Rb-Sr, K-Ar Geochronology and Nd Isotopic Study of Contrasting Granite Massifs in Western Bohemia

W. Siebel GeoForschungsZentrum, 14 473 Potsdam, Germany


I. Wendt GeoForschungsZentrum, 14 473 Potsdam, Germany

K. Breiter Czech Geological Survey, 15 200 Prague 5, Czech Republic

A. Höhndorf Bundesanstalt für Geowissenschaften und Rohstoffe, 30 655 Hannover, Germany

F. Henjes-Kunst Bundesanstalt für Geowissenschaften und Rohstoffe, 30 655 Hannover, Germany

M. René Academy of Sciences of Czech Republic, 18 202 Prague 8, Czech Republic

Within western Bohemia late-Variscan granites form two separate massifs, the Bor massif and the Waidhaus-Rozvadov massif. The granites of the Bor massif intruded along the West Bohemian Shear Zone, which is the contact between the Moldanubian sensu stricto and the Zone of Teplá-Domazlice (ZTT). The intrusion has been interpreted as a consequence of extensional tectonics accompanying the collapse of the ZTT (Zulauf, 1994). The Waidhaus-Rozvadov granites were emplaced as a system of mainly NW trending dikes penetrating Moldanubian gneisses. The present contribution deals with the Rb-Sr, K-Ar and Nd isotopic features of these granite massifs.

The Bor massif consists of an early tonalite, a biotite-monzogranite, which is the most important member of
the massif and a late two-mica monzogranite. For the biotite-monzogranite a Rb/Sr whole-rock isochron was obtained, corresponding to an age of 337 ± 7 Ma (1s with an initial 87Sr/86Sr ratio, ISr, of 0.7073 ± 0.0003 (MSWD = 2.8).
This age agrees, within error, with an U-Pb zircon age determination for this granite of ×330 Ma (W. Dörr, personal communication) and provides an upper limit for the age of crystallization. From all three granite types we have analysed mica separates by conventional K-Ar analyses. Theses dates constrain the 500-450°C cooling period of the Bor massif to 320-315 Ma.

The Waidhaus-Rozvadov massif is composed of three main granite types: Cordierite-Biotite granite (CBG), Rozvadov granite (ROG) and strongly evolved Kreuzstein/Kr˜iz˜ovy´ kámen granite (KKG). Rb-Sr whole-rock dating yields ages of 296 ± 9 Ma (ISr = 0.7156 ± 0.0004, MSWD = 0.16) for the intrusion of the CBG, 304 ± 19 Ma (ISr = 0.7164 ± 0.0040, MSWD = 1.5) for the intrusion of the ROG and 297 ± 2 Ma with a poorly defined ISr of 0.750 ± 0.017 (MSWD = 1.4) for the intrusion of the KKG. A composite Sm-Nd isochron of 322 ± 68 Ma that can be drawn through the CBG and the ROG samples is consistent with the Rb-Sr results. Conventional K-Ar muscovite dates for these granites range from 313 to 307 Ma (CBG), from 310 to 302 Ma (ROG) and from 304 to 301 Ma (KKG) and are in accordance with the relative intrusion sequence predicted by field work (Breiter and Siebel, 1995). K-Ar dates of biotites from these granites range from 303 to 290 Ma indicating that cooling and/or thermal reactivation lasted until Late Stephanian to Early Permian times.

The Bor granite displays high eNd(T) signatures in the range -1.9 to -3.5 which overlap with those of paragneisses of the ZTT (F. Henjes-Kunst, unpublished data) suggesting that the parental magma for this granite could predominantly crustal-derived but, however, may likewise contain a mantle component. These values also coincide with those of the "older granites" in NE Bavaria (Siebel et al., 1995). The CBG and the ROG are isotopically homogeneous with eNd(T) values in the range -4.9 to -5.7 which corresponds to the eNd(T) values of the "younger granites" in NE Bavaria (Siebel et al., 1995). The KKG is characterized by the least radiogenic Nd isotopic signatures, and the most variable eNd(T) values. The similarities in eNd(T) values of the Waidhaus-Rozvadov granites with Moldanubian gneisses suggests a possible derivation from the latter.


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