Anders Lindh

Westward growth of the baltic shield

Abstract At the closure of the orogenic processes in the Svecofennian Orogen roughly 1750 Ma ago, continental crust began to form to the present west of the newly stabilized craton. This western part of the Baltic Shield is here considered as a growth zone. The oldest documented development is the intrusion of alkali-calcic granitoids ≈ 1780 Ma ago within or close to the western margin of the Svecofennian-Archaean craton. This plutonism continued when calc-alkalic granitoids intruded 1.6 to 1.7 Ga ago still farther to the west, where small amounts of older continental rocks are also preserved. However, final cratonization was effected by repeated intrusion of anatectic granites into the growth zone contemporaneously with rupture and mafic intrusions in the cratonized Svecofennian-Archaean continent to the east. With time, the strontium isotope initials of the granitoids in the growth zone increase. This suggests repeated remelting of the continental crust. Multistage periods of folding and metamorphism render the interpretation of some radiometric age determinations uncertain. Lithologically and tectonically differently developed crustal segments within the growth zone are delimited by belts of highly strained rocks - the Protogine and Mylonite Zones.

The influence of recent lead loss on the interpretation of disturbed UPb systems in zircons from metamorphic rocks in southwest Sweden

Abstract Zircons from progressively metamorphosed tonalites and granodiorites from the approximately 1.65-Ga-old, calc-alkalic Amal suite (SW Sweden) have been analyzed with the UPb method. The data are discordant when plotted on concordia diagrams. The regressed discordias give geologically meaningless intercepts with the concordia even though the linear arrangements of the points are good. Zircons separated from rocks from four different localities (20 km apart) yield identical upper intercepts (within limits of error). The isotopic system could have been opened at various times. Extensive magmatic activity occurred in the area about 1.56 and 1.25 Ga ago. Most rocks are polymetamorphic, and all of them are metamorphosed in the amphibolite facies. Conventional RbSr isotopic analyses on whole-rock samples as well as RbSr small-slab studies show that the isotopic system in the Amal granitoids was strongly overprinted approximately 1.4 and 1.0 Ga ago. Analyses of zircons treated with stepwise leaching with phreatic water, HF and with air abrasion strongly suggest that the UPb system in the zircons was influenced by a recent lead loss caused by natural processes. A three-stage mathematical model for zircon evolution is developed. This model is consistent with the intrusion of the Amal suite and crystallization of the zircons at 1.65 Ga ago followed by a thermal event involving new growth of zircons at 1.4 Ga and finally, a recent lead loss. The thermal event about 1.0 Ga ago did not appreciably influence the UPb isotope system in the zircons; however, sphene gives a nearly concordant age corresponding to this event. This model is also consistent with the data obtained by fission-track analyses on apatites and with scanning electron microscope investigations.

40Ar/39Ar geochronological constraints from the southeasternmost part of the Eastern segment of the Sveconorwegian orogen: Implications for timing of granulite‐facies metamorphism

Abstract The Southwest Swedish Granulite Region is characterized by mafic granulite‐ and upper amphibolite‐facies rocks, charnockites, and deformed and migmatized granitic to intermediate gneisses. A 40Ar/39 geochronological study was carried out focusing on the mafic rocks. Three granulites and four amphibolites have been analysed. The obtained ages range between 930 and 1030 Ma. The older 40Ar/39 ages are interpreted to be associated with crustal thickening, while the younger 40Ar/39 ages may be associated with final orogenic uplift. Wang X.‐D., Page, L.M. & Lindh, A., 1996: 40Ar/39 geochronological constraints from the southeasternmost part of the Eastern segment of the Sveconorwegian orogen: Implications for timing of granulite‐facies metamorphism. GFF, Vol. 118 (Pt. 1, March), pp. 1–8. Stockholm. ISSN 1103–5897.

The Mylonite Zone in south-western Sweden (Värmland) A re-interpretation

Abstract The Varmland part of the Mylonite Zone in western Sweden and the surrounding rocks to the west of Lake Fryken have been mapped, in order to study the origin of this lithotectonic complex. The area is predominantly made up of acid-to-intermediate gneisses, most of which are presumably of plutonio origin. The rocks of the Mylonite Zone are commonly intensely deformed. Several periods of deformation have been distinguished. The spatial distribution of petrographically similar rocks indicates that the Mylonite Zone predominantly comprises rocks of the north-eastern Pregothian mega-unit. Rocks of the southwestern Pregothian mega-unit are found in the south-eastern part of the investigated section of the Mylonite Zone. The formation of the Mylonite Zone is later than the folding of the Pregothian rocks, and possibly also later than the intrusion of the hyperites.

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.

