Early Caledonian high-grade metamorphism within exotic terranes of the Troms Caledonides?

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1 Early Caledonian high-grade metamorphism within exotic terranes of the Troms Caledonides? MONA LINDSTRØM & ARILD ANDRESEN Lindstrøm, M. & Andresen, A.: Early Caledonian high-grade metamorphism within exotic terranes of the Troms Caledonides? Norsk Geologisk Tidsskrift, Vol. 72, pp Oslo ISSN X. A tectonic megalens, part of the Nordmannvik Nappe of the Upper Allochthon, is located at Takvatnet in Central Troms. The nappe unit is dominated by upper amphibolite- to granulite-facies gneisses. A Rb-Sr whole-rock dating of a metadiorite within this unit gives an isochron age of 492 ± 5 Ma with an initial Sr-isotopic ratio of O ± 0.()()()()8. The isochron age is interpreted to represent the high-grade metamorphic event associated with a complete resetting of the isotope system. The obtained age thus represents evidence of a pre-scandian metamorphic event for at!east one of the high-grade units within the Upper Allochthon in the Troms Caledonides. Mona Lindstrøm, Department of Geology, University of Trondheim-NTH, N-7034 Trondheim, Norway; A. Andresen, Department of Geo/ogy, University of Oslo, P. O. Box 1947 Blindern, 0316 Oslo 3, Norway. The Scandinavian Caledonides are characterized by a series of ftat- lying nappes emplaced onto the Baltoscandian Craton with its platform cover sediments during Early to Mid-Paleozoic closure of the Iapetus Ocean. Within the northem part of the Caledonides, two distinct tectonothermal events are generally recognized. An early Caledonian event ('Finnmarkian') of Late Cambrian to Early Ordovician age was originally believed to be responsib le for the deformation and translation of the structurally lowest nappes, the Gaissa, Laksefjord and Kalak Nappes in Finnmark {Sturt et al. 1978; Roberts & Gee 1985; Ramsay et al. 1985). This interpretation was partly based on isotopic ages ranging from 540 Ma to 490 Ma on inferred synorogenic intrusives of the Seiland Igneous Province (Sturt et al. 1978, and references therein) situated in the upper parts of the Kalak Nappe Complex. The igneous activity has recently been reinterpreted as being largely rift-related and of Precambrian to Early Cambrian age (Aitcheson et al. 1989; Daly et al. 1990; Krogh & Elvevold 1990), although some isotopic datings within the Kalak Nappe Complex still provide evidence of an Early Ordovician tectonothermal event in this area {Dallmeyer 1988; Mørk & Stabel 1990; Roberts & Sundvoll 1990). The nappes structurally above the Kalak Nappe Complex were emplaced during a late Caledonian event (' Scandian') of Late Silurian to Early Devonian age (Sturt et al. 1978; Roberts & Gee 1985), this phase being recognized throughout the Scandinavian Caledonides as the main event of metamorphism, deformation and nappe transport (Gee 1975; Roberts & Gee 1985; Ramsay et al. 1985). The nature and extent of these two orogenic events within the Caledonian nappe pile is, however, highly uncertain for large portions of the orogen, including the Troms Caledonides. Within the Troms Caledonides, composed of the following nappes in ascending order; the Kalak Nappe Complex/Må lselv Nappe, Vaddas Nappe, Kå fjord Nappe, Nordmannvik Na ppe, Lyngen Nappe and Tromsø Nappe Complex, fossils have been found in only the Vaddas and Lyngen Nappes (Olaussen 1976; Binns & Gayer 1980; Binns & Matthews 1981; Bjørlykke & Olaussen 1981 ). Published isotope ages are limited to a few errorchrons from the Kå fjord Nappe (D angla et al. 1978). As a consequence, little is known about the absolute timing of orogenic evolution in this key area, representing the transitional zone between the type area for the Finnmarkian and Scandian orogenic zones {Roberts & Gee 1985). Based on the findings of Late Ordovician to Early Silurian fossils within the Lyngen Nappe (Bjørlykke & Olaussen 1981) and the Vaddas Nappe (Binns & Gayer 1980) the boundary between the two beits has been placed at the base of the Vaddas Nappe. Andresen et al. {1985), following Binns ( 1978), advocated the view that the high-grade metamorphic Nordmannvik Nappe, sandwiched between the fossil-bearing Vaddas and Lyngen Nappes, acquired its high-grade mineral assemblages during the Finnmarkian event and possibly represented a depositional basement to the fossil- bearing units. Barker ( 1989), on the other hand, argued that there was no evidence of an Early Caledonian orogenic event within the metamorphic allochthons in Troms, excluding the Tromsø Nappe Complex. To test these opposing views, a meta-igneous rock within the Nordmannvik Nappe has been dated by the Rb-Sr whole rock method. Geological setting The nappe stratigraphy in central Troms, based on Andresen et al. { 1985) and Bergh & Andresen ( 1987), is summarized in Fig. l. Greenschist-facies pelites and psammites, most probably representing a slice of the

