Lithospheric Mantle Modification during Cenozoic Rifting in Central Europe: Evidence from the Księginki Nephelinite (SW Poland) Xenolith Suite

Clinopyroxene phenocrysts from the Księginki nephelinite (SW Poland)

“…As for Księginki nephelinite, no significant variation for major elements was noticed in bulk rock composition ( Figure 3 and Table 1), only clinopyroxene and olivine phenocrysts can be used as tracers of evolutionary changes. Puziewicz et al, (2011) pointed only slight variability in the composition of olivine phenocrysts; therefore, clinopyroxene seems to be the best carrier of information on the evolution of the nephelinite melt beneath the Księginki volcanic center. History of clinopyroxene crystals in silica undersaturated melt depends on the PT conditions during magma ascent.…”

“…Experimentally defined line marking the beginning of crystallization of clinopyroxene in the olivine nephelinite has positive slope in the P-T diagram (Bultitude and Green, 1971); thus the clinopyroxene phenocrysts that are crystallised from the host magma at higher pressures should be dissolved during adiabatic transport to the surface or if the heat loss was small. Puziewicz et al (2011) showed that lithospheric mantle beneath the Księginki volcano was subjected to both pervasive and channelised percolation of melt of composition similar to that of the host nephelinite. Crystal accumulation occurred in temporarily stagnant channels, resulting in olivine clinopyroxenites.…”

“…The Księginki nephelinite also contains few centimetres clinopyroxene megacrystals, which were interpreted as high-pressure precipitates from a magma similar to that responsible for origin of olivine clinopyroxenites. Significant load of mantle xenoliths, some reaching 50 cm in diameter (Puziewicz et al, 2011), infers high velocity of magma ascent, which should result in dissolution of the cognate clinopyroxene. At the same time, part of the numerous peridotite and clinopyroxenitic xenoliths was fragmented, providing xenocrystic material to the melt (Puziewicz et al, 2011).…”

“…Significant load of mantle xenoliths, some reaching 50 cm in diameter (Puziewicz et al, 2011), infers high velocity of magma ascent, which should result in dissolution of the cognate clinopyroxene. At the same time, part of the numerous peridotite and clinopyroxenitic xenoliths was fragmented, providing xenocrystic material to the melt (Puziewicz et al, 2011). Therefore, to establish the evolution of Księginki nephelinite, a clear division between xenocryst and possible phenocrysts should be made.…”

“…The lava which formed the Księginki nephelinite migrated through the lithospheric mantle by the channelised flow, causing metasomatism of the peridotites (Puziewicz et al, 2011). During this channelised and pervasive flow in the upper mantle at the depth between 35 and 50 km, crystallisation of the clinopyroxenite cumulates took place ( Figure 8; Puziewicz et al, 2011).…”