The radiometric age of the Proterozoic granite at Sandsjön, western Värmland, Sweden

Abstract Red, microcline rich granites occur in the Amal and western Pregothian tectonic mega-units in south-western Sweden. The Sandsjon granite is the northernmost known occurrence of these rocks in the ‘Western Pregothian’ of Sweden. The Rb-Sr whole-rock dating of the Sandsjon granite fails to give any conclusive age, hypothetically due to either a metamorphic disturbance or to an inhomogeneous distribution of the initial 87Sr/86Sr ratio. The zircon age is 1210+16 −14 Ma (1σ) and is interpreted to correspond to the crystallization of the Sandsjon granite. The emplacement of this granite is thus contemporaneous with previously dated similar granites in the Amal and ‘Western Pregothian’ mega-units.

The Blomskog granite — a possible diapiric structure

Abstract A group of granitic intrusions occurs in western Sweden, showing a regular spacing between consecutive intrusions. Their spacing suggests solid-state doming of granitic material. However the textures of the rock are most easily interpreted in terms of crystallization from a magma. This interpretation is also consistent with contact observations and chemical characteristics of the rock. A hypothesis simultaneously explaining the textures and the spacing is presented, where doming of a light layer of potassium-rich migmatite is suggested to be followed by partial melting of the same rock. The magma collects at the top of the thus-formed sinusoidal waves and later intrudes into the country rock, where it crystallizes before reaching the surface.

The age of the Hinneryd granite ‐ its significance for interpreting the terranes of the southern Baltic Shield

Abstract The adamellitic (monzogranitic) Hinneryd granite occurs in the southern region of the Eastern Segment, which is part of the Southwest Swedish Gneiss Complex. The granite is strongly foliated and pervasively metamorphosed. Near‐concordant zircon fractions (less than c. 15% discordant) define a U‐Pb upper‐intercept age of 1548+10 Ma interpreted as the intrusion age. END(1548) is +0–5 and the depleted‐mantle Sm‐Nd model age is 1.84 Ga. Sm‐Nd isotope equilibrium is not established among the mineral phases. Two mineral isochrons are obtained by using different mineral‐whole‐rock combinations; these isochrons correspond to Sveconorwegian ages, 1065137 and 896±54 Ma (2a). The best‐fit line (1065 Ma) is obtained from phases that define the upper amphibolite‐facies assemblage and the foliation. The metamorphic mineral assemblage suggests temperatures of c. 650°C and pressures of c. 1 GPa. No contemporaneous granite is known from the northern region of the Eastern Segment. This supports earlier suggestions...

U-Pb zircon age of the Görbjörnarp syenite in Skåne, southern Sweden

Abstract Zircons were separated from a syenite body occurring along the Protogine Zone near Gorbjornarp in northeastern Skane, southern Sweden. U-Pb analyes of the zircons yield an almost concordant age of 1204+14 -8 (2σ) Ma. The MSWD is 0.740. The obtained age is interpreted as the intrusion age of the syenite. It is within its limits of error identical to upper intercept U-Pb zircon ages of the Vaggeryd and Onnestad syenites and the Gumlosa-Glimakra granite also found aligned to the Protogine Zone.

Interpreting the Småland-Blekinge Deformation Zone from chemical and structural data

Abstract Chemical and structural data from the Småland-Blekinge Deformation Zone (SBDZ) in south-east Sweden are presented. Earlier published age determinations are included in the discussions. The Transscandinavian (TIB) granitoids immediately to the north of the SBDZ are characterised by anomalously low K/(K+Na+Ca) ratios compared to the majority of TIB granites. The generally more mafic Tving granitoids to the south of the SBDZ are mostly higher in K/(K+Na+Ca) than the neighbouring granites in the north. The TIB granitoids to the north of the SBDZ are characterised by a stronger increase in mafic components with decreasing SiO2 than in most other TIB granitoids. Results suggest differences in source rock compositions on either side of the SBDZ. This does not contradict suggestions of a former coherent terrane across the SBDZ but casts some doubts on them. Rocks in the SBDZ can in some cases be positively identified as deformed and metamorphosed equivalents of rocks occurring on either side of the SBDZ. However, counterparts of mafic rocks occurring within the SBDZ have not been identified in the neighbouring provinces. When approaching the SBDZ, the foliation becomes parallel with the SBDZ-strike direction, 110°. It is either vertical or dips steeply towards the SSW. Rocks occurring within the SBDZ are partly mylonitic, even suggestions of pseudotachylites are observed. Movements are principally vertical with an ‘up-to-the-south’ sense. A sinistral horizontal component is less well constrained and is interpreted to predate the vertical movements. The 1.45 Ga old granites essentially post-date the movements in the SBDZ.