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K Yanite-Grade Metamorphism in the Evenes and Bogen Groups, Ofoten

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K Yanite-Grade Metamorphism in the Evenes and Bogen Groups, Ofoten Kyanite-grade metamorphism in the Evenes and Bogen Groups, Ofoten, North Norway MARK G. STELTENPOHL & JOHN M. BARTLEY Steltenpohl, M. G. & Bartley, J. M. : Kyanite-grade metamorphism in the Evenes and Bogen Groups, Ofoten, North Norway. Norsk Geologisk Tidsskrift, Vol. 64, pp. 21-26. Oslo 1984. ISSN 0029-196X. Directly north of Ofotfjorden in northern Norway, pelitic schists within the Evenes and Bogen Groups contain the mineral assemblage garnet + biotite ± kyanite ± staurolite + white mica + quartz ± plagio­ clase. This assemblage implies metamorphic P-T minima of- 540"C and - 4. 8 kb. The rocks are thus at a higher grade than suggested by previous reports, which placed them in the greenschist facies. This indicates that several metamorphic allochthons in Ofoten, including rocks of the Narvik, Evenes, Bogen, and Niingen Groups, are all at kyanite grade, supporting recent interpretations which on structural grounds concluded that the metamorphic peak outlasted stacking of these allochthons. A proposed correlation of the Evenes Group with the Middle Ordovician-Lower Silurian Balsfjord Supergroup implies that this stacking and associated kyanite-grade metamorphism are post-early Silurian and are related to the Scandian phase of the Caledonian orogeny. M. G. Steltenpohl & J. M. Bartley, Department of Geology, University of North Carolina, Chapel Hill, North Carolina 27514, USA. Our mapping, structural analysis, and petro­ ultramafic rocks of the Narvik Group, which are graphic studies in Ofoten have concentrated on presently at kyanite grade (Foslie 1941, 1949, the structural and metamorphic development of Gustavson 1966, 1972, Hodges 1982a, Tull et al. the Caledonian nappe stack. Figure l shows a in press). The thrust beneath the Narvik Group preliminary interpretation of the tectonostrati­ (Gustavson 1966, 1974, Hodges 1982a) cuts graphic sequence in the Skånland area. Details structurally upward passing northward so that of the lithologic sequence and petrography will the entire Narvik group and the Elvenes con­ be presented in a later paper (Steltenpohl in glomerate are missing north of Ofotfjord. In this prep.). In this note we present field and petro­ northern area, the thrust places Evenes Group graphic observations regarding metamorphism of marbles upon 100-200 m of mica schist and roar­ the Evenes and Bogen Groups, which may have ble, which we will call the 'basal' allochthon. The major implications for the regional geology. 'basal' allochthon directly overlies Precambrian granitic basement. Gustavson (1974) correlated the 'basal' allochthon with either the Narvik or Rombak Groups; we doubt that these rocks cor­ Tectonostratigraphic context relate with the Narvik Group, but are uncertain The Evenes Group and overlying Bogen Group of their affinities pending more detailed study. contain a series of marbles and mica schists, with The contact between the Evenes and Bogen subordinate quartzites and metaconglomerates Groups is characterized by: l) a lithologic break, (Strand 1960). Gustavson (1966) included both from the marble-dominated Evenes Group which the Evenes and Bogen Groups in his Salangen contains rare and_minor mafic intrusions, to the Group. However, two distinct lithologic assem­ calc-schist dominated Bogen Group which was blages, which correspond well to the Evenes and intruded by numerous concordant granite sheets Bogen Groups of Strand (1960), are clearly pre­ prior to most of the penetrative deformation; sent within the Salangen Group in the Skånland and 2) a zone of concentrated strain in which area. We thus use the earlier nomenclature here. units in the underlying Evenes Group are trans­ South of Ofotfjorden (Fig. 1), the base of the posed and locally truncated. Evenes Group is marked by the Elvenes Con­ The contact could be either a thrust or a modi­ glomerate, which was deposited unconformably fied unconformity. A conglomerate present at or upon the schists, calc-schists, amphibolites, and near the structural top of the Evenes Group 22 M. G. Steltenpohl & J. M. Bartley NORSK GE OLOGISK TIDSSKRIFT l (1984) L..of­ lslands • kyanlfe locality O staurollte loctJiity locallty ,. ky8staur lll:�:::sc:���� � LONER ALLOCHTHON(dotted pottem) NARVIK GROUP(Iined pattom) BASEMENT o-.._....;5:..-....