Per-Olof Persson

Relationship between 1.90–1.85 Ga accretionary processes and 1.82–1.80 Ga oblique subduction at the Karelian craton margin, Fennoscandian Shield

Abstract The three main intrusive suites: early calc-alkaline, late I/A-type, and late S-type intrusive rocks in relation to the Svecokarelian orogeny (1.9–1.8 Ga) have been dated at the Archaean craton margin in the Palaeoproterozoic Skellefte district and surrounding areas in northern Sweden. In addition, new SIMS data have been obtained on a calc-alkaline intrusion for which unusually young TIMS ages existed, compared to similar calc-alkaline intrusions elsewhere in the region. Titanite and zircon from a subvolcanic intrusion affected by a major N-S trending shear zone have also been dated to constrain the last ductile deformation in the area. The 1895+14 −12 Ma zircon age for a calc-alkaline intrusion is interpreted as the crystallisation age and is significantly older than the existing 1825 Ma age on titanite from a pyroxene skarn in a marble horizon close to the contact. The latter age is instead interpreted as the age of peak metamorphism in this area. The 1798plusmn;4 Ma age for the S-type granite confirms that the S-type magmatism is largely coeval with the I/A type magmatism previously dated at 1803plusmn;6 Ma. At a larger scale, a zoned belt over 2000 km long with A/I-type magmatism in the west and S-type magmatism in the east can be inferred. Either mafic underplating or Cordilleran type settings can explain the magmatic belt, which trends oblique to the roughly NE-directed subduction that led to the accretion of volcanic arcs onto the older craton between 1.95 and 1.87 Ga. An intimate temporal relationship between the extensive 1.80 Ga magmatism and regional N-S-trending shear zones in the area is confirmed by the titanite age of c. 1.80 Ga from one such shear zone. Kinematics on this shear zone suggest E-W shortening. SIMS data from a calc-alkaline intrusion at Sikträsk indicate that the previously obtained conventional zircon ages of 1.85–1.86 Ga are actually mixed ages of 1.88 Ga magmatic zircons, and c. 1.80 to 1.82 Ga metamorphic overgrowths. This shows that the 1.80 Ga event was not only constrained to shear zones. It is argued that both the 1.80 to 1.82 Ga deformation and metamorphism discussed here is related to E-W shortening and the voluminous magmatism at 1.82–1.80 Ga. This is in contrast to the older c. 1.88 Ga deformation identified to the north and east within the Karelian craton that was related to Svecokarelian accretionary processes.

U-Pb ages of plutonic and volcanic rocks in the Svecofennian Bothnian Basin, central Sweden, and their implications for the Palaeoproterozoic evolution of the Basin

Abstract New U-Pb age data on two early orogenic granodiorites (ca. 1930 Ma) and one felsic metavolcanite (1870 Ma) from the Svecofennian Bothnian Basin area of central Sweden are presented. Combined with literature data they suggest an extended, at least ca. 80 m.y. long, evolution of this part of the Bothnian Basin. Sedimentation started before 1950 Ma, and continued up to at least 1870 Ma. Early-orogenic plutonic activity started at ca. 1950 Ma, perhaps even earlier, and continued up to ca. 1850-1840 Ma. Detrital zircons found in low-grade greywackes of the region range in age from ca. 1880 to 2020 Ma. According to the tentative model suggested here, these zircons may emanate from granitoids intruded into the greywacke sequence of the Bothnian Basin. They were subsequently exposed to erosion and included in upper parts of the sedimentary column. These upper parts could (statistically) be of a lower metamorphic grade than older greywackes, which are generally migmatized.

U-Pb ages of Proterozoic metaplutonics in the gneiss complex of southern Värmland, south-western Sweden

Abstract U-Pb analyses on zircons from three meta-plutonics in the polymetamorphic gneiss complex of south-western Sweden in Varmland have yielded minimum ages of 1609, 1563 and 1249 Ma. Although the metaplutonics have been strongly reworked, the apparent U-Pb ages of the zircons are almost concordant. This is probably due to low uranium contents. The ages show that many rocks in the ‘Pregothian’ are age equivalents of better preserved rocks in the Amal tectonic mega-unit. It is also concluded that the last deformation, metamorphism and mobilization in the investigated area took place during the Sveconorwegian.

