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Polyphase geodynamic evolution of heterogenous mantle beneath the Variscan Orogenic Belt (Moldanubian Zone, Bohemian Massif)

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Year of publication 2022
Type Appeared in Conference without Proceedings
MU Faculty or unit

Faculty of Science

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Description The high-grade Gföhl Unit of the Moldanubian Zone (Bohemian Massif) hosts diverse (ultra)basic rocks derived from different mantle domains during the Variscan continental collision. The significant mantle heterogeneity is documented by the occurrence of contrasting peridotite types (suboceanic vs. subcontinental) closely associated with spinel- and/or garnet-bearing pyroxenites and eclogites within a relatively small subduction-related tectonic mélange (Kubeš et al., 2022). During the Devonian subduction of the Saxothuringian ocean beneath the Teplá-Barrandian-Moldanubian block (?380 Ma; Ackerman et al., 2020), harzburgite was incorporated within the same subduction zone where lherzolite evolved. Concurrently, lherzolite was dragged from the shallow mantle section (?10 kbar) into great depths within subduction channel and experienced UHP conditions (42 kbar, 1100 oC) closely followed by rapid mantle exhumation (~338 Ma; Kubeš et al., 2022). The polyphase mantle refertilization via melt-peridotite interaction included: 1) partial melting of the subducted slab involving sedimentary material that produced parental melts of garnet pyroxenites and eclogites (87Sr/86Sr338 ~0.7109) penetrating and metasomatizing overlaying lithospheric mantle (lherzolite), and 2) decompression-induced melting of the upwelling asthenosphere with a contribution of crustal material generating spinel pyroxenites (87Sr/86Sr338 ~0.7062) during rapid exhumation, which metasomatized subducted fragment of the oceanic plate (harzburgite). The extensive mantle refertilization is well-recorded by major and trace element geochemistry (elevated Al2O3/SiO2, TiO2, FeOtot, HFSE, LREE, Li) and isotopic composition of peridotites (87Sr/86Sr338 ~0.7051). The close spatial association of genetically contrasting mantle-derived assemblages thus provides a unique insight into the history of the Earth mantle that evolved during the Variscan collision within distinct geological settings, from ocean floor to great depths of subduction zone.
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