1916) 1960, P. 188

A GEOLOGICAL TRAVERSE ACROSS THE PYROXENE-GRANULITES OF JOTUNHEIMEN IN THE NORWEGIAN CALEDONIDES M. HUGH BATTEY & WILLIAM D. McRITCHIE Battey, M. H. & McRitchie, W. D.: A geological traverse across the pyroxene granulites of Jotunheimen in the Norwegian Caledonides. Norsk Geologisk Tidsskrift, Vol. 53, pp. 237-265. Oslo 1973. Rocks of the Jotun stem between Tyin and Boverdalen fall into two groups: (a) peripheral gabbros, and granitic rocks of amphibolite facies metamor phism, sometimes retaining igneous textures and (b) a core of ultramafic rocks and pyroxene-gneisses of pyroxene-granulite metamorphism. The core is coextensive with the deep, dense root of the Caledonides on the geophysical model, the peripheral group with a widely-extending shallower sheet. The structure of the two groups is described. In the light of recent work on the surrounding Eocambrian, it is proposed that the peripheral group was in part emplaced magmatically at high level, eroded and unconformably covered by Eocambrian sparagmite, whilst the core rocks were metamorphosed at pressures > 4kb and tectonically elevated from a root in the Faltungsgraben during the Caledonian orogeny. M. H. Battey, Geology Department, University of Newcastle upon Tyne, Newcastle upon Tyne, NEJ 7RU, England. W. D. McRitchie, Department of Mines & Natura/ Resources, Mines Branch, Geological Division, 901 Norquay Building, 401 York Avenue, Winnipeg l, Manitoba, Canada. The principal outcrop of rocks of the Jotun stem of Goldschmidt (1916) forms a north-easterly extending belt 170 km lang, and between 50 and 80 km wide, near the south-eastem border of the Norwegian Caledonides (Fig. 1). The north-western margin of the Jotun tract is fairly straight, but the south-eastern border is sinuous, with extensive lobes and outliers. The rocks included in the Jotun stem are ultrabasic to acid gneisses of the pyroxene-granulite facies of metamorphism, anorthosites, and various gab broic and granitic bodies of more obviously intrusive aspect. The Jotun stem rocks, as a whole, show overthrust relationships to the surrounding metasediments, with extensive mylonitic border zones. They have lang been regarded as a far-travelled sheet, overthrust south-eastwards from an unexposed root zone to the north-west (Holtedahl 1960, p. 188). Gravity surveys (Smithson 1964, Smithson & Ramberg 1970) have more recently shown, however, that the dense Jotun rocks probably extend to a depth of 6 to 13 km below the axial region of the main outcrop. Though there is a considerable amount of data available on the geological structure around the borders of the area of Jotun rocks, little geological work has been done in the interior of this tract. The present contribution presents information on a transect across the main outcrop, from the lake Tyin in the south to upper Boverdalen and Leirdalen in the north (Figs. l 238 M. H. BA TIEY & W. D. McRITCHIE Fig. l. Southern Norway, showing the main outcrop of Jotun stem rocks (diagonal lines) and the area mapped (black). � ,.. � u � ....... " / o\; - - u, ... F "' E � � /\ Q t (� � t ./o ;l ..... 1<:> "' t " (., /'<- t l"' � � ,....(_,,.. t / � �f ..... « o � and 2). It is intended as a preliminary to an account of the petrology of the pyroxene-granulite facies rocks. Structural elements For descriptive purposes the area may be divided into two parts, the region north of the Tyin-Gjende Fault (Fig. 3), largely made of pyroxene-granulite facies rocks, and that to the south, where the rocks belong to the epidote amphibolite facies. The structural elements will be described in the follow ing order: Faults of the northern (pyroxene-gneiss) area. Thrust sheets south of the Tyin-Gjende Fault. Layered structures north of the Tyin-Gjende Fault. FAULTS OF THE NORTHERN (PYROXENE-GNEISS) AREA The faults of the northern area are shown in Fig. 3. The principal faults and some of the subsidiary faults have been located, in part at least, on the GEOLOGICAL TRA VERSE A CROSS JOTUNHEIMEN 239 2. 45 30 W.Oslo 15 61.40'N Fannoråken . 61.30 N sl Skag!'stolst� 5 o 5 10km. Fig. 2. General topography of the area studied. ground. Many of the subsidiary faults, however, have been plotted from features visible on air photos. Tyin-Gjende Fault This fault extends from Breikvamsnosi, N. of the lake Tyin, east-north eastwards to the western end of the lake Gjende (Battey 1965). (Fig. 2 240 M. H. BA TIEY & W. D. McRITCHIE 2" 45 2 30' W.Oslo 2" 15 FAULT PATTERN 32" Principol faults, with dip � Subsidiary faults -·-· Shear zanes N.E. of Tyin � 61"40'N 1"30'N 61"20'N 5 o 5 lO km. Fig. 3. The fault pattern. shows most of the place-names used in the text.) Its line is clearly marked by a topographic feature along most of its length, and the trend of the out crop across the valley Mjolkedola (Fig. 3), as well as featuring on the east face of Breikvamsnosi (Fig. 4), suggest a dip of about 45° to 50° to the NNW. Where the fault crosses the Mjolkedola, its plane dips NNW at 40°, and mylonite layers here, and to the south-west, dip N or NW at angles of GEOLOGICAL TRAVERSE ACROSS JOTUNHEIMEN 241 .. --- __, ---. - --- Fig. 4. Breikvamsnosi, looking eastwards, showing the featuring produced by the northward dip of the Tyin-Gjende Fault and associated shear-planes. The lower ground to the left is underlain by arkose (sparagmite). 