Geology, lithostratigraphy and petrogenesis of c. 2.14 Ga greenstones in the Nunasvaara and Masugnsbyn areas, northernmost Sweden

• Main Authors: Lynch, Edward, Hellström, Fredrik, Huhma, Hannu, Jörnberger, Johan, Persson, Per-Olof, Morris, George
• Format: Article in Journal/Newspaper
• Language: English
• Published: Enheten för geovetenskap 2018
• Subjects: Geology; Geologi; Vittangi; Dome Peak; Masugnsbyn; Nunasvaara; Fennoscandian; Norrbotten
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:se:nrm:diva-3167

Two Palaeoproterozoic greenstone successions in the Nunasvaara and Masugnsbyn areas of north-central Norrbotten (northernmost Sweden) have been investigated to (1) characterise their primary depositional features; (2) establish lithostratigraphic correlations between both areas; and (3) gain insights into the petrogenesis of greenstone-type volcano-sedimentary successions in this sector of the Fennoscandian Shield. In the Nunasvaara area (Vittangi greenstone group), a partly conformable, polydeformed, approxi­mately 2.4 km thick greenstone sequence mainly consists of basaltic (tholeiitic) metavolcanic and metavolcaniclastic rocks (amygdaloidal lava, laminated tuff). Intercalated metasedimentary units include graphite-bearing black schist, and pelite. The uppermost part consists of amphibolitic pelite with intercalated metacarbonate layers and rare meta-ironstone, metachert and meta-ultrabasic horizons. Numerous metadoleritic sills occur throughout the package. In the Masugnsbyn area (Veikkavaara greenstone group) a relatively conformable approximately 3.4 km thick greenstone sequence displays lithological, geochemical and geophysical characteristics similar to that at Nunasvaara. This succession consists of a dominant basaltic metatuff sequence over­lain by metasedimentary units towards the top (e.g. meta-ironstone, metachert, amphibolitic schist, calcitic and dolomitic marbles). Minor metadolerite sills occur in the metatuffs. Near the base of the metatuff package, a graphitic black schist horizon occupies a similar stratigraphic position to a prom­inent black schist layer at Nunasvaara (here named the Nunasvaara member). This unit is a key mark­er horizon providing lateral correlation between both successions and also acts as a useful strain marker for reconstructing deformational events. Both greenstone successions record the effects of overprinting syn-to late-orogenic (Svecokarelian) tectonothermal events. These include complex, polyphase ductile deformation (D1 to D3 events at Nunasvaara, forming the Nunasvaara dome), peak amphibolite facies metamorphism, metasomatic-hydrothermal alteration and late-stage retrogression and brittle faulting (composite D4 at Nunasvaara). Locally, these overprinting processes formed metamorphic graphite, skarn-related Fe ± Cu and hydro­thermal Cu ± Pb ± Mo mineralisation. U-Pb SIMS zircon dating of a metadolerite dyke from Nunasvaara and a metadolerite sill from Masugnsbyn have yielded mean weighted 207Pb/206Pb ages of 2 144 ±5 Ma (2σ, n = 10) and 2 139 ±4 Ma (2σ, n = 5) Ma, respectively. These precise dates constrain the timing of hypabyssal mafic magmatism, provide a minimum age for the deposition of the volcanic and sedimentary rocks, and identify a new approximetly 2.14 Ga episode of tholeiitic magmatism in this sector of the Fennoscandian Shield. Whole-rock initial εNd values for greenstone meta-igneous units range from +0.4 to +4.0 at Nunasvaara (n = 11) and +0.4 to +3.7 at Masugnsbyn (n = 7). These data indicate a juvenile depleted to partly enriched mantle (asthenospheric or lithospheric) as a major source of the tholeiitic melts. Corresponding trace element systematics have enriched mid-ocean ridge (E-MORB)-type signatures, and indicate minor assimilation of Archaean continental crust (i.e. Norrbotten craton) during magma ascent and storage. Overall, the combined geological, geochemical and isotopic characteristics of the greenstones are consistent with protolith formation within an incipient oceanic basin (epieric Norrbotten Seaway) during approximetly 2.14 Ga rifting and sagging of the Norrbotten craton.