as is apparent from Figure 1,
monzonites constitute a small percentage of the complex and are thus overrepresented in the diagram.
Dioritic dikes that intrude and co-mingle with migmatites in the western host rocks plot in a linear array that
is parallel to, but offset from the trend of diorites from
the intrusive complex. We interpret this latter relationship to indicate that the two sets of diorites are unrelated and therefore do not further consider this group of
dioritic dikes.
Figure 9b shows the range of Mg/(Mg+Fe) values compared to CaO. In particular, it shows the wide range of
CaO contents among the dioritic and melasyenitic
rocks. However, despite this range of CaO values, a
group of diorite analyses plot in a sub-linear array that
extends from olivine gabbro compositions to syenite
compositions. We suggest that if a magmatic trend
exists for the intrusive complex, this array is its closest
approximation. Figure 9b also shows that most of the
diorites that are enriched in CaO relative to the proposed magmatic trend were collected from the central
zone of the intrusion, whereas most diorites from the
sheeted zone plot along the magmatic trend.
Mg/(Mg+Fe) values of the granitic dikes within the
intrusive complex overlap with those of the syenites,
but most are lower than Mg/(Mg+Fe) values of diatexitic granites from the host rocks. Although this overlap
Geology of the Hortavær Igneous Complex 201
of Mg/(Mg+Fe) is interesting, it is noteworthy that the
granitic dike compositions do not plot along the proposed magmatic trend as shown in Figures 9b and 9d.
This suggests that a simple crystal–liquid separation
process cannot relate the granitic dike magmas and the
syenitic magmas. The range of Mg/(Mg+Fe) among the
syenites is also essentially identical to that of the melasyenitic rocks. This is consistent with the fact that mafic
minerals in the syenites and melasyenites are compositionally similar.
K2O variation is shown in Figure 9c. Previously identified compositional groups are apparent in this plot.
Variation of K2O is noteworthy for two reasons. First,
the proposed magmatic trend is not linear: K2O is enriched to values of about 8% (at approximately 2%
CaO), and then decreases in the most evolved syenites.
Samples in the K2O enrichment part of the trend also
have Sr contents >500 ppm, whereas samples in the
K2O depletion part have Sr contents <500 ppm. Additional distinctions among the syenites can be seen in a
plot of CaO versus total alkalis (Fig. 9d). In this plot,
the group of Sr-rich syenites shows negative correlation
between CaO and total alkalis and a group of Sr-poor,
quartz-bearing syenites show nearly constant total
alkali content over a range of CaO contents. A third
syenite group (labeled "cumulate") is characterized by
high total alkalis at low CaO contents (Fig. 9d), high
Al2O3 values (most >18.5%), and positive Eu anomalies (see below).
TiO2 contents in the suite increase from the olivine
gabbro to the most evolved dioritic rocks and then
decrease Fig. 9e). In contrast, the compositional array
of the melasyenitic samples crosses the inferred magmatic trend. Strontium abundances increase from olivine gabbro to evolved dioritic compositions (Fig. 9f),
then decrease among the monzonites and syenites. It
appears that the magmatic trend splits at this point,
with a branch that extends to the Sr-rich syenite group
and a branch that encompasses monzonitic compositions. Two distinct groups exist among Ca-rich diorites.
One group contains the most Ca-rich diorites, which
have Sr <1000 ppm; the second group with CaO from
10 to 16 wt% contains Sr contents from 900 to 1500
ppm (Fig. 9f).
Rare earth element data
The rare earth element plot of an olivine gabbro
(91.49H) shows the most primitive pattern of all samples (Fig. 10a) with a negligible negative Eu anomaly, a
shallow negative slope, and minor enrichment of the
light REE. Dioritic samples from the proposed magmatic trend have higher REE abundances, show light REE
enrichment, and small negative Eu anomalies. Rare
earth element abundances are anticorrelated with bulk
202 NORWEGIAN JOURNAL OF GEOLOGY
C. G. Barnes et al.
Fig. 11. Multielement variation diagrams normalized to the primitive composition of Sun and McDonough, 1989). a. Olivine gabbro and dioritic rocks that are part of the proposed magmatic trend. Samples are listed in order of decreasing CaO content. b. Ca-rich dioritic rocks. Samples
70.24 and 91.44H are the "hortites" of Vogt (1916). Sample 93.44H is a nepheline-rich monzodiorite. CaO contents are listed next to sample
numbers. c. Syenitic rocks, distinguished according to Sr content. The shaded field represents the range of gabbroic and syenitic samples of the
magmatic trend. d. Monzonitic patterns are shown with solid lines, whereas patterns for melasyenite and melamonzonite are shown with dashed lines. The shaded field represents the range of gabbroic and syenitic samples of the magmatic trend.
