LEIDSE GEOLOGISCHE MEDEDELINGEN, Deel 49, Aflevering 2, pp. 259-276, 1-9-1973

A preliminary account on the geology of the - area, the region,

Central Norwegian Caledonides

BY

N.Ø. Olesen E. S. Hansen L.H. Kristensen AND T. Thyrsted

Abstract

Sediments and volcanic rocks (ophiolites) all of early Palaeozoic age were metamorphosed, multiply deformed, and intruded by igneous

rocks during the Caledonian orogeny. At least six deformation phases including late faults are recognized. There is no simple correlation between deformation phases and tectonic style. The second deformation phase (D2) is accompanied by Barrovian type metamorphism,

ranging from biotite to sillimanite grade, and transposes earlier surfaces into a new foliation, which is itself folded on a regional scale. The

transposition foliation varies from crenulation cleavage to schistosity. Basic intrusives are rimmed by contact metamorphic aureoles also of

Acid intrusives of Structural and correlations with syn-D2 age. are syn- to post-D2 age. stratigraphic nearby areas are attempted. An Ordovician/Silurianage is suggested for the Gula Schist Group.

INTRODUCTION metamorphosed sedimentary and volcanic rocks, which

divided are into four groups: the Gula Schist Group, the of The Selbu-Tydal area is situated about 75 km ESE Storen Group, the Fundsjö Group, and the Sulamo Trondheim underlain rocks (index map, Fig. 1). It is by Group (Wolff, 1967), all of which were intruded by

of Palaeozoic to the Trondheim acid and early age, belonging syn-orogenic, basic plutonic rocks. 1967, Nappe (Wolff, p. 129).

The purpose of this paper is to present some results of The Gula Schist Group

which initiated 1968 and The Gula Schist the of a mapping programme, was in Group occupies core a major is continuing under the direction of Professor Dr.

H. J. Zwart, Leiden, the Netherlands.

The Trondheim has been of interest Fig. 1. of the region geological Simplified geological map Selbu-Tydal area. Lithological boundaries both certain and uncertain. Strike of S2 for many years. Since the descriptions of Keilhau (1850) foliation indicated of described rocks and by elongation lithological symbols. numerous geologists have structures Section-lines refer to Fig. 2. of the region, resulting in many different interpretations Geology by: BR: Børge Rasmussen; ESH: Erik S. Hansen; LHK: of the structure. The literature has particularly regional Leif H. Kristensen; NØO: Niels Ø. Olesen; TT: Tage Thyrsted; T: been extensively reviewed by Wolff (1967) and Roberts traversed. Names of localities: B: (1967). A useful review of the geology of the northern Bringen Mt.; Bl: Blomlia; Bö: Börsjöen; F: Fongen Mt.; Fi: Finnkoihögda; G: Gjeståen; Ga: Gammel- has recently been published by vollsjöen; Gu: Gudbrandegga; L: Lödölja; M: Melshogna Mt.; R: Robertsetal. (1970). Ristjerna; Ru: Ruten Mt.; S: Storskarven Mt.; Se: Selbusjöen; V: Recent mapping programmes in the Meraker area VålåkleppenMt.; Ö: Öielva. (Chaloupsky & Fediuk, 1967; Roberts, 1967, 1968a,

1971; 1967; Siedlecka & Siedlecki, Siedlecka, 1967; Fig. 2. Sections through the Selbu-Tydal area. Section-lines in-

Wolff, 1967) and in the Röros district (Nilsen, 1971; dicated in Fig. 1. For identification of lithology, see Fig. 1. Arrow direction of younging where indicated Birkeland & Nilsen, 1972; Nilsen & Mukherjee, 1972; in circle shows by primary structures. Further explanationin the text. Rui, 1972) have dealt with rocks similar to those des-

cribed here. Kisch (1962) and Torske (1965) described

areas immediately adjoining the Selbu-Tydal area.

* LITHOLOGY Authors' addresses. - N. 0. O., E. S. H., L. H. K., and

T. T.: Geologisch en Minderalogisch Instituut, Garenmarkt 1 b,

Leiden, the Netherlands. Fig. 1 is a simplified geological of the area studied. map L. H. K. (present address): Geologisk Instituí, Ole Worms Allé, deformed and Aarhus It comprises a sequence of multiply 8000 C, Denmark. 260 261 262

antiformal structure. The deepest exposed rocks outcrop Several discontinuous layers of fine-grained, thinly

around Bringen mountain (all names in this paper are layered amphibolite were observed in the eastern Gula

indicated on Fig. 1). They are calcareous hornblende- Schist Group. They may be equivalent to the Gula green-

biotite schists and garnet-quartz micaschists, interbedded stone of Nilsen & Mukherjee (1972, p. 160). and broad with graphite-quartz schists, they occur in a Along the entire eastern boundary of the Gula Schist which towards the north. zone, tapers Group there is a zone of conglomerate-bearing To the northwest and other east pelitic metasediments, micaschist, which can be traced into the Usmadam-

in of struc- of Kisch varying grades metamorphic transformation, Bukhammerfjell metaconglomerate (1962, p. turally overlie the schists described above (Fig. 2). 52). These conglomerates should probably be correlated Around Börsjöen and Selbusjöen the common rock with the Guda conglomerate to the north (Wolff, 1964). and types are grey green phyllites interlayered with In the area mapped the conglomerate is generally an and metagreywackes metasandstones, which are often oligomict quartzite conglomerate with an amphibolitic

calcareous. In one locality (Blomlia) the phyllite is inter- matrix. However, around Gjestaen it is a polymict con-

bedded with a polymict metaconglomerate, in which glomerate. Quartzite and metabasite pebbles pre-

quartzite and metabasite pebbles predominate. To the dominate, and pebbles of blastoporphyritic metadolerite

east micaschist biotite and marble also garnet-quartz grades into schist, (Fig. 3) occur (Kristensen, 1972, p. 29).

