Early Svecofennian Research Articles

Early Svecofennian rift-related magmatism: Geochemistry, U-Pb-Hf zircon isotope data and tectonic setting of the Au-hosting Uunimäki gabbro, SW Finland

We characterise the geochemistry, zircon Lu-Hf composition, age and the structure of the Uunimäki gabbro (UGB) in south-western Finland to improve the understanding of i) the early Svecofennian (1.92–1.89 Ga) crustal evolution of the central Fennoscandian Shield, ii) the potential role of rift-related magmatism for the build-up of the Paleoproterozoic accretionary orogens and iii) evaluate, which geological features provide the primary control over the localization of an orogenic gold mineralisation. The zircon U-Pb geochronology defines an age of 1891 ± 5 Ma for the UGB, which is slightly older than most mafic intrusions in south-western Finland. The obtained chondritic initial zircon ɛHf values with E-MORB type geochemical affinity suggest a sub continental lithospheric mantle source for the UGB. The overall geochemistry indicates that the UGB magma as well as other E-MORB type rocks in the Pirkanmaa and Häme belts were formed in a rift-related environment in a fore-arc region at 1.89 Ga, predated by arc-type magmatism at ~1.90 Ga and back-arc magmatism at ~1.92 Ga in the Tampere belt. Slab retreat due to roll-back is suggested to cause the extension and related magmatism in the fore-arc region. Moreover, the timing and compositional and isotopic changes of early-orogenic magmatism are broadly compatible with intervals of extension and contraction, i.e., a tectonic switching model, and may provide a perspective to rapid build-up of Paleoproterozoic crust. Structural characterisation provides a framework where gold mineralisations are preferentially located within the high-strain north-eastern domain of the UGB, within fracture networks adjoining the high-strain zones. Our results indicate that neither the geochemical composition nor age of the intermediate-mafic intrusive host rocks play a major role in controlling the formation of gold mineralisation. By contrast, the localization of orogenic gold is controlled by localised structures (shear zones, fractures), and the variation in lithological composition of the intrusive host may contribute to the style of the mineralisation.

Timing of plutonism in the Gällivare area: implications for Proterozoic crustal development in the northern Norrbotten ore district, Sweden

AbstractZircon ion probe (secondary-ion mass spectrometry or SIMS) data from a set of intrusive rocks emplaced in the vicinity of major ore bodies, as well as from large igneous intrusions in the Gällivare area, gave the following results: (1) the Dundret ultramafic–mafic layered complex (1883±5 Ma), the Aitik granite (1883±5 Ma), the Nautanen diorite (1870±12 Ma), the Vassaravaara ultramafic–mafic layered complex (1798±4 Ma), the Aitik dolerite (1813±9 Ma), the Bergmästergruvan and Sikträsk syenites (1795±4 Ma and 1801±3 Ma, respectively) and the Naalojärvi granite (1782±5 Ma). These data broadly fall within the ranges 1.89–1.87 Ga (early Svecofennian) and 1.80–1.78 Ga (late Svecofennian), but geochronologically allow further subdivision into pulses at 1885–1880, 1875–1870, 1800 and 1780 Ma. During these events, large layered ultramafic–mafic and felsic plutonic rocks were generated with distinct overlap in time suggesting coeval felsic–mafic magmatism. Results also indicate the presence of inheritedc.1.87 Ga zircon crystals in the plutonic rocks at 1.78 Ga, supporting reworking of the previous crust. These data indicate the importance of mantle-derived mafic underplating in the process of crustal magma generation in the region. Thec.1.88 Ga event that generated ultramafic–mafic layered complexes is tentatively suggested to have played an important role in the formation of the Aitik Cu–Au porphyry system. The later event atc.1.80 Ga, generating voluminous mafic–felsic units, is suggested to be coupled to the regional iron-oxide-copper-gold (IOCG) overprint.

