Karl-Inge Åhäll

Episodic rapakivi magmatism due to distal orogenesis?: Correlation of 1.69–1.50 Ga orogenic and inboard, “anorogenic” events in the Baltic Shield

The temporal overlap of 1.69–1.55 Ga westward growth of the Baltic Shield and voluminous, episodic, 1.65–1.50 Ga rapakivi magmatism in the Svecofennian domain to the east has long been recognized. New U-Pb data from southwest Sweden indicate that this westward growth occurred in at least three distinct stages involving convergent-margin magmatism (Gothian orogenesis) at 1.69–1.65, 1.62–1.58, and 1.56–1.55 Ga. This improved resolution along the margin reveals an interesting temporal correlation between these events and the intracratonic rapakivi magmatism. Each stage of convergent-margin magmatism was echoed by a phase of bimodal rapakivi magmatism at 1.65–1.62, 1.58–1.56, and 1.55–1.50 Ga. In addition to this temporal correlation, rapakivi suites form distinct north-trending arrays that are subparallel to, but 500–1500 km east of, the active margin. These temporal and spatial links suggest that recurring subduction along oceanward-stepping zones provided first-order control(s) on episodic mantle melting and consequent bimodal rapakivi magmatism in distal, inboard settings. At the very least, emerging constraints from Baltica do not support previous, purely anorogenic models for the 1.65–1.50 Ga rapakivi magmatism or models that singularly implicate lingering effects from the preceding Svecofennian orogeny.

Growth-related 1.85-1.55 Ga magmatism in the Baltic Shield; a review addressing the tectonic characteristics of Svecofennian, TIB 1-related, and Gothian events - A reply

Abstract During the late Palaeoproterozoic, western Baltica was characterised by convergent-margin tectonics that resulted in the formation of N–S trending crustal units: the Transscandinavian Igneous Belt (TIB) and the westward younging Gothian growth zones. Together, these occupy a 200–300 km wide belt between Svecofennian (c. 1.9 Ga) crust in the east and the Skagerrak Sea/Oslo Rift in the west. Spatial and temporal constraints for 83 U–Pb dated rocks previously included in the TIB allow recognition of age groups at 1850, 1813–1766, and 1723–1657 Ma. However, the c. 1.85 Ga rocks should preferably not be included in the TIB because they form a tectonically distinct crustal unit (the Askersund suite). The 1.81–1.77 Ga age group is the major contributor to the TIB and corresponds to TIB 1. Westward younging of the TIB magmatism is demonstrated by the 1.72–1.66 Ga age group. Integration of the petrogenetic and spatial aspects for these rocks suggests an older magmatic phase at c. 1.72–1.69 Ga (TIB 2) and a final phase at c. 1.68–1.66 Ga (TIB 3). Recent structural, geochemical and isotopic data provide spatial and temporal constraints for the Svecofennian and Gothian developments, and for a bridging, c. 1.81–1.75 Ga tectonic stage we term the Smålandian. This tectonic stage includes the N–S trending, 1.81–1.77 Ga TIB 1 magmatism and manifests a marked shift by 1.81 Ga from previous north-directed convergence during the Svecofennian arc-accretionary event to east-directed convergence that also characterised the following Gothian evolution (c. 1.75–1.55 Ga). The early, c. 1.75–1.70 Ga Gothian evolution is still enigmatic but the subsequent development was due to westward stepping orogenesis, resulting in distinct c. 1.69–1.65, 1.62–1.58, and 1.56–1.55 Ga igneous belts between the TIB rocks and the Oslo Rift. The prolonged period of semi-continuous crustal growth during the Svecofennian-Smålandian-Gothian course appears to have ceased after the postulated docking of pre-1.60 Ga crust (“proto-SW Norway”) with Gothian crustal units of Baltica at c. 1.55 Ga. At the very least, the subsequent 1.50–1.20 Ga evolution lacks evidence for continental-margin processes east of the Oslo Rift.

The Gothian and Labradorian orogens: Variations in accretionary tectonism along a late Paleoproterozoic Laurentia‐Baltica margin

Abstract Intermittent crustal growth characterised late Paleoproterozoic development in western Baltica during Gothian orogenesis, and in eastern Laurentia during Labradorian orogenesis. Both regions are inferred to have belonged to the same margin of a supercontinent, but they do not show identical tectonic histories. Long‐lived convergent margin activity associated with successive, oceanward migrating stages of sub‐duction characterized western Baltica during the late Paleoproterozoic, in contrast to the development of a pre‐Labradorian, ca. 1.71 Ga sedimentary depocentre close to the margin of pre‐Labradorian Laurentia that gave way to Labradorian 1.68–1.65 Ga calc‐alkaline magmatism associated with subduction away from cratonic Laurentia. Continued Gothian, ca. 1.62–1.58 Ga continental‐margin calc‐alkaline magmatism and arc accretion has no recognized counterpart in eastern Laurentia, where collision of the outboard microcontinents/arcs resulted in voluminous granitoid magmatism caused by crustal thic...

