Juho Kirs

Radiogenic isotopes of the Estonian and Latvian rapakivi granite suites: new data from the concealed Precambrian of the East European Craton

Abstract The Precambrian crystalline bedrock of the Baltic countries is covered under Phanerozoic sedimentary rocks that flank the Fennoscandian Shield in the south. The covered bedrock consists mainly of Palaeoproterozoic medium- to high-grade metamorphic rocks and unmetamorphosed rapakivi granites and related mafic rocks (mainly gabbros and anorthosites). Our UPb zircon data show that small rapakivi granite plutons in Estonia are 1630 Ma old and felsic and mafic rocks from the Riga batholith of Latvia and westernmost Estonia are 1580 Ma old. The Estonian plutons have ϵNd (1630 Ma) values ranging from −0.5 to −2.5. The mafic and felsic rocks of the Riga batholith have ϵNd (1580 Ma) values between +0.3 and −0.6, except for a pervasively altered silicic volcanic rock on the northern flank of the batholith with an ϵNd value of −4.6. Initial 87 Sr 86 Sr ratios of the mafic rocks are of the order of 0.7036 to 0.7037 and conform to the evolution of average subcontinental mantle. The Pb isotopic compositions of the felsic and mafic rocks (including the low-ϵNd prophyry) are relatively radiogenic with single-stage μ-values of the order of 8.2. The isotopic characteristics of the Estonian and Latvian rapakivi granites are similar to those of the classic rapakivi granites of southern Finland. Our data suggest that the felsic rocks of the Estonian and Latvian rapakivi suites were derived from Palaeoproterozoic (near-chondritic Nd, relatively high-U Pb ) protoliths. The data also imply that the lower crust and upper mantle in this area are devoid of a major Archaean component and that the lithosphere may become more juvenile southward from the Fennoscandian Shield.

ILLITIZATION OF EARLY PALEOZOIC K-BENTONITES IN THE BALTIC BASIN: DECOUPLING OF BURIAL- AND FLUID-DRIVEN PROCESSES

The mineralogical characteristics of Ordovician and Silurian K-bentonites in the Baltic Basin were investigated in order to understand better the diagenetic development of these sediments and to link illitization with the tectonothermal evolution of the Basin. The driving mechanisms of illitization in the Baltic Basin are still not fully understood. The organic material thermal alteration indices are in conflict with the illite content in mixed-layer minerals. The clay fraction of the bentonites is mainly characterized by mixed-layered illite-smectite and kaolinite except in the Upper Ordovician Katian K-bentonites where mixed-layer chlorite-smectite (corrensite) occurs. The variation in expandability plus other geological data suggest that the illitization of Ordovician and Silurian K-bentonites in the Baltic Basin was controlled by a combination of burial and fluid driven processes. The illitization in the south and southwest sectors of the basin was effected mainly by burial processes. The influence of the burial process decreases with decreasing maximum burial towards the central part of the basin. The advanced illitization of the shallowburied succession in the north and northwest sectors of the basin was enhanced by the prolonged flushing of K-rich fluids in relation to the latest phase of development of the Scandinavian Caledonides ≈420–400 Ma.

Late Neoproterozoic Baltic paleosol: Intense weathering at high latitude?

The Neoproterozoic was a time when repeated global cooling events, interrupted by supergreenhouse phases, preceded environmental change toward a modern oxygen-rich atmosphere and the eventual emergence of animal life. Cyclically increased atmospheric CO2 levels intensified weathering of continental silicates, but little is known about the influence of Neoproterozoic climates on soil morphogenesis, which acts as a direct proxy of conditions at the time of formation. However, being typically fragmented in time and space, these mineralic soils (paleosols) had a low preservation potential. An exceptionally well preserved Neoproterozoic deeply weathered paleosol on the Baltica paleocontinent provides new information on weathering during the Precambrian. The kaolinite–Fe-oxyhydroxide composition of this paleosol is indicative of intense weathering in a stable landscape at a time when Baltica was positioned between intermediate to high southern latitudes. It is plausible that this paleosol developed over long steady-state weathering under temperate climatic conditions, or alternatively during an intensified weathering event triggered by elevated CO2 levels and transient greenhouse climatic conditions at the termination of Ediacaran glaciations, or possibly the Ediacaran Shuram-Wonoka isotope event.

Detrital zircon ages of Cambrian and Devonian sandstones from Estonia, central Baltica: a possible link to Avalonia during the Late Neoproterozoic

Detrital zircon U–Pb ages of Lower Cambrian and Middle Devonian have been determined for sandstone in Estonia through LA-ICP-MS (Nu instruments, Wrexham, UK). Both sandstones have a similar zircon age spectrum with distinct age clusters that reflect the basement geology of Baltica, i.e. 2800–2700 Ma (Kola–Karelia), 1900–1700 Ma (Svecofennian), 1600–1500 Ma (Rapakivi) and 1200–1000 Ma (Sveconorwegian). Noteworthy is a cluster at 750–550 Ma, because rocks of such age are absent within the core of Baltica. The present results suggest a possible link between Baltica and Avalonia/Cadomia during the Late Neoproterozoic.

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.