Raimo Lahtinen

Isotopic and geochemical constraints on the evolution of the 1.93-1.79 Ga Svecofennian crust and mantle in Finland

The central part of the Svecofennian domain in Finland is characterized by thick lithosphere with crustal thickness up to 60–65 km. Major collisional events at 1.91-1.90 Ga and 1.89 Ga and a thrusting event at 1.86-1.84 Ga, the latter due either to continuation of shortening or to a separate intracrustal collision, are considered the major causes of crustal thickening. The origin of the Svecofennian domain has been formerly attributed to mixing of depleted mantle melts with variable amounts of Archaean crustal component via subduction, but the new data and reinterpretation of older data imply that, besides juvenile crust, the Svecofennian domain also contains older Palaeoproterozoic crustal components. No Archaean component is found in the 1.93-1.91 Ga gneissic tonalites and related felsic volcanics (eNd (t) + 1 – +4) which occur in the Savo Belt (SB), adjacent to the Archaean craton. The depleted mantle model ages vary from 1.94 Ga to 2.33 Ga with most values ≤2.0 Ga. The analysed syntectonic (post-thickening) granitoids (≤1.89 Ga) in the SB exhibit positive eNd (t) values ranging from +2.8 to +3.0 pointing to a juvenile source. The hypersthene granitoids are similar in age but show epsilon values around −1, which are only slightly lower than found in EM-derived mafic rocks. The isotopic and geochemical data for the Central Finland Granitoid Complex and Tampere Schist Belt indicate the occurrence of evolved thick crust (≥2.0 Ga) and associated lithospheric mantle already at 1.91 Ga. The southernmost part of the Southern Svecofennian in Finland also shows evolved-type crust (≥2.0 Ga). A tentative model for the development of crust and mantle is presented. It includes the occurrence of continental nucleus surrounded by more juvenile island arc accretions. Two successive collisional events thickened both the crust and the subcontinental lithospheric mantle, with the greatest relative amount of thickening occurring in the mobile zones. Convective removal of the lowermost parts of thickened lithosphere caused an upwelling of hot asthenosphere, increase in the geotherm, and melting of both the enriched subcontinental lithospheric mantle and the convective depleted asthenosphere. Mafic magmatism during the post-thickening stage (≤1.89 Ga) thus varied from dominantly enriched- to dominantly depleted-mantle melts and was associated with variable crustal contamination. The thickening of the lithosphere in the central part of the Svecofennian domain was not followed by large extension, suggesting that the thickness was already great 1.88-1.80 Ga ago. This, together with the thick high-velocity lower crust, could have provided the isostatic balance needed to stabilize the thick crust.

Contrasting source components of the Paleoproterozoic Svecofennian metasediments: Detrital zircon U–Pb, Sm–Nd and geochemical data

Abstract The Paleoproterozoic Svecofennian Orogen in the Fennoscandian shield can be regarded as one entity or as a collage of several accretionary units. To test for the possible occurrence of a suture zone dividing the Svecofennian Orogen into Central and Southern parts, we report new ion microprobe data on clastic zircons of nine samples of metasediments. Whole-rock Sm–Nd and geochemical data were acquired to characterize the source components of the metasediments. The Central Svecofennian metasediments in Finland can be divided to lower and upper types with inferred deposition ages at 1.92–1.91 and 1.90–1.88 Ga, respectively. Both types are immature and show a derivation from an unweathered orogenic source. The lower Central Svecofennian metasediments show a dominant Proterozoic age population at 1.93–2.02 Ga and a variation in eNd (1.9) values from about −3 to −0.5, which is mainly explained by variable amounts of Archean detritus (30–45%). The upper Central Svecofennian metasediments have eNd (1.9) at about 0, similar to values obtained from adjacent volcanic and plutonic arc rocks. One meta-arkose is tentatively classified as a molasse-type sediment with a source dominated by earlier Svecofennian sediments. Southern Svecofennian metasediments can be divided into mature and less mature types with dominant deposition ages at 1.90–1.88 Ga and eNd (1.9) values from −3 to +1. Mature quartzites seem to occur at least in two stratigraphic levels where the upper quartzites have maximum deposition ages at 1.87–1.86 Ga. A characteristic feature of some Southern Svecofennian metasediments is a dominant Proterozoic age population at 2.0–2.1 Ga. Coincident deposition of mature and immature sediments and volcaniclastic rocks indicate that different sources, weathered and non-weathered ‘basement’, and syn-depositional volcanic centers, produced simultaneously detritus to a large subsiding basin. Mature pelites and quartzites suggest a long residence time and stable passive margin or cratonic environment. The tectonic setting and possible provenance areas for the Central and Southern Svecofennian metasediments are also discussed. As a conclusion, the metasediments from the Central and Southern parts of the Svecofennian Orogen show different lithological characteristics and have contrasting source components. The presented isotope data indicate a different evolution of these parts prior to 1.89 Ga and the existence of a suture zone between them as proposed in earlier studies.