A regional 1.92 Ga tectonothermal episode in Ostrobothnia, Finland: Implications for models of Svecofennian accretion

The mostly metasedimentary Svecofennian Western Pohjanmaa belt in Ostrobothnia, Finland, can be divided into two stratigraphic groups separated by a major unconformity that reflects deformation following regional metamorphism. The western Lappfors group, interpreted as a Svionian basement complex, has strong W-trending folding and aeromagnetic signatures that contrast with the overlying eastern Evijärvi group, interpreted as lower Bothnian, which has more open N-trending folding and magnetic patterns. Several lines of evidence date the unconformity at ∼1.92 Ga. Detrital zircons from two samples of Lappfors group metasediment, and a sample of the basal Nivala gneisses in the Eastern Pohjanmaa belt, have 1.92–1.91 Ga post-depositional low-Th/U metamorphic overgrowths. The maximum deposition age of the Lappfors sedimentary protoliths, based on detrital zircon ages, is between ∼1.99 and ∼1.95 Ga. Three samples of Bothnian sediments lack pervasive ∼1.91 Ga overgrowths, instead having a variety of detrital zircons as young as ∼1.95–1.91 Ga, reflecting recycling of the underlying basement complex. The maximum deposition age of the Bothnian sedimentary protoliths is inferred to be ∼1.91 Ga. The Niska granitoid, which intrudes the Evijärvi group and is deformed only by the younger tectonic episode affecting that sequence, has a zircon age of 1896 ± 6 Ma. That episode, which established the present relationships between basement and cover, is dated by ∼1.88 Ga metamorphic zircon overgrowths in both the Svionian and Bothnian samples, and by 1878 ± 4 Ma metamorphic monazite from a metasediment from the Savo belt, east of the Nivala district. The post-1.91 Ga volcanic sequences of the Svecofennian Province are unlikely to represent arc accretion. The Svionian metamorphic sequences are probably the remnants of a widespread marginal basin that formed between ∼1.97 and 1.92 Ga, then was accreted to the craton during an Early Svecofennian (∼1.92–1.91 Ga) orogenic phase, forming the basement on which the Bothnian volcano-sedimentary sequences were subsequently deposited.

Preservation of Palaeoproterozoic early Svecofennian structures in the Orijärvi area, SW Finland—Evidence for polyphase strain partitioning

The predominantly migmatitic Palaeoproterozoic Uusimaa belt preserves early lower-grade Svecofennian structures in the Orijärvi area in SW Finland. This study aims at explaining the deformational history responsible for its preservation and also at defining the age of the early Svecofennian deformation. Detailed structural analysis reveals that the preservation was enabled by polyphase strain partitioning, which initiated during the early Svecofennian D 2 deformation, ∼1875 Ma ago, as revealed by ion microprobe U–Pb data on zircons from granodioritic and intermediate syn-D 2 intrusive dykes. The D 2 structures were low-strain upright folds at high crustal levels and sub-horizontal high-strain folds at deeper crustal levels. The sub-horizontal D 2 structures were refolded into upright folds during the subsequent late Svecofennian D 3 deformation, whereas the upright D 2 structures behaved as almost rigid blocks that caused strain partitioning into high-strain zones along the block margins. This accounts for the low cumulative strain in specific parts of the Orijärvi area. Further strain partitioning during D 4 caused reverse dip-slip movements along regional-scale shear zones. Crustal depth controlled the metamorphic grade during D 2, when local migmatisation took place at deep crustal levels. Later metamorphic overprint during D 3 deformation is evident from post-D 2 growth of sillimanite and a second generation of andalusite. Similarities in the structural patterns between the Orijärvi area and the Tampere-Vammala area (∼100 km to the north) suggest that irrespective of the age of the later overprint, subsequent deformation was localised along the margins of the early formed upright domains, while the low-grade rocks within the domains were preserved.

