Elsevier

Precambrian Research

Volume 114, Issues 1–2, 1 February 2002, Pages 121-147
Precambrian Research

Zircon geochronology of migmatite gneisses along the Mylonite Zone (S Sweden): a major Sveconorwegian terrane boundary in the Baltic Shield

https://doi.org/10.1016/S0301-9268(01)00220-0Get rights and content

Abstract

The southern section of the Mylonite Zone (MZ) is a major lithotectonic terrane boundary in the SW Baltic Shield. It separates the parautochthonous Eastern Segment of the Sveconorwegian orogen from the allochthonous Western Segment. Complex zircons in migmatised and banded orthogneisses along the southern MZ were investigated by ion microprobe analyses guided by backscattered electron imaging to directly date the partial melting and associated penetrative ductile deformation in this zone. In the eastern part of the MZ (Eastern Segment), resorbed igneous zircon cores in stromatic orthogneiss are 1686±11 Ma old, whereas extensive overgrowths and abundant newly formed simple zircons are 969±13 Ma old. Migmatised K-feldspar megacrystic granite has 1359±26 Ma igneous protolith zircon cores and abundant 968±13 Ma overgrowths and simple grains. Both rock units contain amphibolitised mafic pods with remnants of garnet-clinopyroxene-bearing, high-pressure granulite facies parageneses. In the western part of the MZ (Western Segment), igneous protolith zircon cores in stromatic orthogneiss are dated at 1585±11 Ma and abundant new growth of zircon at 980±13 Ma. Enclosed mafic rocks have middle amphibolite facies parageneses. Secondary zircon in all three samples occurs as distinct, sub-idiomorphic overgrowths or as simple crystals with low Th/U ratios (<0.07). The morphology and high modal abundance of secondary zircon (25–50% of the total volume of zircon), the absence of early- or pre-Sveconorwegian secondary zircon, and field relations provide evidence for that anatexis and associated penetrative ductile deformation in the southern MZ took place between 980±13 and 968±13 Ma. These late-Sveconorwegian ages contradict previous interpretations of a pre-1.55 Ga age for the principal penetrative ductile deformation and stromatic layering of orthogneisses along the southern MZ. As a consequence, the role of the MZ as an important amalgamation zone for crustal growth in the Baltic Shield during the 1.70–1.55 Ga Gothian orogeny is questioned. Instead, the new data suggest that the MZ is a major Sveconorwegian lithologic and metamorphic terrane boundary along which middle-amphibolite facies supracrustals and orthogneisses of the Western Segment (protolith ages: 1.64–1.58 Ga) were juxtaposed against high-pressure granulite to upper-amphibolite facies orthogneisses of the Eastern Segment (protolith ages: 1.70–1.66 Ga) in late Sveconorwegian time. By implication, the pre-Sveconorwegian relation between crustal units west of the MZ and the margin of continent Baltica remains to be established. Understanding of the late Sveconorwegian tectonic evolution, particularly the amount of displacement along the MZ, is therefore, a pre-requisite for modelling the early and pre-Sveconorwegian tectonic evolution and crustal block configurations in the SW Baltic Shield.

Introduction

Migmatisation of the crust is a key event in the geological record of metamorphic terrains since the melting process requires heating above ∼630 °C (e.g. Johannes, 1985), and commonly occurs close to the thermal peak of a metamorphic P–T path. Partial melting also reduces rock-strength so that rocks undergoing melting may channel large amounts of deformation. Moreover, migmatite veins serve as structural markers to distinguish between different generations of deformation fabrics, and direct dating of migmatisation is, therefore, of prime interest for the understanding of the evolution of high-grade gneiss terrains.

The diffusion of U and Pb in crystalline zircon domains is negligible at temperatures below 1000 °C, i.e. at temperatures appropriate for partial melting under crustal conditions (Mezger and Krogstad, 1997, Cherniak and Watson, 1998). Therefore, the age of newly crystallised zircon formed during partial melting directly dates the mineral crystallisation in the melt and not the cooling along a metamorphic P–T path. Also, unless resorption or recrystallisation takes place, the high closure temperature of U–Pb diffusion in non-metamict zircon domains leaves this isotopic system completely or partly undisturbed through repeated metamorphism (e.g. Black et al., 1986, Watson, 1996, Mezger and Krogstad, 1997). Spot dating of different domains in complex zircon can thus give chronological information of both the igneous and the metamorphic history of rocks complexly reworked at high temperatures (e.g. Black et al., 1986, Schiøtte et al., 1989, Whitehouse et al., 1999).

