Metasomatic effects in the lithospheric mantle beneath the NE Bohemian Massif: a case study of Lutynia (SW Poland) peridotite xenoliths

Spinel lherzolite and spinel harzburgite mantle xenoliths occur in the 4.56±0.2Ma Lutynia basanite in SW Poland. Only one studied xenolith contains minor pargasitic amphibole. Minerals forming the xenoliths are chemically unzoned. Olivine (Fo [(Mg/(Mg+Fe^tot))*100] _89.8-92.4) contains 0.34-0.45wt.% NiO and <780ppm Ca; orthopyroxene is Al enstatite (#mg [(Mg/(Mg+Fe^tot)*100] 0.90-0.92, Al 0.06-0.17a.pfu); clinopyroxene is Al-Cr diopside (#mg 0.91-0.93, Al 0.104-0.197a.pfu). Exsolved orthopyroxene occurs in the clinopyroxene and vice versa. Al-Mg spinel occurs in symplectites with clinopyroxene. Second generation crystals of olivine (Fo_88.0-91.7, Ca up to 1800ppm), clinopyroxene and spinel occur in small interstitial patches containing feldspar. Clinopyroxene II is Al-poor (0.018-0.070a.pfu, #mg 0.921-0.932) when coexisting with alkali feldspar, but Al-rich (0.046-0.261a.pfu; #mg 0.907-0.925) when found with plagioclase. Four types of REE (rare earth elements) patterns are found in the clinopyroxene: (group A) flat HREE (heavy REE) with LREE (light REE) content increasing smoothly; (group B) flat HREE with an abrupt increase of MREE (medium REE) and LREE; (group C) LREE-enriched, flat HREE with negative inflection at MREE; (group D) smoothly LREE-enriched with no flat HREE pattern. The exception is clinopyroxene from xenolith MM30 which is extremely depleted in LREE. Clinopyroxene I from all the xenoliths (excluding MM30) contains high amounts of Th and U.Clinopyroxene trace element compositions record 8-15% of partial melting. Major and trace element compositions of minerals record later cryptic metasomatism induced by a CO₂-bearing alkaline melt. Variable REE patterns for clinopyroxene I from groups A, B and C are due to chromatographic enrichment, with group A peridotites located close to the metasomatic source and group C being the furthest. The pargasite-bearing MM04 was probably the closest to the metasomatic source and thus recorded incipient modal metasomatism. Clinopyroxene-spinel symplectites and ortho-/clinopyroxene exsolutions suggest that the peridotites were transported from the garnet-lherzolite facies into the spinel one where they cooled and equilibrated at 960-1000 °C. The patches formed by the second generation of minerals are the effect of pre-eruption infiltration by the basanitic melt.