rock Mg/(Mg+Fe) and CaO content. Other dioritic
samples show similar REE patterns (Fig. 10b); the samples richest in CaO content have the lowest REE abundances. These samples (70.27 and 91.44H) correspond
to the "hortite" of Vogt (1916).
concentrations and positive Eu anomalies. Three of
these samples are dikes (89.08H, 89.14H, and 93.66H)
and the latter sample is fine-grained. This suggests that
residual magma was able to escape from alkali-feldspar-rich mushes.
The transition from dioritic to monzonitic samples
(Fig. 10c) involves an increase in total REE contents
and generally deeper Eu anomalies. In contrast, the
transition from monzonite to syenite (Figs. 10d and e)
shows variable enrichment of light REE but depletion
of heavy REE. Eu anomalies in the Sr-rich syenites are
similar to those of the monzonites, but many of the syenites with Sr < 500 ppm (Fig. 10e) have deeper negative
anomalies. Four syenite samples have patterns consistent with feldspar accumulation (Fig. 10f): low REE
Granitic dikes from the intrusive complex display a
range of REE patterns and abundances (Fig. 10g). Most
samples have cup-shaped patterns and deep negative
Eu anomalies. Light REE abundances are generally
lower than those of the syenites but heavy REE abundances are similar (~10x chondrites). Granitic rocks
from Kvingra show similar variability in REE abundances (Fig. 10h) but the cup-shaped pattern is absent;
instead the middle and heavy REE show a flat pattern.
With one exception, granitic dikes from the western
Geology of the Hortavær Igneous Complex 203
NORWEGIAN JOURNAL OF GEOLOGY
migmatitic host rocks have patterns similar to Kvingra
samples, but with smaller Eu anomalies. The exception
is a two-mica trondhjemite (sample 91.24H), which has
low REE abundances and a positive Eu anomaly, suggestive of feldspar accumulation.
Multi-element diagrams
Primitive-mantle-normalized multi-element patterns
are shown in Figure 11. Patterns for the olivine gabbro
and dioritic samples that lie on the magmatic trend
(Fig. 11a) show the most consistent patterns, in which
Th, Nb, Ta, REE and K abundances increase with decreasing CaO content. The patterns are characterized by
negative anomalies for Nb, Ta, P, and Ti, and positive
anomalies for K and Sr, and arguably for Cs and Rb.
Such patterns are characteristic of magmas erupted in
supra-subduction zone environments, which is consistent with proposed tectonic setting for the Bindal Batholith (e.g., Stephens et al. 1985, Grenne et al. 1999,
Yoshinobu et al. 2002). Dioritic samples that do not
plot on the inferred magma lineage (Fig. 11b) show
patterns similar to those in Figure 11a but with considerably more scatter. The patterns of syenitic rocks
(Fig. 11c) are characteristic of highly fractionated magmas (e.g., Thompson et al. 1984, Thompson & Fowler
1986), with peaks at Cs, Rb, Th, and K and negative
anomalies at Ba, Nb, Ta, P, and Ti. The monzonites have
patterns intermediate between the syenites and diorites
(Fig. 11d). Nb and Ta abundances in the monzonites
are the same as, or slightly higher than those in the diorites and are tightly clustered compared to the syenites,
in which Nb and Ta abundances vary by an order of
magnitude.
Discussion
Style of intrusion of the Hortavær complex
Although it is common practice to think of plutons as
congealed pools of magma in the crust, the Hortavær
complex cannot be viewed in this way. Field relationships clearly show that the complex was emplaced as a
series of thousands of magma batches, many of which
had sheet-like dimensions. Some of these magma batches were emplaced as dikes, but where they encountered older unsolidified magma batches, they mingled and
locally mixed. In other words, there is little evidence for
giant, homogeneous masses of magma. Instead the
complex was emplaced as small-volume pulses that
varied in composition from dioritic to syenitic throughout the history of the complex. Finally, in its late stage,
magma compositions were predominantly granitic.
