Metasediments of the deep crustal section of Southern Karnataka

A review of the rocks of supracrustal origin in the amphibolite and granulite facies terrane in southern Karnataka was presented. In addition to introducing the metasediments in the field area of the workshop, a review was presented of the common occurrence of metasediments in amphibolite and granulite facies rocks worldwide. Models of granulite metamorphism must include a mechanism for the burial of these sediments to the depths recorded by the geobarometers in granulite metamorphism in addition to their reexposure at the surface. Unfortunately, the common occurrence of supracrustals in granulite facies rocks, sometimes with remarkably little deformation was deemed significant

On the stratigraphic position of kaolin in Väyrylänkylä, South Puolanka area, Finland

Drilling data on the stratigraphic position of recently discovered clay occurrences containing kaolinite are given. Preliminary information on the clay minerals involved is included. The weathering undergone by the Precambrian rocks underlying the clays is described briefly. Secondary enrichment of iron caused by weathering in iron formations as revealed by partial ore analyses is presented. The clays lie between strongly folded Karelidic rocks and Quaternary deposits, and are thus Preglacial in age. Owing to the varying kaolinite content (0–60 %) and the thick Quaternary overburden the clays are not economically exploitable. The kaolin of Pihlajavaara, which some authors have considered to be of Karelian age, is correlated by the present author with Preglacial clays

Rare-earth elements in Precambrian iron formations in Väyrylänkylä, South Puolanka area, Finland

Partial rare-earth element (REE) and ore analyses of 19 samples of oxide-, carbonate-, silicate-, and sulphide-facies rocks from the Superior-type Precambrian (Karelian) iron formations and associated rocks (pelitic metasediments, dolomite, basic tuffite and metadiabase) are given. For comparison one analysis of an Archean (Prekarelidic) iron formation of the Algoma type is also included. The Karelian iron formations are relatively rich in REE and their REE distribution patterns show depletion of Ce. This is due to the regularly occurring apatite, in which these features are multiplied as is indicated by the analysis of the apatite-rich band. This stratum is interpreted as a marine phosphorite interband and, consequently, the apatite in the iron formations as of marine origin. With the exception of Ce, the REE distribution patterns in iron formations and associated pelitic metasediments and dolomite are relatively similar. All these rocks show a clear trend towards relative depletion of the lighter REE and Yb and Lu in comparison with the North American shale (NAS). Metadiabase and basic tuffite are poor in REE and depleted in the lighter REE in the same way as are tholeitic basalts. Archean iron formations seem to have a greater Eu:Sm ratio than the younger iron formations. More REE data from geologically wellknown iron formations are needed, however, before any definitive conclusions can be drawn

The Mesoproterozoic sub-Heddersvatnet unconformity, Telemark, South Norway

The 1.51 Ga Rjukan group,Telemark, S Norway, is divided into felsic volcanic rocks of the Tuddal formation and mafic rocks of the Vemork formation. It is overlain by the sedimentary rocks of the Vindeggen group (1.50-1.17 Ga), starting with the arkosic Heddersvatnet formation.The contact between the Rjukan and Vindeggen groups has been variably interpreted in the literature. New field data indicate that the contact corresponds to an unconformity, corroborating Wyckoff's early observations in 1934.The contact is referred to as the sub-Heddersvatnet unconformity. The nature of the contact varies. Around Lake Heddersvatnet, it most likely represents an angular unconformity with a sharp, erosional surface, whereas near Lake Skjesvatnet a thin in situ palaeoweathering crust developed on a massive Tuddal porphyry defines it, These observations indicate that the sub-Heddersvatnet unconformity represents a deeply weathered land surface cutting diverse folded Tuddal units. How big is the time gap it represents and the nature of the pre-Vindeggen deformation are open questions as the sedimentation age of the Heddersvatnet formation is unknown and the structure of the Rjukan group has not been studied on a regional scale

