Relationships Among Eclogite Bodies and Host Rocks in the Lotru Metamorphic Suite (South Carpathians, Romania): Petrological Evidence for Multistage Tectonic Emplacement of Eclogites in a Medium-Pressure Terrain

• Published in: International geology review 2003-03, Vol.45 (3), p.225-262
• Main Authors: Săbău, Gavril, Massonne, Hans-Joachim
• Format: Article
• Language: English
• ISSN: 0020-6814; 1938-2839
• DOI: 10.2747/0020-6814.45.3.225

The spatially most extensive unit containing pre-Alpine basement rocks in the South Carpathians, the Lotru Metamorphic Suite, hosts numerous eclogite bodies appearing in two different settings. Type I eclogites are mainly metamorphosed MORBs hosted in mica-rich rocks along a major synmetamorphic thrust zone. Calculated P-T conditions are located along an early retrograde array slightly exceeding 700°C at pressures between 1.8 and 2.7 GPa. Type II eclogites occur in a high-grade felsic complex structurally beneath the thrust zone. Peak P-T conditions were 800-900°C, 2.4-2.7 GPa for Mg-rich type IIa and ~600°C, 2.1-2.2 GPa in type IIb eclogites of Fe-Tirich basaltic composition. Differences in peak pressures as high as 1 GPa are documented between eclogites and their host rocks, which have undergone a polymetamorphic history, never reaching eclogitic conditions. The derived P-T histories imply a model of polystage tectonic emplacement of eclogites originating from both subducted oceanic crust and eclogitized mantle cumulates during a subduction-collision event. Eclogite emplacement in upper-crustal rocks resulted from a succession of syn-subduction exhumation of detached slab fragments, ascent of continental mantle fragments in mélanges with a felsic matrix, and eduction of lower-crustal slivers already containing accreted eclogites. During the collisional stage, the passive margin and the eclogite-bearing units were thrust far over the hotter active margin in a flake-tectonics style triggered by horizontal decoupling in the structurally weakened active margin. Weakening resulted from partial melting due to a thermal gradient increase in the back-arc zone. Metamorphic re-equilibration following the tectonic stacking occurred at 0.6 GPa and 600-700°C. The emplacement of the tectonic flakes without significant crustal thickening prevented rapid exhumation of the structure, thereby allowing a delayed unroofing that occurred only during subsequent events.