Hannu Huhma

Svecofennian detrital zircon ages: implications for the Precambrian evolution of the Baltic Shield

Metasediments intruded by 1.90-1.87 Ga old plutonic rocks form the oldest major Proterozoic crustal component in the Svecofennian Domain of the Baltic (Fennoscandian) Shield. Their NdTDM model ages and conventional multigrain zircon UPb ages between 2.4 and 2.1 Ga have previously been interpreted either as mixing ages between ∼ 1.9 Ga old juvenile materials and a minor Archaean component, or as actual rock and protolith ages. To resolve the ensuing controversy, 120 individual detrital zircons from Svecofennian metasediments in Sweden and Finland were analysed using the SHRIMP ion microprobe. The oldest materials in this array are a 3.44 Ga old zircon from the Tampere Schist Belt in Finland and a 3.32 Ga old crystal from southeastern Sweden. About 30% of the analysed crystals are 2.97-2.60 Ga old, while ∼ 65% have ages between 2.12 and 1.88 Ga. Thus there is no evidence of 2.6-2.1 Ga old protoliths, but the age range of the Proterozoic zircons indicates that a major area of 2.1-1.9 Ga old crust was in erosional position 1.9 Ga ago. This implies that the formation of Palaeoproterozoic crust in the Baltic Shield or its one-time close neighbourhood must have commenced 100–200 Ma earlier than hitherto assumed. In conjunction with previously obtained isotopic data, the youngest detritus ages of the present study constrain the age of Svecofennian sedimentation. It can also be concluded that the Archaean zircons found in quartzites from southern Sweden may have been derived from source areas to the southwest of the central-Svecofennian marine depositional basin, the so-called Bothnian Basin, separating southern Sweden from the Archaean craton in the northeastern part of the Shield.

Sm?Nd and Pb isotopic study of mafic rocks associated with early Proterozoic continental rifting: the Perpohja schist belt in northern Finland

The Peräpohja schist belt in northern Finland rests unconformably on Archaean granitoids, and marks the early stages of Proterozoic crustal evolution in the Fennoscandian (Baltic) shield. 2440 Ma old layered mafic intrusions predate the supracrustal , and ca. 2200 Ma old sills of the gabbro-wehrlite association intrude the lowest quartzites and volcanics (Runkaus) of the sequence. The Sm-Nd mineral isochron of the Penikat layered intrusion gives an age of 2410±64 Ma. The initial ɛNd-values of the Penikat intrusion (ɛNd(2440) = −1.6) and the Runkausvaara sill (ɛNd(2200) ≈ 0) suggest that these mafic magmas were contaminated by older crustal material. The Sm-Nd and Pb isotopic results on the 2.44–2.2 Ga old Runkaus volcanics indicate mobility of Pb, fractionation of Sm/Nd during “late” greenschist facies metamorphism, and crustal contamination. The Pb-Pb data provide an age of 1972±80 Ma with a high initial 207Pb/204Pb ratio (μ1 = 8.49), while scattered Sm-Nd data result in an imprecise age of 2330±180 Ma, with an initial ɛNd-value of about zero. Secondary titanite gives an U-Pb age of ca. 2250 Ma. The Jouttiaapa basalts, in contrast, ascended from the mantle without interaction with older crust. These LREE depleted tholeiites mark a break in continental sedimentation, and yield a Sm-Nd age of 2090±70 Ma. Their initial ɛNd = + 4.2 ±0.5 implies that the subcontinental early Proterozoic mantle had been depleted in LREE for a long period of time. The first lava flows are strongly depleted in LREE, suggesting that their source was significantly more depleted than the source of mid-ocean ridge basalts today.

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.

Temporal constraints on the Paleoproterozoic Lomagundi-Jatuli carbon isotopic event

The Paleoproterozoic Lomagundi-Jatuli positive 13C excursion in sedimentary carbonates represents an event whose magnitude and duration is unique in Earth history, although precise absolute chronology of this event remains poorly constrained. In northeastern Fennoscandia, an 1300-m-thick sedimentary-volcanic succession of the Pechenga Greenstone Belt records decline of this isotopic excursion. Zircons from sedimentary rocks that occur within the decline have yielded 207Pb/206Pb dates at 2058 ± 2 Ma (±6 Ma including U decay constant uncertainties) and provide the first maximum age constraint on the termination of the Lomagundi-Jatuli event. Combined with existing constraints, these data indicate an 140 m.y. interval characterized by 13C-rich carbonate accumulation.

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.

A Sm-Nd isotopic study of the South Harris Igneous Complex, the Outer Hebrides

Sm-Nd geochronology may be used to bracket the age of metamorphism in rocks which are difficult to date by other methods. By coupling whole rock Sm-Nd analyses of the principal members of the South Harris Igneous Complex, with Sm-Nd mineral isochrons on two anothositic gabbros, the age of granulite facios metamorphism has been defined.Whole rock analyses of three pairs of closely spaced samples of the anorthosite give consistent ages averaging 2.18±0.06 Gyr, but in general the data from the anorthosite do not define an isochron as a result of variable contamination of the evolving magma chamber. Whole rock data on the tonalite indicate that it is younger than 2.06 Gyr; its mean TCHUR age is 1.86±0.05 Gyr.Garnet-pyroxene-amphibole-plagioclase mineral isochrons on two anorthosite samples give identical 1.87±0.04 Gyr ages which date cooling after the high pressure granulite facies metamorphism. Together with the tonalite whole rock data this defines the age of that metamorphism and confirms Dearnleys original assignment of an early Laxfordian age.

