| Popis |
A variable assemblage of ultrabasic rocks along with minor eclogites investigated on a small area of a few km2 (Gföhl unit, Moldanubian Zone of the Bohemian Massif) reflects incorporation of contrasting mantle domains within a subduction-related tectonic mélange during the Variscan orogeny. Based on mineral composition, whole-rock chemistry, isotopic signatures, and pressure-temperature (P–T) estimates, four principal lithological types have been distinguished (1) spinel harzburgite (2) garnet lherzolite (3) spinel websterite and (4) eclogite. Spinel harzburgite, exclusively associated with HT migmatized gneisses, corresponds to the refractory oceanic lithosphere, as demonstrated by whole-rock composition (low Al2O3, CaO), chemistry of spinel (Cr# <0.4) and Al-rich pyroxene (Al2O3 up to 9 wt%). By contrast, garnet lherzolite, predominantly enclosed in HP felsic granulite, represents a fragment of the subcontinental mantle wedge, considering the strong fertile character (high Al2O3, TiO2, Yb) and spinel chemistry (Cr# >0.5). Websterites likely represent products of decompression partial melting of the asthenospheric mantle with a variable input of crustal component, whereas eclogites correspond to HP crystal cumulates from partial melts migrating through the Variscan mantle wedge. Both peridotite protoliths experienced various degrees of secondary refertilization, recorded as a cryptic metasomatic overprint, due to interaction with subduction-related silicate melts, from which numerous websterite and rare eclogite layers crystallized. The secondary mantle refertilization via melt-peridotite reaction is well-documented by decreasing bulk Mg# along with MgO/SiO2, elevated Al2O3/SiO2, TiO2 and FeOtot contents, and isotopic composition (87Sr/86Sr338 ~ 0.7051). Moreover, the positive correlation between highly fluid-immobile incompatible trace elements (e.g. Ti, Sc, V, Zr, Yb) and distribution of REE and Li is consistent with the melt refertilization trend. During the Variscan subduction, lherzolite experienced UHP conditions (42 kbar, 1100 °C). The presence of Cr-spinel relics preserved in garnet suggests that lherzolite was dragged from the shallow mantle wedge to deeper levels of the subduction zone, probably along with the underlying subducting oceanic plate involving refractory harzburgite. This deep burial during the Variscan subduction was closely followed by rapid exhumation dated by the Lu-Hf age of 338.4 ± 6.3 Ma, corresponding to re-equilibration at lower crustal levels or possibly at greater depths, closer to peak P conditions. Decompression-induced partial melting of upwelling asthenosphere with a contribution of crustal material resulted in derivation of primitive to mildly evolved transient melts (87Sr/86Sr338 ~ 0.7049–0.7062) represented by websterites, percolating through and metasomatizing wall-rock peridotites. The rapid exhumation and following juxtaposition of genetically diverse yet closely associated ultrabasic rocks provide a unique insight into the history of Earth mantle, evolving during the formation of the Variscan orogenic belt in Central Europe within contrasting geological settings, from ocean ridge environment to great depths of subduction zone.
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