Geochemical Evidence for a Rift-Related Origin of Metadolerites Within the Senja Nappe, Troms, North Norwegian Caledonides: a Discussion
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Discussion Geochemical evidence for a rift-related origin of metadolerites within the Senja Nappe, Troms, north Norwegian Caledonides: a discussion R. D. DALLMEYER Dallmeyer, R. D.: Geochemical evidence for a rift-related origin of metadolerites within the Senja Nappe, Troms, north Norwegian Caledonides: a discussion. Norsk Geologisk Tidsskrift, Vol. 68, pp. 133-134. Oslo 1988. ISSN 0029-196X. Bergh & Andresen (1987) have suggested correlation of the Senja Nappe (Troms District, North Norway) with Ordovician-Sihirian sequences of the Balsfjord Group which unconformably overlie mafic-ultra mafic rocks of the Lyngen Nappe. This is inconsistent with c. 475-500Ma post-metamorphic 40Arj39Ar homblende cooling ages previously reported for the Senja Nappe. Field and geochronologic characteristics indicate correlation of the Senja Nappe with other units of the regional Middle Allochthon Scandinavian succession. R. D. Dal/meyer, Department of Geology, University of Georgia, Athens, Georgia 30602, U.S.A. Bergh & Andresen (1987) have presented geo Cumbest & Dallmeyer (1985) and Dallmeyer & chemical analyses of foliated, polydeformed Hames (1986), document a complex, polyor metadolerite dikes within marble and mica schist ogenic tectonothermal evolution for the Senja of the Senja Nappe in the Troms district of the Nappe. This involved initial growth of a middle north Norwegian Caledonides. They interpreted amphibolite facies mineral assemblage which is their results to indicate that protoliths of the dikes only locally preserved as rotated porphyroclasts were MORB basalts with affinities transitional within a penetrative mylonitic foliation. This fab between 'within plate' and 'plate margin'settings. ric developed during emplacement of the Nappe Bergh & Andresen suggested that the protoliths into its present relative structural position on were emplaced into a platformal or miogeoclinal synchronously imbricated, crystalline basement sequence similar to that of the Caledonide Middle units (with geochronological characteristics indi Allochthon. However, because of ' ...extensive cating a Baltic Shield correlation) which comprise car bonate build-ups . ', which Bergh & Andre the Western Gneiss Terrane (WGT) on Senja. sen indicated were '. not characteristic of the These basement rocks locally display proto Middle Allochthon (Kalak Nappe Complex) mylonitic-mylonitic fabrics adjacent to the Senja further east', they proposed correlation of the Nappe tectonic contact. These and the nappe Senja Nappe with dike intruded, Ordovician contact are offsetby a regional, up to 2 km wide, Silurian metasedimentary sequences of the ductile strain zone. Porphyroblastic homblende Balsfjord Group which unconformably overlie overgrowing this strain zone and mylonitic fabrics the mafic-ultramafic (ophiolitic?) igneous com within the Senja Nappe record 40Arj39 Ar plateau plex of the Lyngen Nappe (Upper Allochthon). ages which indicate that post-metamorphic Field, petrologic, and geochronologic charac cooling through appropriate argon closure tem teristics of the Senja Nappe are markedly incon peratures occurred between c. 475 and 500 Ma, sistent with this correlation. requiring an earlier emplacement for the Senja The results of a systematic program of detailed Nappe and development of related mylonitic fab mapping and coordinated geochronological work ric elements. The horn blende 40Arj39 Ar age spec on Senja, presented by Cumbest et al. (1983, tra record variable effects of a c. 380-390Ma 1984), Williams et al. (1985), Clark et al. (1985), thermal rejuvenation. Porphyroblastic chlorite 134 R. D. Dal/meyer NORSK GEOLOGISK TIDSSKRIFT 68 (1988) and muscovite overgrow mylonitic fabrics within References the WGT and the Senja Nappe. The muscovite and dynamically recrystallized biotite within the Bergh, S. G. & Andresen, A. 1987: Geochemical evidence for mylonitic foliation record 380-390Ma 40 Arj39Ar a rift-related origin of metadolerites within the Senja Nappe, plateau ages indicating total rejuvenation (or Troms, North Norwegian Caledonides. Norsk Geologisk Tidsskrift 67, 25-37. growth) during a late Caledonian thermal Clark, A. H., Dallmeyer, R. D. & Andresen, A. 1985: overprint. The Late Cambrian-Early Ordovician Basement/cover relations along the eastern margin of the thermal record within the Senja Nappe precludes Western Gneiss Terrane, Senja, Troms, Norway.Geological correlation with Ordovician-Silurian successions Society of America, Abstracts With Programs 17, l, 11. Cumbest, J. J., Dallmeyer, R. D., Solomon, C. & Andresen, within the Balsfjord Group. A. 1983: Age and origin of ductile fabrics in the Western The internally imbricated character of the Senja Gneiss Terrane of the Norwegian Caledonides (Troms). Geo Nappe precludes a reliable estimate of strati logical Society of America, Abstracts With Programs 15, 6, graphic thicknesses, and thereby limits the use 552. fulnessof lithologic dominance in regionalnappe Cumbest, R. J., Hames, W. E., McKinney, J. B. &Dallmeyer, R. D. 1984: Tectonothermal evolution of the northwestern correlations. Bergh & Andresen's suggestion that basement terrane of the Norwegian Caledonides, Senja, carbonates are not typical within supracrustal Troms. Geologica/ Society of America, Abstracts With Pro units of the Kalak Nappe Complex is not sup gram 16, l, 11. ported by the descriptions of Roberts((1968} and Cumbest, R.J. &Dallmeyer, R. D. 1985: Polyphase Caledonian tectonotherrnal evolution of the Western Gneiss Terrane, Zwaan & Roberts (1978}, which list the Falkenes Senja, Troms, Norway. Geo/ogica/ Society of America, Marble Group as a persistent constituent. The Abstracts With Program 17, l, 14. Late Cambrian-EarlyOrdovician tectonothermal Dallmeyer, R. D. & Hames, W. E. 1986: Polyphase Caledonian record within the Senja Nappe, combined with tectonotherrnal evolution of the Western Gneiss Terrane, the internally imbricated Baltic Shield basement, Senja, Troms, North Norway. Terra Cognita 6, 2, 32. Roberts, D. 1968: The structural and metamorphic history of clearly suggest that that nappe occupies a tec the Lungstrand--Finfjord area, Sørøy, North Norway.Norges tonostratigraphic level similar to that of western geologiske undersøkelse253, 160 pp. segments of the classically defined Scandinavian Williams, T. J., Oark, A. H., Dallmeyer, R. D. &Andresen, A. Middle Allochthon. These characteristics do not 1985: Lithotectonic character of the Western Gneiss Terrane, permit correlation with the Ordovician-Silurian Senja, Troms, Norway. Geo/ogica/ Society of America, Abstracts with Programs 17, l, 70. Balsfjord Group which unconformably overlies Zwaan, K. B. & Roberts, D. 1978, Tectonostratigraphic suc mafic-ultramafic rocks of oceanic affinity witWn cession of the Finnmarkian Nappe Sequence, North Norway. the Upper Allochthon. Norges geologiske undersøkelse 343, 53-71. Manuscript received June 1987. .