Provenance of early proterozoic and archaean metasediments in finland: a SmNd isotopic study

Abstract SmNd isotopic analyses of early Proterozoic and Archaean clastic metasediments from Finland are presented. They have been used to calculate crustal residence ages and to constrain the sedimentary provenance. The mean crustal residence age ( T DM ) for the Svecofennian metagreywackes in the Tampere schist belt is 2.22 Ga, which is about 0.3 Ga in excess of the assumed stratigraphic age. This indicates that the bulk of the sediments were eroded from newly formed crust. Their major source was most likely Svecofennian orogenic material, mixed with a small amount of Archaean-derived detritus. The precursors of many granitoids in the Svecofennian terrain also had similar short crustal residence times. The Kalevian metasediments, which were deposited 2.05-1.9 Ga ago on the edge of the Archaean craton in eastern and northern Finland, have a mean T DM of 2.41 Ga, which is 0.4–0.5 Ga more than the inferred stratigraphic age. Hence their source was, on average, older than that of the Svecofennian metasediments investigated, but definitely not purely Archaean. Neither could crust similar to the bulk of the 1.9 Ga old Svecofennian terrain have been the only source. Various sources contributed at least locally, but the bulk of the sedimentary material probably had a common provenance. Major sources, consistent with existing isotopic data, could have been 2.1 Ga old Jatulian rocks, the c. 2.0 Ga old Granulite Belt in northern Finland, and/or the Svecokarelian early orogenic (arc) terrain mixed with Archaean-derived Nd. The method and data do not allow a distinction to be made between the possible alternatives. The T DM model ages for two analysed Archaean metasediments are slightly over 3 Ga, which is about 0.3 Ga in excess of their assumed stratigraphic age, thus providing no evidence for crust much older than 3 Ga in this area.

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.

P–T–t development of Archaean granulites in Varpaisjärvi, Central Finland: II. Dating of high-grade metamorphism with the U–Pb and Sm–Nd methods

Exposed blocks of lower crustal Archaean granulites separated by Palaeoproterozoic faults occur near the western boundary of the Karelian craton in central Finland. The southwesternmost granulite block in the study area, the Jonsa block, differs from other granulites by having younger zircon U–Pb and Sm–Nd TDM model ages. The U–Pb ages on zircons and monazites from leucosomes and mesosomes of mafic and pelitic granulites in the Jonsa block are ca. 2.63 Ga, which is interpreted as the age of granulite metamorphism. Igneous enderbites, which make up a considerable part of the bedrock in granulite blocks, are ca. 50 Ma older. Outside the Jonsa block both conventional and ion probe zircon ages of mesosomes of migmatitic mafic granulites are 3.2–3.1 Ga, which are the protolith ages. Sm–Nd TDM model ages of the mafic granulites are 2.7–2.9 Ga in the Jonsa block but ca. 3.2 Ga in the other granulites. The age differences are interpreted to represent terrane accretion that juxtaposed 3.2 Ga rocks next to younger rocks. All granulites were metamorphosed in similar PT conditions at 2.63 Ga; granulite metamorphism affected the whole area but was not able to reset zircons in older rocks. In all blocks the Sm–Nd garnet-whole rock ages are younger, ranging from 2.48–2.59 Ga. This reflects relatively low closure temperature of the Sm–Nd system in garnet in these rocks. The U–Pb age of a Palaeoproterozoic dolerite which cuts the granulites is 2.3 Ga, close to the K–Ar age of amphibole in a retrogressed fracture.

Archean zircons from the mantle: The Jormua ophiolite revisited

The Jormua ophiolite represents seafloor from an Early Proterozoic ocean to continent transition zone that mainly consisted of Archean subcontinental lithospheric mantle. These mantle rocks were exhumed as a result of extreme crustal thinning and detachment faulting in association with the opening of the Svecofennian Sea. At the prerift stage of continental breakup, residual lithospheric peridotites became intruded by alkaline melts that formed a diverse suite of clinopyroxenite and hornblendite-garnetite dikes and veins. These Proterozoic dikes contain Archean zircon xenocrysts inherited from deeper sources of the continental mantle. The relatively large spread of 2 0 7 Pb/ 2 0 6 Pb ages between 3106 and 2718 Ma suggests that the zircons are derived from a variety of source rocks. Some xenocrysts have U and Th abundances comparable to zircon in common alkali basalts, whereas a population of Archean high-U and high-Th zircons is similar to those described from mica-amphibole-rutile-ilmenite-diopside-bearing mantle xenoliths. These are the oldest zircons found from upper-mantle rocks anywhere and imply that the Fennoscandian cratonic root was relatively cool and strongly metasomatized by ca. 3.1 Ga.