The development of the Central European continental crust since the Early Archaean based on conventional and ion-microprobe dating of up to 3.84 b.y. old detrital zircons

Abstract

Archaean components up to 3.84 b.y. old have been detected by ion-microprobe dating of zircon from various sediments and metasediments of the European Hercynides (Moldanubian of northeastern Bavaria and the Massif Central). The Early Archaean age of 3.84 b.y. was obtained within the euhedral core of a detrital grain from a Moldanubian paragneiss. The same detrital zircon grain reveals at least three metamorphic overprints at 2.59 b.y. B.P., 1.94 b.y. B.P. and 460 m.y. B.P., and at least two cycles of mechanical abrasion during surface recycling in the Early Proterozoic and after the Pan-African cycle. It is, thus, the crystal with the most complex geological history ever detected. Further Archaean ages for the crystallization of primary magmatic zircons, probably in the course of continental crust formation in the first cycle provenances of the two studied areas, were found at 3.15, 2.9, 2.76 and 2.65 b.y. B.P., and in a few cases between 2.6 and 2.5 b.y. B.P. Early Proterozoic crust-forming events between about 1.75 and 2.15 b.y. B.P. play an important role, especially for the sediments and metasediments of the Montagne Noire at the southern rim of the Massif Central. Middle and Late Proterozoic ages cluster around 1.0 b.y. B.P. and ≈ 600 m.y. B.P. for all analysed samples. Therefore, the deposition of the metasedimentary precursors took place after the Pan-African cycle, which is in line with the Rb-Sr whole-rock systematics of Central European metasediments. The ages of metamorphism in the different source areas could be inferred in some cases by lower intercept ages obtained on single detrital grains. The following sequence coinciding with times of magmatic events has been so far established: 2.59,1.94 and 1.0 b.y. B.P., and 600 m.y. B.P. Thus, ion-probe dating is also capable of defining secondary effects in the respective provenances which are typical for orogenic cycles. The ubiquitous presence of Pan-African detrital zircons in European metasediments is a strong indication that the detrital material was derived from Gondwana. However, from the presence of metamorphic units including eclogitefacies rocks which probably formed at different continental margins and at various times between about 500 m.y. and 330 m.y. ago it is probable that various Gondwana-derived microcontinental plates contributed to the different basin fillings. The Gondwana-derived detritus is also supported by the observation of two periods of relative quiescence or non-activity around 2.3 b.y. ago, and especially around 1.5 b.y. ago, which is a typical feature observed in Gondwana (Cahen et al., 1984) but not in Laurasia, the other possible source of the detritus. The lack of zircons crystallized between 3.84 and 3.15 b.y. ago might have different causes. Thus, the continental crust of the European Hercynides probably developed since the Early Archaean and grew via many crustal accretion events to its present state.