2 376 M. Lindstrøm & A. Andresen NORSK GEOLOGISK TIDSSKRIFf 72 (1992) TROMSØ NAPPE COMPLEX LYNGEN NAPPE (lncludtng the Lyngen Gabbro) NOAOMANNVIK NAPPE - - c::3 KAFJORO ANO VAOOAS NAPPES KALAK NAPPE COMPLEX/ MALSELV NAPPE AUTOCHTHONOUS / PARAUTOCHTHONOUS COVER PRECAMBRIAN BASEMENT,tS Htgh angle A /J normal lault Fig. l. Geological map of western Troms showing the distribution of the main Caledonian allochthons in the region. The inserted box shows the location of the sampled area, enlarged in Fig. 2. Baltoscandian margin, make up the Må lselv Nappe. Andresen ( 1988) included the overlying Vaddas, Kå fjord, Nordmannvik and Lyngen Nappes in the Upper Allochthon and considered them to be exotic terranes relative to Baltica. The Vaddas Nappe is an upper greenschist-/lower amphibolite- facies volcanosedimentary sequence of Late Ordovician to Silurian age ( Binns & Gayer 1980), whereas the overlying Kå fjord and Nordmannvik Nappes represent intensely deformed medium- to highgrade ortho- and para-gneisses of uncertain age and origin. A large ophiolite fragment, the Lyngen Gabbro, unconformably overlain by a thick Late Ordovician- Silurian metasedimentary seq uence (the Balsfjord Group) makes up the Lyngen Nappe. Middle to upper greenschist-facies metamorphism characterizes the Lyngen Nappe, with an increase in metamorphic grade toward the contact with the overlying Tromsø Nappe Complex, where lower amphibolite-facies is locally reached (Andresen et al. 1985; Bergh & Andresen 1985). The latter nappe, representing the Uppermost Allochthon in the area, is composed of three high-grade metamorphic units, of which the upper third includes a dolerite- intruded carbonate sequence now transformed into eclogite lenses in marble (Krogh et al. 1990). The age and origin of the various high-grade units within the Tromsø Nappe Complex is still unclear, even though a pre-scandian age is indicated (Griffin & Brueckner 1985; Krogh et al. 1990). Petrography and field relationships of rocks surrounding the metadiorite The dated metadiorite occurs as a small body in a megalens of the Nordmannvik Nappe at Takvatn (Fig. 2). The area was first mapped by Landmark (1973), who named the high-grade gneisses sandwiched between rocks of clearly lower grade (the Kå fjord/vaddas and Lyngen Nappes) the Heia Gneiss. Elvevold ( 1988), in a study of the P-T history of the high-grade rock, used the Heia Nappe as an informal name for this megalens of the Nordmannvik Nappe. The Heia Nappe is dominated by mylonitic kyanite- bearing gamet-mica gneiss. It locally contains lenses of pyroxene-bearing amphibolite. Thin layers of calc-silicate gneiss, amphibolite, marble and metadolerites also occur (Fig. 2). Small bodies of sagvandite (including the type locality) occur less than l km from the metadiorite. The gamet-mica gneiss dominating the Heia Gneiss displays a complex structural and metamorphic evolution. An early deformational event (pre-dl) is characterized by prograde metamorphism. The main Dl event is characterized by the development of isoclinal folds and a mylonitic fabric, Sl. There is no evidence of granulite facies metamorphic condition during development of the mylonites, indicating that peak metamorphic conditions were reached befare the anset of mylonitization (Dl)

3 NORSK GEOLOGISK TIDSSKRIFT 72 (I992) Early Caledonian, Troms 377 N t LEGE NO lyngen Nappe Nordmonnvik l Heia Hoppe [I] Sagvandite Amphibotite ØJ Amphibolih gneiss 'tllit h lenses of metodiorite E=3=::f= (ale- silicote gneiss [ZJ Harble D Garnet.