Kauko Laajoki

Major Palaeoproterozoic shear zones of the central Fennoscandian Shield

Abstract Employing two recently studied crustal-scale shear zones as type examples, this paper summarizes the major Palaeoproterozoic (Svecokarelian) shear tectonics of the central Fennoscandian Shield and demonstrates that this part of the Shield was not as stable during the Svecokarelian Orogeny as commonly assumed. The collision of the Svecofennian island arc with the Karelian Continent first created numerous NW-SE trending folds and thrusts of stages D1 and D2, which were then modified by successive shearing during stages D3 and D4. Stage D3 built up a system of N-S trending shear zones, here named the SavolappiShearSystem, the type example of which is the Hirvaskoski Shear Zone. This is a dextral strike-slip shear zone at least 150 km long and 10–30 km wide, characterized by blastomylonitic fault rocks and various structures such as hook folds, Z-folds and sheath folds associated with the principal displacement zone, synthetic Riedel shears, and pinnate shears. The traces of the axial planes of F3 en-echelon folds deviate 15°–30° anticlockwise from the plane of the principal displacement zone. Other members of the Savolappi Shear System are the Pajala Shear Zone in northern Sweden and the Russian North Karelia Shear Zone in the east. Stage D4 created a conjugate shear system called the Finlandia Shear System, the type example of which is the Oulujarvi Shear Zone. This is a NE-SW trending sinistral strike-slip shear zone more than 250 km long and 20–30 km wide across its southwestern end. It is composed of a NE-SW trending principal displacement zone, synthetic Riedel shears, and pinnate shears with antithetic Riedel shears in a NW-SE direction. Typical fault rocks within these shears are S-C mylonites. The axial-plane traces of F4 folds of all scales diverge by 20°–40° clockwise from the plane of the principal displacement zone. The Kuopio Shear Zone is a conjugate NW-SE trending counterpart of the Oulujarvi Shear Zone. As a whole, the Finlandia Shear System forms a conjugate network of NW-SE and NE-SW trending shear zones which occupies most of the northern and central Fennoscandian Shield.

The precambrian supracrustal rocks of Finland and their tectono-exogenic evolution

Abstract The Precambrian supracrustal rocks of Finland were produced during four main evolutionary stages: (1) late Archaean (Pre-Svecokarelidic) greenstone-schist evolution on an unknown basement; (2) at least four cycles of early Proterozoic (Karelian) continental-epi/pericontinental sedimentation and volcanism on or next to the cratonized late Archaean basement; (3) early Proterozoic (Svecofennian) eugeosynclinal evolution west of, or in part, on the late Archaean basement and its cratonic cover; and (4) middle to late Proterozoic (Jotnia) continental sedimentation on the stabilized early Proterozoic (Post Svecokarelidic) basement.

Palaeoproterozoic glaciomarine sedimentation in an extensional tectonic setting: the Honkala Formation, Finland

Abstract The Honkala Formation of the Honkajarvi Group in the Kainuu Schist Belt of northern Finland (Central Fennoscandian Shield) provides evidence of Palaeoproterozoic glaciation on the fractured surface of the craton. The diamictites and associated rocks of the Honkala Formation are some 190 m thick in the studied area. They consist of a remnant of the glacigenic sequence deposited during an incipient stage of crustal extension. The rocks rest unconformably on the Late Archaean basement and conformably on rift-related volcanics. The volcanics were erupted during a period of general fragmentation and rifting of the Late Archaean basement ∼ 2.4-2.3 Ga ago. At this time, shallow intracratonic basins and depressions started to form on a continental scale. Subsequently, many of these basins subsided below sea level. The lower part of the Honkala Formation consists of turbiditic, non-glacial mudstones and sandstones, up to 70 m thick. Glacial advance is indicated by ice-rafted deposits forming a succession of some 10 to 50 m. Deposition took place in a glaciomarine rift-basin extending towards the interior of the Archaean Craton. Waning glacial influence is indicated by some 70 m of sandstones. The uppermost sandstones were influenced by intermittent chemical weathering indicating that these deposits were subjected to subaerial conditions and warmer climate. Correlation on a global scale is possible. During the early Palaeoproterozoic, the Fennoscandian and North American cratons were probably situated close together at a high latitude and both were affected by a major continental glaciation. The Fennoscandian Craton then shifted to lower latitudes, whereas the North American cratonic margin recorded further glaciations at high latitudes.

An interlinked system of folds and ductile shear zones—late stage Svecokarelian deformation in the central Fennoscandian Shield, Finland

Abstract The Svecokarelian deformation of the Karelian and Svecofennian Provinces in Finland involved four main phases. Isoclinal folding and associated thrusting were the main processes of D 1 , which was followed by more open folding in D 2 . Subsequent deformation was localized during D 3 and D 4 . This paper focuses on the late stage ( D 4 ) Svecokarelian deformation that created most of the ductile shear zones in the central Fennoscandian Shield and was the last penetrative deformational event to affect the entire Palaeoproterozoic domain. Deformation during D 4 occurred approximately from 1.85–1.80 Ga, and left a greater imprint in the rocks than has commonly been supposed. The shear zones formed during D 4 are referred to collectively as the Finlandia Shear System. The Karelian province is composed of Archaean basement and its autochthonous-allochthonous Palaeoproterozoic sedimentary cover. Svecokarelian tectonics split the basement into separate complexes, the rheological properties of which differed markedly from those of the adjacent Proterozoic supracrustal units. On the northern and southern margins of the Archaean complexes, folding with steep E-W-trending axial surfaces was the main response to D 4 deformation, whereas on the eastern and western margins of the complexes the response to D 4 was the development of sinistral and dextral shear zones trending NE-SW and NW-SE, respectively. On a large scale, these areas of folding and of shearing form an interlinked system, the changes from one style of deformation to the other being gradual. The principal D 4 elements in the area of juvenile Svecofennian crust are folds with E-W-trending axial surfaces.