In light of this style of emplacement, we consider the
Hortavær complex to represent a zone of magma transfer rather than a simple pluton. Intrusive complexes
with this geometry should result in significant subvertical thermal anomalies. In fact, such anomalies should
be larger than thermal aureoles associated with singlepulse plutons because magma that passes through the
system provides more heat than a single magma pulse
(Annen & Sparks 2002, Dufek & Bergantz 2002). In
addition, processes that can be observed at the level of
exposure clearly provide only part of the entire magmatic history, because each magma pulse conceivably
carries its own petrologic history. An example of this
problem at Hortavær concerns the relative proportions
of rocks that represent differentiated magmas (those
belonging to the magma lineage) versus cumulate
rocks. If our interpretation of the magma lineage is correct, then on a volumetric basis, the system lacks the
abundant cumulate rocks necessary to explain the
volume of differentiated rocks. This issue is "solved",
i.e., moved to greater depth, by recognition of the complex as a zone of magma transfer.
Tectonic setting
Trace element patterns (Fig. 11) are consistent with
emplacement of the complex in an arc setting. The 456
Ma titanite age suggests that the Hortavær complex
represents the oldest post-migmatization magmatic
activity in the HNC, followed by 448 to 445 Ma (U-Pb,
zircon) mafic to intermediate plutons in the Velfjord
region (Nordgulen et al. 1993, Yoshinobu et al. 2002).
The tectonic setting of the Velfjord massif and related
plutons was interpreted to be a continental-margin arc
formed near Laurentia during the Taconic orogeny (e.g.
Yoshinobu et al. 2002, Roberts 2003). The Hortavær
complex may be associated with this burst of mafic
magmatism. Arcs are commonly zones of local extension because of trench rollback (Hamilton, 1988; Hawkins, in press). This also fits the Hortavær complex,
which consists of swarms of dikes of locally uniform
orientation. For these reasons, we currently favor the
interpretation that Hortavær magmatism is part of the
Taconic episode.
Magmatic processes
Although the complex shows wide geochemical scatter,
there is a discernible, consistent, magmatic trend. If one
considers the previous discussion concerning mode of
emplacement, it should be clear that "line of descent"
does not imply evolution of a single magma body.
Instead, it implies similar, coincident or parallel lines of
evolution of numerous magma batches in a thick, complex, crustal column. However, the observation of a
magmatic trend, the regular variations in plagioclase,
clinopyroxene, and amphibole compositions, and regular enrichment patters of the incompatible elements
suggests that crystal-liquid separation processes may
have influenced evolution of the magma.
Sample
89.08H 89.14H 91.12H 91.34H
64.29
64.68
62.83
63.22
SiO2
0.02
0.11
0.40
0.24
TiO2
19.76
18.91
16.90
17.00
Al2O3
0.45
0.49
0.38
0.85
Fe2O3
FeO
n.d.
n.d.
3.23
2.01
MnO
0.00
0.01
0.07
0.06
MgO
0.07
0.10
0.16
0.09
CaO
1.82
1.06
3.43
2.88
7.25
4.88
3.82
4.91
Na2O
5.15
8.55
7.87
6.33
K2O
0.01
0.09
0.08
0.02
P2O5
LOI
1.00
0.38
0.35
1.42
Total
99.81
99.26
99.52
99.04
Mg/(Mg+Fet) 0.225
0.291
0.074
0.056
A/CNK
0.95
0.98
0.80
0.84
trace element concentrations in parts per million
Rb
160
361
178
297
Sr
132
210
690
377
Zr
18
54
n.d.
124
Y
0.4
4.0
n.d.
34.0
Nb
b.d.
3
n.d.
9
Ba
152
160
342
477
Sc
0.2
0.8
1.4
0.2
V
n.d.
b.d.
n.d.
b.d.
Cr
b.d.
n.d.
b.d.
b.d.
Ni
n.d.
n.d.
n.d.
n.d.
Cu
b.d.
b.d.
n.d.
b.d.
Zn
4
6
57
54
La
3.2
6.82
31.80
31.64
Ce
3.8
9.28
73.30
61.46
Nd
1.5
3.67
38.80
25.60
Sm
0.25
0.86
6.82
5.59
Eu
0.16
0.65
0.98
1.10
Tb
0.02
0.12
0.64
0.79
Yb
0.07
0.49
1.56
3.40
Lu
b.d.
0.06
0.23
0.52
Co
0.4
0.2
2.8
2.0
Cs
1.13
4.80
0.73
8.87
Hf
0.07
1.25
1.71
3.69
U
b.d.
