Tectonic implications of U-, Mo- and V-enriched graphitic phyllites in the Høgtuva and Nasafjall Windows, Scandinavian Caledonides

JAN-ERIK LINDQVIST

Lindqvist, J.-E.: Tectonic irnplications of U-, Mo- and V-enriched graphitic phyllites in the Høgtuva and Nasafjiill Windows, Scandinavian Caledonides. Norsk Geologisk Tidsskrift, Vol. 68, pp. 187-199. Oslo 1988. ISSN 0029-196X.

Basement occurs in windows in the northern Scandinavian Caledonides, which are distributed in two rows: an easterly row along the national border between Norway and Sweden, including the Nasafjiill Window, and a westerly row in the Glomfjord-Sjona area including the Høgtuva Window. The basement and cover rocks in the eastern windows can be correlated with those of the Baltoscandian Platform. The tectonostratigraphical position of the rocks in the western windows has been debated. This paper presents a study of metasedirnentary cover sequences in the medium-grade rocks of the Høgtuva and Nasafjiill Windows that compare closely with the autochthonous Dividal Group in the eastern thrust front and in the eastem windows. These include U-, Mo- and V-enriched graphitic phyllites; this suggests that the basement of the western windows is also of Baltoscandian affinity.

Jan-Erik Lindqvist, Avd. for Mineralogi och Petrologi, Geologiska Jnstitutionen, Solvegatan 13, S-223 62 Lund, Sweden.

Proterozoic crystalline rocks occur in several win­dows in the Scandinavian Caledonides of the counties of Nordland in Norway and Norrbotten in Sweden. These windows can be divided into two sets: one to the west near the Norwegian coast, including the Høgtuva, Sjona and Glomfjord windows, the other further east along the national border, including the Nasafjlill and Rombak windows (Fig. 1). While the basement of the eastern windows is accepted as an extension of the Baltic Shield, the interpretation of the western windows is highly debated (Stephens et al. 1985). These western windows were previously believed to be Caledonian intrusions. Rutland (1959) demonstrated that they were of pre-Cale­donian origin and inferred that they composed the lowermost structural unit in the area.

There are currently two different views on the tectonostratigraphic position of these western windows (Fig. 2). One interpretation is that they are a part of the Baltic craton (Nicholson &

Rutland 1969; Wilson & Nicholson 1973; Gustavson 1978; Gorbatschev 1985) and form a part of the Parautochthon or Lower Allochthon (cf. Gee & Zachrisson 1979). Different views have been put forward for the tectonostra­tigraphical position of the lower part of the

Rodingfjlill Nappe in this area. This unit was identified as a separate part of the Rodingfjlill Nappe, The Meløy Group, and assumed to retain its original stratigraphical relations (Nicholson &

Walton 1963). For this unit an autochthonous interpretation in relation to the gneisses of the western windows, and a correlation with Vendian sparagmite deposits has been advocated by Rutland & Nicholson (1965) and Nicholson & Rutland (1969), while others include these units in the Caledonian Allochthon (cf. Gustavson & Gjelle 1981; Stephens et al. 1985).

In an alternative model these windows are . thought of as integral parts of the Uppermost Allochthon. This interpretation has been advo­cated by Graversen et al. (1981), who obtained a preliminary Rb/Sr whole rock age of 577 Ma on a metagranite within the Rodingfjlill Nappe. On the basis of structural eVidence they inferred that this metagranite was intruded after the juxta­position of the Rodingfjlill Nappe and the basal gneisses. It has also been argued that the dif­ference in degree of Caledonian penetrative deformation and metamorphism between the Nasafjlill and Høgtuva Windows, pointed out by Nicholson and Rutland (1969), supports the latter interpretation (Stephens et al. 1985). Further-

188 J.-E. Lindqvist

F===J High er nappe l::=::::::J unit s D lower most units

1n the Nasafjall Window

NORSK GEOLOGISK TIDSSKRIFT 68 (1988)

Precambrian • Dividal Group and

crystalline basement equivalent units

Fig. l. Generalized geological map over the northem central Scandinavian Caledonides. The map shows the basement windows and the Lower Allochthon, while the Middle, Upper and Uppermost Allochthons are indicated as higher thrust units. The map is compiled from maps of Kulling (1972) and Gjelle & Gustavson (1978). Abbreviations: G: Gibnotjåkkå, L: Laisvall, H: Hornavan, NaW; Nasafjåll Window, HT: Høgtuva Window, SI: Svartisen Window, GL: Glomfjord Window, SJ: Sjona Window. Locations of the profiles in Fig. 2 (A to B), and of the stratigraphical profiles from the eastern thrust front (G & L) in Fig. 3 are shown.