In this study, ion microprobe spot dating guided by backscatter imaging is applied to complex zircon populations from three migmatites sampled across a major lithological and metamorphic terrane boundary in the Sveconorwegian orogen of the SW Baltic Shield, the southern section of the Mylonite Zone (MZ). The rock units sampled have been used as key localities to characterise and chronologically sub-divide high-grade tectonic fabrics associated with crustal growth of Baltica during the 1.70–1.55 Ga Gothian orogeny (e.g. Åhäll, 1995, Åhäll et al., 1995, Åhäll et al., 1997). Direct dates of the migmatisation and the associated penetrative ductile deformation structures have, however, been lacking.

The objectives of this study are: (1) to directly date partial melting along the southern MZ using U–Pb–Th data and morphological characteristics of complex zircon in migmatised orthogneisses along the zone; (2) to document the metamorphic conditions and structural relations associated with the migmatisation; (3) to examine the role of the MZ as a prominent late-Sveconorwegian lithological and metamorphic terrane boundary of central importance for reconstructing both Sveconorwegian and pre-Sveconorwegian geodynamics in this part of the Baltic Shield

Section snippets

Regional geology

The Sveconorwegian orogen in Scandinavia represents an approximately 500 km wide Proterozoic mobile belt in the southwestern Baltic Shield. It comprises gneiss segments that are separated by roughly N–S-trending Sveconorwegian ductile deformation zones (Fig. 1). The orogen is delimited to the east by the steep, west-dipping Sveconorwegian Frontal Deformation Zone (SFDZ in Fig. 1; Wahlgren et al. (1994)). Sub-vertical structures of the Protogine Zone (PZ), west of, and sub-parallel to, the SFDZ,

Migmatite gneisses across the southern MZ: field geology and zircon chronology

Three migmatised gneisses were sampled on both sides of the southern MZ. These rocks have been used as key lithologies for characterising and bracketing the age of the migmatisation and ductile deformation in the southern MZ and on a regional scale (Åhäll, 1995, Åhäll et al., 1995, Åhäll et al., 1997). Two of the samples were collected in the eastern MZ: (1) a banded orthogneiss at Skene (Skene gneiss), representative of the oldest basement gneisses in the southern ES (Fig. 2, Fig. 3); and (2)

Interpretation and discussion of the zircon ion microprobe data

Ion microprobe spot-analyses of complex zircon populations, selected by detailed examination of BSE images, in three migmatised orthogneisses along the southern MZ demonstrate extensive zircon growth between 980±13 and 968±13 Ma. Pre-Sveconorwegian zircons occur as primary igneous core domains, with significantly higher Th/U ratios (1.08–0.26) than the secondary zircon (≤0.07), and are interpreted to date magmatic crystallisation of the igneous protoliths (Fig. 5, Table 2).

The abundance,

Re-evaluation of the basis for previous structural and tectonic models: three key localities

The Skene, Bua and Torpa gneisses have all been used as key localities for structural and tectonic models inferring that the main penetrative ductile banding and migmatisation of country rock gneisses along the southern MZ took place prior to 1.55 Ga (e.g. Åhäll, 1995, Åhäll et al., 1995, Åhäll et al., 1997). The age data and field relations presented in this study do not support these models; each rock unit and its setting is discussed below, followed by a summary of the regional consequences

Conclusions

  • 1.

    Ion microprobe analysis of secondary zircon of high modal abundance in three migmatised orthogneisses along the southern section of the MZ yield late Sveconorwegian ages at 969±13, 968±13 and 980±13. The secondary zircon is idiomorphic to sub-idiomorphic, has low Th/U-ratios and occurs both as overgrowths on igneous-zoned cores, and as new, non-complex crystals. The late-Sveconorwegian ages of secondary zircon growth date the partial melting, associated metamorphism and the penetrative ductile

Acknowledgements

This study was financially supported by grants from the Swedish Natural Science Research Council (NFR) to Charlotte Möller and Leif Johansson. K. Lindén, T. Sunde, J. Vestin and M. Whitehouse at the NORDSIM ion microprobe laboratory at the Swedish Museum of Natural History in Stockholm are gratefully thanked for technical guidance, discussions and cheerful treatment. Journal reviewers James Connelly and Bernard Bingen are gratefully acknowledged for constructive reviews of the manuscript. This

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