0.8
b.d.
n.d.
Th
0.40
2.29
3.80
n.d.
Ta
0.02
0.09
1.11
0.38
Pb
n.d.
n.d.
n.d.
n.d.
Eu/Eu*
1.49
1.49
0.32
0.38
REE lab
Imperial OSU
OSU
UTEP
major element oxide weight percent
93.67H
63.96
0.04
17.84
0.67
1.72
0.05
0.12
1.84
6.18
6.67
0.03
0.46
99.59
0.086
0.86
146
113
31
6.8
2
43
0.9
b.d.
b.d.
b.d.
b.d.
27
13.20
25.00
11.90
2.27
1.12
0.28
1.20
0.16
1.0
0.66
1.00
b.d.
0.95
0.05
n.d.
0.99
UTEP
93.66H
65.32
0.02
19.51
0.37
n.d.
0.00
0.03
1.15
7.01
5.98
0.01
0.21
99.61
0.122
0.97
117
580
6
1.2
3
203
0.2
b.d.
b.d.
b.d.
b.d.
5
4.40
6.20
2.80
0.44
0.75
0.04
0.18
0.03
0.2
0.26
0.18
b.d.
0.39
0.06
n.d.
3.79
UTEP
syenite
Table 4. Representative major and trace element analyses.
139
113
681
26.8
19
213
5.1
5
b.d.
b.d.
2
56
142.00
254.00
85.80
14.50
1.08
1.09
3.50
0.48
2.4
0.83
14.30
4.4
24.60
0.52
n.d.
0.18
UTEP
93.75H
64.88
0.33
16.78
1.48
2.58
0.08
0.35
1.62
4.27
7.3
0.10
0.19
99.97
0.139
0.94
92
663
119
33.0
23
424
1.7
9
b.d.
b.d.
3
82
41.30
96.30
45.70
9.34
1.41
1.09
2.90
0.39
4.4
0.70
3.44
b.d.
5.40
1.40
n.d.
0.31
UTEP
93.76H
62.01
0.57
17.16
1.45
3.27
0.09
0.34
4.97
5.49
4.41
0.10
0.45
100.31
0.117
0.75
260
275
40
8.0
n.d.
192
n.d.
15
b.d.
b.d.
b.d.
48
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
b.d.
n.d.
n.d.
n.d.
2.60
n.d.
17.9
n.d.
02.16H
63.50
0.09
17.07
2.88
n.d.
0.06
0.22
2.27
4.46
8.51
0.06
0.40
99.52
0.131
0.83
88
2718
265
65.5
26
1666
0.5
15
11
7
b.d.
130
64.50
122.00
63.80
14.70
3.02
1.83
5.70
0.75
12.0
0.42
6.79
1.1
6.60
1.86
n.d.
0.41
UTEP
91.06H
50.94
1.61
12.75
2.87
8.67
0.21
0.45
14.02
2.78
3.60
0.27
1.39
99.56
0.066
0.38
28
1935
215
34.7
32
384
1.0
19
b.d.
b.d.
b.d.
129
60.90
137.00
68.90
13.10
2.31
1.25
3.20
0.38
10.9
0.33
6.18
b.d.
5.70
1.65
n.d.
0.39
UTEP
91.10H
51.52
1.39
15.26
2.71
7.41
0.17
0.54
13.08
4.33
1.15
0.25
1.13
98.95
0.089
0.47
19
794
286
48.0
n.d.
390
n.d.
33
b.d.
b.d.
12
124
6.40
13.90
5.70
1.75
1.26
0.51
2.30
0.38
5.2
2.32
4.63
3.5
1.00
0.69
n.d.
1.09
UTEP
91.20H
45.48
2.99
7.08
16.28
n.d.
0.27
1.72
23.18
1.88
0.88
0.57
2.43
100.33
0.173
0.15
107
1910
216
36.2
32
1968
0.7
21
b.d.
b.d.
5
104
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
UTEP
91.21H
51.14
1.98
10.96
2.15
8.51
0.20
1.05
14.49
2.45
4.33
0.32
1.91
99.50
0.152
0.31
melasyenite/melamonzonite
169
464
226
29.1
23
887
2.7
13
n.d.
n.d.
b.d.
73
33.80
65.10
36.20
7.62
1.71
0.91
2.10
0.31
7.8
0.59
5.28
1.8
3.90
0.89
n.d.
