91(3), pp. This ﬁle was downloaded … · 2020. 11. 6. · Myrland, 1974) recognized the presence of a number of migmatitic rock units in central Helgeland and a later study in

This may be the author’s version of a work that was submitted/acceptedfor publication in the following source:

Barnes, Calvin, Reid, Kristin, Frost, Carol, Barnes, Melanie, Allen, Char-lotte, & Yoshinobu, Aaron(2011)Ordovician and Silurian magmatism in the Upper Nappe, Uppermost Al-lochthon, Helgeland Nappe Complex, north-central Norway.Norwegian Journal of Geology (Norsk Geologisk Tidsskrift), 91(3), pp.121-136.

This file was downloaded from: https://eprints.qut.edu.au/108703/

c© Copyright 2011 C. G. Barnes et al.

This work is covered by copyright. Unless the document is being made available under aCreative Commons Licence, you must assume that re-use is limited to personal use andthat permission from the copyright owner must be obtained for all other uses. If the docu-ment is available under a Creative Commons License (or other specified license) then referto the Licence for details of permitted re-use. It is a condition of access that users recog-nise and abide by the legal requirements associated with these rights. If you believe thatthis work infringes copyright please provide details by email to [email protected]

License: Creative Commons: Attribution 4.0

Notice: Please note that this document may not be the Version of Record(i.e. published version) of the work. Author manuscript versions (as Sub-mitted for peer review or as Accepted for publication after peer review) canbe identified by an absence of publisher branding and/or typeset appear-ance. If there is any doubt, please refer to the published source.

http:// njg.geologi.no/ vol-91-100/ details/ 1/ 76-76

https://eprints.qut.edu.au/view/person/Allen,_Charlotte.html

https://eprints.qut.edu.au/view/person/Allen,_Charlotte.html

https://eprints.qut.edu.au/108703/

http://njg.geologi.no/vol-91-100/details/1/76-76

121

Introduction

The 478–424 Ma Bindal Batholith in north-central Nor-way is an excellent example of a long-lived orogenic mag-matic province in which a complex combination of man-tle melting, crustal melting, magma mixing, and assim-ilation contributed to magma compositions (Nordgulen and Sundvoll, 1992; Nordgulen, 1993; Nordgulen et al., 1993; Barnes et al., 1992; Birkeland et al., 1993; Barnes et al., 2007). Because the Batholith intrudes a number of nappe sheets in the Helgeland Nappe Complex (HNC), an understanding of the timing of regional migmatiza-tion and its relationship to generation and coalescence of plutonic magmas is crucial to reconstruction of the tec-tonic setting and thermal history of the region. Regional migmatization and crustal melting affected parts of the HNC from ~480 to 475 Ma (Yoshinobu et al., 2002; Barnes et al., 2007) and granites emplaced during this time are peraluminous to strongly peraluminous with high initial 87Sr/86Sr values, features taken to indicate pre-dominantly crustal origins (Nordgulen and Sundvoll, 1992; Nordgulen, 1993; Birkeland et al., 1993). Younger magmatic activity in the Batholith (465 to 424 Ma; Nord-gulen et al., 1993; Yoshinobu et al., 2002; Nissen et al.,

2006; Barnes et al., 2007; this paper) was much more diverse and included crust- and mantle-derived magmas in various forms of hybridization (Birkeland et al., 1993; Barnes et al., 1992, 2002, 2005).

Early mapping in the HNC (Kollung and Myrland, 1970; Myrland, 1974) recognized the presence of a number of migmatitic rock units in central Helgeland and a later study in the Velfjord–Tosen area (Thorsnes and Løseth , 1991) showed that these migmatitic units belonged to discrete nappes that were separated from one another by non-migmatitic (lower grade) nappes. It was not clear on the basis of this work whether regional migmatization in different nappes was the result of a single early Ordo-vician metamorphic event, or whether it represented temporally and/or spatially distinct thermal events.

This paper reports a U–Pb geochronologic, thermo-barometric, and petrologic study of a long, nearly continu ous road cut near Tosenfjord that exposes migma titic rocks of a structurally low part of the Upper Nappe of the HNC. The migmatites underwent partial melting at ~480 Ma and then from 448 to 424 Ma were repeatedly intruded by mafic to granitic dikes. Local

Barnes, C.G., Reid, K., Frost, C.D., Barnes, M.A., Allen, C.M. & Yoshinobu, A.S.: Ordovician and Silurian magmatism in the Upper Nappe, Uppermost Allochthon, Helgeland Nappe Complex, north-central Norway. Norwegian Journal of Geology, vol. 91, pp 121-136. Trondheim 2011. ISSN 029-196X.

The structurally lowest parts of the Upper Nappe of the Helgeland Nappe Complex consist primarily of migmatitic quartzofeldspathic gneiss with intercalated calc-silicate gneiss. Detrital zircons in the gneiss indicate an Ordovician depositional age. Migmatization via biotite dehydration reac-tions occurred at ~480 Ma at pressures of ~700 MPa; some migmatites were mobilized into cross-cutting diatexites. Diverse gabbroic to granitic plutons and dikes of the Bindal Batholith were emplaced from 448–424 Ma. They range from calc-alkaline to alkalic and most have initial 87Sr/86Sr <0.707 and εNd from 0 to -4. Pressures of emplacement were ca. 300 MPa. Shear zones with reverse shear sense cut 437 Ma intrusions, suggesting that contraction accompanied magmatism.

Calvin G. Barnes, Department of Geosciences, Texas Tech University, Lubbock, Texas, 79409-1053, USA. E-mail: [email protected]. Kristin Reid Department of Geosciences, Texas Tech University, Lubbock, Texas, 79409-1053, USA. Carol D. Frost, Department of Geology and Geophysics, University of Wyoming, 1000 East University Avenue, Laramie, Wyoming, 82071, USA. E-mail: [email protected]. Melanie A. Barnes, Department of Geosciences, Texas Tech University, Lubbock, Texas, 79409-1053, USA. E-mail: [email protected]. Charlotte M. Allen, Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia. E-mail: [email protected]. Aaron S. Yoshinobu, Department of Geosciences, Texas Tech University, Lubbock, Texas, 79409-1053, USA. E-mail: [email protected].

Calvin G. Barnes, Kristin Reid, Carol D. Frost, Melanie A. Barnes, Charlotte M. Allen & Aaron S. Yoshinobu

NORWEGIAN JOURNAL OF GEOLOGY Ordovician and Silurian magmatism in the Upper Nappe, Uppermost Allochthon, Helgeland

Ordovician and Silurian magmatism in the Upper Nappe, Uppermost Allochthon, Helgeland Nappe Complex, north-central Norway

122 C. G. Barnes et al. NORWEGIAN JOURNAL OF GEOLOGY

64

74

6465

76N

447±32

Ma

445 Ma2

447±3.72

Ma

Velfjord

Tosenfjo

rd

10 km

Lande

65°00'1 2°0 0 '

65°00'

6 5 ° 3 0 '1 4 ° 0 0 '

AtlanticOcean

25 km

Upper Allo

chth

on

Fig. 1C

Vikna

1 4°0 0 '

RødingsfjalletNappe Complex(Uppermost Allochthon)

Upp

erm

ost A

lloch

thon

Central Norway Basement Window

~465-440 Ma Granitoid rocks

~465-440 Ma Diorite & gabbro> 465 Ma Igneous rocks

Ophiolitic rocks

Lower Nappe (pelitic schist & migmatite/marble)

Sauren-Torghatten Nappe

Horta Nappe(carbonate/gneiss/migmatite)

Middle Nappe

Upper Nappe

<440 Ma Granitoid rocks

Undifferentiated metamorphic rocksNormal-sense shear zone

Uppermost AllochthonUpper AllochthonMiddle AllochthonLower Allochthon

Devonian - Permian basin fill

Explanation

200 km

Western Gneiss Region

Fig. 1B

Atlantic Ocean

Parauthoch-thonous rocksProterozoic Baltic cratonal rocks

Nor

weg

ian

Cal

edon

ides

Trondheim

A B

C

Explanation - Helgeland NappeComplex & Bindal Batholith

D

~442 Ma Andalshatten

pluton

Stromatic migmatiteTF-304, melanosome 480.3±1.9 MaTF-204, leucosome 480.5, 479.4, 465.2 MaDiatexitic migmatiteTF-104 480.6±3.8 MaQuartz-diorite plutonTF-404 447.8—1.7 MaNet-veined monzonitic dikeN16.05 436.7±3.5 MaBiotite monzograniteTF-82 ~ 424-470 Ma

StørborjaG.A.S.P. ~ 500-700 MPa, ~800°C

Tosbotn tunnel

430±7 Ma1420 MPa2

Diatexitic migmatiteTF-77B 445.1±6.4 MaGabbroTF-70 436.9±4.4 MaTonalite dikeTF-82 431.9±3.5 MaBiotite monzogranite TF-82 424.7±5.6 Ma

G.A.S.P. ~ 500-700 MPa, ~800°C

Poles to foliations, n = 17, average = 343/60Lineations, n = 3

Figure 1. A. Tectonic map displaying allochthonous units within the Scandinavian Caledonides. B. Regional geologic map of the Helgeland Nappe Complex and Bindal Batholith, located within the Uppermost Allochthon. C. Simplified geologic map of the field area; legend as in B. Dike complex and migmatitic rocks are shown in grey and exposed along route 76 on the northwest shore of Tosenfjord. Regional structural trends are depicted by foliations with filled triangles. SHRIMP U-Pb zircon ages are shown in yellow; LA-ICP-MS U-Pb zircon ages are shown in white. Previously published age and depth measurements are from: 1 = Nordgulen et al., 1993; 2 = Yoshinobu et al., 2002. D. Lower-hemisphere stereonet showing poles to foliations and lineations from road side outcrop.