which commonly contains numerous porphyroblasts of Calcite marble, sometimes micaceous, occurs locally in the staurolite, kyanite, garnet (e.g. the Kvernfjell-horizon, same zone (cf. the Vollfjell limestone, Vogt, 1940, p.

see e.g. Carstens, 1928), and hornblende. The mica- 180;Kisch, 1962, p. 18).

schists are commonly interlayered with calc-silicate The Stören rocks and quartzitic sandstones, apparently equivalents Group An of metabasites of the metasandstones and metagreywackes of the ophiolitic*) sequence (locally with relict mafic western phyllites. pillow structures), metatuffs, meta-agglo-

merates, and some layers of phyllite is separated from At two localities good graded bedding was observed.

the rocks of the Gula Schist Group by a distinct One occurrence is in metagreywacke at the outlet of

Garbergselva, and the other is in micaschist at Rotla river *) The term 'ophiolite' is used here in the sense of geosynclinal, one km west of the Fundsjö Group. The younging direc- mafic, volcanic mainly rocks, not implying that they represent tions at these localities are indicated on 2. Fig. ancient oceanfloor.

3. Fig. Polymict metaconglomerateat Gjeståen. Large pebbles in upper left part are deformed blastoporphyritic metadolerite. 263

sequence of quartz phyllite. The latter is equivalent to rounded by tuffs and pelitic sediments, which com-

the quartz schist of Torske (1965), and typically dis- monly grade laterally into one another. This pattern is

plays an interlayering on the cm scale of light quartzite believed to be of primary origin.

and dark phyllite. The nature of the Fundsjö Group changes drastically

The outcrop pattern is dominated by N-S trending towards the east, pelitic and semi-pelitic metasediments mafic synformal structures, the cores of which are occupied by with intercalations of tuffs being the main rock

the ophiolites. types.

The Fundsjö Group The Sulåmo Group

Metabasites, locally with relict pillow structures (Fig. 4), East of the Fundsjö Group, around Finnkoihögda and and metatuffs, mafic as well as felsic, meta-agglomerates, Gammelvollsjöen, grey and green phyllites, locally the domi- some layers of locally graphitic metapelites are graphitic or calcareous, are intercalated with metagrey- The difference from the metasandstones and of nant rock types. only major wackes and some layers con-

rocks of the Storen Group is a higher grade of meta- glomeratic greenschist. The sequence structurally under- morphism. Intrusive blastoporphyritic metadolerites and lies the rocks of the Fundsjö Group.

metaquartz-keratophyres are locally prominent. By In one locality south of Finnkoihögda distinct graded

virtue of their association with the pillow lavas they are beds were observed in metagreywacke, indicating an up-

believed to be syn-depositional, high-level intrusives, side-down disposition of the rocks (Fig. 2). of metadolerites with the exception some occurring in The conglomerates are polymict, containing pebbles of

the vicinity of the Fongen-Melshogna igneous complex metabasite, quartz-keratophyre and quartzite, and

(see p. 264). should probably be correlated with the Lille Fundsjö

These rocks form a unit, the eastern major dominating conglomerate (Chaloupsky & Fediuk, 1967, p. 12). of the also within part investigated area. They outcrop Towards the south the phyllites are porphyroblastic the Gula Schist in isolated which Group occurrences, (biotite, garnet, and occasionally Ca-amphibole), resem- the result of scale of probably are large folding early age. bling the Stuedalsschists of Reusch (1890, p. 31; 1896, The lavas metamorphosed pillow occur as 'wedges' sur- p. 24).

Fig. 4. Weakly deformed metabasite with relict pillow structures. Locality from the river Rotla. Looking NW. 264

Intrusive Rocks occupies the core of an asymmetrical synform. The

addition to the mafic and felsic In syn-depositional in- rocks of the complex vary in composition from perido-

trusive rocks of the ophiolites described above, syn- tites, through pyroxene-gabbros, partly olivine-bearing,

igneous is evidenced the of to diorites, orogenic activity by presence all of which are in places cut by numerous acid, intermediate, and basic intrusive rocks. Most of the biotite or hornblende pegmatites. A major part of the rocks shows well acid, plutonic are trondhjemitic types (Gold- complex a developed primary, rhythmic

schmidt, 1916, p. 77). They occur partly as large bodies layering, which is commonly graded. Occasionally at and and (e.g. Börsjöen, Öielva, Ristjerna), partly as primary structures such as slumping are developed. In

or sills, which in occur as swarms. In the one on the dykes places locality eastern slope of Fongen mountain an the latter case trondhjemites may even be the dominant erosional disconformity in the rhythmic layering (Fig. 5) rock at show indicates that the type (e.g. Tydal). They some petrographic body in general lies in a right-side-up variation well (see e.g. Kisch, 1962), as as varying struc- disposition (Fig. 2). However, it is inverted locally since

tural relationships. Some trondhjemites cut across the it is folded into an overturned 1 s y n c i n e . Fig. 1 and have the regional schistosity original igneous fabric shows that the intrusive body is discordant to the litho- These boundaries throughout. obviously postdate any penetrative logical and it is also discordant to an early deformations associated with the of the development schistosity (see Timing of metamorphism). The igneous schistosity. Most regional trondhjemites, however, rocks are to a varying extent transformed to hornblende- cut the them- and though they across regional schistosity, are metagabbros, locally have attained a secondary folia-

foliated to selves parallel this surface. Intrusion of these tion concordant with the regional schistosity. An exten- rocks therefore was contemporaneous with the develop- sive contact metamorphic aureole is developed around

ment of the the regional schistosity. This establishes the body (see p. 270, and Olesen, 1972). intrusive trondhjemitic activity as syn- to post-meta- A large number of sills and dykes of blastoporphyritic

morphic (see Timing of metamorphism). metadolerite, showing structural relationships similar to

