U-Pb systematics of detrital zircons from some unmetamorphosed to slightly metamorphosed sediments of Central Europe

Abstract

In an attempt to find the premetamorphic discordance pattern of detrital zircons extracted from Central European metasediments, unmetamorphosed or only slightly metamorphosed sediments were collected from two areas: (1) from the Montagne Noire, the southernmost part of the French Central Massif and (2) from the innerbohemian Algonkian (= Proterozoic) in the CSSR.

The generally accepted hypothesis that zircons from Central European metasediments must have plotted close to or at the corresponding upper intercept between discordia trajectory and concordia curve prior to the metamorphism of the host rocks could not be supported. The zircon populations from sediments of both areas are similar in discordance to those of the numerous populations extracted from metasediments of the Central European basement complexes. However, in contrast to the latter, the data points of size fractions scatter considerably and reliable intercept ages cannot be calculated.

In the case of the Cambro-Ordovician sand- and siltstones of the Montagne Noire, the ages of detrital muscovites strongly argue for a Cadomian (550–700 m.y.) provenance of the detritus. Thus, the strong discordance of the analyzed fractions most probably is caused by zircons newly formed and/or partly or completely reset during a Cadomian event in the provenance of the detritus. In addition, lattice unit parameters indicate that the detrital zircons must have been recrystallized after their primary formation more than 1.7 b.y. ago.

The Algonkian sediments of Bohemia (CSSR) can be taken as the very low-grade metamorphic equivalents of the Moldanubian paragneisses from which discordia trajectories between about 2 b.y. and 460–320 m.y. are known (Gebauer and Grünenfelder, 1974; Grauert et al., 1974). Nevertheless, all analyzed zircon fractions are strongly discordant indicating that they probably recrystallized during the Assyntian (=Cadomian) very low-grade metamorphism of the host rock loosing most of their accumulated radiogenic lead. If such an interpretation is correct, the low-temperature recrystallization model of Gebauer and Grünenfelder (1976) can be applied to metamict zircons in host rocks formed at temperatures as low as 300 ° C. In our 1976-paper we gave temperatures of 350–400 ° C for the maximum temperature necessary to recrystallize metamict zircons in chlorite-grade quartzphyllites in agreement with the experimental results of Pidgeon et al. (1973).

In contrast to the zircons of the Montagne Noire it can be shown that the U-Pb systems of the Algonkian zircons must have been re-opened in post-Assyntian time, probably recently or in the Tertiary. However, no plausible explanation can be given to account for this.