Joakim Mansfeld

Geological, geochemical and geochronological evidence for a new palaeoproterozoic terrane in southeastern Sweden

The geological and geochronological relationships between a rhyolite, belonging to the Transscandinavian Igneous Belt (TIB), and an intrusive suite (the Backaby intrusion), southeast Sweden, have been investigated by means of bedrock mapping, geochemistry and radiometric age determinations (UPb on zircons). Field relations suggest that the rhyolite rests unconformably on the Backaby intrusion, which previously has been correlated with ∼ 1.9 Ga granitoids within the Svecofennian Domain. The rhyolite yielded an age of 1800 ± 8 Ma and a tonalite belonging to the Backaby intrusion basement yielded an age of 1834 ± 3 Ma. The age of the rhyolite is slightly older (10–15 Ma) than adjacent granitoids belonging to the Transscandinavian Igneous Belt, and is considered as the earliest magmatic expression of TIB in the area. The age and the geochemical trends for the Backaby intrusion is significantly different from that of the surrounding TIB granitoids and indicate a rock forming event at about 1830–1840 Ma in southeastern Sweden represented by the Oskarshamn-Jonkoping Belt. This event is significantly younger than the older Svecofennian rocks within the Svecofennian Domain proper, indicating a hitherto unidentified geological terrane in southeastern Sweden.

Relations between 1.19–1.13 Ga continental magmatism, sedimentation and metamorphism, Sveconorwegian province, S Norway

The Sveconorwegian and Grenville orogenic belts display widespread 1.19–1.13 Ga Early Grenvillian continental magmatism including A-type granitoids. In the Sveconorwegian province, S Norway, bimodal 1.17–1.14 Ga metavolcanic rocks of the Telemark sector are part of this magmatism. Volcanic rocks in low- to medium-metamorphic grade are interlayered with immature and locally conglomeratic clastic metasediments and covered by a thick metasedimentary sequence. Minor unconformities are reported. New zircon U–Pb data are presented and integrated in a revised stratigraphy of the Telemark supracrustal rocks. A metarhyodacite at the base of the Nore group yields a crystallisation age of 1169±9 Ma and displays 1.7–1.5 Ga inherited zircon grains (SIMS data). A metarhyolite situated below sandstone of the Heddal group yields a crystallisation age of 1159±8 Ma. In the cover sequence, a metasandstone of the Heddal group has detrital zircon grains in the intervals 2.86–2.41 and 1.94–1.11 Ga (34 analysed grains) and a metasandstone of the Kalhovd formation in the intervals 2.85–2.74 and 2.00–1.05 Ga (41 analysed grains). These metasediments were deposited after 1121±15 Ma and 1065±11 Ma, respectively and were transformed by 1.01 Ga Late Sveconorwegian deformation and metamorphism. The metasedimentary rocks contain a significant amount of regionally derived clasts. Two deformed A-type granite metaplutons yield zircon U–Pb intrusion ages of 1146±5 Ma (Eiddal) and 1153±2 Ma (Haglebu, ID–TIMS data). The 1.19–1.13 Ga magmatism is distributed in the western part of the Sveconorwegian province, in the Telemark, Bamble and Rogaland–Vest Agder sectors, indicating that these sectors were part of a single plate at that time, which is characterised by a thin lithosphere today. The A-type geochemical signature of the felsic magmatism and the continental lithosphere signature of the associated mafic volcanism point to a continental non-compressional tectonic regime. The overlap in time between widespread 1.19–1.13 Ga continental magmatism, intermontane basin formation and Early Sveconorwegian 1.15–1.12 Ga granulite-facies metamorphism recorded in the Bamble sector suggest a thermal pulse linked to upflow of asthenospheric mantle. Deposition of the cover of clastic sediments between 1.12 and 1.01 Ga possibly reflects thermal subsidence after the 1.19–1.13 Ga event and before the Late Sveconorwegian (1.03–0.95 Ga) orogenic phases. An analogy between the 1.19–1.13 Ga evolution of the Sveconorwegian province and the Cenozoic formation of the Basin and Range province in USA is discussed.