The Svecofennian orogen: a collage of microcontinents and island arcs

Abstract Based on an integrated study of geological and geophysical data, a tectonic model for the Palaeoproterozoic evolution of the Svecofennian orogen within the Fennoscandian Shield at the northwestern corner of the East European Craton is proposed. The Svecofennian orogen is suggested to have formed during five, partly overlapping, orogenies: Lapland-Savo, Lapland-Kola, Fennian, Nordic and Svecobaltic. The Svecofennian orogen evolved in four major stages, involving microcontinent accretion (1.92-1.88 Ga), large-scale extension of the accreted crust (1.87-1.84 Ga), continent-continent collision (1.87-1.79 Ga) and finally gravitational collapse (1.79 and 1.77 Ga). The stages partly overlapped in time and space, as different processes operated simultaneously in different parts of the plates. In the Lapland-Savo and Fennian orogenies, microcontinents (suspect terranes) and island arcs were accreted to the Karelian microcontinent, which itself was accreting to Laurentia in the Lapland-Kola orogeny. The formation of the Svecofennian orogen was finalized in two continental collisions producing the Nordic orogen in the west (Fennoscandia-Amazonia) and Svecobaltic orogen in the SSW (Fennoscandia- Sarmatia). The collisions were immediately followed by gravitational collapse.

Palaeoproterozoic accretionary processes in Fennoscandia

Abstract Accretionary processes contributed to major continental growth in Fennoscandia during the Palaeoproterozoic, mainly from 2.1 to 1.8 Ga. The composite Svecofennian orogen covers c. 1×106 km2 and comprises the Lapland–Savo, Fennia, Svecobaltic and Nordic orogens. It is a collage of 2.1–2.0 Ga microcontinents and 2.02–1.82 Ga island arcs attached to the Archaean Karelian craton between 1.92 and 1.79 Ga. Andean-type vertical magmatic additions, especially at c. 1.89 and c. 1.8 Ga, were also important in the continental growth. The Palaeoproterozoic crust is the end product of accretionary growth, continental collision and orogenic collapse. Preserved accretional sections are found in areas where docking of rigid blocks has prevented further shortening. The Pirkanmaa belt represents a composite accretionary prism, and other preserved palaeosubduction zones are identified in the Gulf of Bothnia and the Baltic Sea areas. In the southern segment of the Lapland–Savo orogen collision between the Archaean continent (lower plate) and the Palaeoproterozoic arc–microcontinent assembly (upper plate) produced a special type of lateral crustal growth: the Archaean continental edge decoupled from its mantle during initial collision and overrode the arc and its mantle during continued collision.

Archaean-Proterozoic transition: geochemistry, provenance and tectonic setting of metasedimentary rocks in central Fennoscandian Shield, Finland

The central part of the Fennoscandian Shield in Finland is composed of the Palaeoproterozoic Svecofennian domain and the Archaean Karelian craton with a Palaeoproterozoic allochthonous and autochthonous cover. A cryptic suture separating these areas and another tentative suture dividing the Svecofennian into central and southern parts have been proposed. The chemical composition of sedimentary rocks (N=300) within the study area, including the effects of palaeoweathering, hydraulic sorting, depositional environment and post-depositional processes, have been studied in order to delineate sediment source components. The main proposed source components for the Archaean sedimentary rocks are weathered 3.0–3.2 Ga greenstone+granite±TTG and local 2.7 Ga sources. Autochthonous 2.2–1.9 Ga cover rocks were mainly derived from a mixture of chemically weathered palaeosol (2.2–2.35 Ga), sedimentary rocks derived from the palaeosol, and mafic dykes and plateau volcanics (mainly 2.2–2.1 Ga) although in places locally derived non-weathered Archaean sources dominated. Archaean crust and 2.0–1.92 low-K bimodal rocks from a primitive island arc are the proposed source for the allochthonous Western Kaleva cover rocks. These formed in a subsiding foredeep during initial collision from orogenic detritus in the same oblique collision zone. The central Svecofennian sedimentary rocks can be divided into local arc-derived rocks (≤1.89 Ga) and older (≥1.91 Ga) rocks from a mixture of Western Kaleva sources and a 1.91–2.0 Ga mature island arc/active continental margin source. Rifting followed by increased subsidence during initial collision in the NE and subsequent arc reversal caused rapid erosion from the mountain belt, exposing diverse source compositions as seen in the large variation of Th/Sc (2–0.5), and deposition into an oblique hinterland basin further developing into a subduction related foredeep. Mature greywackes from the southern Svecofennian in the study area resemble passive margin sediments with a source dominated by inferred alkaline-affinity complexes and Archaean rocks. Less mature rocks also occur and had sources dominated either by island arc/active continental margin rocks or local picritic rocks. In the sedimentary record the Archaean–Proterozoic transition up to 2.1 Ga was dominated by input of mainly mafic plateau-type volcanic contribution to the Archaean detritus. Palaeoproterozoic sediments having a crustal component (≤2.1 Ga) show higher Th/Sc, Th/Cr, and lower Sm/Nd and Eu/Eu* relative to the Archaean rocks but locally low Th/Cr ratios complicate the situation. Ba depletion relative to K, Rb and Th is a characteristic feature of the sedimentary rocks of the central Fennoscandian Shield indicating high amounts of Ba lost from the clastic record during 2.3–1.9 Ga and further recycled back to the mantle forming a subduction component and an enriched mantle component. Ba depletion seems to have been especially characteristic of chemical weathering during 2.35–2.2 Ga under CO2-rich and low-O2 atmosphere. Whether this strong Ba depletion is characteristic of the Archaean–Proterozoic transition and quiet supercontinent stages in general remains to be determined.