-mico gnein O A 6 Vaddas Nappe / lit holagical boundories 11Y Orientotion of moin foliotion Sample locality Fig. 2. Geological map of the Nordmannvik (Heia) Nappe in Central Troms (Elvevold 1988). (Elvevold 1988). Bergh & Andresen (1985) on the other hand recorded peak (granulite) metamorphism at a late stage of the Dl event in a high-grade unit correlative to the Nordmannvik Nappe northwest of Takvatnet. The peak metamorphism in the Takvatn area is recorded in the porphyroclasts within the mylonitic gneiss, according to Elvevold ( 1988). Geothermobarometric studies on relict equilibrium mineral assemblages from the porphyroclasts show that upper amphibolite- to granulite facies conditions with P-T estimates of 9.2 ± 1.0 kbar and 715 ± 30 C were reached (Elvevold 1988). The mylonitic foliation dominating the gneiss is characterized by a medium amphibolite- facies metamorphism. This mylonitic fabric is in the eastern part of the nappe unit truncated by the thrust separating the Nordmannvik and the underlying Kå fjord/vaddas Nappe (Fig. 2). The dated metadiorite is situated within a 200 m wide strongly foliated pyroxene-bearing amphibolite gneis (Fig. 2). The metadiorites occur as lens-like bodies of variable size, generally less than lom across, within the gneiss. The lenses are variably deformed. Some lenses show a gradual transition from an equigranular metadiorite to gneiss over a distance of lom. Other lenses are almost angular and have a sharp contact against the surrounding gneiss. Field observations and detailed petrographical studies suggest that the metadiorite represents the protolith to the mylonitic pyroxene-bearing amphibolite gneiss (Elvevold 1988). The metadiorite is composed of plagioclase, biotite, clinopyroxene, amphibole and garnet, with granoblastic to heteroblastic texture in the least deformed samples. Carbonate, epidotefclinozoisite and opaques occur in accessory amounts. Bent twin lamellae in plagioclase and kinked biotite occur locally. A porphyroclastic texture, with porphyroclasts of feldspar, garnet, biotite and amphibole in a fine- grained matrix of recrystallized quartz, biotite and opaques characterizes the more deformed varieties. Elvevold ( 1988) interpreted the mineral assemblages and texture to have a metamorphic rather than magmatic origin. Sampling and analytical methods The analyzed samples ( 3-5 kg) were collected at the artillery post next to E-6 on Heia (Fig. 2), the UTMlocalization is given in Table l. Only samples without a mesoscopic foliation and alteration were collected. Rb-Sr content was determined on the Phillips PW 1400 X- ray ftuorescence spectrometer at the University of Tromsø, following the methods of Norrish & Chappel (1967). Chemical separation of Rb and Sr was carried out following conventional HF-HN03-HC1 dissolution procedures and ion exchange techniques. Mass spectrometry was performed on the VG Micromass MS 30 at the Geological Museum in Oslo. Variable mass discrimination in the Sr-isotopic ratios was corrected by normalizing the 86 Sr/ 88 Sr-ratio to (Faure & Hurley 1963). The 87Rb decay constant used was 1.42 x w- ;years (Steiger & Jager 1977) and the data were regressed by the technique of Y ork ( 1969). All errors are quoted as 2u errors. Table l. Whole rock analytical data from the metadiorite Sam p le Rb Sr Rb/Sr 87Rb/86Sr 87Sr/86Sr SE AADN ± AADN o ± AADN ± AADN ± AADN ± AADN o ± Map reference: 1533 III Takvatnet (l :50,000) UT M

4 378 M. Lindstrøm & A. Andresen NORSK GEOLOGISK TIDSSKRIFf 72 (1992) Results and interpretation The Rb, Sr and Sr-isotope ratios obtained from the Heia metadiorite are presented in Ta ble l, and the data are plotted in Fig. 3. The six data-points give a well-defined isochron age of 492 ± 5 Ma with an initial 8 7 Sr/8 6 Sr-ratio of ± The scatter of the data points about the best-fit line (MSWD l.ol) can be assigned to analytical errors only (Brooks et al. 1972). The obtained age can be interpreted either as the primary crystallization age of the rock, or s the timi? g of the high-grade metamorphism recorded m the gnetss unit. There is no evidence to believe that the obtained age has anything to do with the mylonitization dominating the country.rock, as textures related to this event are absent in all the analysed samples. A rift-origin of the metadiorite is most probable, and is supported by the fact that its composition locally is gabbroic (E. Krogh, pers. com.). One would thus exp ct a source within the upper mantle or lower crust wtth little or no contamination of country rocks, which is indicated by the low ppm-amounts of Rb (Table 1). The Sr-initial ratio of ca. O Iies well a bo ve the presumed initial ratio of the upper mantle/lower crust at about 500 Ma (Faure & Powell 1972), and favours the interpretation of a resetting of the isotope system during the high-grade metamorphism. One would also expect a greater scatter of the data points than indicated by the Iow MSWD of 1.01 if the isochron age reflected the crystallization age of the original