HT

R Crystalline � basement

Profile 1

Profile 2

NaW ..... - .....

( .......... l .....

50 kilometres

11 Ba_sal sedimentary r771 Caledonian

umt L{Lj nappes

Ett

2 km

o

2 km

o

Fig. 2. Profiles illustrating the two tectonostratigraphical models for the western windows. In profile l the western windows form

a western continuation of the Baltic Platform. The basal sedimentary unit here includes the Dividal and Mierkenis Groups and their possible equivalents in the Høgtuva Window. Abbreviations: HT: Høgtuva Window, NaW: Nasafjall Window, Etf: eastern thrust front. Profile 2 shows a model where these windows form an integral part of the Uppermost Allochthon. The location of

the profile is shown in Fig. l.

NORSK GEOLOGISK TIDSSKRIFT 68 (1988) Tectonic imp/ications of U, Mo and V minera/ization 189

more, Gabrielsen et al. (1981) showed that while there is a significant gravity low associated with the Nasafjlill and Rom bak Windows, such a nega­tive anomaly is lacking in the Sjona- Glomfjord area. In their opinion, this indicates that these western windows are integral parts of the nappes.

Arguments for a correlation with the Bal­toscandian Platform are based on similarities in lithology and age of the crystalline basement. The basement rocks in the Glomfjord Window and in the N asafjlill Window gave Rb/Sr whole rock ages of 1695 ± 71 Ma and 1742 ± 42 Ma, respectively (Wilson & Nicholson 1973 recalculated using Å 87Rb = 1.42 10-11 a-1). Also, the appearance of quartzite and gniphitic schists in the contact with the basement gneisses in the Høgtuva Window (Søvegjarto personal communication in Stephens et al. 1985) supports a correlation with the Bal­toscandian Platform.

The two models have been discussed in a review article by Stephens et al. (1985), who concluded that more detailed information on the basement­cover contact relations in these western areas was needed if this important question was to be resolved. The present paper concentrates on the metasedimentary cover sequences in the Høgtuva and Nasafjlill Windows as these units are critical to this interpretation.

Throughout the eastern thrust · front of the Scandes and within many eastern windows the crystalline basement rocks are overlain by a thin metasedimentary cover sequence in both the Autochthon-Parautochthon and Lower Allochthon. These metasediments are referred to as the Dividal Group (Føyn 1967; Thelander 1982) in northern Scandinavia and as the Mier­kenis Group in the Nasafjlill Window. The Divi­dal Group passes south into lower units of the Jlimtland Supergroup in the central Scandinavian Caledonides (Gee 1975). The latter rocks were deposited in late Precambrian to Silurian times. In this paper these metasedimentary units will be referred to as the Basal Sedimentary Unit (BSU).

The sedimentary basin extended over a large area of the Baltic Shield. The lower sediments of the Dividal Group in Norrbotten were deposited in a nearshore environment of the Baltoscandian Platform (Willden 1980). Subsequently, the sea transgressed over the Baltic Shield and black shales were deposited over extensive areas (Andersson et al. 1985; Gee 1980).

The correlation of the basement rocks in the eastern windows with those in the Baltic Shield

autochthon is strengthened by the appearance in the windows of BSU cover sequences which correspond to the autochthonous occurrences in the eastern thrust front (Gee 1980). The BSU is an important diagnostic unit, as emphasized by

Kautsky (1953) who referred to these units as the Hyolithus zone. In the windows, these rocks are most! y strongly deformed and no fossils have been found. The correlation is made on the basis of similar stratigraphy, lithology, and, particularly, the geochemistry of the black phyllites occurring in these units. This is the same black shale as the metal-rich shale unit in Ranstad, Sweden, which is characterized by high contents of U, V and Mo (Armands 1972; Andersson et al. 1983).