0.46
UTEP
91.45H
59.60
1.12
17.28
1.05
3.74
0.08
0.76
4.06
4.45
6.22
0.15
0.19
98.70
0.224
0.81
196
1275
227
19.0
n.d.
507
n.d.
12
b.d.
b.d.
6
123
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
13.0
n.d.
n.d.
n.d.
7.00
n.d.
n.d.
n.d.
93.45H
52.89
0.60
20.75
7.21
n.d.
0.15
0.31
5.28
7.79
4.94
0.12
1.90
100.04
0.078
0.75
154
863
77
34.9
18
770
2.8
13
14
28
b.d.
89
42.30
83.60
35.70
7.39
1.32
1.01
3.20
0.44
7.7
0.47
2.13
b.d.
3.70
1.15
n.d.
0.35
UTEP
93.70H
57.74
0.56
17.13
2.82
4.31
0.14
1.10
4.26
4.91
5.78
0.32
0.35
99.41
0.222
0.78
monzonite
74
522
237
42.0
n.d.
362
n.d.
58
25
13
7
136
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
37.8
n.d.
n.d.
n.d.
4.80
n.d.
10.4
n.d.
02.11H
47.80
1.86
15.98
13.43
n.d.
0.20
4.09
8.58
3.43
3.45
0.43
0.82
100.07
0.376
0.64
204 NORWEGIAN JOURNAL OF GEOLOGY
C. G. Barnes et al.
93.52H
48.59
1.13
17.82
9.99
n.d.
0.18
4.78
10.98
5.09
1.34
0.21
0.92
100.11
0.487
0.60
68
548
307
44.0
n.d.
169
n.d.
62
8
32
16
149
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
33.0
n.d.
n.d.
n.d.
20.00
n.d.
n.d.
n.d.
93.44H
43.04
1.04
17.02
1.71
7.59
0.19
2.10
15.64
6.03
1.93
0.40
3.61
100.30
0.291
0.42
93
1100
258
45.4
24
134
1.3
20
26
138
8
134
44.03
78.60
34.90
8.14
1.67
1.16
4.55
0.69
22.0
7.13
4.59
n.d.
n.d.
1.51
n.d.
0.39
UTEP
97
720
196
42.8
14
361
3.0
32
b.d.
144
5
127
39.78
74.73
34.80
7.90
1.71
1.09
3.97
0.60
27.0
2.05
4.41
n.d.
n.d.
1.00
n.d.
0.42
UTEP
93.58H
50.59
1.44
18.16
1.51
9.90
0.19
2.92
6.87
5.14
2.23
0.36
0.55
99.86
0.316
0.78
67
1284
225
26.0
n.d.
443
n.d.
29
b.d.
3
12
101
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
33.0
n.d.
n.d.
n.d.
7.00
n.d.
n.d.
n.d.
93.72H
44.63
1.08
18.89
11.66
n.d.
0.18
3.99
10.93
5.41
2.1
0.34
1.57
99.21
0.404
0.61
8
1073
237
42.0
n.d.
213
n.d.
18
b.d.
b.d.
b.d.
116
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
16.1
n.d.
n.d.
n.d.
14.90
n.d.
12.3
n.d.
02.14H
53.02
1.31
16.86
9.01
n.d.
0.15
0.88
12.55
4.72
0.77
0.38
0.17
99.82
0.162
0.54
69
448
138
26.0
n.d.
b.d.
n.d.
107
45
75
14
93
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
36.8
n.d.
n.d.
n.d.
10.10
n.d.
13.8
n.d.
02.32H
48.52
1.11
17.03
9.85
n.d.
0.16
6.50
11.09
3.67
1.54
0.16
1.42
101.05
0.567
0.61
97
752
189
26.0
n.d.
319
n.d.
54
b.d.
38
9
93
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
34.5
n.d.
n.d.
n.d.
12.30
n.d.
14.5
n.d.
02.45H
50.88
0.93
18.04
9.40
n.d.
0.14
4.86
9.23
4.49
2.04
0.19
0.70
100.90
0.506
0.68
47
1345
195
38.0
n.d.
506
n.d.
19
b.d.
b.d.
6
82
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
21.2
n.d.
n.d.
n.d.
11.00
n.d.
12.4
n.d.
02.57H
52.73
1.81
15.25
8.55
n.d.
0.15
1.58
12.82
3.60
1.92
0.39
1.25
100.05
0.268
0.49
69
223
223
15.6
11
255
4.6
19
b.d.
n.d.
b.d.
