Amphibolites from the Szklarska Poręba hornfels belt, West Sudetes, SW Poland: magma genesis and implications for the break-up of Gondwana
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- Ilnicki, S. Int J Earth Sci (Geol Rundsch) (2012) 101: 1253. doi:10.1007/s00531-011-0727-2
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Amphibolites from the Szklarska Poręba hornfels belt (northern part of the Karkonosze-Izera Massif) represent rocks of alkali-basalt composition metamorphosed during Variscan times. Despite the intense thermal influence of the Karkonosze granite superimposed on the effects of regional amphibolite-facies metamorphism, the geochemical affinities of the Szklarska Poręba amphibolites are well preserved. They are similar to alkaline OIB basalts derived from an enriched (undepleted) sub-lithospheric source in the garnet stability field at depths ca 80–120 km. Trace-element characteristics and geochemical modelling indicate that the source was not modified by metasomatism in a supra-subduction zone or by alkali (silicate, carbonatitic) infiltration. Subsequent intra-crustal fractional crystallization involved olivine and clinopyroxene, and subordinate spinel and, presumably, plagioclase. The chemical composition of the rocks is most similar to that of modern magmas generated in an extensional setting (intra-continental rift). Neither geochemical characteristics nor estimated mantle temperatures only slightly higher than those of ambient mantle convincingly attest to the involvement of deep-mantle plume activity. Instead, decompression melting of passively upwelling asthenosphere beneath opening fractures in fragmented lithosphere is invoked. The origin of the amphibolite protolith was presumably associated with the Early Palaeozoic rifting of northern Gondwana, well documented throughout the Karkonosze-Izera massif. Locally rifting must have ceased earlier (immature rift) as reflected by mafic dykes exposed in the northern part of the massif, i.e., in the Szklarska Poręba hornfelses, and by the Izera gneisses and the Stara Kamienica metapelites. A passive rift system controlled by lithosphere extension provides a plausible explanation for the origin of mafic rocks in the Karkonosze-Izera Massif and sheds light on possible mechanisms involved in the break-up of Gondwana.