It has long been recognized that Scandinavian black shales are high in U, Mo and V. Subsequent research has shown that although black shales are commonly metal-rich, the geochemical pattern of the Cambro-Ordovician black shales in Scan­dinavia is highly unusual (Dahlman & Eklund 1953; Swanson 1961; Cave 1965; Vine & Turtelot 1970; IUREP 1980).

Platform sediments including black shales were deposited over extensive areas on the European side of the lapetus Ocean while the Ame ri can side was dominated by limestones (Gee 1975). This lea ds to the hypothesis that if the western windows in Nordland belonged to the Baltoscandian Plat­form it might be expected that they would have a BSU cover, including radioactive black shales. The aim of the present work was to search for such westerly BSU occurrences and compare them with eastern ones.

Eastern thrust front The stratigraphy of the Dividal Group occur­rences in the Autochthon and Lower Allochthon in the county of Norrbotten is known from the Laisvall mine (Lilljequist 1973; Willden 1980) and from well-exposed sections (Fig. 3), Luopakte, Laisviksberget and Gibnotjåkkå (Kulling 1982; Thelander 1982). Palaeoenvironmental inter­pretations of this paper follow those published by Willden (1980) and Thelander (1982).

The lowermost part of the Dividal Group com­prises conglomerates, feldspathic sandstones and siltstones. These were deposited in Vendian times when the sea transgraded over the deeply weath­ered peneplane of the Baltic Shield. Locally, diamictites and laminated shales occur. Some of

190 1.-E. Lindqvist

Femfjellet (FE)

Aldajaureh (AW)

m

30

20

10

m

20

10

m

50

40

30

20

............... 10

:::;:;:;:;:;:;:::::::::;:;:;: �

Gibnotjåkkå

(G)

Melfjellet (ME)

Aldatjåkkå (AE)

Laisvall

(L)

m

40

30

20

10

m

90

60

30

NORSK GEOLOGISK TIDSSKRIFT 68 (1988)

m

60

40

20

Mierkenisvagge

(M)

legend

Not exposed

Greywacke

Alumshale

Siltstone

Phyllite

Quartzite

Limestone

Pebbly phyllite

Diamictite

Arkose

Basement

Fig. 3. Stratigraphy and tectonostratigraphy of the cover sequences in the eastern thrust front, Nasafjall Window and the Høgtuva Window. Locations of the sections are shown in Figs. l, 4 and 5. The Laisvall section is from Willden (1980), Aldajaureh is from Lindqvist (1984), Mierkenisvagge is compiled from Clausen, Lindqvist & Nordgren in prep. The sections in the windows are

strongly deformed and original stratigraphy as well as thickness is tectonically disturbed.

NORSK GEOLOGISK TIDSSKRIFT 68 (1988) Tectonic implications of U, Mo and V mineralization 191

these have been interpreted as glacial deposits which are likely to correlate to the Vendian tillites in Finnmark, northem Norway (Kulling 1951; Willden 1980; Thelander 1982). For the diamic­tites of the Vakkejokk Breccia in the Tometriisk area, an interpretation as a mass fiow deposit from a basement island is favoured by Kulling ( 1972) and Thelander (1982).

These lower units are covered by a sequence of quartzites and shales deposited in tida! fiats and lagoons. The quartzites are generally overlaid by a phosphorite conglomerate probably deposited

Fig. 4. Map of the Nasafjall Window indicating the location of the stratigraphic sections shown in Fig. 3. (M, A W and AE). The position of the profile (A-B) in Fig. 5 is also shown. Abbreviations K: Kammeren, M: Mierkenisvagge, SW: Silvergruvan Window, BW: Bolna Window, RW: Raudfjellet Window, KW: Krokstrand Window, ST: Storselet Window, A W: Aldajaureh West, AE: Aldajaureh East. The map is compiled from Du Rietz (1949), Kulling (1972), Gjelle & Gustavson (1978), Gjelle (1978) and Thelander et al. (1980). Locations for the samples in Table l are indicated.