High-resolution X-ray computed microtomography: A holistic approach to metamorphic fabric analyses

An intrinsic limitation of studying microstructures in thin section is that their spatial (three-dimensional, 3-D) distribution, shape, and orientation have to be inferred by combining 2-D data from different sections. This procedure always involves some degree of interpretation that in some cases can be ambiguous. Recent advances in high-resolution X-ray computed microtomography have made possible the direct imaging in 3-D of volumes of rock to centimeter scale. This rapidly evolving technology is nondestructive and provides a holistic approach of microstructural analysis that eliminates interpretative procedures associated with 2-D methods. Spatial images can be generated through any part of the rock sample and used as virtual petrographic sections. Our application of this technique to an oriented drill core sample from the classic Orijarvi metamorphic region of southern Finland reveals a number of in situ 3-D aspects, including: (1) the spatial distribution and shape of andalusite porphyroblasts, (2) the geometry of a matrix foliation anastomosing around the porphyroblasts, (3) a millimeter-scale compositional layering that controlled the oscillation of porphyroblasts and sulfide mineralization, and (4) distinct inclusion trail patterns characterizing porphyroblast core versus rim zones. The combined data indicate that the steeply dipping bedding-subparallel foliation that characterizes the Orijarvi area formed by bulk north-south crustal shortening and associated vertical stretching.

Three-dimensional textural and quantitative analyses of orogenic gold at the nanoscale

Ore textures provide direct clues for tracking ore-forming processes. In this regard, most of our knowledge is generally based on two-dimensional (2-D) image analyses, leaving a considerable gap in comprehending three-dimensional (3-D) in-situ textural settings. Recent advances in lab-based and synchrotron radiation–based X-ray computed microtomography and nanotomography have made it possible to visualize and quantify rock volumes in a 3-D space. In this study, we first analyzed microscale textures in oriented drill cores from the world-class Suurikuusikko orogenic gold deposit of northern Finland using lab-based X-ray computed microtomography. The technique revealed a kinematic history and a number of in-situ 3-D quantitative aspects including size, shape, spatial distribution, and geometrical orientation of arsenopyrite and pyrite in a highly altered host-rock matrix. For 3-D nanotomography, the experimental procedure known as holotomography was adopted. Individual arsenopyrite crystals were separated and scanned with voxel sizes ranging from 50 nm to 150 nm using the X-ray nanoprobe beamline (ID16B) at the European Synchrotron Radiation Facility, France. This ultrahigh-resolution technique illustrated the 3-D distribution of micron- to nanoscale gold inclusions, mostly associated with primary rutile or along secondary microfractures inside arsenopyrite. The workflow, from micro- to nanotomography, outlined in this study offers an indispensable new technique in quantifying and characterizing 3-D textural settings of ores, which is otherwise impossible with conventional 2-D imaging devices. The method can also be highly useful in evaluating the amenability of ores to treatment with different processing options.

Evolution of the Bedrock of Finland: An Overview

The Precambrian bedrock of Finland forms the core of the Fennoscandian Shield. The Archean area includes exposed Archean rocks (23%) and Archean crust overlain by Paleoproterozoic supracrustal and plutonic rocks (30%). The Proterozoic area is composed of the Paleoproterozoic Svecofennian Province (41%) intruded by rapakivi granites (4%).