The mesoproterozoic sub-Lifjell unconformity, central Telemark, Norway

The sub-Lifjell unconformity subdivides the traditional Seljord group of the Telemark supracrustals, south Norway, into the Vindeggen and Lifjell groups. It is defined by an in situ weathering breccia and an angular unconformity above quartzites of the 1155 Ma old Vindeggen group and by a volcaniclastic palaeoregolith developed above the 1155±2 Ma old porphyry of the Brunkeberg formation. Due to the complex deformation of the Vindeggen and Lifjell groups this unconformity has often been sheared or cut by faults, which impedes its use as a lithostratigraphic boundary. Dated porphyries under and above the Lifjell group define the age of the sub-Lifjell unconformity between 1155±2 Ma and 1145±4 Ma indicating that the part of the unconformity developed above the Brunkeberg formation represents a relatively short time gap (<10 Ma). The part of the unconformity developed above the Vindeggen group represents a substantially larger time gap, for the Vindeggen group was folded before the extrusion of the Brunkeberg porphyry. This time duration cannot be, however, approximated more closely as the ages of the sedimentation and folding of the Vindeggen group are not known. In the terms of sequence stratigraphy, the sub-Lifjell unconformity defines the lower bounding surface of an extensive Mesoproterozoic beach-shallow self sequence

Additional observations on the Late Proterozoic Varangerfjorden unconformity, Finnmark, northern Norway

This paper is complementary to the author’s recent publication, which provided several new pieces of evidence for the glacial abrasion of the late Proterozoic Varangerfjorden unconformity. Additional descriptions of striations and diverse pits and embedded clasts on the unconformity surface at Bigganjargga are given, and it is stressed that the primary glacial features of the unconformity have been destroyed by secondary pits and imprints, most severely just west of the Bigganjargga diamictite, where the intra-Smalfjord hiatus joins the unconformity. This point represents the axial zone of the channel, which eroded the Bigganjargga diamictite and provided a favourable environment for diagenetic and possibly later fluids to attack the unconformity surface. Small grooves visible as positive casts at the base of the Smalfjord Formation are reported from Handelsneset. The general regional features of the unconformity at Vieranjarga and Ruossoai’vi and syn-Smalfjord erosional features at Skjåholmen are described. Roches moutonnées of either late Proterozoic or Pleistocene age that had developed on a Precambrian basement gneiss are reported at Karlebotn. Pleistocene striations and pits on the Veinesbotn quartzite are described for comparison purposes

The Central Puolanka Group - a Precambrian regressive metasedimentary sequence in northern Finland

Quite recently, an amphibolite-facies metasedimentary sequence at least 2500 m thick, named the Central Puolanka Group (CPG) was recognized underlying the traditional Jatuli-type sequences at the western margin of the Early Proterozoic Kainuu Schist Belt in Central Puolanka. To the west, towards the Archaean basement of the Pudasjärvi ‒ Vaala area, the primary sedimentary and stratigraphic features of the metasediments have been destroyed by severe metamorphism and migmatization to such a degree that the lowermost and the western parts of the CPG, have inverted into gneisses, called collectively the West Puolanka Paragneiss (WPP), within which it is no longer possible to map the rocks in terms of paleosedimentology and whose sedimentation basement has not yet been found. The zone between the CPG proper and the WPP is underlain by the Kettukangas paragneiss which may locally show turbidite features and which grades stratigraphically upwards into the CPG. The CPG itself consists of three formations called, from the oldest to the youngest, the Puolankajärvi Formation (PjF), the Akanvaara Formation (AvF) and the Pärekangas Formation (PkF). In terms of primary sediments, the lower and middle part of the PjF is made up of turbidite sands and muds deposited either at the outer parts of a shelf or in a deep water fan of an ancient continental margin. The upper part consists of cross-bedded or rippled silts and fine sands with interbeds of cross-bedded sands whose amount increases upwards towards the AvF. The latter is made up almost entirely of cross-bedded sands deposited either in an inner shelf or in deltaic environments. They are overlain abruptly but conformably by the dominant muds and silts of the PkF showing evidence of a tidal environment. This regressive CPG sequence is overlain unconformably by the fluvial gravels and sands of the first of the transgressive Jatuli-type sequences which form the bulk of the Kainuu Schist Belt. The CPG can be mapped as a coherent lithostratigraphic unit from Puolanka to Paltamo. The preliminary mapping of the paragneiss areas west of the Kainuu Schist Belt indicates, however, that the sediments of this sequence once covered a wide area which extended from Puolanka to at least Koillismaa in the north and to Otanmäki, at least, in the south and they may represent the same sedimentaryvolcanic cycle which produced the Middle and Upper Lapponian sediments and volcanics of Lapland

