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-along the north-western margin of the Tepla-Barrandian microplate and consists of metagabbro, amphibolite and eclogite, with subordinate amounts of serpentinite, felsic gneiss and calc-silicate rocks. The MLC is interpreted as a metaophiolite complex that marks the suture zone between the Saxothuringian rocks to the north-west and the Tepla-Barrandian microplate to the south-east. Sm-Nd geochronology of garnet-omphacite 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, implying that the suite of samples may not be cogenetic. Eclogites do not have correlated epsilon(Nd) values and initial Sr-87/Sr-86 ratios. Five of the eight eclogite samples have high epsilon(Nd) values (+10.2 to +7.1) consistent with derivation from a MORE-like source, but variable Sr-87/Sr-86 ratios (0.7033 to 0.7059) which probably reflect hydrothermal seawater alteration. Three other eclogite samples have lower epsilon(Nd) values (+5.4 to -0.8) and widely variable Sr-87/Sr-86 ratios (0.7033 to 0.7096). Such low epsilon(Nd) values are inconsistent with derivation from a MORE source and may reflect a subduction or oceanic island basalt component in their source. The MLC is an important petrotectonic element in the Bohemian Massif, providing evidence for Cambro-Ordovician formation of oceanic crust and interaction with seawater, Late Devonian (Frasnian-Famennian) high- and medium-pressure metamorphism related to closure of a Saxothuringian ocean basin, Early Carboniferous (Visean) thrusting of the Tepla terrane over Saxothuringian rocks and Late Visean extension.