Basic rocks occur in the those of the igneous principally Fongen- Fongen-Melshogna complex, occur parti- in the rocks of Melshogna igneous complex, which is a layered, sheet- cularly the contact aureole. They are like body, attaining a maximum thickness of at least probably contemporaneous with the main body. 5 km. It thickest A small sheet-like of is apparently towards the west, and body pyroxene-gabbro, largely thins towards the east It is folded and transformed to rapidly (Fig. 2). hornblende-metagabbro, outcrops on

Fig. 5. Erosional disconformity in rhythmic layering of the Fongen-Melshogna igneous complex. Eastern slope of Fongen mountain. 265

which surface a foliation, is recognized Vllakleppen mountain (Kristensen, 1972, p. 45). It is prominent

the The foliation can be also associated with dioritic rock types, is discordant throughout Selbu-Tydal area. observed in and be traced from out- with the country rocks, and has a contact aureole. most outcrops can

out- to outcrop in morphology, by orienta- Finally a group of hornblende-albitemetagabbros crop by similarity and the main of tion, by a relationship to crop east of Tydal (the Kistafoss metagabbros Kisch, contemporaneous sills regional metamorphism (see Timing of metamorphism). 1962, p. 56). They occur as major in the phyllites. short Local cross-cutting relationships with the schistosity Although similarity exists over distances, a regional

and Kisch variation in and orientation is (see were observed in the area mapped, (op. cit., p. morphology apparent

discordant the 266 and This foliation will be referred to as 57) also reports apophyses. However, p. p. 268).

foliated to the S Folds and other structures or S are bodies are also locally parallel regional 2 . pre- syndating 2 schistosity. collectively referred to as Group I, while those post- Thus II All the basic and associated intermediate rocks, dating S-> are referred to as Group II. Group I The described here, thus show identical structural relation- structures consistently overprint Group structures. do constitute fold ships, suggesting contemporaneous intrusion. groups not, however, generations, as each superimposed fold patterns occur within group.

STRUCTURAL SEQUENCE Thus there are folds with S as their axial plane surface 2 folding earlier folds; both by definitionare Group I folds Introduction and D of Table I). Within Group II kink-bands, (Dj 2 Most rocks of the Selbu-Tydal area are metamorphic occurring partly as conjugate sets, form a very distinct

with faults and retro- tectonites. Prominent exceptions are the majority of style group, which is associated

syn-orogenic intrusive rocks, which, as described above, grade metamorphism. These structures consistently over- Also of of the from these are only locally deformed penetratively. parts print other structures group. Apart

of and late kink-bands and faults, a number of II the ophiolitic sequence the Storen Fundsjö large Group folds and have been into three Groups, particularly the metabasites, were competent remain, they separated and enough to avoid penetrative deformation. sets on the basis of orientation of axial plane sense

The following terminology is used: S denotes an s-sur- of asymmetry. Due to overprinting relationships (of

face, the surface defined dimensional the sets of folds are to e.g. by preferred varying confidence) assigned sepa- orientation of minerals, or by a crenulation cleavage. rate deformation phases. and the The minerals defining schistosity may concomitantly Table I shows the structural age sequence cor-

define linear fabric. Such lineation denoted L. It is evident that with the a a is responding style groups. ex-

Quartz-rods and elongation direction of deformed ception of kinks any individual style group is not restric- deforma- pebbles are also denoted by this symbol. Fold axes are ted to one deformationphase. Nor is a specific

denoted by F. Subscript numerals are added to denote tion phase always characterized by a specific style. relations between and relative S overprints and a Similar discordant tectonic style age relationships, e.g. 2 S], described Means and of symbols with a common subscript (e.g. S L fold generations are by (1966) group 2 , 2 , and F structural elements of a defor- Williams (1970). 2 ) comprises single mation phase (D2 ).

Structural age sequence Style groups Basis for proposed structural sequence

The rocks indicate of deformations. The a long sequence Kink-bands & faults (3)+ 4 observed the be subdivided many folds, in area, may on ? ? Selbu-Tydal phase 3 the basis of tectonic style (Turner & Weiss, 1963, p. Group II Knnkoihogda phase 2+3 78—79) into four style groups: isoclinal folds Gudbrandegga phase (1)+ 2 + (3) style group 1 comprises tight to

with axial 1 + 2 an plane schistosity, D 2 Group I 2 to isoclinal folds style group comprises tight Dl 1

with an axial plane crenulationcleavage,

3 folds with style group comprises open to tight Table I. () refer to minor occurrences.

no axial plane foliation, which style group 4 comprises kink-bands, I partly occur as conjugate sets. Group

Numerous superimposed foldpatterns demonstrate that In the lower grade parts of the Selbu-Tydal area a few

the folds belong to a number of fold generations. It has mesoscopic, isoclinal folds with axial plane schistosity have been observed. been possible, however, to demonstrate that there is no (Sj) Most mesoscopic folds, how-

to isoclinal with axial straightforward correlation between tectonic style and ever, are tight plane crenulation

the fold The latter folds the generations. cleavage (S 2 ). consistently overprint

of tectonic the folds and other and S is of the Independently style former, 2 developed by transposition

be of older Thus there at least two structures may subdivided with a high degree con- S] schistosity. are genera-

fidence into two reference tions of I and D major age groups, using as Group folds, Dj 2 respectively. 266

Fig. 6. Tight Group I folds with S2 axial plane schistosity, overprinted by folds and crenulation cleavage (parallel to pencil) of Gudbrandegga phase (lithology: biotite schist). Locality from eastern part of Gula Schist Group at river Rotla. Looking NW.

