U-Pb Geochronolgy of Granulite Facies Rocks:
Implications from BSE Study of Zircons

J. Kos˜ler Department of Geochemistry, Charles University, Prague, Czech Republic

M. Svojtka Geological Institute, Czech Academy of Sciences, Prague, Czech Republic

E. Jelínek Department of Geochemistry, Charles University, Prague, Czech Republic

Granulite facies rocks in the Moldanubian zone of the Variscan Bohemian Massif contain two distinct morphological populations of zircons: (1) elongate, near euhedral grains (with dominant 100 prisms and 111, 101 and 211 pyramides) and (2) equant (near-spherical) grains with poorly expressed crystal faces. The microprobe and backscattered electron study of the previously dated samples (van Breemen et al., 1982; Aftalion et al., 1989) has revealed the presence of
(1) primary growth structures (oscillatory zoning) in elongate and most of the equant grains, (2) limited occurrence of inherited cores in both the elongate and equant zircons and (3) secondary growth structures (diffusion zoning) in equant zircons. The zones and domains within zircon crystals as seen on BSE images differ in Zr/Hf ratios as well as in the trace element contents (Th, U, REE, Y).

Presence of elongate near-euhedral zircons with oscillatory zoning in the studied granulite facies rocks is consistent with their having an igneous protolith for which a U - Pb zircon upper intercept age at ca 370 Ma has been established (Wendt et al., 1994), while presence of cores in some of the grains reflects inheritance of older component in the protolith of the granulite facies rocks. Proterozoic age of this component may be reflected by whole-rock Sm - Nd data from granulite facies rocks in the central part of the Blansky les massif (Fryda et al., 1995). The formation of equant grains is best explained as resulting from partial dissolution of zircons, probably also in response to extraction of anatectic melt from the protolith under granulite facies conditions (Vavra et al., 1994), for which both the field and geochemical evidences have been reported from perpotassic granulite facies rocks in the Blansky les massif (Vrána, 1989). The lack of metamorphic zircon overgrowth in response to granulite facies event is compatible with the grossly granitoid (Zr-poor) composition of the protolith and zircon saturation temperatures well below the peak of granulite facies.

The morphology of zircons and their BSE images suggest that the most likely mechanism which can potentially result in zircons reflecting true metamorphic age is a complete loss of radiogenic Pb* by volume diffusion in response to granulite facies event. Although the diffusivities of Hf, U, REE and Pb in zircon lattice are poorly constrained, on the basis of ionic radii, the diffusion rate of Pb4+ is much slower than those of the main zone-forming elements in zircon: Hf4+, U4+ and REE3+. Accordingly, the preservation of zoning patterns in zircons suggests that the grains have retained most of their radiogenic Pb*. In the case of studied zircons, only the spherical to oval grains which show no growth patterns are likely to yield the metamorphic (lower intercept) age, while the grains which have preserved oscillatory zoning or its relics should lie on the discordia with the upper intercept corresponding to the protolith magmatic age. However, the equant grains which contain older inherited cores would lie on different discordias whose slopes and positions are dependent on the core/magmatic overgrowth proportion and core composition. As a result, the morphological population of equant grains separated from a granulite facies rock will yield a lower intercept age which may be in excess to the true age of granulite facies event and should only be interpreted as a maximum age of granulite facies event.