Composition of lithospheric mantle beneath southern margin of East European Craton evidenced by peridotitic xenoliths from Scania, S Sweden.

Southern Sweden (Scania region) is located in the peripherical parts of the East European Craton (EEC). In the Mesozoic, up to three pulses of volcanic activity took place between 191 and 110 Ma (Bergelin et al., 2006, IJES; Tappe et al., 2016, GCA). Some of the alkali basaltoids carry ultramafic, mafic and felsic xenoliths (Rehfeldt et al, 2007, IJES). In this study, we focused on the evolution of the lithospheric mantle sampled by anhydrous, spinel-facies lherzolites, harzburgites, and subordinate dunites.

Based on the Fo content in olivine, the peridotites were classified into three groups. Group X peridotites are characterized by Ca-rich olivine (890-1470 ppm) with Fo=91.1-91.7.  Enstatite has Mg#=91.5-91.9 and Al=0.16-0.22 atoms per formula unit (apfu), while the Cr-augite has Mg#= 90.8-91.2 and Al=0.21-0.28 apfu. Clinopyroxene is chemically homogenous in terms of trace elements and is LREE-enriched with positive Eu-anomaly. The Nd and Sr isotopic ratios in clinopyroxene are ¹⁴³Nd/¹⁴⁴Nd=0.512548 (εNd=2.63) and  ⁸⁷Sr/⁸⁶Sr=0.704237, respectively. Olivine in group Y peridotites is Ca-poor (<951 ppm) and has Fo=89.5-91.1, enstatite has Mg#=89.7-91.7, and Al content of 0.084-0.169 apfu. The Cr-diopside has Mg#=90.8-93.5 and Al=0.118-0.232 apfu. Trace element patterns in clinopyroxene allow subdivision of this group into two subgroups: subgroup Y1 – with heterogeneous LREE-enriched clinopyroxene, and subgroup Y2 – with homogenous LREE-enriched clinopyroxene; both groups are characterized by a positive Eu anomaly, but in subgroup Y1 it is significantly more pronounced. The Nd and Sr isotopic ratios in clinopyroxene from subgroup Y1 are ¹⁴³Nd/¹⁴⁴Nd=0.512624–0.512644 (εNd=4.13-4.52) and ⁸⁷Sr/⁸⁶Sr=0.703027–0.703100, therefore significantly more depleted than group X. In group Z peridotite the Fo content in olivine is 88.1-89.1, the Mg# in enstatite is 89.1-89.5 and its Al content is 0.19-0.20 apfu. The Mg# of Cr-diopside is 88.5-89.4 and the Al content is 0.24-0.25 apfu. The trace elements contents in clinopyroxene is homogenous and the REE pattern is flat at values double that in the primitive mantle.         

 The highest equilibration temperatures were estimated for the group X xenoliths, where T_WES=1101-1110 °C (Witt-Eickschen and Seck, 1991, CMP) and T_BK=1214-1241 °C (Brey and Köhler, 1990, JoP).  The temperatures calculated for group Y xenoliths are T_WES=875-1033 °C and T_BK=872-1027 °C and do not significantly differ between subgroups. Temperatures recorded by the group Z sample are T_WES=1040-1056 °C and T_BK=1065-1081 °C.

The composition of group X peridotites suggests their metasomatism by a high-temperature mafic melt resembling the basaltoids from Scania. Alternatively, they may represent high-pressure cumulates, as suggested by their coarse-grained texture. The group Y peridotites record cryptic metasomatism of a significantly depleted peridotite (melt extraction ranging typically between 25 and 30%) by a carbonatitic melt. The carbonatitic metasomatic agent was fractionating chromatographically from REE-, Th- and U-rich in subgroup Y2 to -poor in those elements in subgroup Y2. The group Z peridotite possibly represents depleted peridotite which was further metasomatized by a mafic melt. The lithospheric mantle beneath the marginal part of EEC has a complex composition, which is however different from a typical cratonic mantle.

 

Founded by Polish National Science Centre grant no. UMO-2016/23/B/ST10/01905 and WTZ PL 08/2018.