-D

M ierken is

Group

Higher

thrust

units

in an exposed beach environment. These are in turn followed by siltstones of the Grammatjukku Formation (Laisvall)/Upper Siltstone Member (Luopakte) (Willden 1980; Thelander 1982). The Grammatjukku Formation comprises the base of the Cambrian (Vidal 198 1; Ahlberg 1983). In the top of the autochthonous Dividal sequence in Norrbotten, metalliferous black shales of the Alum Shale Formation occur. Limestones are occasionally present between the Grammatjukku Forma ti on and the Alum Shale Formation. Along the Silverviigen road, the alum shales are overlain

+++++ +++++ +++++ +++++ +++++

D

Basement

granite

Gargatis·

Nappe

------ Main road

o Sample point

192 J.-E. Lindqvist

A

Legend

Raudljell Window

l r-========��=�====:=::=-=-7 / li

, , l;

//

NORSK GEOLOGISK TIDSSKRIFT 68 (1988)

B

Bolna Window

EIJ ���t�er nappe ��� Mierkenis Groupt::::::3 Porphyry H! Granite • AmphibolitP

Fig. 5. Profile over the Bolna and Raudfjellet Windows. The geometry of these windows is determined by the post-thrusting, D2-D3 folding. Location of the profile is shown in Fig. 4.

by thick sequences of greywacke of the Ringselet Formation (Clausen, Lindqvist & Nordgren in prep.).

The Nasafjall Window The NasafjlHI Window is a composite structure consisting of the Gargatis Nappe thrust over a parautochthonous granite with a thin cover of quartzite and black phyllite (Figs. 4 and 5). The Gargatis Nappe is composed of crystalline rocks with a metasedimentary cover of the Mierkenis Group. In the southeastern part of the window the crystalline rocks are composed of porphyries. In the northwestern areas, they are dominated by coarse-grained gneisses of granitic composition. The parautochthonous granite occurs in windows within the Nasafjall Window (Du Rietz 1949; Thelander et al. 1980).

Precambrian crystalline rocks of the Gargatis Nappe: The porphyries of the Gargatis Nappe are of rhyolitic composition with phenocrysts of potassium feldspar, plagioclase and quartz. The gneisses are coarse-grained, with potassium feid­spar, plagioclase and quartz and sparse epidote, biotite, chlorite and muscovite. The gneisses of the Gargatis Nappe were dated using Rb- Sr whole rock methods to 1742 ± 42 Ma (Wilson &

Nicholson 1973; recalculated using Å 87Rb = 1.42 10-11 a-1). For the crystalline rocks of the Gar­gatis Nappe, a correlation with the Transscan­dinavian Granite Porphyry Belt is likely (Gorbatschev 1985).

Mierkenis Group: Metasediments referred to as the Mierkenis Group (Kulling 1972, 1982; Thel­ander et al. 1980) occur on the crystalline rocks of the Gargatis Nappe and the parautochthonous granite. These metasediments are preserved along most of the basement cover contact. The sedimentary sequence is dominated by graphitic phyllite and quartzite. The graphitic phyllite has been correlated with the Alum Shale Formation of the Dividal Group (Thelander et al. 1980). The Nasafjall occurrences are rich in vanadium (380-1730 ppm), uranium (25-150 ppm) and Mo (70-140 ppm; Table 1). In Mierkenisvagge, thick sequences of biotite phyllites and greywackes occur together with graphitic phyllite and sub­ordinate quartzite. At one locality, a thin lime­s tone horizon has also been found in the lower part of the stratigraphy. This greywacke sequence forms a westerly continuation of the Ringselet Formation in the Lower Allochthon farther east (Clausen, Lindqvist & Nordgren in prep). In the Aldajaureh area in the southeastern part of the window, diamictites and laminated shales occur in the lowest part of the sedimentary sequence (Lindqvist 1983). These diamictites occur in E-W trending synforms and are only slightly deformed. Sedimentary structures and primary contacts to the underlying crystalline rocks are preserved in the lower part of the sequence. The sedimentary contact to the strongly weathered crystalline basement is also preserved (Marklund 1952; Lindqvist 1983). The Aldajaureh sequence com­pares well with the autochthonous sequence in Laisvall.