Two metamorphic stages in the Svecofennian domain: Evidence from the isotopic geochronological study of the Ladoga and Sulkava metamorphic complexes

The Ladoga, Russia, and adjacent Sulkava, southeastern Finland, metamorphic complexes are the two largest “granulite” provinces of the Svecofennian domain. In this area, the domain is composed of outer and inner zones. Sulkava is situated in the inner zone, which principally can be compared to the accretionary arc complex of Southern Finland. Ladoga is situated in the outer zone, which is correlated with the accretionary arc complexes of central and Western Finland. The complexes contain different metamorphic assemblages, which are caused by the different composition of the sedimentary protoliths: the rocks of the Sulkava metamorphic complex are higher in Al and K than those of the Ladoga Complex. Pb-Pb step leaching dating was used to determine the age of prograde sillimanite from both complexes. The dates thus obtained constrain metamorphic peaks for the Sulkava and Ladoga complexes at 1799 ± 19 Ma and 1878 ± 7 Ma, respectively, which is consistent with the U-Pb monazite ages of gneisses from both of the complexes. The differences in the ages of the metamorphic minerals from these complexes reflect the Early Svecofennian (1.89–1.86 Ga) and Late Svecofennian (1.83–1.79 Ga) metamorphic stages in the Fennoscandian Svecofennides.

Origin of chemically distinct granites in a composite intrusion in east-central Sweden: geochemical and geothermal constraints

A few tens of millions of years after the intrusion of the Early Svecofennian (∼1.87–1.85 Ma) granitoids in central Sweden, a renewed magmatic activity resulted in the emplacement of the Late Svecofennian granites, the tectonic setting of which remains obscure. S-type granites dominate this group, but both I-type and transitional granites are common. This study deals with one of these intrusions in east-central Sweden; a composite pluton that is insignificantly deformed and hosts both I- and S-type granites. One of the I-type granites shows a compositional trend from granodiorite to granite, which is uncommon among the Late Svecofennian granites. Major element and incompatible trace element compositions and ɛNd data show that two different sources, one igneous and one sedimentary, were involved. An important conclusion is that nearly coeval granites derived from different sources are found in close connection. The granites are suggested to have formed by partial melting in a thickened continental crust that was formed in an early stage of the Svecofennian event. Thermal models suggest that the slightly older, high-temperature I-type granite (granodiorite) was formed deeper in the crust than the S-type granite. The coexistence of essentially pure I- and S-type granites, rather than transitional mixtures, reflects the relative depths of the proposed sources and the varying thermal parameters of the lithologic units in the Svecofennian crust.

Petrogenesis of Mesoproterozoic (Subjotnian) rapakivi complexes of central Sweden: implications from U–Pb zircon ages, Nd, Sr and Pb isotopes

ABSTRACTU-Pb zircon geochronology of Mesoproterozoic (Subjotnian) rapakivi complexes in central Sweden yields: 1526 ± 3 Ma (Mullnäset), 1524 ± 3 Ma (Mårdsjö), 1520 ± 3 Ma (Nordsjö) and 1497 ± 6 Ma (Rödön). Together with complexes further S in Sweden, they constitute the westernmost, youngest (1·53−1·47 Ga) belt of rapakivi magmatism in the Fennoscandian shield.The low initial εNd values (−8·9 to −4·8) of all studied Subjotnian basic, intermediate and silicic rocks, require an input from an old (Archaean) low-radiogenic source component, as evidence for Palaeoproterozoic protoliths in the age range 2·5−2·1 Ga is lacking in this region. Crustal, early Svecofennian + Archaean (roughly 30−40%) sources are suggested for the Subjotnian A-type granites and syenites, where the granites derive from undepleted, granodioritic, and the syenites from monzodioritic (±depleted crustal) protoliths. The basic rocks originate from a depleted mantle acquiring the enriched Nd isotopic signatures during interaction with an Archaean lower crust (20−40%), largely depleted after rapakivi melt extraction. Pb isotope data from feldspars (207Pb/204Pb to 15·018−15·542) support the presence of Archaean components in the magmas.The results indicate that an Archaean basement is underlying relatively wide areas of Svecofennian formations in central Sweden. This old basement section was most likely rifted off the Archaean craton in the NE in Palaeoproterozoic times.

