Petrogenesis and Lu-Hf Dating of (Ultra)Mafic Rocks from the Kutna Hora Crystalline Complex: Implications for the Devonian Evolution of the Bohemian Massif
Abstract
The Lu-Hf isotope system and Sr-Nd-Hf-Os isotope systematics of mantle rocks are capable of unravelling the early processes in collision belts, especially in a hot subduction context where the Sm-Nd and U-Pb systems in crustal rocks are prone to resetting owing to high temperatures and interaction with melts during exhumation. To improve models of the Devonian-Carboniferous evolution of the Bohemian Massif, we investigated in detail mafic and ultramafic rocks (eclogite, pyroxenite, and peridotite) from the ultrahigh-pressure and ultrahigh-temperature Kutna Hora Crystalline Complex (KHCC: Uhrov, Becvary, Doubrava, and Spaeice localities).
Petrography, multiphase solid inclusions, major and trace element compositions of rocks and minerals, and radiogenic isotopic data document contrasting sources and protoliths as well as effects of subduction-related processes for these rocks. The Uhrov peridotite has a depleted composition corresponding to the suboceanic asthenospheric mantle, whereas Beevary and Doubrava peridotites represent lithospheric mantle that underwent melt refertilization by basaltic and SiO2-undersaturated melts, respectively.
Multiphase solid inclusions enclosed in garnet from Uhrov and Beevary peridotites represent trapped H2O +/- CO2-bearing metasomatizing agents and Fe-Ti-rich melts. The KHCC eclogites either formed by high-pressure crystal accumulation from mantle-derived basaltic melts (Uhrov) or represent a fragment of mid-ocean ridge basalt-like gabbroic cumulate (Spaeice) and crustalderived material (Doubrava) both metamorphosed at high P-T conditions.
The Lu-Hf age of 395 +/- 23 Ma obtained for the Uhrov peridotite reflects garnet growth related to burial of the asthenospheric mantle during subduction of the oceanic slab. By contrast, Spaeice and Doubrava eclogites yield younger Lu-Hf ages of similar to 350 and 330 Ma, respectively, representing mixed ages as demonstrated by the strong granulite-facies overprint and trace element zoning in garnet grains.
We propose a refined model for the Early Variscan evolution of the Bohemian Massif starting with the subduction of the oceanic crust (Saxothuringian ocean) and associated oceanic asthenospheric mantle (Uhrov) beneath the Tepla-Barrandian at >= 380 Ma, which was responsible for melt refertilization of the associated mantle wedge (Beevary, Doubrava). This was followed by continental subduction (similar to 370-360 Ma?) accompanied by the oceanic slab break-off and incorporation of the upwelling asthenospheric mantle into the Moldanubian lithospheric mantle and subsequent coeval exhumation of mantle and crustal rocks at similar to 350-330 Ma.