NORSK GEOLOGISK TIDSSKRIFf 68 (1988) Tectonic implications of U, Mo and V mineralization 193

Parautochthon: The lowermost tectonostrati­graphical units in the Nasafjiill area occur in subordinate windows below the Gargatis Nappe. These are the Storselet and Silvergruvan Win­dows in Sweden (Thelander et al. 1980) and the Krokstrand (Du Rietz 1949), Bolna (Gjelle 1978) and Raudfjellet (this paper) Windows in Norway. The Graddasjaure Window (Thelander et al. 1980) was later reinterpreted to belong to the Gargatis Nappe (Thelander written com­munication, Uppsala 1986). The outcrop pattem of the windows is controlled by gentle folding and doming. The Silvergruvan Window and the Bolna Window belong .to the same approximately.E­W trending antiform. The Raudfjellet Window occurs in a parallel antiform. The basement rocks of the Raudfjellet Window are composed of amphibolite with a thin and discontinuous meta­sedimentary cover. It thus differs from the other windows, which are composed of granite although with a similar metasedimentary cover.

Mineralogically, the granite occurring beneath the Gargatis Nappe is composed mainly of microcline, with a low con tent of plagioclase and quartz. Biotite and sphene occur as accessory minerals. This granite varies from strongly de­formed to almost undeformed.

The cover on the parautochthonous crystalline rocks comprises radioactive graphitic phyllite and, locally, quarzite. These metasediments are generally no more than a few metres thick and in the most deformed parts the sediments are absent and the contacts often mylonitic. In other parts, the sedimentary sequence is thickened through folding and thrusting, up to a few tens of metres. This leads to tectonic repetition of the different rock units.

Biotite schists occur locally in the thrust zones separating the Gargatis Nappe from the under­lying unit. These are formed through strain­induced retrogression of amphibolite during the overthrusting. Such retrogradation of amphibole assemblages has been described among other areas from the Lewisian Complex in Scotland (Beach 1980), and is a common feature in basement windows within the Scandes. Although the crystalline rocks are often micaceous, true phyllonites are rarely developed in the thrust zones in the Nasafjiill Window. An exception is

Kammaren in the north em part of the window. Hydrothermal quartz is found locally in the

thrust zone. In the Silvergruvan Window, the Nasafjiill silver mineralization occurs associated

with hydrothermal quartz (Zenzen 1927). This occurrence has been mined for silver rich galena. Lead isotope studies have suggested that the lead was deri ved from the granitic bedrock (Johansson 1980).

Structures The pervasive foliation and isoclinal folds with NW-SE trending axes were developed during the thrusting-related D1 phase of deformation. A further two phases of plastic deformation occurred giving open folds with east-west and north-south trending axes. These fold the thrust contacts of the Gargatis Nappe. The outcrop pat­tem of the parautochthonous basement is deter­mined by the D2D3 folds.

The Høgtuva Window Precambrian crystalline rocks in the Sjona Glomfjord area occur in a system of antiforms which are overtumed to the west (G. Qvale & H. Qvale in Ramberg & Stephens 1981). The core of the Høgtuva Window is composed of fine­to coarse-grained gneisses, probably of volcanic origin. The dominating minerals are quartz, pot­assium feldspar, plagioclase and biotite. The foli­ation in the gneisses is concordant with the con­tacts with the overlying cover. For the western windows in the Glomfjord area, geochemical similarities with the rapakivi granites have been pointed out by Cooper & Bradshaw (1980). Gor­batschev (1985) suggested a correlation with the Transscandinavian Granite Porphyry Belt. Fur­thermore, the uranium content of the gneisses in Høgtuva is very high. This, together with a similarity in uranium mineralizations, led Lindahl (1983) to propose that the Høgtuva Window was a westerly continuation of the Arjeplog-Arvidsjaur uranium province (Adamek & Wilson 1979) in Sweden. This province continues from the eastem foreland westwards beneath the Caledonian Allochthon. However, the uranium rich granites within the Caledonian windows differ from those east of the thrust front in their lower 87Sr/86Sr ratios (Wilson 1981), their peralkaline chemistry and unusually high contents of rare metals includ­ing Be, U, Sn, Th, Li and REE (Lindahl & Grauch 1986). This may strengthen a correlation with the rapakivi granites, which show similar geochernistry (Nurmi & Haapala 1986).