The sub‐Jotnian strömsbro granite complex at gävle, Sweden

The Strömsbro granite complex in eastern south‐central Sweden occurs on the north side of the Gävle graben, which is filled with Jotnian (Mesoproterozoic) sandstones. It has the form of a half oval of which the southern part is covered by the Jotnian rocks, thus rendering the sub‐Jotnian label adequate. The granites are undeformed, red, and coarsely porphyritic. A few dolerite dykes are interpreted as associated with the granite. The Strömsbro granites show considerable geochemi‐cal similarity with the rapakivi granites of the Fennoscandian Shield. They are metaluminous, within‐plate, A‐type granites. The dolerites are alkali‐rich quartz tholeiites showing marked similarities with other rapakivi‐associated basic rocks. The U‐Pb zircon age of the granite massif is 1500119 Ma (MSWD=1.7), which is similar to other sub‐Jotnian complexes in Sweden. The low initial ϵNd values of the Strömsbro rocks, and especially the dolerite, suggest the involvement of Archaean crustal material in the magmas. The granites are interpreted as derived by melting of non‐depleted, tonalitic granodioritic, early Svecofennian (and partly Archaean), lower crustal protoliths in response to heat brought by mantle‐derived magmas, now represented by the dolerites. Andersson, U.B., 1997: The sub‐Jotnian Strömsbro granite complex at Gävle, Sweden. GFF, Vol. 119 (Pt. 2, June), pp. 159–167. Stockholm. ISSN 1103–5897.

Granitoid episodes and mafic-felsic magma interaction in the Svecofennian of the Fennoscandian Shield, with main emphasis on the ∼1.8 Ga plutonics

Three major episodes of felsic plutonism can be identified in the Svecofennian of the Fennoscandian Shield. Each of these episodes is associated with coeval (but not comagmatic) mafic mantle-derived plutonism. The first episode comprises the early Svecofennian (∼1.90-1.86 Ga) granitoids, which formed the first continental crust, together with associated supracrustals, in most of the Svecofennian by a separation from the mantle at that time. They are dominated by tonalitic-granodioritic compositions and contain frequent examples of mafic-felsic magmatic interaction. The second episode (∼1.83-1.77 Ga), which is of main concern here, includes the Transscandinavian Igneous Belt (TIB), a huge N-S trending batholith of granitoids, mafics and some associated volcanics, separating the Svecofennian from the Southwest Scandinavian Gneiss Domain, and the smaller massifs and plutons of the Within Svecofennian Episode 2 (WSE2) granites. The TIB contains several examples of extensive homogenized hybrids of mafic-felsic magma-mixing, related to large aeromagnetic anomalies. The hybrids generally grade into the coarse porphyritic TIB-granites, and have intermediate mineralogy and chemistry. The geochemistry of the hybrid rocks is consistent with mixing trends in the range between ≈55% SiO 2 and 65–70% SiO 2 , which represent the approximate end members. Mixing and mingling on a local scale is also commonly observed. The WSE2-granites have a more limited felsic composition, higher contents of incompatible trace elements, and usually lack evidence of mafic-felsic interaction, consistent with a generation by a lower degree of partial melting of the same composite, early Svecofennian crust compared to the TIB-granites. The third episode corresponds to the rapakivi granites (∼1.65-1.56 Ga). These are intimately associated with coeval mafic magmatism. Interaction between magmas has been identified in composite dikes, but is also proposed to have formed the dark, olivine- and pyroxene-bearing, more mafic rapakivi varieties, which are very similar to the homogenized hybrids of the TIB.