123NORWEGIAN JOURNAL OF GEOLOGY Ordovician and Silurian magmatism in the Upper Nappe, Uppermost Allochthon, Helgeland

after exhumation to ~400 MPa (Barnes and Prestvik, 2000).

The focus of this study is on basal parts of the Upper Nappe (Fig. 1C; Thorsnes and Løseth, 1991). The Middle Nappe–Upper Nappe contact is ~1.6 km west of our study area, and is marked by the abrupt change structur-ally upward from layered, amphibolite facies, non- migmatitic schists of the Middle Nappe to intercalated migmatitic and calc-silicate gneiss of the Upper Nappe. The latter rock types are characteristic of the Upper Nappe along Tosenfjord from Lande to Tosbotn (Fig. 1C), with lesser amounts of quartzofeldspathic gneiss and marble. Thus, the Upper Nappe contains rock types nearly identical to those of the Lower nappe, although in different proportions.

The nappes are juxtaposed along east-dipping faults that were originally interpreted as thrusts (Thorsnes and Løseth , 1991). Although initial reverse motion is likely on these faults, Yoshinobu et al. (2002) showed that final motion on the fault separating the Lower and Middle Nappes was in a normal sense. The Sauren-Torghatten, Lower, Middle, and Upper nappes were intruded by the Andalshatten pluton (Fig. 1B; Nordgulen et al., 1992). High-precision TIMS dating of the Andalshatten pluton to 442 Ma (Anderson et al., 2008) places a minimum age on terrane amalgamation. A recent study of zircon inherit ance in Lower Nappe plutons (Barnes et al., 2007) indicates that terrane amalgamation was even earlier, and was complete by ~478 Ma.

Plutonic rocks of the Bindal Batholith span an age range of 478–424 Ma. The oldest plutons are primarily per-aluminous to strongly peraluminous granites, have Sr, Nd, and Pb isotopic features consistent with origins by crustal melting (Nordgulen and Sundvoll, 1992; Birke-land et al., 1993; Marko et al., 2007), and locally con-tain abund ant enclaves of metasedimentary rocks (Nord gulen, 1993; Marko et al., 2008). These plutons are referred to as “anatectic granites” (e.g., Nordgulen, 1993). Magmatism continued with emplacement of the ~466 and ~465 Ma Horta vær and Svarthopen intrusions, respectively (Barnes et al., 2007). Then at about 450 Ma, a prolonged pulse of magmatism began with emplace-ment of the Troholmen pluton near Rødøy (Barnes et al., 2007), followed by the Velfjord plutons (Yoshinobu et al., 2002), the Andalshatten pluton (~442 Ma; Ander-son et al., 2008), and then a broadly distributed, diverse range of plutons from c. 440 to 424 Ma. The 450–424 Ma magmatism encompasses alkali-calcic to calc-alkalic and gabbroic to granitic magmas (Nordgulen and Schouen-borg, 1990; Nordgulen et al., 1993; Nissen et al., 2006; Barnes et al., 2007; this paper). The isotopic composi-tions of these rocks are quite variable and are broadly consistent with mixed crustal and mantle sources (Nord-gulen and Sundvoll, 1992; Birkeland et al., 1993).

deformation and development of metamorphic mineral assemblages in these dikes hint at a more or less continu-ous thermal pulse from ~437–424 Ma. Apparently, this exposure provides, in a km-long section, a significant portion of the Caledonian thermal history of the Upper Nappe and an important location by which to compare other parts of the Upper Nappe and the rest of the HNC.

Geological setting

Regional setting

The Helgeland Nappe Complex is the structurally high-est nappe complex of the Uppermost Allochthon (Fig. 1A; Roberts et al., 2007). In the Tosen–Velfjord–Brøn-nøy region, the HNC consists of four named nappes (Thorsnes and Løseth, 1991; Yoshinobu et al., 2002) and at least one additional nappe (Barnes et al., 2007). From structurally lowest to highest, these nappes are the informally named Horta nappe that is host to the Hortavær intrusion (Barnes et al., 2007), the Sauren- Torghatten, Lower, Middle, and Upper Nappes (Fig. 1A, B). Basal slivers of metaperidotite (serpentinite) and gabbro character ize the Sauren-Torghatten and Middle Nappes; these slivers are unconformably over-lain by cover sequences of greenschist to amphibo-lite facies metasedimentary and metavolcanic rocks. The cover sequences include conglomerates and clas-tic metasediment ary rocks whose provenance ranges from volcanic to continental shelf (Thorsnes and Løs-eth, 1991; Frost et al., 2006), sug gestive of rapidly chang-ing sediment sources. Moreover, the Sauren-Torghatten and Middle Nappes are evidently not overturned, as indi-cated by the east- dipping contact between the ultramafic and meta sedimentary rocks which was interpreted to be an unconformable depositional contact by Thorsnes and Løseth (1991) and Yoshinobu et al. (2002).

In contrast, the Lower and Upper Nappes contain a large proportion of high-grade semi-pelitic schist and gneiss, quartzofeldspathic gneiss, and marble (Gustavson, 1981; Thorsnes, 1987; Thorsnes and Løseth, 1991; Nord gulen and Sundvoll, 1992). Most of the quartzofeldspathic gneissic rocks are migmatitic, with migmatite fabrics that range from stromatic (layered) and nebulitic (pod-like leucosomes) to diatexitic. Diatexitic migmatites are those in which original structure has been disrupted, generally by magmatic flow. In the Lower Nappe, which is domi-nated by quartzofeldspathic gneiss and marble, with lesser calc-silicate gneiss, two migmatization events have been recognized. The older event (>477 Ma; Yoshinobu et al., 2002) was of regional extent and produced mildly to strongly peraluminous alkali-calcic granites (Barnes et al., 2002). The younger event was restricted to contact aureoles around mafic to intermediate plutons emplaced from 448 to 445 Ma (Barnes et al., 2002; Yoshinobu et al., 2002). This so-called contact migmatization began when the Lower Nappe was at ~700 MPa pressure, but ended


Table 1: Table 1. Summary of rock units and ages.

Rock unit Rock types Age (Ma)*gneissic metamorphic rocks

migmatitic gneiss stromatic (banded) gneiss with tonalitic leucosomes 480.3 ± 1.9

calc-silicate gneiss blocks in migmatite: diopside, garnet, plagioclase, calcite

diatexitic (disrupted) migmatite massive–weakly banded garnet biotite tonalite migmatite 480.6 ± 3.8

pegmatitic leucogranite dike -----

eastern pluton augite biotite hornblende qtz diorite/qtz monzodiorite 447.8 ± 1.7

gabbroic to granitic dikes

gabbroic blocks in leucogranite hornblende pyroxene gabbro 436.9 ± 4.4

net-veined monzonitic dikes monzonite (enclaves) cut by leucomonzonite veins 436.7 ± 3.5

homogeneous & mingled dikes homogeneous monzodiorite -----

composite tonalite (host)--qtz diorite (enclaves) -----

composite melagranite (host)--qtz diorite (enclaves) -----

granitic & tonalitic dikes muscovite-biotite granodiorite/tonalite 431.9 ± 3.5

biotite monzogranite ~424

N16.05435.1 ± 2.8 Ma

TF-204TF-304480.2 3.6 Ma±

diatexiticmigmatite

mingled (”net veined”) diorite

mingled (”net veined”) monzonite/diorite

TF-77B 443.0 4.9 MaTF-104 480.6

: ±: ± 3.8 Ma

net-veined meta-diorite

stromaticmigmatite

N17.05

biotite tonalitewith stromatic migmatiteenclaves

stromaticmigmatite

stromatic migmatite

TF-82431.9 3.5 Ma±biotite leucogranite

TF-81424.7 ± 5.6 Ma

mingleddike

TF-75

A

A’’A’

A’

west

east

Figure 2.Figure 2. Composite photograph of the northern side of the road cut with a sketch showing the various rock units and their crosscutting relationships. Dated units are labeled and the specific locations of dated samples are shown, where known. Shear zones are shown with red lines.