Mineralogy, geochemistry and metamorphism of the early Proterozoic Vähäjoki iron ores, northern Finland

The Vähäjoki iron ores occupy the uppermost part of the Karelian (Early Proterozoic) quartzite-dolomite sequence deposited on the late Archaean basement in southern Lapland. The ores occur as magnetite veins brecciating dolomite and as magnetite disseminations in chlorite and mica schists. Their average iron content is 40%, and they contain up to 3.8% P2O5 features which together with the division of the ores into 14 small bodies make them uneconomic. The only main ore mineral is magnetite, with which there occur accessory ilmenite, haematite, pyrite, chalcopyrite, pyrrhotite, arsenopyrite and cobaltite with small gold inclusions. The main gangue minerals classified by microanalysis are dolomite or Fe-dolomite in the dolomites, ferri-tremolite, tremolite, ferri-actinolite and ferrian actinolite, cummingtonite, magnesio-hornblende and tschermakitic hornblende in amphibole-bearing hosts rocks, green Mg-rich biotite, brown Fe-rich biotite and Ba-bearing (BaO 0.18‒7.9%) biotite, and chlorite in chlorite and mica schists. Major and trace elements were analyzed in 18 samples. The dolomites contain 24‒53% CaO and 3‒20% MgO, while the amphibole rocks, amphibole schists, chlorite schists and mica schists are chemically rather similar, containing about 13‒27% Fetot and 7‒21% MgO, the amphibole-bearing varities being a little richer in these elements than the phyllosilicate-bearing ones. The ore samples from magnetite matrix of the breccia type contain 71‒85% Fe2O3 and 0.05‒0.2% P2O5. The garnet-biotite and calcite-dolomite geothermometers and the mineral chemistry of the Ca-amphiboles suggest that the Vähäjoki iron ores were metamorphosed under greenschist ‒ amphibolite facies conditions (T = 465 °C and P = 2‒4 Kbar). Flunctiation in metamorphic grade is reflected in the change in amphibole composition from tremolite-actinolite to tschermakitic hornblende. The Vähäjoki iron ores were remobilized and enriched during early Proterozoic regional metamorphism and deformation, forming an epigenetic iron ore

Lithostratigraphy of the Mesoproterozoic Vemork formation, central Telemark, Norway

The Vemork formation forms a c. 2 km thick volcanic-sedimentary unit above the 1510 – 1500 Ma old Tuddal felsic volcanite formation, the oldest unit of the Mesoproterozoic Telemark supracrustals in southern Norway. It is dominated by basaltic metalavas with sedimentary interunits of variable thicknesses. Its lower contact with the major Tuddal volcanite body is considered conformable, but an angular unconformity is also possible. The change from the felsic Tuddal volcanism to the basaltic Vemork volcanism is interbedded as the flow-banded Skardfoss and Homvatnet metarhyolite members occur in the lower part of the Vemork formation. Zircon U-Pb dating of the Skardfoss metarhyolite constraints the beginning of deposition of the Vemork formation at around 1495 ± 2 Ma. In the north, the Vemork basalts are overlain by volcaniclastic arkosite and quartzites of the Vindeggen group, whereas in the south the Venutan member of diverse felsic volcanite rocks occupies the uppermost part of the Vemork group. It is mingled with the Vemork basalts, but seems to pass via a felsic vocaniclastic conglomerate and arkosites to the Gausta quartzite of the Vindeggen group. Because of the Sveconorwegian deformation and metamorphism and uneven outcrop distribution individual Vemork units cannot be followed laterally for any longer distances and vertical sections are incomplete. Consequently, the lithostratigraphy for the Vemork formation can be established only tentatively. In the Frøystaul type section, the 2 km thick sequence comprises at least 10 basaltic units separated by epiclastic units with variable amounts of both felsic and mafic volcanic material. The nature of the upper boundary of the Vemork formation with the quartzite-dominated Vindeggen group is problematic as the rocks within the contact zone are intensely foliated and mostly unexposed. Sudden dying of volcanism and input of extrabasinal epiclastic material into the lower part of the Vindeggen group indicate that a significant tectonic change took place at the Vemork/Vindeggen boundary