A granodioritic rock and its relationship to the surrounding biotite-gneiss in the metamorphic basement complex of northeastern Bavaria (Moldanubikum) was studied. Field work and microscopic investigations show strong evidence for metasomatic origin of the para-granodiorite. A detailed study of zircon and apatite both in the para-granodiorite and the gneiss is made in order to obtain further criteria. Morphology, elongation ratios, and the grain-in-grain fabric of zircon point to non-magmatic origin of both rocks. Size frequency distribution of zircon in gneiss is lognormal and probably due to sedimentary sorting. The skewed distribution curve of zircon in para-granodiorite is explained by metamorphic growth of sedimentary zircons which may have been lognormally distributed. Apatite in para-granodiorite on the other hand shows lognormal distribution of grain size. As crystallization of apatite is posttectonic, it seems that random growth conditions in the metamorphic stage may also produce lognormal size frequency distributions. Thin section study is emphasized as some data cannot be obtained from mounted zircon concentrates.

Résumé Dans la série du gneiss en forme de perles, fréquent dans la Bayerischer Wald du Sud (Bavière), on peut trouver entre Deggendorf et Regen un interlit de roche granodioritique. Les recherches géologiques sur le terrain et au microscope rendent vraisemblable l'origine métamorphique de cette Paragranodiorite. Pour obtenir d'autres caractéristiques lithogénétiques on a fait appel à la texture morphologique, aux relations de longueur à largeur, à la texture grain-en-grain et à la granulométrie statistique du zircon et de l'apatite. Il se dégage ainsi d'autres arguments pour une origine non-magmatique de la Paragranodiorite.

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