Sorosite, Cu(Sn,Sb), a new mineral from the Baimka placer deposit, western Chukotka, Russian Far East

Abstract Sorosite, ideally Cu(Sn5Sb), is a new mineral species from the Baimka gold-platinum- group mineral placer deposit, Chukotka, Russian Far East. It occurs as large subhedral to euhedral crystals (0.1-0.4 mm in length), hexagonal in cross section, minute crystals (≤ 15 μm, also hexagonal), and anhedral grains. Sorosite forms inclusions in Sb-bearing native tin, is often intergrown with stistaite (Sn1.12 - 1.13Sb0.87 - 0.88) and occurs with herzenbergite (SnS), native lead, and trace cassiterite. Sorosite is brittle with a microhardness VHN40;50 = 443.7 kg/mm2 (n = 3). No cleavage is observed. In reflected light, the large crystals are nearly white with a pinkish tint, whereas the microcrystals show a pronounced pinkish tint. Bireflectance is variable. The average of nine electron microprobe analyses gave Cu 35.33, Fe 1.18, Sn 58.18, and Sb 4.77, sum 99.46 wt%, corresponding to (Cu1.00Fe0.04)∑1.04 (Sn0.89Sb0.07)∑0.96. The powder pattern is close to those of natural Cu(Sn,Sb) and synthetic η-Cu6Sn5; it was indexed for a hexagonal cell, with a = 4.217(4) Å, c = 5.120(6) Å, and V = 78.85 Å3. For Z = 2, the calculated density is 7.6 g/cm3. The strongest lines in the pattern are at 2.970 (011), 2.112 (110), and 2.094 Å (012). The sorosite-bearing mineral assemblage apparently formed under low fO₂ and fS₂ conditions.

Primary platinum-bearing copper from the Lesnaya Varaka ultramafic alkaline complex, Kola Peninsula, northwestern Russia

SummaryThe Lesnaya Varaka ultramafic alkaline complex in the northeastern Fennoscandian Shield (Kola Peninsula, NW Russia) is a concentrically-zoned intrusion with a dominantly dunitic core (Fo85-92) surrounded by clinopyroxenites. The complex resembles an Alaskan-type intrusion, but differs in its strong alkaline affinity. Native copper occurs as small (5 to 15 μm), subhedral to euhedral crystals, isolated within titanomagnetite, in a dunite containing abundant titanomagnetite-perovskite mineralization (up to ∼ 30 modal %). Nickel-rich (4.1–4.5 wt.% Ni) tetraferroplatinum is also present as minute (up to ∼ 5 gmm) subhedral crystals, enclosed by titanomagnetite. They are typically partially rimmed by rhodian pentlandite (∼ 6 wt.% Rh). The copper crystals contain 0.6 to 10.1 wt.% Pt, 2.1 to 3.0 wt.% Ni, and essential Fe (approximately 2 to 3 wt.%). There is a wide variation in the Pt content between individual crystals, but its distribution within single crystals is fairly constant. Compared with Cu-Pt alloys from other localities, solid solution of Cu with Pt in the Lesnaya Varaka native copper is low. Unlike most occurrences in ultramafic rocks, the crystalline copper at Lesnaya Varaka appears to be a primary phase, which formed under moderately oxidizing conditions and at very low sulphur activities.ZusammenfassungDer ultramafisch-alkalische Lesnaya Varaka Komplex im nordöstlichen Fennoskandischen Schild (Halbinsel Kola, NW Rufland) ist eine konzentrisch zonierte Intrusion mit einem Dunitkern (Fo85-92) umgeben von Klinopyroxeniten. Der Komplex ähnelt einer Alaskan-Typ Intrusion, unterscheidet sich aber durch seine stark alkalische Affinität. Gedigen Kupfer tritt in einem Dunit, der verbreitet eine TitanomagnetitPerovskit Mineralisation führt, in Form kleiner (5 bis 15 μm), sub- bis euhedraler Kristalle isoliert im Titanomagnetit auf. Nickel-reiches (4.l-4.5 Gew. % Ni Tetraferroplatin kommt ebenfalls in Form winziger (bis zu ca. 5 μm subhedraler Kristalle im Titanomagnetit vor. Typischerweise sind diese Kristalle zum Teil von Rhodium-führendem Pentlandit (ca. 6 Gew.% Rh) umgeben. Die Kupferkristalle führen 0.6 bis 10.1 Gew.% Pt, 2.1 bis 3.0 Gew.% Ni und beträchtliche Gehalte an Fe (ca. 2–3 Gew.%). Die Pt-Gehalte zwischen verschiedenen Kristallen streuen stark, während sie innerhalb einzelner Kristalle ziemlich konstant sind. Im Vergleich mit Cu-Pt Legierungen von anderen Lokalitäten ist die Mischbarkeit von Cu und Pt im Kupfer von Lesnaya Varaka gering. Im Unterschied zu den meisten anderen ultramafischen Gesteinen, scheint das kristalline Kupfer von Lesnaya Varaka eine primäre Phase zu sein, die sich unter mäßig oxidierenden Bedingungen und bei sehr niedrigen Schwefelaktivitäten gebildet hat.