diorite. Although rocks can undergo considerable metamorphism without resetting of the isotope system (e.g. Jiiger & Hunziker 1979), resetting is favoured in recrystallized rocks. The dated samples were collected over a relatively small area, and isotopic homogenization may well have been completed within the area. A metamorphic age of 492 ± 5 Ma implies that the diorite originally intruded at an earlier stage, probably in early Caledonian or Late Precambrian time. A model age o ± 5 M.. I.R. Ø ± MSWD o.s 1,0 l.s 2. o Fig. J. Rb-Sr isochron diagram of the Nordmannvik Nappe metadiorite. calculation was performed on the isotopic data, taking a presumed upper mantle value of the Sr-initial ratio in Late Precambrian time (0.703) (Faure & Powell 1972). This calculation indicates a primary age of 788 Ma. Discussion and conclusions An age of 492 ± 5 Ma supports the idea of Binns ( 1978) and Andresen et al. ( 1985), who proposed, without presen ting any evidence that the high-grade nappes located structurally between the lower-grade Vaddas and Lyngen Nappes bad a pre-scandian meta orphic h st? ry : Pre Scandian mylonitization of the Heta Nappe 1s mdtcated by a truncation of the mylonitic foliation in the nappe. by the thrust fault separating the unit from the underlymg Kåfjord/Vaddas Nappe. The thrust fault is related to the main Scandian nappe transport (Andresen et al. 1985; Elve old 1988). The overlying Lyngen Nappe contains evidence of a two-stage tectonic history represented by an obducted ophiolite fragment and a conformably overlying Scandian, fossil-bearing sedimentary sequence (Minsaas & Sturt 1985; Andresen et al. 1985). A high-grade metamorphic event at ca. 500 Ma (Early Ordovician) is recorded at several places within the north and central Caledonides. Previous 40ArP9Ar and Sm/Nd works from Upper Allochthon (Seve Nappe) in the Norrbotten area (Sweden) have shown that parts of these rocks underwent high-grade ( eclogite) metamorphism at Ma (Dallmeyer & Gee 1986; Mørk et al. 1988). These ages and associated high-grade metamorphism were interpreted by Dallmeyer & Gee ( 1986) in terms of westward subduction of the thinned, rifted and dike-intruded western continental margin of Baltica during an early Caledonian orogenic event. The age of the Heia Metadiorite is also comparable with recently obtained ages from the Kalak Nappe Complex in Finnmark. Mørk & Stabel ( 1990) recorded a high-grade metamorphic age of 502 ± 28 Ma from a mafic granulite within the Seiland Igneous Province. Dallmeyer ( 1988) recorded 40Arf39Ar ages of hornblende from within the Ka1ak Nappe Complex of 490 ± 5 Ma, interpreted as post-metamorphic cooling ages. This was taken as evidence for significant tectonothermal activity along the Baltoscandian margin during Early Ordovician. The obtained age demonstrates that the 'Scandian' nappes in the Troms area include tectonic elements with a pre-scandian tectonometamorphic history. Although comparable in age with the Early Ordovician metamorphic ages recorded in the Seve Nappes no straightforward correlation exists owing to the inferred suspect origin of the Nordmannvik Nappe compared with the Baltoscandian affinity of the Seve Nappes. One possible model, and the one favoured here, is that the early Caledonian tectonothermal events recorded in the Seve and Nordmannvik Nappes are related to two entirely different subductionfcollision zones as proposed by Stephens & Gee ( 1985) from the Central Scandinavian

5 NORSK GEOLOGISK TIDSSKRIFT 72 (I992) Early Caledonian, Troms 379 Caledonides. Within such a scenario one could very well envision the Nordmannvik Nappe as a tectonic fragment of a mature volcanic are or micro-continent onto which the Lyngen Gabbro (Ophiolite) was emplaced. An alternative view would be to consider the Nordmannvik Nappe as a fragment of the Seve Nappes which at a late stage in the Scandian collisional process was tectonically emplaced, higher up in the tectonostratigraphy. by an out-of-sequence thrust, Acknowledgements. - Field and analytical work carried out in this study forms a cand. scient. thesis (M.L.) and was supported by NAVF grant D to A.A. We thank the staff at the Mineralogical Geological Museum, University of Oslo for help and guidance with the analytical work. References Manuscript received May 1991 Aitcheson, S. J., Daly, J. S. & Cliff, R. A. 1989: A late Proterozoic orogen in the North Norwegian Caledonides. Terra Abstracts /, 15. 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