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