In the of the isoclinal higher grade part area tight to It should be noted here, that the transposition of Sj mesoscopic folds with axial plane schistosity (S ) into S is in in the of 2 2 observed, general, only hinge-zones abound, particularly in the metasedimentsand metatuffs D whereas in the limbs of these folds one 2 folds, only relics of folded earlier (Fig. 6). Microscopic a schistosity s-surface can be recognized. It is believed to be parallel

(Sj) and a few occurrences of superimposed fold to the older Sj, but is progressively rotated towards the patterns, e.g. at Gjestaen (Fig. 7), reveal that two fold S orientation, and has been 2 generally considerably generations are again present and D ), and that the modified further and (Dj 2 by flattening recrystallization.

S is formed by transposition of the older schistosity. In the higher grade of the area the S schistosity 2 Sj part 2 If no superimposed fold pattern is observed it is not is lineated. The lineation (L ) is defined by preferred 2

to ascribe a fold to either D dimensional orientations of always possible given Dj or 2 mica, hornblende, kyanite in the because both have the and and field, same style orien- etc. generally accompanied by a prominent quartz- tation, since the Dj axial planes (Sj) have been trans- rodding.

into the S surface. relics of folded In addition to I posed 2 Microscopic a common mesoscopic examples, Group

Sj in many of the folds, however, demonstrate that the folds also occur as macroscopic structures. Thus the out-

majority of Group I folds in the higher grade part of the crop pattern of the Storen Group is dominatedby N—S

area are of D in the of which a 2 age. trending synforms (Fig. 2), hinge-zones Because of the S schistosity the higher grade part of well developed S crenulation cleavage is observed, in- 2 2

the area can be traced into the S crenula- D On the northern of continuously 2 dicating a 2 age. slope Bringen tion of the lower that cleavage grade parts, it is seen mountain a major recumbent fold (Fig. 2) is also inter- these two S surfaces are equivalent (this is the S used as of D because meso- 2 2 preted being 2 age, recognizable, reference as surface as mentioned above), and that Dj D folds on the limbs are congruous with the scopic 2 and D in the the Thus the structure. The isolated of the 2 respective areas are same. major occurrences Fundsjö

morphology of the S changes from a crenula- the Gula Schist indicate the existence of 2 gradually Group in Group tion in the lower of the cleavage grade parts area to a major isoclinal folds (Fig. 2), which have axial planes

medium- to in the folds are 'con- coarse-grained schistosity higher grade parallel to S . D 2 Many mesoscopic 2 similar transition is described White with the folds How- part (a by e.g. gruous' major as regards asymmetry.

(1949)). ever, the majority of these mesoscopic folds plunge 267

variation in orientation of S which will be regional 2 , described in the following section.

Group II

The structural elements of Group I are commonly ob-

served to be deformed by later folds (Fig. 6), which vary

in size from the mesoscopic to the macroscopic scale. of Apart from the late kink-bands and faults, three sets

folds are recognized. One deformation phase produces

folds of regional scale, which have contributed signi- of ficantly to the present configuration lithology, struc-

tures, and metamorphic zones. This phase is termed the of folds refer- Selbu-Tydal phase. The other two sets are

red to as the Gudbrandegga and Finnkoihögda phases. and show distinct They are of less regional importance folds of the non-congruous relationships to the major

of these two Selbu-Tydal phase. The age relationships

phases to the Selbu-Tydal phase are not fully established

evidence. How- due to a lack of mesoscopic overprinting

of axial of the two ever, variation in orientation planes

the Selbu- phases suggests that they are postdated by

Tydal phase (see below). It is because of this uncertainty of that these have been age relationships phases given

names of localities in which the folds are most inten-

sively developed rather than labelling them as chrono-

logical deformationphases.

Gudbrandegga phase. - This phase is mostly represented

by (sub-)angular folds of mesoscopic scale, mostly

associated with a crenulation cleavage (Fig. 6), which in

few localities be traced into a can a schistosity (post-S 2

schistosity). In the eastern part of the Gula Schist Group

the axial directions he around N-S, the axial planes are

and the folds are Z-folds 1964, Fig. 7. D2 folds overprinting D1 folds at Gjeståen (lithology: mostly steep, (Fleuty, p.

layered psammitic and pelitic schist). The latter folds are excep- 476) as viewed towards the north. The folds are most the D tionally well preserved, as folds are unusually open. 2 intensely developed at and around Gudbrandegga, where Looking SW. they occur as macroscopic structures, which are clearly

outlined by the curving boundary between the Gula

towards while the Schist and the moderately to steep SSW (see below), Group Fundsjö Group. They correspond of the folds the Hilmostöten-Tveraa anticline of Kisch fold axes major are sub-horizontal, as to (1962, p.

deduced from the geological map. Due to this discre- 118) to the south.

the of these I folds is still Some folds with the but dif- pancy age major Group a mesoscopic same style ferent matter of dispute. orientation occur in grey and green phyllites in the north-western of the Gula The geometrical pattern of the fold axes, the schisto- part Schist Group. They

sity/cleavages, and the lineations is altered by later are isolated from the Gudbrandegga phase folds sensu

