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

Abstract

Spinel lherzolite and spinel harzburgite mantle xenoliths occur in the 4.56 ± 0.2 Ma 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 0.34–0.45 wt.% NiO and < 780 ppm Ca; orthopyroxene is Al enstatite (#mg [(Mg / (Mg + Fe tot) * 100] 0.90–0.92, Al 0.06–0.17 a.pfu); clinopyroxene is Al–Cr diopside (#mg 0.91–0.93, Al 0.104–0.197 a.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 1800 ppm), clinopyroxene and spinel occur in small interstitial patches containing feldspar. Clinopyroxene II is Al-poor (0.018–0.070 a.pfu, #mg 0.921–0.932) when coexisting with alkali feldspar, but Al-rich (0.046–0.261 a.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 CO 2-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.