A REGIONAL STUDY OF δ18O IN ECLOGITES AND ASSOCIATED ROCKS OF THE BOHEMIAN MASSIF

The histories of orogenic eclogites in the high-pressure Bohemian Massif, Czech Republic are debated. Oxygen isotope ratios help determine if they crystallized in the mantle, or within oceanic or continental crust. Using 55 new laser fluorination values and 9 published δ¹⁸O values of garnet from eclogites, host gneisses, and ultramafic rocks from the Bohemian Massif, comparisons were made to δ¹⁸O(Grt) from 1055 new and published values from other eclogites, HP mafic rocks, and oceanic crust to attempt to distinguish protolith. Of all Bohemian eclogite samples, 40% fall in the normal range for high-T equilibrium with typical mantle (δ¹⁸O(Grt)=5.3±0.6‰); 37% are <4.7‰, and 23% are >5.9‰. Eclogites found in gneiss (external eclogites) from the Bohemian Massif range in δ¹⁸O(Grt) from 1.4‰ to 7‰ indicating that eclogite protoliths interacted with heated meteoric water or seawater before garnet growth. External eclogites are not equilibrated with their host gneisses at meter-scale. No negative δ¹⁸O(Grt) values were found and the range of δ¹⁸O values of external eclogites are similar to that of δ¹⁸O(WR) of hydrothermally altered oceanic crust and to δ¹⁸O(Grt) of oceanic crust-derived Alpine eclogites. Exchange with heated meteoric water in a continental setting cannot be ruled out by the new δ¹⁸O(Grt) data, but would imply fortuitously low water/rock ratios for all samples studied. Tectonic models exist that allow for both the subduction of continental crust to HP and the interlayering of oceanic crust with continental crust. Eclogites associated with ultramafic rocks (internal eclogites), have a smaller range in δ¹⁸O(Gt) from 3.1 to 7.8‰ also indicating interaction with the crust. The low δ¹⁸O(Gt) values suggest that internal eclogites inherited δ¹⁸O from lower oceanic crust by metamorphism or assimilation, while values above 6‰ suggest interaction with upper oceanic crust.