Significance of Zircon Ages for Southern Bohemian Granulites; Evidence for Late Zircon Growth as a Consequence of Long-Term Melt Residence During Retrograde Metamorphic Evolution

Malcolm P. Roberts Institut für Mineralogie, Universität Salzburg, Hellbrunnerstraße 34, A-5020 Salzburg, Austria

Fritz Finger Institut für Mineralogie, Universität Salzburg, Hellbrunnerstraße 34, A-5020 Salzburg, Austria

A major unresolved problem concerning the tectonic evolution of the Bohemian Massif, is the timing of Variscan granulite-facies metamorphism. This affected a number of bodies of mostly calc-alkaline I- to S-type granitoid rocks at peak PT conditions estimated as 16 kb/1000 °C (O'Brien and Carswell, 1993). Following this HP event the granulites underwent decompression and were included in the Variscan nappe piles, where they recrystallised under the PT conditions of normal Moldanubian regional metamorphism (about 7-8 kb/ 700-800 °C) (O'Brien and Carswell, 1993). The age of this regional metamorphism is well constrained at about 335 to 340 Ma (Friedl et al., 1994). The age of the HP granulite event is commonly considered as having occurred not much earlier, at roughly 340-350 Ma (van Breeman et al., 1982; Kröner et al., 1988; Aftalion et al., 1989; Wendt et al., 1994). This narrow time interval between the two metamorphic events may be difficult to rationalise geologically. Age dating of the HP event is based on a number of high quality U-Pb concordant to slightly discordant zircon ages that cluster around 340-350 Ma (van Breeman et al., 1982; Kröner et al., 1988; Aftalion et al., 1989; Wendt et al., 1994). These ages were obtained mostly from zircons with distinct equant morphologies, commonly interpreted as being typical for granulite-facies zircon growth. In granulites from the Blansky les massif, a second zircon forming event has been recorded at roughly 370 Ma from grains with a magmatic appearance (euhedral, elongate and oscillatory zoned) (Wendt et al., 1994). Using these magmatic growth features it was argued that the 370 Ma event represented the age of igneous protolith formation. In felsic granulites, the authors found in addition, significantly older zircon components interpreted as an inheritance within the granite protolith.

We discuss whether the zircon forming event recorded at 370 Ma (Wendt et al., 1994) could not be interpreted equally well as the age of HP granulite metamorphism. While, the 340-350 Ma ages may represent a late stage in the granulite decompression path. The presence of magmatic growth features in the 370 Ma zircons is not in conflict with such an interpretation.

We have developed a model for granulite-facies metamorphism in Southern Bohemia based on experimental data for partial melt-forming reactions and partial melt volumes (Pöschel-Otrel, 1995; Svojtka and Kosler, 1995). Considering the high temperatures envisaged for the HP event (O'Brien and Carswell, 1993), it is clear that silicate melt must have been present. Our modelling of the fluid-absent partial melting of a quartzo-feldspathic rock with 3 to 8 vol% biotite shows that melt volumes lie between 5 and 20 vol%. This hot, partial melt may hold as much as 1300 ppm Zr before becoming saturated in zircon (Watson and Harrison, 1983). It could potentially dissolve most of the zircons in the protolith. In more felsic protoliths, with low partial-melt volumes, old zircons could preferentially survive, and this explains why high old Pb components have been found mainly in leucocratic granulites Wendt et al., 1994). Likewise, it can be assumed that zircon crystallisation would have occurred during the very early stages of the retrograde path, but still close to the PT peak, under magmatic conditions.

Our model shows that throughout much of the retrograde evolution of the granulites, using the published PT estimates (O'Brien and Carswell, 1993), a silicate melt phase was present at low volumes down to the PT conditions of about 820°C/6 kb. Using experimental data on zircon solubility (Watson and Harrison, 1983), it is possible that roughly half of the granulite zircons crystallised during this late, medium pressure melt evolutionary stage. For more mafic protoliths with higher partial melt volumes, the end result will be abundant zircons with relatively young ages. Back-scattered electron imaging indicates that southern Bohemian zircons have magmatic growth histories throughout (Pöschel-Otrel, 1995; Svojtka and Kosler, 1995). Our ongoing research will focus on the relationship between U-Pb isotope systematics, whole-rock geochemistry, zircon typological populations, their internal zoning patterns and elemental distribution, and comparison with other accessory minerals. Without a thorough and systematic study of this kind, the potential of zircon age data and their significance in the tectonic evolution of the southern Bohemian granulites can never be fully exploited.


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