*Ages reported for migmatites are interpreted to date partial melting. All other ages interpreted to date magma emplacement.


Local settingThe study area is located along highway 76 west of Tosenfjord and 1.4 km northeast of Lande (Fig. 1C). The main focus of research is a ~500-meter-long set of double road cuts into high-grade metamorphic rocks cross-cut by mafic, felsic, and composite dikes. The western end of the road cut has the following UTM coordinates: zone 33, 397370 easting, 7239419 northing (WGS84). The sequence of crosscutting relationships involves three dis-tinct metamorphic units and a number of crosscutting intrusive units diagrammatically illustrated in Figure 2 and summarized in Table 1. Photographs in Figure 3 show some of these relationships in greater detail and additional detail is presented in Appendix 1. The vari-ous rock units are described below in order of decreasing age, as determined by U–Pb (zircon) dating and inferred from crosscutting relationships.

Migmatitic and calc-silicate gneiss. The oldest rocks in the road cut exposures are isoclinally folded semi-pelitic gneiss, migmatitic gneiss, and associated calc-silicate gneiss. These migmatites are banded (stromatic), with distinct peraluminous, tonalitic leucosomes and biotite -rich, garnet- and sillimanite-bearing melanosomes (Fig. 2; Appendix 1). A sample of the stromatic migmatite was collected for geochronology and was split into a leuco-some fraction (TF-204) and a melanosome fraction (TF-304). Migmatitic rocks exposed from the base of the Upper Nappe to Tosbotn are broadly similar to those in the road cut, but in some cases also contain staurolite and cordierite. Calc-silicate rocks are intercalated with the migmatites and are present as boudins or schollen in stromatic migmatite. They typically consist of variable proportions of diopside, garnet, plagioclase, and calcite.

In the central part of the road cut, stromatic migmatite is crosscut by medium-grained, foliated diatexitic mig-matite (Figs. 2, 3A). The diatexite is peraluminous, tonal-itic to quartz dioritic in composition and contains biotite, garnet, sillimanite, and muscovite. Garnet is similar in composition to that in the stromatic migmatite (Appen-dix 1). The diatexite encloses blocks (m- to dm-scale) of stromatic migmatite and calc-silicate gneiss (Fig. 2) and is therefore interpreted to be intrusive. Two samples for U–Pb dating were collected (TF-104 and TF77B; Fig. 2).

A

B

D

diatexite

stromaticmigmatite

C sample N16.05

Figure 3

Figure 3. Outcrop photographs. A. Stromatic migmatite intruded by diatexite. Hammer handle is 38 cm long. B. Blocks of foliated, banded diorite (e.g., TF-70) intruded by leucogranite. Pen is 13 cm long. C. Net-veined diorite–monzonite intrusion cutting stromatic migmatite, showing the location of dated sample N16.05. Pen is 13 cm long. D. Circa 435 Ma mingled dike. Hammer is 59 cm long.


dating of a sample of banded diorite (TF-70) indicates that this is not the case (see below), at least for some of the mafic rocks.

Fine- to medium-grained net-veined monzonitic dikes cut diatexite and stromatic migmatite (Fig. 3C). The dikes consist primarily of biotite hornblende augite mon-zonite, which occurs as enclaves surrounded by thin veins of hornblende ± augite monzonite/quartz monzo-nite. The enclave-dike margins are commonly interdig-itated. Where the net-veined dikes are in contact with migmatite, 8–10 cm-wide zones of “hybrid” quartz-bearing hornblende monzonite are present. A sam-ple of enclave material from the largest net-veined dike (N16.05) was collected for U–Pb dating (Figs. 2, 3C). One of the net-veined dikes is deformed by a through-going, top-to-the-west shear zone (Fig. 2).

Homogeneous and mingled dikes. The net-veined dikes are crosscut by dikes of fine-grained, dark gray, homoge-neous hornblende monzodiorite and by fine- to medium-grained, pillowed, composite dikes. There are two types of mingled dike: pale gray, biotite tonalite with enclaves of dark gray, hornblende biotite quartz diorite, and pale gray, porphyritic, biotite quartz diorite with enclaves of hornblende biotite melagranite (Fig. 3D). Deformation of these dikes is variable, from undeformed to strongly deformed and mylonitic.

Granitic and tonalitic dikes. The stromatic migmatite, diatexite, and homogeneous and net-veined monzo-nitic dikes are crosscut by dikes of white, fine- to coarse-grained, biotite monzogranite and pale gray muscovite-bearing biotite granodiorite/tonalite. The largest exam-ple of the granodiorite/tonalite is exposed near the east-ern end of the road cut and reaches at least 15 meters in width (Fig. 2); it was sampled for U–Pb dating (sample TF-82). The granodiorite/tonalite is cut by a pale gray, medium-grained biotite monzogranitic dike in a splayed array with both curved and angular contacts (Fig. 2). Two samples of this dike were dated (TF-81 and N17.05).

Geochemistry

Analytical methods.

Analytical methods are presented in Appendix 2. In brief, mineral compositions were determined by elec-tron microprobe at the University of Wyoming, major element rock compositions were measured by induc-tively-coupled plasma optical emission spectrometry at Texas Tech University, rare earth and other trace element concentrations were determined by inductively- coupled plasma mass spectrometry (ICP-MS) at Washington State University, and isotope ratios by isotope dilution thermal ionization mass spectrometry at the Univer-sity of Wyoming. U–Pb ages of zircon were determined by Sensitive High Resolution Ion Micro Probe-Reverse

Structural development within the migmatitic and calc-silicate gneiss is dominated by a moderate to steeply east-dipping, axial planar metamorphic foliation and out-crop-scale isoclinal folds. Gneissic banding and foliations are penetrative within the migmatitic rocks (Fig. 3). Dis-creet, cm- to m-scale shear bands occur throughout the road cut. In many cases, these shear bands are localized on leucosomes, indicating melt-present deformation. Three outcrop-scale shear bands approximately 3–10 cm thick are shown on Figure 2 and exhibit top-to-the west sense of displacement along east-dipping planes. How-ever, asymmetric kinematic indicators were not widely observed within the outcrop; a few normal-sense (down-to-the east), cm-scale shear bands were observed. A lim-ited number of lineations defined by recrystallized aggre-gates of quartz, feldspar, and biotite plunge moderately to the east-northeast along northwest-trending foliation planes (Fig. 1D), consistent with west-directed thrusting (e.g., Thorsnes and Løseth, 1991; Yoshinobu et al., 2002).

Pegmatitic leucogranite dikes. Near the western end of the road cut, white, pegmatitic leucogranitic dikes crop out. Grain size increases from medium-grained cores to very coarse-grained margins and the proportion of alkali feldspar increases toward the margins. Biotite reaches 4 cm in length and is oriented perpendicular to dike mar-gins (Fig. A1). These dikes are interpreted to be accumu-lations of leucosome magma from adjacent migmatites, but have not been dated.

Pluton east of main road cut. Approximately 100 m east of the section studied in detail, a small, medium- to coarse-grained pluton of augite biotite hornblende quartz dio-rite/quartz monzodiorite intrudes the stromatic migma-tite. It contains rounded mafic magmatic enclaves and xenoliths of the host rocks. A sample of this pluton (TF-404) was collected for U–Pb dating.

Gabbroic to granitic dikes. The remaining intrusive rocks in the road cut range from gabbro to granite. They dis-play an array of grain size, intrusive shapes (Fig. 2), min-eral assemblages, chemical composition, deformation, and degree of metamorphic fabric development (see Appendix 1 for detailed descriptions and photographs). Many of the dikes are mingled and some show evidence of physical and chemical hybridization. U–Pb dating (see below) and field relationships show that they were emplaced from 437 to 424 Ma, and most were emplaced between 437 and 432 Ma.

Green, fine-to coarse-grained mafic rocks crop out near the west-central part of the road cut where they form blocks and boudins in white leucogranite (Fig. 3B). The blocks range from coarse-grained and massive to medium-grained, foliated, and banded (Fig. 3B); they contain diopsidic pyroxene, poikilitic hornblende, bio-tite, and plagioclase (Appendix 1). In the field, these mafic blocks were interpreted to represent mafic intru-sions emplaced prior to migmatization; however U-Pb


probability plot at ~420 and ~460 Ma. These zircons have a relatively large range of U contents (Appendix Table 1), but there is no correlation between age and abundance of U. Using zircons in the 420–460 Ma range, an age of 436.7 ± 3.5 Ma is obtained (MSWD = 1.20).