The direction of F axes varies stricto. The axial directions are also the axial folding (see Group II). 2 N—S, the and the folds like the L lineation, the latter being N-S with planes dip gently to east, are also grossly 2 Z-folds viewed towards the north. These folds low plunge in the west, gradually swinging through SW as are

to the to W and NW with increasing plunge in the east. It tentatively interpreted as belonging Gudbrandegga the difference of axial should be noted, however, that this similar geometry phase, in orientation planes being well due later the between F and L is statistical, as as as to folding by Selbu-Tydal phase. 2 2 only large

small between F and L are commonly observed, angles 2 2

addition is also - This is also when the rocks are studied closely. In it Finnkoihögda phase. phase represented by

within to observe D folds of folds of scale, associa- possible a single outcrop 2 (sub-)angular mesoscopic mostly with with identical shape and orientation of axial plane, but ted a crenulation cleavage. The axial directions lie

90°. A ex- the NW—SE the axial axes differing in orientation up to possible in quadrants, planes are sub-

for this is that the D folds in fact constitute horizontal or towards NE or planation 2 dipping moderately SW, and the folds S-folds viewed towards NW. more than one generation. are as They observed the half of the The variation of L is by a are only in eastern regional 2 accompanied investigated 268

Fig. 8. Interpreted three-dimensional shape of major folds of Selbu-Tydal phase. The folded surface is the S2 foliation. Major folds of

also in Gudbrandegga phase are shown south-central part.

and fold indicate that where the sheet area, superimposed patterns they gabbro apparently occupies only the

postdate the Gudbrandegga phase. Variation in orienta- western, overturned limb of the synform.

tion of the axial planes is interpreted as being due to Mesoscopic folds ofthis phase seem to be scarce. North

later the of few folds folding by Selbu-Tydal phase. and north-east Bringen mountain a open with direction N-S axial and steep axial planes are inter- As mentioned above this of this North of Selbu-Tydal phase. set preted as belonging to phase. Fongen folds of E-W sections the is major importance. through mountain some mesoscopic open folds are also inter- show the variation in attitude investigated area (Fig. 2) preted as belonging to this phase. of boundaries and the S foliation. Con- flexural fold mechanism lithological 2 Assuming a slip an unfolding

S two structures are apparent, an anti- of the folds reveals more sidering 2 , major Selbu-Tydal phase a simple the form to the west (Selbu antiform) and a synform to geometrical pattern of the Group I structural elements.

east (Tydal synform). The folds trend roughly N-S, and L remains N—S to the west and to E.g. 2 gradually swings show considerable variation 8 shows they in profile. Fig. NW SE in the east. At least a part of this variation

the interpreted shape of the folds. could be primary. Furthermore this unfolding also

To the south the Selbu antiform is a broad, flat-topped rotates the axial planes of the folds of the Gudbrandegga and structure with a sub-vertical western flank an over- phase into a higher degree of parallelism. This also turned undula- eastern flank. The flat 'top' has minor applies for the folds of the Finnkoihögda phase, sup-

but towards the west. tions is generally dipping gently porting the age relationships given above. Towards the north the 'top' consists of several smaller

- macroscopic folds with a sub-horizontal enveloping sur- Kink-bands and faults. Kink-bands, partly in conjugate

face. In the extreme north the 'top' dips towards the sets, are extensively developed particularly in the

east, and both flanks are overturned. vicinity oflate faults.

The Tydal synform is overturned towards the east. The A number of faults with sub-vertical fault planes of the is influenced the wrench faults shape structure strongly by striking NNW-SSE, are interpreted as

presence of the Fongen-Melshogna igneous complex. because of the geometry of the associated conjugate Close to the intrusion the fold remains kinks. The of dextral horizontal 'open', apparent- apparent magnitude a

due to ly the presence of the competent igneous body, shift is known in some cases, the maximum determined

and it is believed to become tighter towards the north, being 0.5 km. where the no gabbro sheet is present, and to south, A fault can be followed from north and east of Fongen 269

Fig. 9. Map of metamorphic zones of the Selbu-Tydalarea.

1: of Regional metamorphism: biotite zone (g: isolated occurrence of garnet); 2: garnet-hornblendezone (k: isolated occurrence kyanite);

3: staurolite-kyanite zone; 4: sillimanite zone.

Contact metamorphism: 5: andalusite zone; 6: sillimanite zone; 7: cordierite zone.

Unbroken lines are isograds determined with reasonable precision, while broken lines refer to uncertainty due to transitional nature or

unconvenient lithology.

text. Major syn-orogenic intrusives are indicated (see Fig. 1). Further explanation in the

Mt. where the Barrovian 9 shows the distribution of to Tydal, it joins up with the Heina zone of type. Fig.

Kisch (1962, p. 44) and the 'western thrust fault' of Rui metamorphic zones. The isograds have been drawn to delineate where index-minerals (1972, p. 8). The faultplane is dipping towards the west. areas, given are of in Isolated It is interpreted as a reverse fault, also because of the common occurrence appropriate lithologies. mineral outside geometry of the associated conjugate kinks. It is occurrences corresponding zones are in- of minor dicated by letter symbols. probably displacement, as it does not, ap- parently, offset the previously established metamorphic The pattern. 1) biotite zone. Garnet occurs locally.

2) The garnet-hornblende zone. In Brecciation and retrograde metamorphism accompany the and the north-western Gula Schist the lower limit of faults, in the case of the reverse fault pseudo- Group

the zone is delineated on the basis of the tachylites cut across the schistosity of the country rocks. mainly occur-

rence of hornblende.