1.83-1.82 Ga formation of a juvenile volcanic arc - implications from U-Pb and Sm-Nd analyses of the Oskarshamn-Jönköping Belt, southeastern Sweden

Abstract The Oskarshamn-Jönköping Belt in southeastern Sweden is a geographically well-defined area comprising calc-alkaline intrusions and volcanic rocks together with units of coarse-grained clastic metasedimentary rocks. Deviating from the general composition is the Fröderyd Group with basalts of MORB character. The belt is surrounded by the 1.81-1.77 Ga Transscandinavian Igneous Belt. A conglomerate clast from the central part of the Oskarshamn-Jönköping Belt yielded a U—Pb zircon age of 1829±8. This age confirms the c. 1.83-1.82 Ga formation age of the belt. Sm—Nd whole rock analyses of various rock types throughout the Oskarshamn-Jönköping Belt show that no substantially older (>100 m.y.) continental material contributed to these rocks. Furthermore, the high positive εNd values of many of the analysed rocks points to the depleted mantle as the main component of their source. The Sm—Nd analyses also suggest that while some of the felsic units were formed by a high degree of magmatic fractionation from a mantle derived melt others were formed as the result of remobilisation of older Svecofennian crust. The new U—Pb data together with previously published ages imply that the formation of the Oskarshamn-Jönköping Belt was a rather quick process, possibly completed in c. 10 m.y. The most likely model of genesis for the area, based on the new U—Pb and Sm—Nd data, is formation at a continental margin subduction zone to the present south-west of a slightly older Svecofennian continent. In such a model the Fröderyd Group represents either a fore-arc setting or a back-arc rift.

Characterization of the Paleoproterozoic Mantle beneath the Fennoscandian Shield: Geochemistry and Isotope Geology (Nd, Sr) of ~ 1.8 Ga Mafic Plutonic Rocks from the Transscandinavian Igneous Belt in Southeast Sweden

Geochemistry and Sr-Nd isotope geochemistry of ca. 1.8 Ga, mafic intrusions of the Transscandinavian Igneous Belt (TIB-1) in the Fennoscandian shield were studied in southeastern Sweden. These rocks show LILE-LREE-enriched, HFSE-depleted, calc-alkaline, continental arc signatures in the north, grading to slightly less enriched, oceanic affinities southward. εNd(1.80) values range from +2.0 to +0.7 and 87Sr/86Sr(1.80) from 0.7022 to 0.7029 (with one outlier at 0 and 0.7033), without correlation to fractionation (e.g., Mg#) or crustal contamination, indicating sources that are mildly depleted. The most depleted ratios occur in the south, trending with the geochemistry toward more enriched compositions northward. The sources represent depleted mantle wedge material that was subjected to enrichment not long before (TDM ca. 2.0 Ga), i.e., during the preceding arc subduction (2.1-1.82 Ga), and/or during the TIB-1 magmatism itself, by hydrous fluids with a sediment and/or melt input increasing northward. The TIB-1 magmatism occurred above a south(west)ward-retreating subduction zone along the continental margin of the juvenile Svecofennian continent at 1.81-1.76 Ga.

U-Pb age determinations of Småland-Värmland granitoids in Småland, southeastern Sweden

Abstract Two postorogenic Smaland-Varmland granitoids were analysed with the U-Pb method on zircons. One sample was collected at Ramnebo in eastern Smaland and one sample was collected at Savsjo in central Smaland. The Ramnebo sample gave an upper intercept of 1802±4 Ma and the Savsjo sample gave an upper intercept of 1786+6 -5 Ma; both are suggested to represent crystallization ages of the Smaland granitoids. These ages support the view that the Smaland-Varmland granitoids were formed during the period 1810–1770 Ma, contemporaneous with the late orogenic granites of the Svecofennian orogeny.

Geology, geochemistry and age of a Cu-Mo-bearing granite at Kabeliai, southern Lithuania