Stratigraphies and depositional ages of Svecofennian, Palaeoproterozoic metavolcanic rocks in E. Svealand and Bergslagen, south central Sweden

Abstract A belt of Palaeoproterozoic, dominantly felsic metavolcanic rocks occurs in south-central Sweden. The volcanic rocks comprise volcaniclastics, lavas and subvolcanic intrusions. The volcanic pile is more than 8 km thick and overlies an unexposed basement. Two new U-Pb zircon age determinations provide precise control on the age of stratigraphically and volcanologically well-constrained volcanic rocks in the belt. One age determination from a pyroclastic flow deposit at Uto in the easternmost part of the volcanic belt yielded an age of 1904±4 Ma. Stratigraphic and depositional facies analyses show that this rock marks the onset of volcanism in the area. The igneous activity was coeval with a change from deep water sedimentation to shallow water and subaerial sedimentation in a broadly continental setting. Another determination of zircons from a similar pyroclastic flow deposit at the lowest known stratigraphic level east of Hallefors in W. Bergslagen in the westernmost part of the volcanic belt gav...

Granitoids from the Äspö area, southeastern Sweden ‐ geochemical and geochronological data

Abstract The bedrock on and around Aspo island, north of Oskarshamn is predominated by Smaland granitoids belonging to the Trans‐scandinavian Igneous Belt (TIB). A second magmatic event was the formation of anorogenic granites (e.g. the Gotemar granite) 1.4 Ga ago, represented by massifs of coarse‐grained granite cut by fine‐grained granitic dikes. The Smaland granitoids constitute a suite of medium‐grained quartz monzodiorites (Aspo diorite) to granites (Avro granite) penetrated by fine‐grained granites. They are classified as alkalic to alkali‐calcic, metaluminous to peraluminous I‐granites. A U‐Pb dating on zircon and titanite of the Aspo diorite yielded an age of 1804±3 Ma. Two darings of the fine‐grained Smaland granite gave ages of 1794+16 ‐12 and 1808+33 ‐30 Ma. The fine‐grained, felsic Smaland granites are distinguished chemically from the similar looking fine‐grained anorogenic granites through, e.g., differing Fe/Mg, alkali contents and HREE‐pat‐terns. Kornfalt, K.‐A., Persson, P.‐O. & Wikman, H...

A U–Pb dating of the Askersund granite and its marginal augen gneiss

Abstract U–Pb zircon ages of 1848±15 and 1842+23 -13 Ma have been obtained for the coarse porphyritic Askersund granite (‘Filipstad type’) and for its marginal augen gneiss, respectively. The results indicate that the onset of magmatism in the Transscandinavian Igneous Belt (TIB) took place in connection with the late Svecofennian orogenic development. They also support a structural model implying that regionally deformed and folded marginal parts of older granitoid intrusions in the TIB have similar ages as the isotropic and discordant central parts. The deformation is reflected in the morphology of the zircons but does not affect the isotopic ages.

Geochronology and structural setting of the 1.38 Ga Torpa granite; implications for charnockite formation in SW Sweden

Abstract In the Varberg region of the Southwest Scandinavian Domain in SW Sweden, the megacrystic Torpa granite forms a sheet‐like, partly charnockitic intrusion in intensely migmatised gneisses. The granite has been interpreted as part of an igneous suite, termed the Varberg Charnockite‐Granite Association. The gneisses are made up of ca. 1.75–1.55 Ga old crust that has been subjected to high‐grade Sveconorwegian‐Grenvillian metamorphism. A U‐Pb zircon age of 1380±6 Ma for a charnockitic portion of the Torpa granite is interpreted as the emplacement age and is also suggested to represent the age for the entire Varberg Charnockite‐Granite Association. Together, this age and the field relationships demonstrate that the regional gneiss‐forming event(s) in the Varberg region is older than 1.38 Ga, and that the pervasive deformation of the Mylonite Zone, a major N‐S trending shear zone and tectonic boundary in the region, is younger than 1.38 Ga and therefore probably Sveconorwegian in age (1.1–0.9 Ga). The s...