38
6.40
13.90
5.70
1.75
1.26
0.51
2.30
0.38
5.2
2.32
4.63
3.5
1.00
0.69
n.d.
1.09
UTEP
91.24H
72.68
0.43
14.33
0.58
2.17
0.03
0.77
2.76
3.59
1.85
0.11
1.00
100.32
0.336
1.11
Notes: b.d., below detection limits. n.d., not determined. REE lab: Imperial College, London, OSU: Oregon State Univ.; UTEP: Univ. of Texas-El Paso.
Sample
91.48H 91.49H 93.39H 93.40H
48.90
47.94
49.59
49.73
SiO2
0.51
1.03
1.42
1.46
TiO2
25.01
16.20
17.64
16.94
Al2O3
3.66
1.73
2.23
2.24
Fe2O3
FeO
n.d.
5.80
8.30
8.20
MnO
0.06
0.13
0.20
0.18
MgO
2.38
8.67
4.18
3.42
CaO
14.52
15.33
8.71
7.81
2.89
2.38
4.99
4.89
Na2O
1.06
0.38
1.93
2.57
K2O
0.11
0.08
0.26
0.32
P2O5
LOI
0.98
0.51
0.88
0.80
Total
100.08 100.18 100.33 98.56
Mg/(Mg+Fet) 0.563
0.677
0.420
0.374
A/CNK
0.77
0.50
0.67
0.68
trace element concentrations in parts per million
Rb
37
15
120
124
Sr
1536
479
603
600
Zr
87
105
230
258
Y
17.3
29.6
43.5
44.5
Nb
5
b.d.
14
17
Ba
159
112
340
423
Sc
5.0
19.7
6.4
5.0
V
32
97
49
40
Cr
9
36
b.d.
b.d.
Ni
59
118
26
8
Cu
17
21
19
12
Zn
25
55
113
115
La
n.d.
8.18
29.65
36.13
Ce
n.d.
24.10
56.86
69.76
Nd
n.d.
14.40
28.40
34.00
Sm
n.d.
3.83
7.11
8.08
Eu
n.d.
1.20
1.65
1.79
Tb
n.d.
0.65
1.11
1.18
Yb
n.d.
2.23
4.22
4.44
Lu
n.d.
0.34
0.64
0.68
Co
n.d.
37.3
32.0
28.0
Cs
n.d.
0.57
8.44
4.83
Hf
n.d.
3.36
4.99
5.83
U
n.d.
b.d.
n.d.
n.d.
Th
n.d.
1.57
n.d.
n.d.
Ta
n.d.
0.19
1.03
1.13
Pb
n.d.
n.d.
n.d.
n.d.
Eu/Eu*
n.d.
0.57
0.43
0.42
REE lab
OSU
UTEP
UTEP
gabbro, diorite, monzodiorite
major element oxide weight percent
Table 4. Continued Representative major and trace element analyses.
412
15
163
22.9
13
8
0.1
n.d.
b.d.
n.d.
b.d.
44
8.50
21.00
7.60
1.88
0.09
0.36
2.80
0.64
0.2
12.70
7.53
15.5
50.90
0.50
n.d.
0.08
UTEP
91.59H
76.39
0.04
13.30
0.36
0.60
0.03
0.05
0.27
4.74
4.44
0.00
0.17
100.39
0.090
1.02
469
38
277
22.7
26
65
0.5
b.d.
b.d.
3
b.d.
115
31.57
56.69
16.60
3.31
0.30
0.34
2.19
0.37
1.1
26.90
7.36
n.d.
n.d.
0.58
n.d.
0.19
UTEP
93.43H
73.11
0.06
14.20
0.41
0.88
0.02
0.06
0.41
5.55
4.59
0.03
0.32
99.62
0.074
0.96
granitic rocks
134
120
407
67.0
21
404
10.7
49
28
35
b.d.
62
47.74
100.50
49.50
10.67
1.76
1.54
5.53
0.85
9.5
4.38
10.40
n.d.
n.d.
1.30
n.d.
0.31
UTEP
93.84H
69.38
0.81
14.77
0.77
4.00
0.07
1.24
2.00
3.49
3.08
0.11
0.85
100.57
0.320
1.16
255
43
74
9.0
n.d.
b.d.
n.d.
14
b.d.
b.d.
b.d.
40
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
b.d.
n.d.
n.d.
n.d.
25.80
n.d.
