GEOCHRONOLOGY AND GEOCHEMISTRY OF ECLOGITES FROM THE MARIANSKE LAZNE COMPLEX, CZECH-REPUBLIC - IMPLICATIONS FOR VARISCAN OROGENESIS

The Marianske Lazne complex (MLC) is located in the Bohemian Massif-al ong the north-western margin of the Tepla-Barrandian microplate and co nsists of metagabbro, amphibolite and eclogite, with subordinate amoun ts of serpentinite, felsic gneiss and calc-silicate rocks. The MLC is interpreted as a metaophiolite complex that marks the suture zone betw een the Saxothuringian rocks to the north-west and the Tepla-Barrandia n microplate to the south-east. Sm-Nd geochronology of garnet-omphacit e pairs from two eclogite samples yields ages of 377 +/- 7, and 367 +/ - 4 Ma. Samples of eclogite and amphibolite do not define a whole rock Sm-Nd isochron, even though there is a large range in Sm/Nd ratio, im plying that the suite of samples may not be cogenetic. Eclogites do no t have correlated epsilon(Nd) values and initial Sr-87/Sr-86 ratios. F ive of the eight eclogite samples have high epsilon(Nd) values (+10.2 to +7.1) consistent with derivation from a MORE-like source, but varia ble Sr-87/Sr-86 ratios (0.7033 to 0.7059) which probably reflect hydro thermal seawater alteration. Three other eclogite samples have lower e psilon(Nd) values (+5.4 to -0.8) and widely variable Sr-87/Sr-86 ratio s (0.7033 to 0.7096). Such low epsilon(Nd) values are inconsistent wit h derivation from a MORE source and may reflect a subduction or oceani c island basalt component in their source. The MLC is an important pet rotectonic element in the Bohemian Massif, providing evidence for Camb ro-Ordovician formation of oceanic crust and interaction with seawater , Late Devonian (Frasnian-Famennian) high- and medium-pressure metamor phism related to closure of a Saxothuringian ocean basin, Early Carbon iferous (Visean) thrusting of the Tepla terrane over Saxothuringian ro cks and Late Visean extension.