SummaryA Cu-Mo-bearing granitoid belonging to the concealed Precambrian crystalline basement of Lithuania has been dated by the U-Pb zircon method and investigated geochemically. chemically. The granitoid is located at Kabeliai in southernmost Lithuania and forms part of a granitoid complex recognized as the Marcinkonys batholith. The Kabeliai granite is composed of quartz, plagioclase, microcline and biotite and shows a granitic to adamellitic peralummous/metaluminous composition with dominantly I-type chemistry. U-Pb dating of zircon yields an age of 1505 ± 11 Ma, which is considered to reflect the crystallization age of the granite.The Kabeliai granite displays several similarities in terms of geological setting, chemistry and age with certain granitoids in northeastern Poland (Mazury complex) and northwestern Belorussia (Mostovsky, Kamensky and Vydgodsky plutons), which are considered “rapakivi-like” in the literature. It is, however, uncertain whether proper rapakivi granites really exist in these areas as none of these granitoids displays the common characteristics of rapakivi granites (A-type chemistry, wiborgitic textures, associated Sn-Be-Pb-Zn-Cu mineralizations). We speculate that the 1.4-1.5 Ga granites southeast of the Baltic Sea rather might be correlated with granites of comparable age and geochemical character in southwestern Sweden. Another possible alternative is that the granitoids in Lithuania may not be correlated with any part of the Fennoscandian Shield.ZusammenfassungEin Cu-Mo-führender Granitoid, der zum präkambrischen, kristallinen Grundgebirge Litauens gehört, wurde mittels der U-Pb Methode an Zirkonen datiert und geochemisch untersucht. Der Granit repräsentiert einen Teil des Marcinkonys Batholithes, und tritt in Kabeliai, im südlichsten Litauen auf. Der Kabeliai Granit besteht aus Quarz, Plagioklas, Mikroklin und Biotit und zeigt adamellitische, peraluminöse/metaluminöse Zusammensetzung mit vorwiegend 1-Typ Charakter auf. U-Pb Datierung der Zirkone ergibt ein Alter von 1505 ± 11 Mio Jahren, das als Kristallisationsalter anzusehen ist.In seinem geologischen mit Granitoiden im nordöstlichen Polen (Mazury Komplex) und Grant Ånlichkeiten mit Granitoiden im nordöstlichen Polen (Mazury Komplex) und Grant Ånlichkeiten mit Granitoiden im nordöstlichen Polen (Mazury Komplex) und im nordwestlichen Weißrussland (Mostovsky, Kamensky und Vydgodsky Plutone), die in der Literatur als “Rapakivi-ähnlich” angesehen werden. Es ist jedoch unischer ob echte Rapakivi-Granite in diesen Regionen existieren. Keiner dieser Granite zeigt nämlich die für Rapakivi-Granite typischen Kennzeichen (A-Typ Chemie, Wiborgit Texturen, assoziierte Sn-Be-Pb-Zn-Cu-Mineralisationen). Wir vermuten, daß die 1.4 1.5 Mia Jahren alten Granite südöstlich der Ostsee eher mit Graniten ähnlichen Alters und von ähnlichem geochemischen Charakter im südwestlichen Schweden zu vergleichen sind. Eine weitere Alternative wäre, daß die Granitoide Litauens mit keinem Granit im Fennoskandischen Schild korreliert werden können.

Palaeoproterozoic rifting and formation of sulphide deposits along the southwestern margin of the Svecofennian Domain, southern Sweden

Abstract A mineralized Palaeoproterozoic volcanic sequence has been identified southwest of Vetlanda in southern Sweden. The sequence, which is defined as the Froderyd Group, is surrounded by 1.78 Ga granitoids and may be considered as the southernmost segment of Svecofennian crust in Sweden. The Froderyd Group has a bimodal geochemical character and is dominated by pillow basalts and associated gabbros but rhyolites, thin intercalations of marble and stratiform sulphide ores do also occur. The carbonate intercalations and the stratiform sulphide ores are interpreted as submarine exhalites formed syngenetically with the volcanic activity. The mafic rocks have a typical mid-ocean ridge basalt geochemistry with flat rare earth elements patterns. The lead isotopic compositions of the mafic rocks and the associated stratiform sulphide ores support such a close affinity to juvenile sources with mantle character. In contrast, the rhyolites display more evolved geochemical signatures and isotopic patterns and are interpreted to represent remelted products of slightly older sialic crust. A model for the formation of the Froderyd Group is presented, suggesting rifting along a continental margin which may have occurred in the final stage of the Svecofennian crust-forming period.

A compilation of metamorphic pressure–temperature estimates from the Svecofennian province of eastern and central Sweden

Abstract Here, we present a compilation of 44 metamorphic pressure–temperature (P–T) estimates from 31 localities in the Svecofennian province of eastern and central Sweden. Based on these P–T estimates, which were obtained using the average P–T method of the computer programme THERMOCALC, we calculated an apparent metamorphic field gradient of 54 ± 4 °C/km for the Svecofennian province. This is typical for low-medium P/T (Buchan) metamorphism and supports tectonic models that imply Svecofennian crustal growth by accretion of volcanic arc systems. In general, estimated P and T conditions range from 0.2 to 0.6 GPa and from 400 to 800 °C, respectively; i.e., from greenschist to granulite facies conditions. Metamorphic grade is generally higher, reaching upper amphibolite or granulite facies in northern and southwestern parts of the Svecofennian province, whereas metamorphism in Bergslagen was at greenschist to lower amphibolite facies conditions. The higher metamorphic temperatures recorded by rocks in the southwestern part of the province might relate to magmatic activity associated with the Transscandinavian Igneous Belt (TIB). Higher pressure, epidote amphibolite facies metamorphic conditions in the western part of the province probably reflect Sveconorwegian overprinting. Finally, local upper amphibolite and granulite facies conditions probably reflect contact metamorphism.