194 1.-E. Lindqvist

Within the basement gneisses there are sheets of gamet amphibolites and biotite schists. The contacts to these sheets are tectonic and con­cordant with the regional foliation. No shear zones have been found to subdivide the window in to several tectonic levels and no metasediments have been found within the gneisses.

The window is tectonically overlain by thrust sheets of psammitic gneisses (Figs. 6 and 7). These gneisses occur in the lower part of the Rodingfjiill Nappe and have been identified as a separate unit, the Mevik Group (Nicholson & Walton 1963; Rutland & Nicholson 1965). These authors

Fig. 6. Map covering the southeastern part of the Høgtuva Window showing the

metasedimentary cover sequence inferred to be equivalent to the Dividal

Group. The thickness of this unit is strongly overemphasized. The map also shows the extension of the psammitic thrust sheets in the lowermost part of the Rodingfjall Nappe. Location of the profiles in Fig. 3 is indicated by ME: Melfjellet and FE: Femfjellet. The pi'Ofile in Fig. 7 is also indic:ated.

N

o

Metasediments

Amphibolite

Basal gneiss

Sample point

NORSK GEOLOGISK TIDSSKRIFT 68 (1988)

argued that the gneisses were deposited onto the underlying gneisses, and compared them with the sparagmite nappes of the Middle Allochthon. The psammitic gneisses are mineralogically dominated by potassic feldspar, quartz, plagioclase and biotite. The gneisses also contain gamet, zoisite, calcite and muscovite. These gneisses differ from the basement in their higher feldspar content. Within the gneisses, amphibolitic sheets occur with thicknesses up to a few tenths of metres. The amphibolites are composed of amphibole, plagioclase, and quartz. Horizons of micaceous quartzite, sometimes graphite-bearing, with a

NORSK GEOLOGISK TIDSSKRIFT 68 (1988) Tectonic implications of U, Mo and V mineralization 195

m 500 -

400 -

300 -

Melfjellet

,,,,, Psammitic ,,,,, ,,,,, gneis ses

• Limestone

Bl Graphitic micaschist

D Quartzite

[ill -

D .

Flaggy,

Tj!lrn­rasta

quartz rich gneiss

Ouartz rich gneiss

� Basal gneisses '

• Amphibolite

Fig. 7. Protile over the basement cover contact on the eastern side of the Høgtuva Window. The profile shows the lithological variation in the lower part of the ROdingfjåll Nappe (Meløy Group of Nicholson & Walton 1963).

thickness of up to ten metres, occur within the gneisses. Horizons composed of quartzite, marble and graphitic phyllite with low radioactivity and uranium contents, can be found in between these thrusts sheets overlying the window. The thick­ness of these metasedimentary units reaches up to a few metres. All contacts within this unit are of tectonic nature and are concordant with the regional pervasive foliation. The repetition of the sedimentary horizons is inferred to refiect imbri­cation of this unit, possibly in a duplex structure. It rna y, however, be due to large-scale isoclinal folding.

These lithologies can be followed high up in the ROdingfjall Nappe. The present observations do not support the view that the gneisses above

the Høgtuva basement represent an original stra­tigraphical sequence that compares with the Vendian sparagmite nappes. Both the occurrence of amphibolites within the sequence and the occurrence of uranium-rich graphitic phyllite in the base of the sequence are foreign to the Vendian rocks of the Baltic Shield. No separate units comparable to the Lower Allochthon in the eastern front of the Caledonides can thus be identified above the Høgtuva Window. The psammitic gneisses are inferred to have a thrusting relation to the underlying gneisses of the Høgtuva Window. The tectonic setting of the se high er nappe units has been discussed more extensively by Gustavson & Gjelle (1981) and Stephens et al. (1985).

Table l. Chemical composition of the graphitic phyllites in the NasafjåU and Høgtuva Windows. The localities are shown in Figs. 4 and 5. The Mo, V and U contents of average alum shale Upper Member in Billingen are 270 ppm, 680 ppm, 206 ppm (Armands 1972) and average black shale Mo 10 ppm, 150 ppm, U 8 ppm (Vine & Tourtelot 1970, the U value is from Swanson 1961).