Tonalitic dike. SHRIMP-RG analysis of sixteen zircons from the biotite tonalite (TF-82) yielded dates from 425 to 448 Ma (Fig. 4H). This population yielded an average age of 431.9 ± 3.5 Ma.

Late biotite monzogranite. Laser ablation ICP-MS anal-ysis of zircons from sample N17.05, the biotite monzo-granite that cuts the biotite tonalite dike (TF-82), showed a large variation in U content, from < 130 to >26,000 ppm. Moreover, within the normal limits used to define concordance, many zircons were classified as discor-dant. Figure 4I shows the age spectrum obtained when slightly less stringent limits were placed on concordancy. In this case, two zircons have ages at 424.0 ± 1.2 Ma, two at ~444 Ma, and a cluster of 8 analyses between 458 and 470 Ma. There is no correlation between age and U con-tent. Six zircons from a second sample of this body (TF-81) were analyzed by SHRIMP-RG. The average age for all six analyses is 425.2 ± 9.7 Ma and if two extreme sam-ples (441 and 404 Ma) are rejected, the remaining four crystals yield an age of 424.7 ± 5.6 Ma. We interpret the SHRIMP and LA-ICP-MS data to indicate an age of ~424 Ma for this monzogranite, with inheritance at ~444 Ma as well as older ages. The small number of circa 424 Ma zircons is consistent with the monzogranite having crys-tallized from a low-T magma in which most zirconium in the sample was sequestered in inherited zircon. This conclusion is supported by zircon-saturation tempera-tures (Watson and Harrison, 1983) of 654–691 °C.

Summary of U–Pb ages. We interpret the LA-ICP-MS ages determined for samples of the stromatic and diatex-itic migmatites to indicate that crustal melting occurred in this part of the Upper Nappe at ~480–481 Ma. The discrepancy between the ca. 480 Ma age determined for diatexite sample TF-104 by LA-ICP-MS and the ~445 Ma age determined for TF-77B by SHRIMP is problematic. Some of the most intricate and difficult intrusive rela-tionships exposed in this outcrop are leucocratic veins that appear to emanate from stromatic or diatexitic mig-matite and then mingle with or cut various dike rocks. In the field, these relationships were interpreted to indi-cate that the migmatites were partly molten at the time of later dike emplacement. This observation was par-ticularly important with regard to the net-veined mon-zodiorite that intrudes the diatexite (e.g., N16.05; Fig. 3C), because some rocks at the margin of this intrusion were interpreted to be hybrids of the net-veined dike and migmatite. However, the 436.7 ± 3.5 Ma age of sample N16.05 is interpreted to be an intrusive age because the dated zircons were separated from the mafic part of the dike. Because of this result, the ca. 445 Ma date obtained from diatexite sample TF-77B may be interpreted as (1)

Geometry (SHRIMP-RG; Stanford University) and laser ablation (LA) ICP-MS at the Research School of Earth Sciences, Australia National University. The geochrono-logic data are given in Appendix Tables 1 (LA-ICP-MS) and 2 (SHRIMP-RG).

U–Pb dating

Stromatic migmatite melanosome. Zircons from sample TF-304 were dated by LA-ICP-MS. Two crystals gave concordant ages at 717 and 773 Ma (Appendix Table 1). The remaining concordant zircon ages range from 467 to 498 Ma (Fig. 4A). An average of all ages in this range yields 480.2 ± 3.6 Ma (MSWD = 2.43). Exclusion of the three youngest and four oldest zircons results in a sta-tistically identical age of 480.3 ± 1.9 Ma (MSWD = 0.73; Fig. 4A).

Stromatic migmatite leucosome. The leucosome fraction (sample TF-204) yielded few zircons. Two zircons yielded concordant ages of 480.5 and 479.4 Ma with a third at 465.2 Ma. Five older concordant ages were obtained at 974, 1014, 1060, 1407, and 1735 Ma.

Diatexitic migmatite. Two samples of diatexitic migma-tite were dated, one (TF-104) by LA-ICP-MS (Appen-dix Table 1) and the other (TF-77B) by SHRIMP-RG (Appendix Table 2). Sample TF-104 yielded 13 concor-dant ages in the range 472–492 Ma (Fig. 4B) and oth-ers at 649, 796, 1021, 1401, and 1412 Ma. Within the age range 470–495 Ma, an average age of 480.6 ± 3.8 Ma was obtained. SHRIMP-RG data for sample TF-77B resulted in five approximately concordant detrital zircon ages of 602, 772, 1280, 1348, and 1565 Ma, along with two dis-cordant ages of 463, and 473 Ma and a group of younger zircons whose average age is 445.1 ± 6.4 Ma (Fig. 4C & D). We call attention to the discrepancy in these ages compared to sample TF-104 at the end of this section.

Pluton east of main road cut. The quartz dioritic intrusion sampled east of the main study area (TF-404) yielded 20 concordant dates by LA-ICP-MS (Fig. 4E) that aver-age 447.3 ± 2.8 Ma. If the five youngest and four oldest dates are excluded, an average of 447.8 ± 1.7 Ma (MSWD = 0.27) is obtained. No inherited zircons were observed.

Banded gabbro. SHRIMP-RG dating of the banded gab-bro sample TF-70 yielded a range of concordant ages from 420 to 458 Ma (Fig. 4F). With the possible excep-tion of the single date at ~458 Ma, no distinct break in the zircon age data is apparent, therefore all of the data were used to obtain an age of 436.9 ± 4.4 Ma. As with quartz diorite sample TF-404, no inherited zircons were observed.

Net-veined monzonitic dike. Enclave material from a net-veined dike that cuts diatexite (N16.05) yielded zir-cons with LA-ICP-MS dates that span a range from ~415 to 479 Ma (Fig. 4G), with changes in the slope of the


possible if the middle crust was sufficiently heated by magma emplacement at that time (e.g., Velfjord plutons). However, the nearest large plutons of this age crop out 10 km from the field area. The latter two explanations are quite possible, particularly in view of the intricate veins and dikes that were emplaced from 437–424 Ma. Fur-ther detailed geochronologic study of the Upper Nappe is needed to constrain better its thermal history.

After migmatization, the next identifiable igneous event in the area is emplacement of the quartz dio-ritic pluton (TF-404) at 447.8 Ma. This is the same age

mis-sampling in the field, (2) evidence that the diatexite remained in a partly molten state for more than 40 m.y., (3) resetting of part, but not all of the zircons in the dia-texite at ca. 447 Ma, (4) evidence that the migmatite was remelted during magma intrusion at ~437 Ma, permit-ting hybridization, or (5) contamination of the migma-tite by younger leucocratic magmas. At present, we dis-count the first possibility because the chemical com-position of TF-77B is similar to other migmatitic rocks (see below), and discount the second possibility because there is no evidence for similar long-term high tempera-tures elsewhere in the HNC. Resetting at ca. 447 Ma is

460

465

470

475

480

485

490

495

500

-3 -2 -1 0 1 2 3stdev

da

te (

Ma

)

TF-104diatexitic migmatite480.6 ± 3.81.17 (n=13)

460

465

470

475

480

485

490

495

500

-3 -2 -1 0 1 2 3stdev

da

te (

Ma

)

TF-304 stromatic migmatitemelanosome

entire range: 480.2 ± 3.62.43 (n=21)

480.3 1.90.73 (n=15)

±

420

400

440

460

480

da

te (

Ma

)

480460

440420

0.050

0.054

0.058

0.062

0.066

12.6 13.0 13.4 13.8 14.2 14.6 15.0 15.4238U/ 206Pb

207Pb/

206Pb

420

425

430

435

440

445

450

455

460

465

470

-3 -2 -1 0 1 2 3stdev

da

te (

Ma

)

TF-404 quartz diorite

entire range447.3 ± 2.8 Ma1.24 (n=20)

447.8 ± 1.7 Ma0.27 (n=11)

TF-77B diatexite445.1 ± 6.43.4 (n=13)

TF-77B diatexite

A B

CD

E Figure 4


Major and trace element compositions

Migmatites. The migmatite samples vary in SiO2 content from ~53 to 73% (Appendix Table 3; Fig. 5). Two samples of diatexite have SiO2 contents near 55%, whereas a bulk sample of the stromatic migmatite and two analyzed leu-cosomes range from 65 to 73% SiO2 (Fig. 5). All of the analyzed migmatites are strongly peraluminous, with A/CNK > 1.1 (Fig. 5A). The melanosome has the highest A/CNK value (~2.1), which is indicative of its high bio-tite content. The migmatite samples have quite uniform values of FeO*/(FeO*+MgO) of ~0.74 (Fig. 5B). Such uniformity is not characteristic of magmatic suites (e.g.,

as emplacement of voluminous dioritic to granitic plu-tons of the Velfjord area to the west (Fig. 1; Yoshinobu et al., 2002; Anderson et al., 2008). In the study area, there evidently was a magmatic lull until about 437 Ma. From ~437 to ~424 Ma, the Upper Nappe was intruded by mafic (gabbroic and dioritic), tonalitic, monzonitic, and granitic magmas, many of which were mingled. This time range is very similar to the ages of the large felsic plutons that characterize the eastern part of the Bindal Batholith (Nordgulen et al., 1993; Nissen et al., 2006).