METAMORPHISM 3) The staurolite-kyanite zone. In

addition to staurolite and kyanite, garnet and horn-

Areal distribution blende of are widespread occurrence. Diopside and gros- The of the sular-rich in calc-silicate regional metamorphism investigated area is of garnet occur some rocks. 270

It is noted that the regional metamorphism is asymmetri-

related to cally the regional, large-scale structures, e.g. the eastern flank of the Selbu antiform is ofhigher grade

than the western flank. This is due to the fact that the

folds of the Selbu-Tydal phase deform a pile of rocks, in

which a metamorphic zonation with discordantrelations

to lithological boundaries and regional schistosity was already established(see Timing of metamorphism).

The regional metamorphic zonation pattern here des-

cribed does not differ essentially from that established Fig. 10. Schematic illustration of some microstructural changes byGoldschmidt(1915). involved in the transition from lower to higher grade rocks. The eastern transition from higher- to lower-grade

with crenulations of the A: S1 D2 are typical biotite zones. metamorphism is substantially influenced by contact the crenulations. Biotite porphyroblasts overgrow D2 metamorphism around the Fongen-Melshogna igneous B: Gradual transitions from rocks containing S1 to rocks con- complex. tainingan S2 schistosity are common particularly in or close to The contact zones are the garnet-hornblende zones. Hand-specimens may contain both following metamorphic present:

schistosities:

a: an schistosity develops preferentially in layers relatively S2 5) The andalusite zone. Garnet is of wide- micas. poor in spread occurrence. Staurolite occurs locally. The rocks b: in layers rich in white mica and/or graphite an S1 schistosity are mostly well foliated (schists, amphibolites). The with or without D2 crenulations may persist into the higher grade zones. andalusite and garnet isograds coincide in the north-

C: In the higher grade zones the schistosity is mostly ofD2 of age. eastern part the area. To the south-east the zone is Relics of S1 may be preserved in e.g. biotite porphyroblasts. defined the of by occurrence garnet, as andalusite is lacking.

4) The sillimanite zone. Scattered forma- 6) The sillimanite zone. Garnet is of

tion of white mica + sillimanite. Kyanite, staurolite, widespread occurrence. Staurolite occurs locally. and hornblende of Unstable relics of andalusite garnet, are widespread occurrence. are locally present. Ortho- The in association with of clase the where also sillimanite zone occurs spatial a swarm occurs innermost in zone,

also trondhjemitic dykes. occurs as prismatic crystals up to 5 cm long (see

Fig. 11. A white mica with biotite nicol. From in fine-grained schistosity (S1) D2 crenulations, overgrown by porphyroblast. One locality western of in the part Gula Schist Group river Garbergselva (spec. LHK 69—358). The western biotite zone. 271

Kisch, 1962, p. 53, and Nilsen, 1971, p. 339). In the relative age of the porphyroblasts is not well established.

outer parts of the zone sillimaniteoccurs as fibrous pods Comparisons with neighbouring areas suggest that the

those described Roberts included is an or an S The resembling by (1968a, p. 172). schistosity Sj early 2 . por-

The rocks are mostly foliated (schists and amphibolites) phyroblasts are spatially related to the hornblende-albite that of and commonly show migmatitic structures. metagabbros, suggesting they are a contact meta-

7) The cordierite zone. Garnet is of wide- morphic origin. into the schists also spread occurrence. The rocks are commonly rich in Moving higher grade zones many contain biotite include enstatite/hypersthene, anthophyllite/gedrite, or cum- porphyroblasts. They commonly mingtonite/grunerite. Sillimanite, orthoclase, and stauro- relics of Sj, occasionally with helicitic crenulations (as defined and the S deflects lite are also The rocks of the zone are by e.g. Spry, 1969, p. 257), present. mostly 2 massive hornfelses and commonly show migmatitic around the porphyroblasts (Fig. IOC, and Fig. 13). The Hornfelsic of and hornblende the structures. rocks similar mineralogy occur same applies to garnet in garnet-

as xenoliths within the complex. hornblende zone. In the highest grade zones garnet,

hornblende, staurolite, and kyanite have also been

A possible outermost biotite zone is not traceable, partly observed with relics of Sj with or without helicitic

because the contact metamorphism overprinted rocks crenulations. Garnet, hornblende, staurolite, and kyanite

already regionally metamorphosed, and partly because

the regional metamorphism continued after the intrusive

event (see p. 274).

Around the intrusive complex of Valakleppen cor- dierite and sillimanite zones are developed, but the andalusite The zone is missing. cordierite zone is only a

few meters wide, and is not shown in Fig. 9.

Some of the bigger trondhjemite intrusions show con-

tact metamorphic effects, such as a change of fabric to

more isotropic and coarse-grained rocks, as well as a

change of mineralogy. Hornfelses containing grossular-

garnet, tremolite/actinolite, clinozoisite, and zoisite are

developed around the Börsjöen trondhjemite, and

diopside in the vicinity of the Öielva trondhjemite is also

believed to be ofcontact metamorphic origin.

Timing of metamorphism

The growth of metamorphic minerals can be dated

relative to the schistosities and cleavages of the rocks

(see e.g. Zwart, 1963). As described above two schistosi-

ties (of and D are in the Dj 2 age respectively) present

Selbu-Tydal area. It is possible with the aid of micro-

structural investigations to follow a gradual transition

from a of a crenulation S predominance cleavage type 2 the in peripheral and lower grade areas to the west and

east (Fig. 10A, and Fig. 11), through transitional stages

and to the (Fig. 10B, Fig. 12), central higher grade areas,

where the regional well developed schistosity is of D 2

age and where relics of S are (Fig. IOC), t only preserved

as inclusions in porphyroblasts (Fig. IOC, and Fig. 13),

and in graphite-rich schists, in which the microstructures

did not readjust so much during the following progres- sive metamorphism.

