Väino Puura

Rapakivi-granite–anorthosite magmatism — a way of thinning and stabilisation of the Svecofennian crust, Baltic Sea Basin

In the Palaeoproterozoic, a 55–80 km thick layer of crust was formed in the Baltic Sea region during the Svecofennian orogeny at 1.9–1.8 Ga. Today, the remaining crustal thickness is 50–65 km. In the marginal parts of the 1,000,000 km2 Svecofennian Domain, the Moho depth reached 40–75 km, which exceeds the 40 km crustal thickness of the southwestern edge of the Karelian Archaean Domain to which Svecofennia was accreted. The 1.65–1.50 Ga rapakivi magmatism of the Fennoscandian Province was limited to the Svecofennian Domain. The rapakivi igneous structures are confined to areally isolated subprovinces, which each have distinct 20–60 Ma long age spans of formation. The petrologic sequences of the subprovinces are alike, although similar petrological events occur at different times in the various subprovinces. The internal structure of a subprovince generally consists of a main igneous polyphase unit in a central position, with smaller felsic intrusions, as well as mafic dike swarms, spread over the peripheries of the subprovince. The rapakivi magmatism started in juvenile crust which was in a late stage of erosional thinning, 150–300 Ma after the period of maximal thickening. The most extensive rapakivi igneous activities were associated with crustal thinning down to 45–50 km. As a result, the thinner marginal parts of Svecofennia and the large interior rapakivi subprovinces were of similar thicknesses as the crust. The primary thickness of the original crust was maintained only in areas void of rapakivi magmatism. No major events destroyed the Svecofennian and rapakivi-related crustal structures subsequent to emplacement. Thus, it can be concluded that the extensive rapakivi igneous activity substantially thinned and stabilised the overthickened portions of the Svecofennian crust.

Rapakivi-related basement structures in the Baltic Sea area; a regional approach

Abstract The largest massifs of rapakivi granites and related rocks in the western part of the East European Craton are located within the junction area of the Baltic Sea and the Gulfs of Bothnia, Finland and Riga, and along the Gulf of Finland. The rapakivi massifs are of Palaeoproterozoic to Mesoproterozoic (Subjotnian) age, ranging from about 1.67 to 1.5 Ga. In the framework of the Palaeoproterozoic basement tectonics, all major and minor occurrences of rapakivi magmatism are areally limited to within the boundaries of the Svecofennian juvenile crustal domain and its frontiers. The most voluminous of the intrusive bodies are located within the originally thickest (55–65 km) primary part of Svecofennia, where they are coupled with crustal thinning to 40–45 km and mafic underplating anomalies. Only minor rapakivi bodies have intruded the originally thinner (40–45 km) peripheral Palaeoproterozoic crust. The largest igneous subprovinces, namely the Wiborg and Riga-Åland rapakivi massifs, are furthermore connected to specific regional Bouguer gravity anomalies. These anomalies exhibit complex patterns due to the rapakivi-related reworking of the upper mantle and the crust. The bimodal rapakivi and related upper crustal magmatism was systemised in such a way that satellite bodies with minor stock and dike suites became located concentrically around the main igneous bodies. Considering the different ages and locations of the igneous suites, an internal subprovincial distribution occurs in the province. The time of formation for each subprovincial cluster is no longer than 50 m. y. The timing of birth and extinction of the different subprovinces rarely coincide with each other.

Settings of Meteorite Impact Structures in the Svecofennian Crustal Domain

The Svecofennian Domain, located on the western part of the East European Craton, is a uniform crustal segment with an age of the basement of ~1.9 Ga. Together with younger (1.5 – 1.8 Ga) transition zones it occupies an area of ~1.56 × 106 km2. The domain unites shield and platform areas including the submerged Baltic Sea. It includes 22 confirmed, 6 possible, and tens of supposed impact structures that survived recurrent erosional epochs during the long geological history. Signatures of impact-induced tectonics depended much on the syn-impact structure of the target, i.e., presence and thickness of sedimentary deposits covering the basement. Due to differentiated tectonic movements, post-impact scenarios of structures of even the same age differ from place to another within the domain. The present paper analyses the up to 1.2 Ga long post-impact histories of impact structures in accord with the geological development of the domain, i.e. past vertical movements of the crust, which predicted formation and destruction of the sedimentary cover, and variable distribution of shield and platform areas. A great number of at least small- and medium-size craters, which have been located within the past sedimentary cover on top of the crystalline rocks of the Fennoscandian Shield and sediments of the Russian Platform, have been eroded completely or partially. However, between the time intervals with low cratering rates, the Early Palaeozoic is extremely rich in survived structures. It gives hints for unequal erosion history within the Domain and the existence of many hidden (buried) structures. Therefore, we can expect that there is still a large number of partially eroded craters, presently hidden under Quaternary, and also under or within Phanerozoic deposits, to be discovered in the future.

Åvike Bay — a 10 km Diameter Possible Impact Structure at the Bothnian Sea Coast of Central Sweden.