420

410

430

440

450

460

date

(M

a)

TF-82 tonalitic dike431.9 ± 3.5 Ma0.89 (n=16)

420

410

430

400

440

450

460

470d

ate

(M

a)

TF-70 banded diorite436.9 ± 4.4 Ma2.4 (n=22)

410

420

430

440

450

460

470

480

-3 -2 -1 0 1 2 3

stdevd

ate

(M

a)

N16.05net-veined monzodiorite dike

436.7 ± 3.5 Ma1.20 (n=19)

410

420

430

440

450

460

470

480

-3 -2 -1 0 1 2 3

stdev

date

(M

a)

N17.05 granitic dikes424.0 ± 1.2

F G

H I

Figure 4

Figure 4. Results of U–Pb dating of zircon. A. Probability plot of results for stromatic migmatite melanosome TF-304. Uncertainties are 1 σ. The shaded area represents analyses selected to interpret the age of migmatization. B. Probability plot for diatexitic migmatite TF-104. C. Concordia plot for diatexite sample TF-77B. D. Age distribution of concordant zircons younger than 480 Ma in TF-77B. See text for discussion. E. Probability plot for quartz diorite pluton sample TF-404. The shaded area represents analyses selected to interpret the age of emplacement. F. Age distribution of zircons from banded diorite sample TF-70. G. Probability plot of net-veined monzodiorite sample N16.05. The shaded area represents analyses selected to interpret the age of emplacement. H. Age distribution of zircons from tonalitic dike TF-82. I. Probability plot for granitic dike N17.05. The shaded area represents analyses selected to interpret the age of emplace-ment. Data in panels A, B, E, G, and I were determined by LA-ICPMS, for panels C, D, F, and H by SHRIMP-RG. See text and Appendix 1 for details.


448 Ma pluton. Two samples of the ~448 Ma quartz dio-ritic to quartz monzodioritic pluton east of the main road cut are metaluminous, magnesian, and span the bound-ary between calc-alkalic and alkali-calcic (Fig. 5). They have moderate REE slopes (normalized La/Lu = 33) and slight negative Eu anomalies (Fig. 6B).

Younger intrusive rocks. Field relationships and U–Pb ages suggest that all of the post-migmatite intrusive rocks in the main road cut have ages from 437–424 Ma, and most range from ~437 to 432 Ma. Therefore, we have not attempted to subdivide these units into narrower age groups and instead discuss them as a single suite. Sam-ples with SiO2 contents < 60 wt% are metaluminous, whereas those with higher SiO2 contents lie on or just

Frost et al., 2001). Instead it is suggestive of uniform compositions of ferromagnesian phases in melanosomes, leucosomes, and diatexites.

Among the migmatites, the melanosome has the high-est rare earth element (REE) abundances (Fig. 6A) and in particular the highest abundances of heavy REE. Diatexite sample TF-104 has a similar pattern to the melanosomes; however, the pattern of diatexite TF-77B is distinct from the melanosome pattern in having much lower heavy REE abundances. Leucosomes are characterized by heavy REE concentrations, ~10x chondrites (Fig. 6A). One leucosome has lower light REE abundances than the other samples and a positive Eu anomaly, all of which suggest that this sample contains cumulate plagioclase.

0

0.5

1.0

1.5

2.0

2.5

A/C

NK str

leudtx

dtx leu

mel

enB

enB

vnA

A=net-veinedB=pillowed dike

0.4

0.5

0.6

0.7

0.8

0.9

FeO

*/(F

eO

*+M

gO

)

45 50 55 60 65 70 75 80

SiO2

SiO2

str

leudia

dia104leumel

hostB

hostB

hostB

hostB

enB

enB432

-10

-5

0

5

10

15

Na2O

+K

2O

-CaO

45 50 55 60 65 70 75 80

SiO2

str

leu

dia dialeu

mel

enB

432

ferroan

alkalic

alkali-c

alcic

calc-alkalic

calcic

magnesian

0

500

1000

1500

2000

2500 ppm

Sr

0 1 2 3 4 5 6 7 8 9

CaO

str

leu

dia

dia

leu

mel

vnA

vnA

enA

enA

enA

enA

432

432

enAz

enAz

enAzenAz

A

B

C

D

Figure 5.

migmatite

448 Ma

437-432 Ma~424 Ma

Figure 5. Representative geochemical variation diagrams, with samples grouped according to approximate age, where known. Black triangles with dashed tie lines connect coexisting enclave (enA, enAz) and host vein (vnA) compositions in the 437 Ma net-veined dike. Black triangle labeled 432 is dated sample TF-82, the tonalite/granodioritic body at the eas-tern end of the road cut. Orange triangles connected by dashed tie lines represent a mingled (pillowed) dike with quartz-dioritic/tonalitic host (vnB) and melagranitic enclaves (B). Abbreviations associated with migmatitic samples are: str = stromatic migmatite; dtx = diatexite; mel = melanosome; leu = leucosome. A. weight percent SiO2 vs. A/CNK, where A/CNK is molar Al2O3/CaO + Na2O + K2O. B. All igneous samples plot in the magnesian field of Frost et al. (2001). C. Samples of igneous rocks range from calc-alkalic to mildly alkalic according to the modified alkali-lime index (Frost et al., 2001). D. Plot of CaO versus ppm Sr showing the wide range of Sr contents, particularly among the most mafic sam-ples. Note the lack of correlation of Sr content with CaO in the mafic and intermediate samples.


Figure 6C shows REE patterns for mingled dikes and related rocks. All show similar patterns, with slight nega-tive Eu anomalies. In two of the enclave-host pairs, the net-veined dike (TF 74) and the pillowed dike (TF-84), the REE abundances of the host are higher than in the enclaves. In contrast, for the mingling zone at the west end of the exposure (samples TF-64 & 66; see Appendix 1), the REE patterns of the enclave and host cross, with higher light REE and lower heavy REE in the host rock (Fig. 6C). The hybrid rock associated with TF-64 (sample TF-66; Fig. 6C) has REE abundances that are generally intermediate between the enclave and host.

Figure 6D shows REE patterns for 437–424 Ma felsic dikes. Samples TF-67 and TF-72 have patterns similar

above the metaluminous–peraluminous boundary (Fig. 5A). All samples plot in the magnesian field of Frost et al. (2001), although two samples associated with the 437 Ma net-veined unit plot near the magnesian–ferroan bound-ary (Fig. 5B). In the modified alkali-lime index diagram (Fig. 5C), compositions vary from calc-alkalic to alkalic. The single sample that plots in the calcic field is a 437 Ma banded gabbro that is probably an augite cumulate. This suite of dikes is characterized by relatively high Sr con-tents (Fig. 5D). Most dikes with less than 3% CaO have >1000 ppm Sr and in this compositional range, no cor-relation exists between the Sr and CaO contents. In con-trast, the older 448 Ma quartz diorite has lower Sr con-tents than the younger dikes (~750 ppm; Fig. 5D) and the ~424 Ma granitic dike has <500 ppm Sr.

1

10

100

1000

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

stromatic migmatite

leucosome

leucosome/melanosome pair

diatexite (TF-104/TF-77B)

1

10

100

1000


TF-17

TF-404

TF-79 (~435 Ma)

1

10

100

1000


TF-74

TF-64

TF-84

mingled dikes(437-432 Ma)

1

10

100

1000


comb-layered dike (TF-71)

TF-82 (431 Ma)

TF-81 (424 Ma)

TF-72 (�424 Ma)

TF-67 (~435 Ma)

tonalitic to granitic rocks

A B

C D

encl

ave

hyb

rid (

TF

-66)

fels

ic c

om

ponent

448 Ma}

sam

ple

/chondri

tes

Figure 6

TF-73

Figure 6. Chondrite-normalized REE dia-grams. A. Migmatitic rocks and related (?) comb-layered dike. B. Samples from the 448 Ma pluton east of the main road cut and monzodioritic sample TF-79 (~437 Ma). C. Mingled dikes. D. Evol-ved dikes ≤ 435 Ma.