The phyllites of the western biotite zone characte-

Fig. 12. A fine-grained white mica schistosity with ristically contain many biotite porphyroblasts, which (S1) D2 crenulations. New micas smaller both (larger grains: biotite, grains: overgrow a fine-grained schistosity, defined by white mica) grow parallel to the planes of crenulation cleavage white mica and minor biotite (S¡ ), and the crenulations (S2). Transitional stage between crenulation cleavage type S2 of D and 2 age (Fig. 10A, Fig. 11). and schistosity type S2. The figure illustrates that two mecha-

The eastern biotite almost confined to the nisms to be active in the of (a) zone is seem process transposition: a bodily rotation of S1 grains into the planes, and (b) nuclea- Sulamo Group. The phyllites are biotite-porphyroblastic S2 tion and growth of new grains of mica oriented parallel to S2. towards the south. The porphyroblasts exhibit planar One nicol. From locality in western part of Gula Schist Group inclusion trails, and the schistosity of the rocks deflects ca. 5 km ESE of Selbu (spec. BR 68—23). The western garnet- to around the porphyroblasts. Due a lack of D folds the hornblende zone. 2 272

to Fig. 13. A medium-grainedwhite mica schistosity of D2 age (section perpendicular to L2; S2 parallel long dimension offigure). Biotite porphyroblast is partly recrystallized (during D2), and includes relics ofan S1 with D2 crenulations (indicatedby graphite trails). One nicol.

From locality in eastern part of Gula Schist Group west of Gudbrandegga(spec. NØO 69—446). The staurolite-kyanite zone.

inclusion trails Fig. 14. Staurolite porphyroblast with S-shaped inclusion trails, indicative of syn-kinematic growth. Continuity between of rotation One nicol. From and the external medium-grained biotite schistosity of D2 age (section parallel to L2). Angle exceeds 130°. The locality in eastern part of Gula Schist Group at river Rotla (spec. NØO 68—286). staurolite-kyanite zone. 273

From locality in eastern of Fig. 15. Garnet porphyroblast postdating a medium-grainedbiotite schistosity of D2 age. One nicol. part Gula

The Schist Group ca. 2 km N of river Nea (spec. NØO 68—156). staurolite-kyanite zone.

also S white mica and chlorite porphyroblasts are overgrow 2 during flattening, commonly involving Retrograde rotation of the growing porphyroblasts (Fig. 14). Finally locally plentiful in the higher grade zones, where these minerals form the of staurolite it is commonly observed that rims or occasionally whole commonly at expense

porphyroblasts of these minerals post-kinematically and kyanite.

S It is here that the indicate overgrow 2 (Fig. 15). emphasized investigations The formation of regional metamorphic sillimanite is that the higher grade rocks are the result of a temporal of relatively late age. Aggregates of coarse-grained white progression of metamorphism, which is directly compar-

mica containing sheaves of fibrolitic sillimanite and able with the present zonal progression, e.g. the silli-

occasionally grains of partly resorbed kyanite lie as manite postdates staurolite and kyanite, which in turn

lenses in the schistosity. Though generally unaffected, include garnet and replace biotite.

staurolite observed in thin section be south-eastern of the was one to In the part area most porphyro- replaced by white mica + sillimanite. The rims of the blasts are believed to be of contact metamorphic origin.

and S while a weak The relative of intrusion of the aggregates overgrow postdate 2 , age Fongen-Melshogna deflection may be observed around the central part of igneous complex can thus be established, using identical

the deflection andalusites and of outer aggregates. This may reflect flattening techniques. E.g. garnets the around preexisting porphyroblasts. aureole are observed in their cores to include an Sj

In the higher grade zones the peak of regional meta- partly with incipient crenulations of D2 age, so that the thus coincides with and the morphism move- intrusion must an of D However, postdates postdate early stage 2 .

ments, responsible for the formation of S The higher the intrusion was followed a continued 2 . by development

grade minerals, staurolite, kyanite, and white mica + of S still under contact metamorphic conditions, and 2 , deformed the II the hornfelses transformed sillimanite, are by Group structures, the igneous body and were in thin although in one section kyanite is observed to over- part to amphibolites and schists, respectively. crenulationof grow a the Gudbrandegga phase.*) Thus there seems to be partial contemporaneity between the kyanite-bearing regional metamorphism and *) Recent microstructural observations indicate, that there is an the andalusite-bearing contact metamorphism. However, at least partial contemporaneitybetween the formation of white the of the time mica + sillimanite and an early stage of the Gudbran- peak regional metamorphism postdates degga phase folding. of intrusion of the Fongen-Melshogna complex (Fig. 16). 274

SSW, where it apparently joins up with the axial plane

trace of the Selbu antiform. As described above, how-

ever, the Selbu antiform is a late structure deforming the

and the axial planes of and D regional schistosity Dj 2 of the folds, suggesting a misinterpretation Stjördalen anticline.

Apart from these discrepancies in the correlation of

major structures, problems in the correlation of meso-

scopic structures abound. Dealing with.the higher grade

zone of the Stjördalen profile a comparison of tectonic style suggests that the mesoscopic Fj structures of Roberts the (1967, p. 96) correlate with Group I struc-

tures of the Selbu-Tydal area (despite the above men- Fig. 16. Schematic correlation of structural, metamorphic, and tioned correlation of macroscopic structures), and parti- in syn-orogenic igneous events the Selbu-Tydal area. cularly it is believed that his F¡ structural elements (schistosity, mineral-lineation) correspond to S and L 2 2

In the eastern part of the aureole the contact meta- of the higher grade parts of the Selbu-Tydal area. This is of morphism was throughout the cooling history higher supported by a microstructural study of a few thin

grade than the regional metamorphism, which explains sections from the Gula Schist Group of the Stjördalen

the absence of overprinting of contact metamorphic profile (Zwart, pers. comm.), showing the schistosity of

mineral assemblages. In contrast, these rocks also to be an S assemblages by regional 2 .