Avike Bay is a 270° degree wide near-circular, 114 m deep bay on the Swedish coast of the Bothnian Sea, northeast of Sundsvall. The structure has a diameter of about 10 km. It was classified as a probable impact structure because of its extraordinary circular topography in the overwiew of impact structures in Fennoscandia. Recent studies lend further support to this interpretation. The structure has a submarine central mound, which is elevated some 40 m above the adjacent sea floor. It has a very distinct tangential and radial on-shore fracture pattern as seen in the topographic map. Along the southwestern shore of the Bay, an enigmatic quartzite breccia of unknown age occurs as part of a larger outcrop of polymict breccia with clasts of crystalline rocks and quartzite of unknown age. In thin section, planar fractures can be observed in quartz and feldspar grains. A detailed investigation showed that in a few cases the quartz grains contained microdeformation features closely resembling PDFs.

A complex model of Mesoproteozoic sedimentary and igneous suites in a graben setting north of Gotland, Baltic Sea

Abstract Marine geological and geophysical investigations have revealed the complex nature of the deep Mesoproterozoic Strombus and Admete basins in graben settings. The Strombus Basin extends in a northwestern direction from northern Gotland to the Landsort Trench. In the northeast a less prominent tectonic basin, named the Admete Basin, flanks it. The Jotnian sandstone layer forms the bulk of the basin infill. In the Strombus Basin the thickness of it increases from 400 to 1600 m towards the southwest, but is stable c. 500 m in the Admete Basin. Regional gravity surveying has revealed distinct gravity lows confined to the basins. According to the geophysical modelling, the Strombus Basin gravity low fits best with the model of the 500 to 1500 m thick Jotnian sandstone layer underlain by an approximately 5 km thick minor rapakivi pluton. The pronounced linear magnetic high, which conforms to the western and southwestern boundaries of the basin, can be interpreted as a large mafic dike or sheet connected to the fault delimiting the basin. A distinct magnetic high of about +800 nT indicates the presence of highly magnetic rocks northeast of the Strombus Basin. In their origin and age of the mafic intrusions may be related to the Subjotnian rapakivi-related gabbro-anorthosite. Alternatively, they may be related to the Jotnian mafic intrusions. The grabens filled with Jotnian have inherited their positions from deep-seated rapakivi intrusions. They are probably derived from the extensional faulting causing the Postjotnian diabase magmatism.

PETROGRAPHY AND MINERALOGY OF THE ATTARAT UM GHUDRAN OIL SHALE, CENTRAL JORDAN

In the authors’ recent papers on oil shale chemical composition and geochemical variability, as well natural gamma radiation, the significantly variable layered lithological structure of the up to 90 m thick oil shale (OS) unit of the Muwaqqar Chalk-Marl Formation (MCM), Central Jordan, was described in detail for the first time. In this work, the original results of detailed comparative petrographic and mineralogical studies of the unit and its separate layers are presented. The studied drill cores represent an area about 73 km out of the large Attarat Um Ghudran (AUG, Attarat) deposit Maastrichtian in age. Oil shale with primary depositional structure and texture dominates. Laterally, layers, beds and interbeds of uniform composition and conditions of accumulation continue over the exploration area. Significant layering-dependent variations of chemical and mineral composition in the vertical succession of the oil shale unit occur. Petrographically, the dominating thick finely laminated uniform siliceous-carbonate mudstone (MS) oil shale intercalates with variable intervals of grain-bearing oil shale, in which interbeds of proper mudstone alternate with laminae, lenses and thin interbeds of wackestone (WS). The dominating biogenic compounds are: (i) calcite as < 5 mμ micrite forming the groundmass mudstone, and both micrite of the matrix and > 5 mμ to 1 mm grains – skeletal particles (shells and their broken fragments) in wackestone, (ii) silica as < 5 mμ particles belonging to the groundmass of mudstone and wackestone matrix, (iii) organic matter (kerogen) in the mudstone and matrix of wackestone, (iv) phosphate skeletal fragments in certain interbeds of wackestone and very fine apatite in groundmass. The clay minerals are the only possible terrigenous admixture in certain intervals. In accordance with the negative correlation between CaO and SiO2 the layers of * Corresponding author: e-mail vaino.puura@ut.ee Petrography and Mineralogy of the Attarat Um Ghudran Oil Shale, Central Jordan 111 the dominant calcite or silica (quartz, tridymite, cristobalite) occur, whereas in MgO-rich barren interlayers dolomite may prevail and in certain P2O5enriched beds/interbeds apatite or in Al2O3-enriched layers clay minerals occur. In the vertical succession of the oil shale unit, also quantitative proportions of principal mineral and organic (kerogen) components vary a lot. Distinct thin interlayers of carbonates with low organic matter (OM) and silica contents reflect temporary breaks and imminent recoveries of oil shale accumulation. The data serve for a further assessment and commercial development of oil shale deposits.