30.2
n.d.
02.54H
75.52
0.04
13.28
1.00
n.d.
0.02
0.04
0.65
5.08
3.57
0.01
0.17
99.38
0.073
0.99
293
218
246
30.5
22
287
2.0
12
b.d.
b.d.
b.d.
61
42.97
80.44
33.10
6.90
0.87
0.82
2.75
0.44
5.8
7.84
6.08
n.d.
n.d.
1.54
n.d.
0.26
UTEP
93.31H
64.43
0.53
16.24
1.42
2.30
0.10
0.69
2.03
4.98
5.1
0.15
1.81
99.78
0.255
0.93
Kvingra
NORWEGIAN JOURNAL OF GEOLOGY
Geology of the Hortavær Igneous Complex 205
206 NORWEGIAN JOURNAL OF GEOLOGY
Furthermore, the effects of assimilation of carbonate
rocks in the magmas are undeniable. This conclusion
arises from a number of observations. For our purposes
the first is the presence of minerals commonly associated with carbonate assimilation, such as calcite, scapolite, nepheline, grossular-andradite garnet, and idocrase. Not only do these minerals display igneous textures, they exist in medium- to fine-grained dikes, which
must represent, at the very least, melt-rich magmas.
The second observation is that layered calc-silicate
rocks have been completely "made over" into rocks that
have igneous assemblages, minerals with igneous compositions, and igneous textures (e.g. the melasyenitic
rocks).
These conclusions do not imply that assimilation was a
simple process of dissolution of carbonate or calc-silicate minerals into the magmas. It is well known that
assimilation of pure calcite in closed systems is of limited extent (e.g. Tilley 1949, 1952), primarily due to low
solubility of CO2 (e.g. Watkinson & Wyllie 1969). Even
if such assimilation were possible, there is no reason to
think that assimilation of calcite should result in a Carich magma. In fact, the opposite might be true. A simple reaction that could characterize assimilation in the
Hortavær complex in dioritic magmas might be:
olivine + calcite + melt 1 =
clinopyroxene + melt 2 +CO2.
In this reaction, calcium provided by the assimilated
carbonate phase is taken up in a solid silicate phase (clinopyroxene). This differentiation process is not a cotectic one, but rather a peritectic one in which the magma
is modified by dissolution of one or more minerals as
others precipitate (much like the olivine-enstatite reaction in basaltic magmas). Thus assimilation will not
raise the Ca content of the melt, instead it will have the
effect of depletion of the melt in elements necessary for
crystallization of clinopyroxene.We suggest that in the
dioritic magmas of the Hortavær complex, a principal
and early major-element effect of carbonate assimilation was enrichment of alkalis, such that rocks in the
suite range from calcic to alkaline. This process would
suppress fractionation of olivine (consumed in the assimilation reaction) and probably of plagioclase (because
Ca is sequestered in clinopyroxene). Therefore, clinopyroxene-dominated alkali enrichment would be accompanied by limited silica enrichment, as recognized by
Meen (1990). Assimilation of carbonates would result in
cumulate rocks with excess clinopyroxene, interstitial
plagioclase, and accessory Ca-rich phases such as scapolite, titanite, and calcite. Such cumulate rocks are the
"hortites" of Vogt (1916).
This type of reaction cannot explain the transition
from nepheline normative and nepheline-bearing
mafic and intermediate compositions to the evolved
C. G. Barnes et al.
quartz syenitic compositions observed at Hortavær.
Direct evolution by fractional crystallization and/or
carbonate assimilation is not possible because of the
low-P thermal divide between silica under- and oversaturated magmas. Thus, the presence of quartz-bearing
syenites suggests either that the quartz syenites are
unrelated to the rest of the Hortavær complex or that
quartz is present due to assimilation of quartzofeldspathic metasedimentary material (e.g. Foland et al.
