D. R. Bowes

Geochronological studies of the Bohemian massif, Czechoslovakia, and their significance in the evolution of Central Europe

U–Pb zircon and Rb–Sr whole-rock analyses from various gneisses and plutonie rocks of the Moldanubian and Moravo-Silesian zones and the stable foreland of the Hercynian (Variscan) orogenic belt indicate that most of the crust in Central Europe was first formed during the Cadomian orogeny which straddles the Precambrian–Cambrian boundary. Zircons, however, have a memory of older ages which correspond with those of events known in Fennoscandia. The new radiometrie data are consistent with the stratigraphie record in that they do not provide any evidence for a major early Palaeozoic tectonothermal event between the Cadomian and Hercynian orogenies. Granulites from two localities in the Moldanubian zone yield U–Pb zircon ages of 345 ± 5 Ma; discordant zircon data points indicate that the granulite facies metamorphism was not of long duration. Tectonic units containing these high grade rocks were emplaced amongst amphibolite facies rocks during an event of widespread shearing which has been dated at 341 ± 4 Ma on the basis of a lower U–Pb zircon intercept age from one of the sheared gneisses and 338 ± 3 Ma U–Pb ages from monazites. Rb–Sr muscovite ages of 331 ± 5 Ma from pegmatites axial planar to asymmetrical folds date the last stage of SE-directed simple shear. A Rb–Sr whole-rock isochron of 331 ± 4 Ma from a principal magmatic type of the Central Bohemian pluton confirms the field evidence that the large NE-trending plutons of the Moldanubian zone were emplaced during a late stage of the deformation. The strong disturbance of the U–Pb zircon isotopic system in the sheared gneisses suggests U loss while a high U/Th ratio in monazite from one of these tectonised rocks suggests the simultaneous passage of hydrothermal fluids. Thus a crustal source is indicated for the uranium deposits of the Moldanubian zone. Critical to any plate tectonic model for the development of the Middle European Hercynides was the existence of an ocean in Early Devonian times which separated a North European continent from a South European continent(s). The northward movement of the South European continent over a shallowly-dipping subduction zone and subsequent continental collision can explain the high T–low P metamorphism and the imbricated tectonic style of the Moldanubian zone and adjacent Moravo-Silesian zone along the southeastern Hercynian foreland. The temporal separation of granulites and granites implies distinct conditions of formation and it has been suggested that the plutonism, following on from the imbrication of the Cadomian crust, was initiated by the subduction of wet oceanic sediments.

Late Proterozoic Charnockites in Orissa, India: A U-Pb and Rb-Sr Isotopic Study

Charnockite formation in the Angul district of Orissa took place between 1088 + 26/- 17 Ma, the U-Pb zircon upper intercept crystallization age of a leptynite neosome, and

Timing of Early Proterozoic Collisional and Extensional Events in the Granulite-Gneiss-Charnockite-Granite Complex, Lake Baikal, USSR: A U-Pb, Rb-Sr, and Sm-Nd Isotopic Study

957 + 8/ - 4-956 \pm 4

Rb-Sr whole rock isotopic studies of Lewisian metasediments and gneisses in the Loch Maree region, Ross-shire

Ma, the U-Pb zircon-monazite upper intercept and U-Pb monazite crystallization ages of a granite. Confirmation of the Proterozoic age of the charnockites is given by (1) a U-Pb zircon upper intercept 1159 -H 59/- 30 Ma age and a Rb-Sr whole-rock

Evidence for basement of late Precambrian age in the Caledonides of western Ireland

1080 \pm 65

40 Ar/ 39 Ar isotopic evidence for mid-Devonian post-metamorphic pegmatite emplacement in the Mariánské Lázně Complex, Bohemian Massif, Central European Hercynides

Ma age for an augen gneiss which pre-dates the leptynite, and (2) U-Pb monazite ages of