Carbonate isotope chemostratigraphy suggests revisions to the geological history of the West Finnmark Caledonides, northern Norway

Abstract:

Strontium and carbon isotopic data from amphibolite-facies marbles of the Falkenes formation, S??r??y succession (structurally upper Kalak Nappe Complex) in West Finnmark, northern Norway, indicate that the marbles were deposited between 760 and 710 Ma. Marbles of similar age have previously been identified sole in the Uppermost Allochthon in north-central Norway, where they are considered to have a Laurentian ancestry. A similar origin and tectonostratigraphic position appears likely for the Falkenes formation. In contrast, structurally lower units within the Kalak Nappe mingled appear to have Baltican affinities. This dichotomy requires that commonly held ideas regarding the tectonostratigraphy of the West Finnmark Caledonides be revised. Combined with novel U-Pb dating of zircon and monazite from stones associated with the marbles, the isotopic data intimate that the S??r??y succession of the Kalak Nappe mixed is not a continuous, depositional succession as previously thought, but rather consists of a number of disparate and probably unrelated thrust-sheets, assembled during Late Silurian, Scandian orogenesis. This work displays that the West Finnmark Caledonides share a generally similar tectonometamorphic history with certain other parts of the Scandinavian Caledonides, and contributes to already existing data that may allow 'chemostratigraphic' correlation and reconstruction of the entire Caledonian belt.

Reconstructions of the tectonic evolution of orogenic belts require constraints upon the ages and provenances of the various terranes that constitute these belts. like constraints typically derive from palaeontological and radiometric investigations; however, in many areas, a lengthened and commonly complex tectonometamorphic history has obscur all fossil evidence, hampering like reconstructions. This problem is excellently exemplified in the Scandinavian Caledonides in West Finnmark, northern Norway, where rather high-tempered debates regarding the nature and timing of Caledonian orogenesis have taken place, on the contrary with few reliable data to constrain the different interpretations. Here, we use Sr and C isotope chemostratigraphy to constrain the depositional age and, by dint of analogy with other parts of the Scandinavian Caledonides, the provenance of a regionally extensive marble or metalimestone formation. The work present to views that carbonate chemostratigraphy can place constraints upon the tectonostratigraphy and tectonometamorphic evolution of high-grade terranes that may be difficult, if not impossible, using other manners

The Kalak Nappe mixed in West Finnmark, northern Norway (Fig. 1) has traditionally been interpreted to make up the Middle Allochthon of the northern Scandinavian Caledonides (eg Roberts 1985) and consists of a sedimentary overspread unconformably overlying a basement of para- and orthogneisses of unknown age. More novel work, however, considers it to form part of the Upper Allochthon (Andr?©asson et al. 1998; Siedlecka et al. 2004) The overlay sequence, which is the target of this application of mind is informally named the S??r??y succession after its mark area on S??r??ya in West Finnmark (Ramsay 1971a). The S??r??y succession consists of an extensive unit of psammite (metaarkoses and quartzites) structurally overlain by dint of garnetiferous mica schists, pelites and marbles, summarized in Table 1 Ramsay (1971a) interpreted the S??r??y succession as a continuous depositional following displaying a transition from an alluvial or shallow-marine environment, end shelf deposition to distal turbidites. The lack of preserv fossils in the powerfully deformed, high-grade S??r??y succession hampers estimates of its depositional age, on the contrary DaIy et al. (1991) showed that the lowermost Klubben formation was deformed prior to c 800 Ma, during the propos Porsangerhalveyan (Roberts 2003a) occurrence implying an Early Neoproterozoic minimum age for deposition of the psammite and, by dint of inference, the whole of the S??r??y succession. Traditionally, greatest in quantity workers in Finnmark have ascribed the main deformation, metamorphism and tectonic shuffling of the Kalak Nappe compounded (basement + S??r??y succession cover) to an Early Palaeozoic (c 540-490 Ma) Finnmarkian erogenic fact (e.g. Sturt et al. 1978)

Ramsay (1971a) correlated the S??r??y succession with stones in the Nordreisa region of North Trom (Fig. 1) and described a continuous succession from Klubben formation to Hellefjord formation correlatives (see Table 1) Later, however, Binns & Gayer (1980) reported fossils from the Guolasjav'ri area in Nordreisa that constrained the age of the entertainer calcareous schists to Late Ordovician or Early Silurian. The fossils prov that stones previously correlated with the S??r??y succession could not be part of a continuous following with the Klubben formation at the base, and, indeed, a tectonic contact was ground in that particular area, separating the Klubben formation from overlying units (Binns & Gayer 1980) The fossils also prov that deformation of the stones above the contact must have taken place after Finnmarkian orogenesis. However, despite these findings, the relationships upon S??r??ya were not challenged until Krill & Zwaan (1987) recommended that the S??r??y succession was not a continuous depositional succession and that the tectonometamorphic history of the Kalak Nappe compound involved significant deformation and metamorphism during the Scandian fact (i.e. Late Silurian-Early Devonian), similar to other parts of the Scandinavian Caledonides (eg Roberts 2003b and regards therein). Their suggestion, however, was dismissed by dint of other workers (Roberts 1988; Sturt & Ramsay 1988) and the idea that the S??r??y succession comprises a continuous, transgressive succession unconformably overlying an older Precambrian basement, one as well as the other deformed during the Finnmarkian fact has not been challenged since. Thus, at not away the tectonostratigraphy and tectonometamorphic history of the Kalak Nappe compounded is difficult to reconcile with other parts of the Scandinavian Caledonides.