Geochronology of porphyries and related rocks in northern and western Dalarna, south-central Sweden

Abstract Volcanic rocks and granites belonging to the Transscandinavian Igneous Belt (TIB) form important constituents of the bedrock in the northern and western parts of the Province of Dalarna, south-central Sweden. The volcanic rocks comprise rhyolitic to quartz-trachytic porphyries as well as mainly intermediate to mafic porphyrites, and contain intercalations of sedimentary rocks. U-Pb zircon datings of 11 porphyries, 2 porphyrites, and 3 granites suggest a chronostratigraphic subdivision into two major magmatic events, at 1.8 and 1.7 Ga. We propose that the porphyries of these two events should be called Older and Younger TIB porphyries, respectively. Some of the porphyries and granites representing the younger event have a distinct rapakivi affinity, as defined by both chemical and textural criteria, whereas in others such features are lacking or only weakly developed. The rocks formed during the older event lack rapakivi characteristics. Our new chronological sub-division conforms with the age pat...

A comparison of the geochronology and geochemistry of plagioclase-dominated granitoids across a major terrane boundary in the SW Balitic Shield

Abstract To make reconstructions of the Proterozoic palaeogeography meaningful, the terranes making up the individual shields must first be identified and the timing of terrane accretion determined. New chemical and isotopic data confirm the terrane-separating character of a major shear zone, the Mylonite Zone, in the Southwestern Scandinavian Gneiss Complex. These data relate to plagioclase-dominated granitoids to the east of the Mylonite Zone. Two zircon upper-intercept age determinations result in 1674+24−19 and 1688+10−10 Ma. Two separate crystals from the 1688-Ma rock give two single-zircon evaporation ages: one crystal gives 1646 ± 172 Ma (2σ) and the other 1654 ± 46 Ma (2s). Earlier investigations have pointed to vast amounts of similar looking granitoids to the west of the Mylonite Zone. However, these rocks are younger, approximately 1.60 Ga. The granitoids to the west of the Mylonite Zone are typical calc-alkaline rocks with a pronounced increase in the content of mafic minerals concurrent with a decrease in quartz. The trend of the eastern rocks is different, mainly involving changes in the quartz/total feldspar ratio. At the same SiO2 content, the eastern rocks are lower in MgO and CaO and higher in (Na2O + K2O) and especially in Ba than their western counterparts. Also other elements like Ni show differences between the two suites. We conclude that these similar looking rocks belong to two different suites that are different in age and have different chemical signatures, probably caused by different source-rock chemistries. ϵNd1.68-values in the eastern suite range from + 2.0 to 2.5, suggesting that assimilation of large amounts of old continental rocks did not take place. Results from Sr-isotope determinations support this suggestion. The present results suggest, together with older data, that no granitoid rocks older than approximately 0.9 Ga are common to the two terranes separated by the Mylonite Zone. There is thus no compelling evidence suggesting that the two terranes formed one entity before the Sveconorwegian (Grenvillian) orogeny.

U-Pb age evidence for repeated Palaeoproterozoic metamorphism and deformation near the Pajala shear zone in the northern Fennoscandian shield

Abstract This paper presents U-Pb ages for an extended period of geological evolution between about 1860 and 1740 Ma in the northern part of the Fennoscandian Shield. It involved magmatism, ductile deformation and metamorphism along the border between Sweden and Finland. The study area separates into a western and an eastern domain roughly along a north-south trending structural boundary defined by the western margin of the Pajala shear zone. Approximately 1860-1850 Ma zircon ages from two granitoid rocks record contrasting post-emplacement deformation histories on either side of this boundary. While metamorphic monazite records c. 1850 Ma events in rocks of the western domain, metamorphic monazite and titanite as well as zircon overgrowths in the eastern domain verify deformation and high-grade metamorphism in the 1820-1780 Ma interval. This post-1820 Ma phase is most probably related to shear zones and to nearby intrusions. A titanite age at c. 1740 Ma in the eastern domain is suggested to exemplify a tectonic phase, which elsewhere in northern Fennoscandia is marked by fracturing, generation of pegmatites, and hydrothermal activity.