Westward accretion of the Baltic Shield: implications from the 1.6 GaÅmål-Horred Belt, SW Sweden

Abstract Three new U-Pb zircon age determinations are reported from the Horred region, south-southeast of Goteborg, SW Sweden. This is a region of the Southwest Scandinavian Domain, within which a major N S trending shear zone and tectonic boundary, the Mylonite Zone, juxtaposes comparatively weakly migmatised lithologies in the west against more intensely migmatised gneisses in the east. West of the Mylonite Zone, a metavolcanic rock (the Mjosjodacite) yields an age of 1643 ± 29 Ma, whereas a cross-cutting plutonic rock (the Idala tonalite) has an age of 1584 ± 15 Ma. Together with a recent age for a volcanic rock from theAmal region farther north (1.61 Ga, Lundqvist and Skiold, 1992 ), these ages help to establish the existence of a coherent calc-alkaline igneous belt of 1.6 Ga age for which the nameAmal-Horred Belt is proposed. East of the Mylonite Zone, a presumably metavolcanic rock (the Grimmared gneiss) yields an age of ∼ 1.61 Ga. The obtained age and the compositional similarity of rocks on each side of the Mylonite Zone indicate that more deformed and more strongly metamorphosed equivalents of the rocks in theAmal-Horred Belt may occur also to the east of the Mylonite Zone in what is termed the Eastern Segment of the Southwest Scandinavian Domain. The new results establish theAmal-Horred Belt as a major geological unit younger than most other crustal components in southern Sweden such as theOstfold-Marstrand Belt (∼ 1.76 Ga), the Eastern Segment gneisses (> 1.66 Ga) and the three age groups of the Transscandinavian Igneous Belt (∼ 1.81 – 1.65 Ga; Larson and Berglund, 1992 ). The configuration of the crustal units in SW Sweden appears to necessitate more complex Proterozoic models than those with a persistent younging from the present east to the west. The present concept of the “Gothian orogeny” must be revised since at least two different orogenic episodes at ∼ 1.7 and 1.6 Ga can now be distinguished.

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...

Crustal units and role of the Mylonite zone system in the Varberg‐Horred region, SW Sweden

Abstract Late Gothian (c. 1.58 Ga) and Sveconorwegian (1.1–0.9 Ga) structures outline a 35 km long, NNE‐oriented, open gneiss synform in the Varberg‐Horred region of SW Sweden. This is a region of the Southwest Scandinavian Domain, within which a major shear zone and tectonic boundary, the Mylonite Zone, forms a branching shear zone system which converges in the eastern part of the synform. A subdivision between the Gothian and Sveconorwegian events is made by using the intervening anorogenic intrusions as structural markers. This, and the non‐recognition of a previously assumed orogenic event, results in a geodynamic model which is similar for the crustal segments on both sides of the largely N‐S trending Mylonite Zone, except for the higher grade Sveconorwegian metamorphism to the east. The evolution is characterised by one or more major Gothian gneiss‐forming events, followed by intermittent anorogenic magmatism and a later Sveconorwegian development that, outside discrete shear zones, gave rise to mod...

Transitioning from Svecofennian to Transcandinavian Igneous Belt (TIB) magmatism in SE Sweden: Implications from the 1.82 Ga Eksjö tonalite

Abstract The Eksjö intrusion forms a major tonalitic body in the Oskarshamn-Jönköping belt (OJB), SE Sweden. This is a NW–SE unit of dominantly intermediate granitoid intrusions, bound by felsic rocks of the Transscandinavian Igneous Belt (TIB). Previous U–Pb data for the Eksjö tonalite (1754±10 Ma) implied that subduction-related magmatism occurred in SE Sweden during a period of time when other data required that such magmatism had already moved further west. New U–Pb zircon dating establishes that the Eksjö tonalite crystallised at 1823±2 Ma and that subduction-related, granitoid magmatism of the OJB occurred in the 1834–1823 Ma interval. Field observations and geochemistry implies fractional crystallisation in either a continental arc or a mature island arc system. Although major portions of the Eksjö intrusion are well preserved, it is cut by NW-trending shear zones attributed to block-related movements that remained active until at least 1.80 Ga. Available constraints for the OJB support models of recurring convergent-margin tectonism along the southwestern margin of Baltica between 1.85–1.82 Ga and, hence, prior to onset of voluminous TIB 1 magmatism (1.81–1.77 Ga). Such a scenario accounts for recurring deformation in SW Finland (1.84–1.80 Ga), broadly coeval deformation in south-central Sweden, and apparent steps in Moho depths between the OJB and the pre-1.85 Ga margin of Baltica.

The 963 Ma Vinga intrusion and post-compressional deformation in the Sveconorwegian orogen, SW Sweden

Abstract In Scandinavia, the 1.15-0.90 Ga Sveconorwegian orogen is transected by major N-S trending shear zones separating lithotectonic segments. East of the Oslo Rift, the Idefjorden terrane hosts lithologically distinct, but spatially related post-compressional intrusions emplaced along similar WNW trends. The Vinga intrusion yields a 963±17 Ma age, which combined with other U-Pb results from this terrane, permits recognition of discrete post-compressional igneous episodes at 1041-1030, 984, 963, and 920 Ma. After final Sveconorwegian convergence and accompanying metamorphism by 969 Ma, gravitational collapse and related extension allowed ascent of the Vinga magma into a WNW fracture. We note that extensional movements with a sinistral strike-slip component confined to the N-S trending shear zones that bound the Idefjorden terrane, provide an emplacement mechanism for post-compressional magmas into the resulting WNW-trending fracture system.