85001 85002 E0150 E0192 E0213 340 81281A 81281H 85005 85006 326 139

Si02 65.85 58.02 57.45 82.56 68.99 63.90 Al20, 14.34 16.11 16.53 7.60 13.60 17.22 TiOz 0.80 0.93 0.81 0.46 0.75 0.70 Fe20, 5.22 2.93 7.63 2.13 3.39 5.80 Mg O 1.73 2.20 3.79 0.56 1.87 2.66 Ca O 1.35 4.76 3.66 1.18 0.71 0.61 Na20 2.37 0.61 0.96 0.46 0.42 0.65 K20 3.03 4.23 3.94 2.11 4.89 4.82 Mn O 0.08 0.05 0.06 0.01 0.05 0.15 P20s 0.18 0.24 0.17 0.14 0.15 0.13 LO! 4.71 10.25 4.70 2.70 4.93 3.33 Tot 99.66 100.33 99.71 99.92 99.75 99.%

Mo 79 140 68 64 62 u 146 29 29 49 25 5.0 35 26 7 17 14 2.5 V 588 1728 384 242 27� Th <l <l

196 1.-E. Lindqvist

In the basement-cover contact, thin horizons of quartzite and graphitic schist occur, as shown on the preliminary map by Gustavson & Lunøe (1976). Quartzites are almost ubiquitous on Melfjellet. Graphitic phyllites are frequent although thin and mostly strongly weathered (Figs. 3 and 6). Phyllonitic rocks can be found locally along the contact. Marble, which is fre­quent in the contacts between the thrust sheets above the basement, is lacking in the basement­cover contact. In the southwestern part of the window, at Femfjellet, the metasedimentary cover sediments are well preserved. Here, an up to about 30 m thick sequence of radioactive graphite-rich schists interbanded with light yellow or white micaceous quartzite (Fig. 3) can be traced for more than l km along the basement cover contact. The graphite-rich schists contain quartz, white mica, plagioclase, biotite and sphene. These graphitic schists are distinctly more radioactive than those in the overlying nappes. They are metalliferous (Table l) and fall within the range of chemical variation displayed by the graphitic schist in the Nasafjåll Window. The units are tectonically repeated and the contacts are of tec­tonic nature. Rocks that might be thrust basement slices occur in the upper part of the section. Thin horizons of garnet-bearing schist and basic schist can also be found. The basement gneiss directly underlying the metasediments is mostly slightly deformed, but thin zones of intense deformation occur a few metres below the contact to the gra­phitic phyllite. This metasedimentary sequence and its contact relations compare well with those in the Nasafjåll Window and other eastern win­dows (Lindqvist 1984). This favours the structural interpretation of the Høgtuva Window in analogy with the eastern windows and the eastern thrust front.

The appearance of black schists with distinct differences in radioactivity in the Sjona­Glomfjord area has been pointed out previously. At Reindalsverket, in the central part of the area, a strongly uraniferous black schist occurs (Lindahl 1983). The uranium con tent of this schist reaches up to 180 ppm U, with an average of 45 ppm. V and Mo, are on average found at levels of 450 ppm and 86 ppm, respectively (Lindahl 1983). This black schist is found in association with quartzite on top of a granite that is exposed in the centre of an antiform (Skjeseth & Sørensen 1952). An interpretation in accordance with the black shale at Femfjellet in the Høgtuva Window seems

NORSK GEOLOGISK TIDSSKRIFT 68 (1988)

probable. For the uraniferous black schist at Rein­dalsverket, a Cambro-Ordovician age has been proposed (Skjeseth 1958).

Qvale, Riis & Ramberg (p. 82 in Ramberg & Stephens 1981) described quartzites which they interpreted as a cover sequence on the basement gneiss of the Sjona Window. They tentatively suggested a correlation with the Mierkenis Group for this quartzite.

Structures of the Høgtuva area D1 gave isoclinal syn schistosity folds, F1 (F2 of Graversen et al. 1981). The F2 folds gave the large-scale pattern with the basal gneiss antiforms that are overturned to the west (Qvale et al. in Ramberg & Stephens 1981). F1 and F2 are coaxial with east-west trending fold axes. The earlier structures are refolded by the open F 3 folds with N-S to NE-SW trending axes.

Metamorphism In the eastern thrust front illite crystallinity inves­tigations have indicated maximum temperatures of the anchizone or possibly slightly lower (Rick­ard et al. 1979; Lindqvist preliminary results). Further west, in the Jiickvik area, biotite occurs in the metasedimentary rocks of the the Lower Allochthon.