20° to 50°. North of Geitho, halfway between Mjolkedola and Gjende, mylonites dip 30° to 40° to NW. On the air photos this fault appears to continue west-south-westwards from Breikvamsnosi for a further 8 km; but this extension has not been followed on the ground. Over the length from Mjolkedola to Gjende, the Tyin-Gjende Fault sepa rates gabbro with relict igneous texture to the south, from a belt, up to l Yz km wide, of variable rocks, collectively called feldspathic crush-rock, to the north (Fig. 5). These crush-rocks are pink-coloured, quartzo-feldspathic types, with epidote and chlorite as dark minerals, and they enclose and form veinlets in fragments of pyroxene-gneiss and gabbro. They have crystallized under epidote-amphibolite facies conditions, and represent an episode of retrograde metamorphism, and possibly of acid intrusion, later than the rocks on either side of them. The feldspathic crush-rock is followed north ward by a large area of pyroxene-granulite facies rocks. It is noteworthy that the line of the Tyin-Gjende Fault is closely followed by the -50 mgal contour on the Bouguer anomaly map (Smithson 1964) and lies at the lip where the lower surface of the Jotun rocks plunges down towards its deep root on the geophysical model. This suggests that the Tyin Gjende Fault may be a feature of major importance in defining the form of the hidden part of the pyroxene-gneiss massif. Utladalen Fault This fault extends from the western edge of the glacier Smorstabbreen, east of the upper part of the river Bovra, on a bearing of about 200° along the 242 M. H. BA TIEY & W. D. McRITCHIE 2 45' 2• 30' W. Oslo LITHOLOGICAL FACIES Arkose (Sporagmite) CJ Marginal gabbro with [Y;?J relict igneoustex ture UL:.!.J 61.40' Sheared gabbro k- :-d , -:·... , - . 61.30'N 61.20'N Feldspothic cr ush-rock and E=:=J Retrograded pyroxene gneisses � U\tramafic bodies (black) in r7:'] 5 o 5 lO km. pyroxene gneisses (blank) � Fig. 5. Distribution of lithological facies. eastem side of vetle Utladalen, and then along Utladalen at least as far as Hjelledalen. In the north it separates an area of gabbroic rocks in the upper reaches of the Bovra, that in part retain their original igneous texture, from pyroxene-gneisses that build Loftet and Smorstabbtindane (Fig. 5). Along its course is a zone of intense disturbance, accompanied by the emplacement of pink, feldspathic fault-rock, with feldspar porphyroblasts sometimes pro- GEOLOGICAL TRA VERSE ACROSS JOTUNHEIMEN 243 ducing augen-structure. It thus shows marked similarities, overall, to the Tyin-Gjende Fault in the south. The gabbros with relict igneous texture east of the Bovra, between the river and the Utladalen Fault, pass, west of the Bovra, into a tract of highly-sheared, aften amphibolised gabbro-type rocks on Rundhaugen. These sheared rocks show analogies with the highly-sheared epidote-amphibolite facies rocks south of the Tyin-Gjende Fault. This em phasises the symmetrical arrangement of zones either side of the pyroxene gneiss core. Followed south into vetle Utladalen, the disturbance caused by the fault continues, with augen-gneiss on Fugleberg and conspicuous masses of feld spathic rock along the Utla. From here to the junction of vetle Utladalen and store Utladalen the fault has not been followed on the ground, but the air photos show it clearly marked by an erosional trench across the south-west spur of Hillerho, just north of store Utladalen. From this point it has been traced on the ground, as a zone of intensely-sheared amphibolic rocks with slickensides and great contortion, through the western tip of Skogadalsnosi to Uradalen. Thence its trace on the air photos continues on a bearing of 200°, to cross the Utla 1 km above the Midtmaradalen confluence. It crosses back again 1 km above Vetti. The dip, calculated from the swing of its outcrop between these two crossings of the Utla, is 32° to ESE. Along the section from store Utladalen to Uradalen, the pink feldspathic rock seen in the upper reaches of the Bovra is not prominent. Little is known of the rocks west of the Utla, but examination of Goldschmidt's thin sections of rocks from Hurrungane sug gests that, in the middle reaches of the Utla, the pyroxene-gneisses on either side of the fault are essentially similar.

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