However, temperatures estimated on this basis result in values that are below the wet solidus (ca. 640°C), which indicates that biotite compositions were reset during cooling. Temperature was also estimated on the basis of Ti-in-zircon thermometry (Watson et al., 2006) with activities of SiO2 and TiO2 set to unity. A decrease in the activity of TiO2 to 0.9 would increase estimated T by 10–20°C. No pressure correction was applied (see discus-sion in Fu et al., 2008). Temperature was calculated for each zircon that was used in the U–Pb age determin-ations. Nineteen zircons separated from melanosome of the stromatic migmatite yielded an average temperature of 811 ± 24 °C and 11 zircons from the diatexite gave an average of 793 ± 30 °C.

Some migmatitic samples contain the assemblage garnet + sillimanite + plagioclase + quartz, which permits the use of the GASP thermobarometer. Figure 8 shows the results of GASP calculations for five samples of the Upper Nappe: two from the Tosen road cut, two from Upper Nappe rocks sampled along Tosenfjord, and one col-lected ~20 km to the north in Størborja (Fig. 1C). If it is assumed that the Ti-in-zircon temperatures determined for the ~480 Ma zircons are representative of the T of melting, then intersection of this temperature range with the GASP equilibrium curves should provide an estimate

to those of the felsic parts of mingled dikes TF-74 and TF-84 (Fig. 6C), whereas sample TF-81 has lower heavy REE, similar to the felsic part of mingled dike TF-64 (Fig. 6C). The pattern for the dated granodiorite (TF-82) shows low REE abundances and a positive Eu anomaly which are typical for plagioclase accumulation.

Nd and Sr isotope data

All three samples of migmatitic rocks analyzed for Nd and Sr isotopic compositions (Appendix Table 4; Fig. 7) have low initial εNd values (-7 to -8) and high ini-tial 87Sr/86Sr values (0.714–0.718). The diatexitic migma-tite sample has distinctly lower initial 87Sr/86Sr than the stromatic migmatite and the leucosome. In view of the concentrations of Sr in all migmatite samples (332–622 ppm) this variation is probably related to isotopic hetero-geneity in the sedimentary protolith rather than differ-ences in the growth of radiogenic Sr after migmatization.

The 448 Ma quartz dioritic pluton east of the main road cut has initial εNd and 87Sr/86Sr values of -4.8 and 0.71043, respectively (Appendix Table 4). These isotope ratios are quite distinct from those of the 437–432 Ma dike rocks, which have εNd from -0.2 to -2.4 and initial 87Sr/86Sr between 0.706 and 0.708. This rather narrow range of iso-topic values encompasses rocks from 50 to 65 wt% SiO2 and 1.2–8% MgO. Within this group, there is no corre-lation between initial 87Sr/86Sr and SiO2 or 1/Sr, although the two samples with the highest 1/Sr values also have the highest initial 87Sr/86Sr. In contrast, a weak negative cor-relation exists between εNd and 1/Nd (and SiO2 content). The youngest sample analyzed for isotope ratios (424 Ma leucogranite TF81) has isotopic compositions nearly identical to the 448 Ma quartz diorite (Fig. 7).

In the preceding field descriptions, we noted field evi-dence that suggested significant interaction between migmatitic rocks and younger dikes. One specific case was the possible interaction of magma of the net-veined dike with adjacent diatexite (Fig. 2). The isotopic com-positions of the enclave and the vein material from this dike are nearly identical, and show no evidence of a mix-ing line with the adjacent migmatite (field A on Fig. 7). In a broader sense, isotopic compositions of the major-ity of the 437–432 Ma dike rocks are difficult to explain as hybrids of a mafic end member and migmatitic rocks (or magmas). This is because such hybridization should result in a decrease in the Sr content of the igneous rocks, with corresponding negative correlation of Sr and initial 87Sr/86Sr with SiO2. Neither correlation is observed.

Pressure and temperature estimates

Migmatitic rocks.

We attempted to use Fe-Mg exchange equilibria between garnet and biotite to estimate temperature (Reid, 2004).

-8

-6

-4

-2

0

2

0.705 0.710 0.715 0.720

87 86initial Sr/ Sr

� (t)Nd

A

hostB

enB

strleu

dia

424 Ma

448 Ma

Figure 7.

migmatite

448 Ma

437-432 Ma<425 Ma

Velfjordplutons

Figure 7. εNd (t) plotted versus initial 87Sr/86Sr. Symbols as in Figure 5. The unpatterned field (A) is the range of com-positions in the net-veined dike (TF-74A, 74B, N16.05). Samples from pillowed dike (B) are connected by a tie line. The horizontally-hachured field is the range of composi-tions of the Velfjord plutons (Barnes et al., 2002).


of plagioclase, K-feldspar, quartz, hornblende, biotite, and Fe-Ti oxide (Hammarstrom and Zen, 1986; Hollis-ter et al., 1987). For these calculations, rim compositions of amphibole and plagioclase were used with the pres-sure calibration of Anderson and Smith (1995) and the edenite–richterite T calibration of Holland and Blundy (1994).

Sample TF-17 is from the 447 Ma quartz dioritic pluton at the east end of the outcrop. Hornblende-plagioclase pairs yielded a P estimate of 329 ± 44 MPa and a T estimate of 712 ± 19°C. Monzonite sample TF-73 is thought to be a hybrid between the monzonitic host and enclaves of the net-veined dike dated at 437 Ma (dated sample N16.05; Fig. 4G). The estimated pressure was 365 ± 37 MPa and the T estimate was 723 ± 12°C. This temperature estimate is consistent with the Ti-in-zircon temperature calculated from magmatic zircons from sample N16.05 of 747 ± 77 C. The peach-colored box in Figure 8 indicates the most likely P and T conditions of hornblende, plagioclase, and zircon equilibrium among these younger intrusive rocks.

Discussion and Conclusions

Timing and conditions of magmatism

We interpret the U-Pb data for the migmatites to indi-cate that partial melting of the Upper Nappe occurred at ~480 Ma. Melting occurred at ca. 500–700 MPa pres-sure (ca. 20–28 km depths) and ~800°C (Fig. 8), condi-tions consistent with biotite-dehydration melting (e.g., Vielzeuf & Holloway, 1988; Patino Douce & Johnston, 1991; Vielzeuf & Montel, 1994; Montel & Vielzeuf, 1997). Under these conditions, enough melt was produced to allow for mobilization of melt-rich parts of the migma-tite to form cross-cutting diatexites. It is noteworthy that the diatexitic rocks are chemically more refractory than the stromatic migmatites (i.e., diatexites have lower SiO2 and higher TiO2, CaO, total Fe). This chemical difference indicates that although the diatexites may have initi-ally had larger proportions of melt than the stromatic migma tites, melt was lost during or after emplacement. There is no field or geochemical evidence that migmati-zation was accompanied by mafic magmatism, at least at the level of exposure. This leaves the source of heat for ca. 480 Ma crustal melting in the Upper Nappe, and else-where in the HNC, an unanswered question.

The next magmatic event recorded in the study area was emplacement of the small quartz dioritic pluton at 448 Ma. The age of this pluton is identical to larger plutons that intrude the underlying nappes of the HNC, specifi-cally the Velfjord plutons ~10 km to the west and north-west (Fig. 1B, C; Barnes et al., 1992, 2004). The quartz diorite is distinct from these plutons in having signifi-cantly higher initial 87Sr/86Sr values (Fig. 7) and also in its calc-alkalic (versus alkali-calcic to alkalic) compo-sitions at comparable SiO2 contents. These distinctions

of the pressure of melting. A further constraint on pres-sure is the observation that cordierite is present in many of the migmatite samples from the Upper Nappe, which restricts melting conditions to be at P lower than the biotite-sillimanite-garnet-cordierite-liquid equilibrium (Fig. 8). We conclude that partial melting of the Upper Nappe migmatites occurred between 500 and 700 MPa and ~800°C. These conditions are appropriate for biotite- dehydration melting (e.g., Patiño Douce and Johnston, 1991; Vielzeuf and Montel, 1994; Montel and Vielzeuf, 1997).

Younger intrusive rocks.