In the eastern part of the Nasafjåll Window, the mineral assemblages in the phyllitic rocks of the Mierkenis Group are indicative of the greenschist facies. In the western and south­western part of the Nasafjåll Window the meta­morphism reached amphibolite facies (Lindqvist 1987). Here almandine-rich garnet coexists with pargasitic amphibole in basic schists. Kyanite occurs in aluminous rocks in association with the thrust zones within the window (Gjelle & Gustavson 1978). The medium-grade assem­blages are formed syn- to post-kinematic in relation to D1 but pre-kinematic in relation to D2• White mica assemblages in the phyllitic rocks break down to plagioclase, potassium feldspar and almandine-bearing assemblages. In garnet­free phyllitic rocks, the anorthite content of the plagioclase increases from An 25 in the eastern part of the window, to An 50-75 in the western part. Phengite barometry applied on the assem­blage potassium feldspar, biotite, phengite and

NORSK GEOLOGISK TIDSSKRIFT 68 (1988) Tectonic implications of U, Mo and V mineralization 197

quartz formed after the last plastic deformation indicate that the retrograde path passed through the andalusite stability field. In the southwestern part of the window this metamorphism is associ­ated with intense penetrative Caledonian deformation. Post-thrust folding has overturned the basement-cover contact in the western part of the Nasafjiill Window (Fig. 5).

In the Høgtuva area almandine garnet and amphibole assemblages occur in the amphibolites within the window. The amphiboles are coexist­ing cummingtonite and ferroan pargasitic horn­blende. The garnets retain an early fabric as in­clusion trails and are inferred to have grown prior to the D2D3 phase of deformation. Plagio­clase in the rocks have an An content of 10-20%, while plagioclase of amphibolites in the Rodingfjiill Nappe (Meløy Group) have an An con tent of 30-40%. In the metasedimentary cover rocks and overlying gneisses the white mica assemblage breaks down to plagioclase, pot­assium feldspar, biotite and white mica assemblages. Some garnets in the gneisses above the window display a sigmoidal inclusion pattern. Both the basement and overlying gneisses meta­morphically recrystallized in a post-kinematic phase. The growth of amphiboles and the break­down of white micas reftect this late phase.

Applying garnet biotite thermometry (Ferry & Spear 1978) on pairs that have equilibrated at a late stage and kyanite-garnet-plagioclase bar­ometry (Newton & Haselston 1981) on assem­blages formed after the last plastic deformation give temperature and pressure of 515°C and 7 kb. Although this barometer is calibrated at higher temperatures, these results indicate upper greenschist to lower amphibolite facies of medium pressure after the last phase of plastic defor­mation.

These observations indicate that no significant difference in metamorphic grade exists between parautochthonous and Gargatis Nappe units in the westernmost part of the Nasafjiill Window and rocks in the Høgtuva Window. The westward increase of Caledonian deformation in the basement may partly be due to a slight increase in metamorphism, as proposed by Nicholson & Rutland (1969). It may also be due to the pro­longed low-grade metamorphism in the western windows compared with the eastern ones. The latter is indicated by reported lower biotite Rb­Sr ages of samples from the Glomfjord Window compared with the Nasafjiill Window; these are

350 Ma and 380 Ma, respectively (Wilson & Nicholson 1973).

Conclusions The cover sequences in the Høgtuva and Nasafjiill Windows show striking similarities. The occur­rence of analogous cover sequences has also been described from other western windows in the Glomfjord and Sjona areas (Skjeseth & Sørensen 1952, Qvale et al. in Ramberg & Stephens 1981). This uniformity in cover sequences strongly sug­gests that the western windows form a westerly continuation of the Baltic craton. This view is supported by the similarities in lithology and age of the basement rocks (Wilson & Nicholson 1973) and uranium mineralizations (Lindahl 1983).

Acknow/edgements. - I thank Per Gunnar Andreasson (Lund) and David Gee (Uppsala) for their critical reviews of the manu­script and Christine Andreasson for doing the dratting work. Dr. H. Sheppard (Lund) corrected the English text and this is kindly acknowledged.

Manuscript received September 1986

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