U-PB ZIRCON AND RB-SR MINERAL DATING OF ECLOGITES AND THEIR COUNTRY ROCKS - EXAMPLE - MUNCHBERG GNEISS MASSIF, NORTHEAST BAVARIA

In order to evaluate the usefulness of the UPb zircon suite method to date eclogitic rocks, zircon populations have been analyzed from two eclogites and one cogenetic metagabbro. Samples were taken from the Münchberg Gneiss Massif (northeast Bavaria) containing the largest B-type eclogite body [1] of the non-Alpine part of Central Europe. All nine analyzed grain size and magnetic fractions plot linearly on a U/Pb evolution diagram and yield intercept ages at 525-31+40 m.y. and 380-22+14 m.y., respectively, indicating a Lower to Middle Devonian eclogitization of a Cambrian tholeiitic protolith. The degree of discordance of the zircon fractions of the eclogites cannot be correlated with either U content, grain size and/or magnetic properties. Instead, new formation of zircon in the course of transformation of the host rocks into eclogite seems to be the governing factor for the degree of discordance. This can be shown to hold true for zircons of the relictic metagabbro which show - in contrast to those of the eclogites - only few signs of new formation and which are therefore much less discordant than the zircon populations of the eclogites (ca. 20% vs. 80%). From the presence of numerous eclogitic layers interbedded with paragneisses it is concluded that the premetamorphic source rocks must have been basaltic lava flows and/or tuffs [2]. In this case the upper intercept age does not only reflect the time of formation of the tholeiitic source rocks but also indicates the time of deposition of the metasedimentary country rocks in the Cambrian. Thus, the UPb zircon suite method applied to such rocks could be an extremely useful tool to unravel depositional ages of highly metamorphosed metasediments. RbSr analyses have been carried out on the post-eclogitic mineral paragenesis, hornblende and phengite, in order to find out whether or not it belongs to the same Acadian cycle (ca. 380 m.y.) as the high-pressure assemblage. Hercynian ages could be expected as the Palaeozoic metasediments surrounding the Münchberg Gneiss Massif were metamorphosed about 320 m.y. ago. However, the data - together with a muscovite-whole-rock age of the immediate country rocks - indicate that hornblende and phengite were formed immediately after formation of garnet and omphacite in the course of the Acadian metamorphic cycle. An initial Sr isotopic ratio of 0.7031 can be calculated for the tholeiitic protolith of the eclogites which is in agreement with a primary magmatic mantle origin. © 1979.