in the western part of the aureole, conditions of higher In the lower grade zones the correlation may be the

contact outlasted the = and F = D grade regional metamorphism straightforward Fj Dj, 2 2 (Roberts, 1967, such metamorphism, resulting in replacement textures, p. 71; Robertsetal., 1970, p. 138).

as andalusite -*■ kyanite, and andalusite -*■ white mica Fig. 2 demonstrates that the rocks of the Fundsjö

+ staurolite. Birkeland & Nilsen (1972, 19) noted p. Group occupy the core of the Tydal synform. A similar

similar textures at the Hyllingen gabbro (the southern structure is not described to the north, but to the south

continuation of the com- his indicates Fongen-Melshogna igneous Rui (1972, Fig. 11) in profile a comparable

plex). structure at Holtsjöen (note, however, that his strati-

The contact metamorphism around the Valakleppen graphy is not comparable with that of the Selbu-Tydal

metagabbro is likewise overprinted by the regional meta- area. Rui (op. cit.) and Nilsen (1971) interpret the schists around the the morphism (Kristensen, 1972, p. 53). Hyllingen gabbro as belonging to

Thus in the north-eastern part of the Selbu-Tydal area Gula Schist while this these rocks Group, in paper are of sillimanite have been two generations recognized, (a) considered as part of the Fundsjö Group). Fig. 2 'older' and an contact metamorphic, (b) a 'younger' obviously points to a correlation of the Gula Schist which in regional metamorphic generation, are separated Group (or parts of it) with the Sulamo Group, and time of by a period kyanite-staurolite grade regional indeed the lithology of the structurally higher parts of

metamorphism. the Gula Schist Group is not essentially different from 16 is to show the struc- Fig. an attempt schematically the lithology of the Sulamo Group as described in this tural, metamorphic, and igneous relationships described paper, and by Chaloupsky & Fediuk (1967), and Sied- above. 17 shows P-T trends of Fig. tentative regional lecka & Siedlecki (1967), when the diffe- and contact rocks the main meta- metamorphic during rent grades of metamorphism are

to morphism of age. Such correlation to be syn- post-D2 neglected. a appears a

paradox, as the Gula Schist Group is considered to be of DISCUSSION Cambrian age, and the Sulamo Group of Middle Ordo-

vician age (see e.g. Wolff, 1967). The results described above are in some respects in con- Considering, however, the available evidences within flict with results from the northern previously published the Selbu-Tydal area of the tectono-stratigraphic posi- Trondheim region. A few points of major interest are tion of the Gula Schist Group, namely younging direc- discussed below. briefly tions as indicated in Fig. 2 (note, however, that the Recent structural of the interpretations regional struc- locality in the eastern Gula Schist Group is of no

ture of the Trondheim were region (Roberts et al., 1970) significance, as it lies in the core of a major Group I based of the particularly on a study Stjördalen profile fold), and the occurrences of metabasite pebbles in

(Roberts, 1967, 1968b). According to Roberts a central conglomerates, structurally underlying the ophiolitic anticline of (Stjördalen anticline), Fj age containing sequences, a major stratigraphic inversion might be rocks of the Gula Schist Group, is flanked by partly This suggested. is supported by Morton (1972, p. 314), overturned of F The axial synclines/synforms age. who on the basis of the at 2 sulphide ore sequence plane trace of the Stjördalen anticline is indicated by Rödhammeren Mine (west of Hyllingen gabbro, the

Roberts et al. (1970, Fig. 2) to continue towards the southern continuation of the Fongen-Melshogna igneous 275

Fig. 17. Tentative P—T trends of regional and contact metamorphic rocks of central higher grade part of Selbu-Tydal area. Dotted arrows show possible post-intrusive trends of contact metamorphic rocks, due to regional metamorphic overprinting. Experimental data from: (1)

Althaus (1969); (2) Luth et al. (1964); (3) Hirschberg & Winkler (1968). (Note that the observed contact metamorphic zonation does not

is correlate fully with that inferred from the experimental data, as a garnet-freecordierite zone not observed).

complex) considers it most probable that these rocks are sequence west of these localities - including the Gula

towards the Schist — of younging west. As seen from Nilsen (1971, Group is actually Ordovician/Silurian age. By

Fig. 2), the rocks at Rödhammeren Mine appear to this interpretation the above mentioned discrepancy in

correlate stratigraphically with the Dictyonema locality age between the Gula Schist Group and the Sulamo (Vogt, 1889). It is therefore suggested that the rock Group would be eliminated. 276

ACKNOWLEDGEMENTS We are grateful for assistance from Norges Geologiske

Undersökelse, Trondheim, particularly from stategeolo-

We are indebted to Professor Dr. H. J. Zwart for con- gists F. Chr. Wolffand Dr. D. Roberts.

stant stimulation and help during the course of this N. 0. O., E. S. H., and L. H. K. acknowledge financial

study. We have benefited from discussions with Profes- support from Carlsbergfondet, Copenhagen; E. S. H., and Dr. L. H. and T. T. from Under- sor Dr. H. J. Kisch, Dr. C. J. L. Wilson, K., Norges Geologiske

P. F. Williafns. The latter also kindly corrected the sökelse, Trondheim; N. 0. O. from Statens Naturviden- and and english text. We acknowledge the permission from skabelige Forskningsrad, Copenhagen; N. 0. 0. E. S. H. from Delft. B. Rasmussen to use his unpublished field results. Stichting Molengraaff-Fonds,

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