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

Magdalena Matusiak-Małek (1) , Jacek Puziewicz (1) , Theodoros Ntaflos (2) , Michel Grégoire (3) , Hilary Downes (4)

1 UWr - Institute of Geological Sciences [Wrocław]
2 Department of Lithospheric Research [Wien]
3 DTP - Dynamique terrestre et planétaire
4 Department of Earth and Planetary Sciences [UCL/Birkbeck]

Spinel lherzolite and spinel harzburgite mantle xenoliths occur in the 4.56 ± 0.2 Ma Lutynia basanite (SW Poland). Only one contains minor pargasitic amphibole. Minerals forming the xenoliths are unzoned. Olivine (Fo89.8 – 92.4) contains 0.34 – 0.45 wt.% NiO and <780 ppm Ca, orthopyroxene is Al enstatite (#mg 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 (Fo88.0–91.7, 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 (An4 -14, Ab49-71), or with plagioclase (An37-46, Ab51-58) it is Al – rich (0.046 – 0.261 a.pfu; #mg 0.907 – 0.925). Four types of REE patterns are found in the clinopyroxene: (group A) flat HREE with LREE content increasing smoothly; (group B) flat HREE with an abrupt increase of MREE and LREE; (group C) LREE- enriched, flat HREE with negative inflection at MREE; (group D) smoothly LREE enriched and with no flat HREE pattern. The exception is clinopyroxene from xenolith MM30 which is extremely depleted in LREE. Cpx I from all the xenoliths (excluding MM30) contain high amounts of Th and U. Cpx trace element compositions record a degree of partial melting event from 8 to 15% (in MM30 >27%). Major- and trace-element compositions of minerals record further cryptic metasomatism induced by a CO2-bearing alkaline melt. Variable REE patterns for Cpx I from groups A, B and C are due to chromatographic enrichment, with group A peridotites located closest 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 regime mantle facies into the spinel one where they cooled and equilibrated at 960 – 1000 ºC. The patches formed by the second generation of minerals are an effect of pre – eruption infiltration by the basanitic melt.

Mots clés: Lherzolite–harzburgite xenoliths; Upper mantle; Chromatographic metasomatism; SW Poland; Cenozoic basanite

Domaines: Pétrographie

Magdalena Matusiak-Małek, Jacek Puziewicz, Theodoros Ntaflos, Michel Grégoire, Hilary Downes. Metasomatic effects in the lithospheric mantle beneath the NE Bohemian Massif: a case study of Lutynia (SW Poland) peridotite xenoliths. Lithos, 2010, 117, pp.49-60. ⟨10.1016/j.lithos.2010.02.005⟩. ⟨hal-00535791⟩