Pressure-temperature estimates for intrusive rocks were made on the basis of Al-in-hornblende thermobaro-metry (Anderson and Smith, 1995). All of the analyzed amphibole has calculated Fe3+/(Fe3+ + Fe2+) > 0.20, which Schmidt (1992) considered acceptable for use with horn-blende barometry. Anderson and Smith (1995) suggested that amphibole outside the Fetot/(Fetot + Mg) range of 0.40-0.65 should not be used for hornblende baro metry. Two samples (TF-73, TF-17) contain amphibole that meet these criteria and contain the equilibrium assemb lage

500 700 900

A

leucosome TTi in zircon T

Al-

in-h

orn

ble

nde P

(A

& S

)

600 800

200

400

600

800

1000

oT ( C)

P (MPa)

Crd Bt V

kyanite

andalusite

sillimanite

Opx As

Grt Crd

Grt Sil

Spl Crd

Bt G

rtO

px

Crd

L

Bt A

s

Grt

Crd

L

Ms

Ab

As

Kfs

L

Ms Ab

As Kfs V

Bt Grt

Opx Crd V

L

Kfs Ab Grt

Bt G

rt Opx As LNaKFMASH

0.8 = Fe/(Fe + Mg) ofgarnet in assemblage:

Grt+Bt+As

Grt+Bt+Crd

Grt+Crd+As

0.9

0.8

0.7

0.6

0.6

0.7

0.9

0.8

Ms

Ab V

As

L

Grt K

fs L

Bt A

s

Grt As LCrd Kfs

Grt K

fs L

Crd

Bt

VF201

VF199A

VF215

Figure 8.

Figure 8. P-T estimates. P-T grid for fluid-absent partial melting of pelitic compositions, after Spear et al. (1999). Colored curves are equilibrium conditions for five Upper Nappe samples with the GASP (garnet-sillimanite-silica-plagioclase) assemblage calculated using TWQ (Berman, 1988, 1990). The gray box indicates estimated conditions of migmatization determined on the basis of Ti-in-zircon thermometry of ~480 Ma zircons and the results of GASP calculations. The pink-shaded box indicates the results of Al-in-hornblende barometry and hornblende-plagioclase thermometry on 448 and 437 Ma intrusive rocks. Phase abbreviations are: Ms, muscovite; ab, albite; As, alumino-silicate; Kfs, K-feldspar; Bt, biotite; Grt, garnet; Opx, ort-hopyroxene; Crd, cordierite; Spl, spinel; Sil, sillimanite; L, silicate melt; V, aqueous fluid.


et al., 2007). Evidently, middle Ordovician metamorphism and local crustal melting occurred throughout the HNC in response to nappe amalgamation (Barnes et al., 2007).

Although anatexis in the Upper Nappe occurred at ~700 MPa pressure, by the 448 Ma emplacement of the quartz dioritic pluton the Upper Nappe rocks were at a pressure of ~300 MPa. Similar amounts of exhumation have been documented in the Velfjord region (Barnes and Prest-vik, 2000) and at Vega (Marko et al., 2007; Oalmann, 2010). In Velfjord, exhumation was interpreted to occur during and after emplacement of ca. 448–445 Ma plu-tons (Barnes and Prestvik, 2000; Yoshinobu et al., 2002), whereas exhumation preceded the 448 Ma magmatism at Tosenfjord. The age of exhumation at Vega is uncertain, but may have been as old as 475 Ma (Marko et al. 2007). Evidently, although high-grade metamorphism was coeval across the HNC, exhumation was diachronous.

The work of Nordgulen and colleagues (Nordgulen, 1993; Nordgulen et al., 1993; Yoshinobu et al., 2002; Barnes et al., 2007) have shown that the earliest plutons in the Bindal Batholith (478–475 Ma) are restricted to the western HNC and that plutons from 467–444 Ma crop out primarily in the central and western parts of the nappe complex. In contrast, plutonic rocks in the 440–430 Ma age range crop out across the HNC, from the westernmost exposures (e.g., Sklinna pluton; Barnes et al., 2007) to the easternmost ones (Nissen et al., 2006). This change from early, relatively focused magmatic activity to later, widespread magmatism in the HNC may be related to changes in tectonic environment (contrac-tion versus extension), dip of a subducting slab (older steep dip to younger shallow dip), or to the development of a slab window in early Silurian time and initiation of the Scandian collision. The possibilities can only be resolved with further detailed petrological and geochro-nological study of the HNC.

Acknowledgments. - We thank Øystein Nordgulen for bringing this exceptional exposure to our attention and for his geologic insights on HNC geology. S. Swapp provided able assistance with the microprobe analyses. We also thank H. Schiellerup and R. Larsen for their thought-ful reviews. This research was supported by NSF grant EAR9814280 and by the Norwegian Geological Survey.

Electronic Appendix Tables

Table 1. U-Pb age data determined by LA-ICP-MSTable 2. U-Pb age data determined by SHRIMP-RG.Table 3. Representative major and trace element analysesTable 4. Nd and Sr isotope data.Table 5. Representative garnet analyses.Table 6. Representative hornblende analyses.Table 7. Representative biotite analyses.Table 8. Representative plagioclase analyses.Table 9. Clinopyroxene analyses.

suggest that the quartz dioritic magma either had a dis-tinct source or underwent significant crustal contamina-tion compared to the Velfjord plutons.

The various dikes with ages from 437–432 Ma are note-worthy because of the wide range of bulk compositions, from gabbroic to granitic. The abundance of mingled dikes among the 437–432 Ma dikes shows that mafic and felsic magmas coexisted in this time range. Overall, these rocks are metaluminous to weakly peraluminous and have low initial 87Sr/86Sr compared to the migmatites and 448 Ma quartz diorite (Figs. 5, 7). There is no corre-lation between the chemical composition (e.g., SiO2 con-tent) and isotope ratios and among the rocks with SiO2 >71 wt %, i.e. rock types that range from tonalite to gran-ite (2.5 to 7.0 wt % K2O), with no correlation of SiO2 with K2O. Nevertheless, most samples have small negative Eu anomalies (Fig. 6) and mafic–intermediate rocks have high (>1000 ppm) Sr contents (Fig. 5). These observa-tions suggest that the various 437–432 Ma dikes are not related by differentiation of a single, evolving magma, but instead arise from a deep-seated source region in which isotopic compositions vary within a narrow range but major element concentrations can vary significantly. Specifically the combination of low initial 87Sr/86Sr, high Sr contents and lack of correlation between Sr and dif-ferentiation among intermediate and mafic rocks, small, uniform Eu anomalies, and low alumina saturation index is suggestive of magma generation or differentiation under conditions where plagioclase fractionation was minor, consistent with lower crustal conditions.

In contrast, the young 424 Ma granitic dike has much higher initial 87Sr/86Sr and lower εNd than the 437–432 Ma dikes, suggesting a distinct source for this youngest magma batch. The low HREE of this granite (Fig. 6D) indicates residual garnet in the source, which is in con-trast to nearly all of the older dikes. It is noteworthy that although the 424 Ma granite displays a garnet REE signa-ture, it is only slightly peraluminous, which rules out an origin by partial melting of typical metapelitic rocks in the HNC (e.g., Nordgulen and Sundvoll, 1992; Barnes et al.2002).

Several of the ca. 437–432 Ma dikes are foliated, includ-ing mingled dikes in which mafic enclaves underwent sub-solidus deformation (Appendix 1), and the ~437 Ma net-veined rocks are cut by a shear zone with reverse-sense (top-to-west) displacement. In contrast, the 424 Ma granitic dikes show no evidence of sub-solidus defor-mation. Evidently, magma emplacement at ca. 435 Ma was accompanied by local penetrative deformation, at least some of which was contractional.

Relationship to regional geology

The age of migmatization of the Upper Nappe at~ 480 Ma is essentially identical to that of migmatization in the Horta and Lower Nappes (Yoshinobu et al., 2002; Barnes


Kollung, S. & Myrland, R. 1970: Beskrivelse til geologisk kart over Norge – Velfjord, Norges geologiske undersøkelse, 1:100,000.

Marko, W.T., Barnes, C.G. & Yoshinobu, A.S. 2007: Magma chamber construction in the mid to lower crust: observations from a large S-type intrusion, southern Nordland, Norway. Sixth International Hutton Symposium abstracts, 127–128.

Marko, W., Barnes, C., Frost, C. & Vietti, L. 2008: Cambrian–Ordovi-cian anatexis in the Helgeland Nappe Complex, north-central Nor-way, 33rd International Geological Congress, Oslo, 194.

Montel, J.-M. & Vielzeuf, D. 1997: Partial melting of metagreywac-kes, Part II. Compositions of minerals and melts. Contributions to Mineralogy and Petrology 128, 176–196.

Myrland, R., 1974, Beskrivelse til geologisk kart over Norge – Bindal, Norges geologiske undersøkelse. 1:100,000.

Nissen, A. L., Roberts, D. & Gromet, P. 2006: U-Pb zircon ages of a tonalite and a granodiorite dyke from the southeastern part of the Bindal Batholith, central Norwegian Caledonides. Norges geolo-giske undersøkelse Bulletin 446, 5–9.

