Geochronology and geochemistry of eclogites from the Mariánské Lázně Complex, Czech Republic: Implications for Variscan orogenesis

Abstract

The Marianske Lazně 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 calcsilicate 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 ɛNd values and initial 87Sr/86Sr ratios. Five of the eight eclogite samples have high ɛNd values (+10.2 to +7.1) consistent with derivation from a MORB-like source, but variable 87Sr/86Sr ratios (0.7033 to 0.7059) which probably reflect hydrothermal seawater alteration. Three other eclogite samples have lower ɛNd values (+ 5.4 to −0.8) and widely variable 87Sr/86Sr ratios (0.7033 to 0.7096). Such low ɛNd values are inconsistent with derivation from a MORB, 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.