-'o;;..._,.:15kmo Figure l. Tectonic map and simplified tectonostratigraphic column of the Skånland area, showing kyanite and staurolite localities. Tectonic units modified from Gustavson (1974). suggests that the contact may be an inverted view that the Evenes and Bogen Groups are unconformity. The absence of granitic intrusions separate nappes which were thrust together. in the structurally lower Evenes Group could be The structurally highest nappe preserved in interpreted to indicate that it was deposited upon Ofoten comprises rocks of the Niingen Group, a the Bogen Group after intrusion of the granite scarcely studied lithologic assemblage broadly sheets, and then the sequence was tectonically similar to the Narvik Group (Gustavson 1966). inverted. This would imply that the conglomer­ The contact between the Bogen Group and over­ ate at the top of the Evenes Group is equivalent lying Niingen Group is a low-dipping zone of to the Elvenes conglomerate at its base, and that intense foliation which separates contrasting a major structural repetition occurs within the suites of metasedimentary rocks. lntrusions in Evenes Group. The lithologies of these two con­ the Niingen Group are generally pegmatitic, glomerates are dissimilar, and we see no evi­ while those in the Bogen Group are fine grained dence for such a repetition. Further, interpreta­ (Gustavson 1966, Steltenpohl in prep.). We rec­ tion of the contact as an unconformity implies ognize no retrograde metamorphism along this that the Bogen Group is structurally equivalent contact; mineral assemblages in the highly to the Narvik Group, which it does not lithologi­ strained rocks at the contact are the same as cally resemble. Combined with the presence of those away from the thrust. No evidence of trun­ concentrated strain and local truncations of rock cation or transposition of the principal metamor­ units at the contact, this leads us to favour the phic fabrics across this or the other probable NORSK GEOLOGISK TIDSSKRIFT l (1984) Kyanite-grade metamorphism in North Norway 23 nappe boundaries has been found in the Skån­ sion is strongest in rocks affected by late fold­ land area. If this contact reflects major tectonic phases which post-date the metamorphic peak juxtaposition, it would appear to predate the (F2, F3 of Gustavson 1972, F3, F4 of Bartley metamorphic peak. 1981, in press, and Steltenpohl 1983, in prep.). Retrograde metamorphism does not appear to be restricted to or concentrated at any particular Prograde metamorphism structural leve!. In some retrograded samples, greenschist-facies mineral assemblages replace Nine occurrences of kyanite and/or staurolite amphibolite-facies minerals along late-stage have been located in the Evenes and Bogen spaced cleavage planes (Fig. 2b ). The typical Groups (see Fig. l for geographic and tectono­ retrograde mineral assemblage in pelites is stratigraphic locations). The mineral assem­ blages are (Fig. 2a): chlorite + biotite ± white mica ± epidote ± quartz. l) Kyanite+ garnet+ biotite 2) Staurolite+ garnet+ biotite Other samples contain these retrograde phases 3) Kyanite+ staurolite+ garnet+ biotite without development of a late-stage cleavage. It is unclear whether garnet was stable during The rocks also contain quartz, white mica, and the retrograde metamorphism. In many samples, plagioclase. In the idealized pelitic system garnet is partly replaced by chlorite; however, (Thompson 1957), assemblages l and 2 are divar­ this may only reflect a shift in the stable garnet iant, while assemblage 3 is univariant. It is likely composition during cooling. Some garnets show that the actual variances are higher due to signifi­ inclusion-free overgrowths over 'snowball' cores cant amounts of Ca O and MnO, although assem­ (Gustavson 1966, Juve 1967, Steltenpohl 1983). blage 3 alternatively may indicate internal buffer­ In similar garnets from the Narvik Group, elec­ ing of fH o (Rumble 1978, Hodges & Spear 2 tron microprobe analyses suggest that rims grew 1982). The absence of staurolite in assemblage l during cooling and may relate to retrogression implies metamorphic P and T above the intersec­ (Hodges 1982b). tion of the quasi-univariant reactions: staurolite+ muscovite+ quartz = kyanite+ garnet+biotite + H20 (l) kyanite = sillimanite (2) Discussion This intersection was placed by Carmichael The metamorphic map of southert;� Troms and (1978) at 540°C and 4.8 kb, based on experimen­ Ofoten by Gustavson (1966) shows the b),Ilk of tal calibrations and field observations. If we as­ the Salangen Group (i.e., the Evenes and Bogen surne that additional components have stabilized Groups) to be within the garnet zone of the staurolite in assemblages 2 and 3, then assem­ greenschist facies. Part of the Salangen Group on blage l is probably the most reliable indicator of Rolla (see Fig. l for location) is shown to be grade because it more closely approaches the within the staurolite and kyanite zones (amphi­ simplified systems used in experimental calibra­ bolite facies), as is the overlying Niingen Group. tion. In this case, Carmichael's (1978) P-T values Gustavson (1966) shows a 'thrust plane (?)' for the intersection of the univariant equilibria which places kyanite-grade Niingen Group rocks are minima for the metamorphic conditions
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