Nordgulen, Ø. 1993: A summary of the petrography and geochemistry of the Bindal Batholith. Geological Survey of Norway, Report 92.111.

Nordgulen, Ø. & Schouenborg, B. 1990: The Caledonian Heilhornet pluton, north-central Norway: geological setting, radiometric age and implications for the Scandinavian Caledonides. Journal of the Geological Society, London 147, 439–450.

Nordgulen, Ø. & Sundvoll, B. 1992: Strontium isotope composition of the Bindal Batholith, Central Norwegian Caledonides. Norges geo-logiske undersøkelse Bulletin 423, 19–39.

Nordgulen, Ø., Bickford, M. E., Nissen, A. L. & Wortman, G. L. 1993: U-Pb zircon ages from the Bindal Batholith, and the tectonic his-tory of the Helgeland Nappe Complex, Scandinavian Caledonides. Journal of the Geological Society, London 150, 771–783.

Nordgulen Ø, Fjeldheim, T., Ihlen, P.M., Nissen, A.L. & Solli, A., 1992, VEVELSTAD 1826-3, Foreløpig Berggrunnskart, Norges Geologiske Undersøkelse, 1:50,000.

Oalmann, J. 2010: Geology, plutonism, and metamorphism at the island of Ylvingen, Nordland, Norway, MS thesis, Texas Tech Uni-versity, Lubbock, 224p.

Patiño Douce, A. E. & Johnston, A. D. 1991: Phase equilibria and melt productivity in the pelitic system: implications for the origin of peraluminous granitoids and aluminous granulites. Contributions to Mineralogy and Petrology 107, 202–218.

Roberts, D., Nordgulen, Ø. & Melezhik, V. 2007: The Uppermost Allochthon in the Scandinavian Caledonides: From a Laurentian ancestry through Taconian orogeny to Scandian crustal growth on Baltica. In: Hatcher, R. D., Jr., Carlson, M. P., McBride, J. H. & Martínez Catalán, J. R. (eds.) 4-D Framework of Continental Crust: Geological Society of America Memoir. Geological Society of Ame-rica, 357–377.

Reid, K. 2004: Magmatic processes in the Tosenfjord region, north-central Norway: Implications for the evolution of the Helgeland Nappe Complex, MS thesis, Texas Tech University, Lubbock, 111 p.

Schmidt, M. W. 1992: Amphibole composition in tonalite as a function of pressure: an experimental calibration of the Al-in-hornblende barometer. Contributions to Mineralogy and Petrology 110, 304–310.

Spear, F., Kohn, M. & Cheney, J. 1999:. P-T paths from anatectic peli-tes. Contributions to Mineralogy and Petrology 134, 17–32.

Thorsnes, T. & Løseth, H. 1991: Tectonostratigraphy in the Velfjord-Tosen region, southwestern part of the Helgeland Nappe Complex, Central Norwegian Caledonides. Norges geologiske undersøkelse Bulletin 421, 1–18.

Vielzeuf, D. & Holloway, J. 1988: Experimental determination of the fluid-absent melting relations in the pelitic system: Consequences for crustal differentiation. Contributions to Mineralogy and Petro-logy 98, 257-276.

Vielzeuf, D. & Montel, J. M. 1994: Partial melting of metagreywackes. Part I. Fluid-absent experiments and phase relationships. Contribu-tions to Mineralogy and Petrology 117, 375–393.

References Cited

Anderson, H.S., Yoshinobu, A.S., Chamberlain, K., Nordgulen, Ø. & Barnes, C.G. 2008: Magma Emplacement and the Development of Ghost Stratigraphy in the Andalshatten Pluton, Central Norway. Geological Society of America Abstracts with Programs, 40, # 6, 251

Anderson, J. L. & Smith, D. R. 1995: The effects of temperature and fO2 on the Al-in-hornblende barometer. American Mineralogist 80, 549–559.

Barnes, C. G. & Prestvik, T. 2000: Conditions of pluton emplacement and anatexis in the Caledonian Bindal Batholith, north-central Norway. Norsk Geologisk Tidsskrift 80, 259–274.

Barnes, C. G., Prestvik, T., Nordgulen, Ø. & Barnes, M. A. 1992: Geo-logy of three dioritic plutons in Velfjord, Nordland. Norges geolo-giske undersøkelse Bulletin 423, 41–54.

Barnes, C., Yoshinobu, A., Prestvik, T., Nordgulen, Ø., Karlsson, H. & Sundvoll, B. 2002: Mafic magma intraplating: anatexis and hybridi-zation in arc crust, Bindal Batholith, Norway. Journal of Petro-logy 43, 2171–2190.

Barnes, C. G., Prestvik, T., Barnes, M. A. W., Anthony, E. Y. & Allen, C. M. 2003: Geology of a magma transfer zone: the Hortavær Igneous Complex, north-central Norway. Norwegian Journal of Geology 83, 187–208.

Barnes, C.G., Dumond, G., Yoshinobu, A. S. & Prestvik, T. 2004: Assi-milation and crystal accumulation in a mid-crustal magma cham-ber: The Sausfjellet pluton, north-central Norway. Lithos 75, 389–412.

Barnes, C. G., Prestvik, T., Sundvoll, B. & Surratt, D. 2005: Pervasive assimilation of carbonate and silicate rocks in the Hortavær igne-ous complex, north-central Norway. Lithos 80, 179–199.

Barnes, C. G., Frost, C. D., Yoshinobu, A., McArthur, K., Barnes, M. A., Allen, C. M., Nordgulen, Ø. & Prestvik, T. 2007: Timing of sedi-mentation, metamorphism, and plutonism in the Helgeland Nappe Complex, north-central Norwegian Caledonides. Geosphere 3, 683–703.

Berman, R. G. 1988: Internally-consistent thermodynamic data for minerals in the system Na2O-K2O-CaO-MgO-FeO-Fe2O3-Al2O3-SiO2-TiO2-H2O-CO2. Journal of Petrology 29, 445–522.

Berman, R. G. 1990: Mixing properties of Ca-Mg-Fe-Mn garnets. American Mineralogist 75, 328–344.

Birkeland, A., Nordgulen, O., Cumming, G. L. & Bjorlykke, A. 1993: Pb-Nd-Sr isotopic constraints on the origin of the Caledonian Bindal Batholith, central Norway. Lithos 29, 257–271.

Frost, B. R., Barnes, C. G., Collins, W. J., Arculus, R. J., Ellis, D. J. & Frost, C. D. 2001: A geochemical classification for granitic rocks. Journal of Petrology 42, 2033–2048.

Frost, C., Vietti, L., McArthur, K., Barnes, C., Nordgulen, Ø. & Mere-dith, M. 2006: Norwegian Caledonides: Nd isotopic data and impli-cations for Ordovician tectonic evolution. Geological Society of America Abstracts with Programs 38.

Fu, B., Page, F.Z., Cavosie, A.J., Fournelle, J., Kita, N.T., Lackey, J.S., Wilde, S.A., & Valley, J.W. 2008: Ti-in-zircon thermometry: appli-cation and limitations. Contributions to Mineralogy and Petrology 156, 197–215.

Gustavson, M. 1981: Beskrivelse til geologisk kart over Norge, 1:250,000, Mosjøen. Norges geologiske undersøkelse, 1:250,000.

Hammarstrom, J. M. & Zen, E. 1986: Aluminum in hornblende: An empirical igneous geobarometer. American Mineralogist 71, 1297–1313.

Holland, T. & Blundy, J. 1994: Non-ideal interactions in calcic amphi-boles and their bearing on amphibole-plagioclase thermometry. Contributions to Mineralogy and Petrology 116, 433–447.

Hollister, L. S., Grissom, G. C., Peters, E. K., Stowell, H. H. & Sisson, V. B. 1987: Confirmation of the empirical correlation of Al in hornblende with pressure of solidification of calc-alkaline plutons. American Mineralogist 72, 231–239.


Watson, E. B. & Harrison, T. M. 1983: Zircon saturation revisited: temperature and composition effects in a variety of crustal magma types. Earth and Planetary Science Letters 64, 295-304.

Watson, E. B., Wark, D. A. & Thomas, J. B. (2006). Crystallization ther-mometers for zircon and rutile. Contributions to Mineralogy and Petrology 151, 413–433.

Yoshinobu, A. S., Barnes, C. G., Nordgulen, Ø., Prestvik, T., Fanning, M. & Pedersen, R. B. 2002: Ordovician magmatism, deformation, and exhumation in the Caledonides of central Norway: An orphan of the Taconic orogeny? Geology 30, 883–886.

Documents

91(3), pp. This ﬁle was downloaded … · 2020. 11. 6. · Myrland, 1974) recognized the presence of a number of migmatitic rock units in central Helgeland and a later study in