1993, Landoll et al. 1994). We find the former unlikely
because quartz-bearing syenites do not occur in a geographically restricted part of the complex. Instead, we
suggest that the principal assimilate in many Hortavær
magmas was not pure carbonate, but instead was calcsilicate rock with both carbonate and quartzofeldspathic components. This would explain the observed
decrease in silica activity from gabbroic to monzonitic
compositions followed by an increase in silica activity
from monzonite to syenite. In the mafic magmas, the
abundance of Mg and Fe allowed the carbonate component of calc-silicate rocks to be consumed (to form clinopyroxene: see reaction above). In contrast, the evolved magmas lacked sufficient Mg and Fe to form significant clinopyroxene and therefore could not assimilate
much carbonate component. Instead, these evolved
magmas assimilated silicate components of the calcsilicate contaminants. This is certainly consistent with
the abundance of layered calc-silicate rocks as screens
in the Hortavær complex. It is also consistent with the
observation that marble screens at Hortavær are essentially untouched by reaction with host magmas, whereas calc-silicate screens are nearly completely converted to layered melasyenite/melamonzonite. Finally, assimilation of calc-silicate rocks is consistent with Nd,
oxygen, and carbon isotope ratios, which are documented in Barnes et al. (2002 and in review).
It is unlikely that the widespread granitic dikes that
make up the youngest parts of the complex are directly
related to the syenites, because of the lower alkali contents of the granites. It is possible that the granites
represent magmas that experienced less carbonate assimilation than those of the Hortavær trend, although
assimilation of carbonate in granitic magma is unlikely
owing to the limited solubility of CaO (e.g. Tilley
1949). It is also possible that the granites originated by
partial melting of the host rocks. If so, the compositions of the granitic dikes are quite distinct from those of
granites in the host rocks (Figs. 8 and 9). Alternatively,
the granites are petrologically unrelated either to the
Hortavær syenites or to the migmatitic host rocks.
One of the interesting and unanswered questions concerning the Hortavær complex is the site of differentiation of the magmas. We argue for emplacement in a
transfer zone, yet also show evidence for seemingly in
situ assimilation, fractionation, and crystal accumulation. The former implies long-term magma storage
Geology of the Hortavær Igneous Complex 207
NORWEGIAN JOURNAL OF GEOLOGY
deeper in the crust, the latter implies that magmas
remained chemically reactive for long periods at the
level of emplacement. Final resolution of this problem
awaits further research. However, we note that isotopic
variation within the complex is consistent with in situ
assimilation of host calc-silicates (Barnes et al. in
review). We take this to suggest either that similar calcsilicate rocks exist at depth or that the collection of
dikes in the Hortavær transfer zone retained enough
heat to permit local, commonly extensive, host-rock
assimilation.
As noted above, closed-system assimilation of carbonate rocks is limited by the small solubility of CO2. This
constraint is removed if CO2-rich fluid escapes from
the site of assimilation. The fact that assimilation was
occurring in a magma transfer zone, and probably in a
zone of extension, suggests a resolution of this problem. As CO2 was enriched to the point of saturation, it
exsolved as a separate fluid phases (very probably with
H2O), and migrated upward, away from the magma
system. As long as the system remained open to loss of
CO2, and contained enough latent heat to promote
chemical reactions, assimilation could proceed.
Conclusions
The Hortavær intrusive complex represents a Caledonian-age (456 Ma) intrusion that evolved from calcic to
alkalic compositions (gabbro to alkali feldspar syenite).
The complex was emplaced as thousands of dikes and
dike-like bodies into a sequence of metasedimentary
rocks characteristic of the Helgeland Nappe Complex.
Magma mingling was widespread and magma mixing
occurred locally. Trace element patterns indicate an arc
setting for the complex.
Magma evolution was dominated by crystal-liquid
separation processes and heavily overprinted by assimilation of calc-silicate host rocks. We interpret differentiation to alkalic (nepheline-bearing) compositions as
the result of carbonate-dominated assimilation in a
magma system open to loss of CO2. Peritectic assimilation of carbonate phases resulted in stabilization of clinopyroxene relative to olivine and plagioclase. This
resulted in enrichment of the alkalis over a small range
of SiO2. As a CO2-rich fluid phase formed in the system, it was lost upward. This fluid loss, combined with
episodic emplacement of new, hot magma batches, permitting continued assimilation. Further magma evolution to quartz syenite requires assimilation of the quartzofeldspathic component of the host rocks.
Acknowledgements: - We are grateful to Øystein Nordgulen for his interest and input during all stages of this project, to J. G. deHaas and
Ingrid Vokes for XRF analyses, and to Susan Swapp for assistance with
microprobe analysis. Field work was ably assisted by Jostein Hiller, Reidar Berg, and Arne Lysø. We thank Bernard Barbarin, Jean-Paul Liegois, and Øystein Nordgulen for thoughtful reviews. This research was
initially funded by a grant from Nansenfondet to Prestvik and received
partial support from National Science Foundation grant EAR-9814280.
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