Incompatible element-enriched mantle lithosphere beneath kimberlitic pipes in Priazovie, Ukrainian shield: volatile-enriched focused melt flow and connection to mature crust?

Major, minor and trace element compositions of mantle xenocrysts from Devonian kimberlite pipes in the Priazovie give an insight into the mantle structure beneath the SE Ukranian Shield and its evolution. Garnets yield low temperature conditions as determined by monomineral thermobarometry. The mantle lithosphere is sharply divided at 4.2 GPa, marked by a high temperature Cpx-Ilm-Phl trend, eclogites and changes in pyrope geochemistry. Seven layers are detected: I^st layer at 2.5–1 GPa is enriched mantle (Fe#Ol ~ 0.11 − 0.14) with Gar- pyroxenites and Sp peridotites; II^nd at 2.5–3.2 GPa – Gar-Sp (Fe#Ol 0.08 − 0.10) peridotite. III^d at 4.3–3.2 GPa is formed of Archaean- Proterozoic peridotites with Fe#Ol ~0.07 − 0.095. IV^th at 3.2–5 GPa- contains pyroxenitic Gar with higher Ca, eclogites, Chr and Cpx (Fe#Ol ~0.10 − 0.125); V^th at 5.8 − 5 GPa is marked by sub-Ca garnets, Cr-rich chromites and Mg-Cr ilmenites; VI^th layer at 5.8–6.8 GPa contains Fe-enriched pyropes, almandines and Cr-Mg ilmenites near the lithosphere base; VII^th layer > 6.8 GPa consists of ‘hot’ Fe-rich garnets. Garnets show increasing enrichment in LREE, LILE, Hf, Zr with decreasing pressure. Primitive garnets have round REE patterns; depleted ones have S-type patterns inflected at Nd. Garnets from 6.5 to 3 GPa show increasing La/Yb_n, Zr-Hf, LILE. Peridotitic clinopyroxenes have inclined linear trace element patterns rounded from La to Pr with high LILE and HFSE levels. The Fe-rich group (reacted with eclogites) shows bell-shaped irregular patterns with LILE close to the LREE levels. A possible reason for LILE (HFSE and) enrichment of the upper part of the mantle is subduction metasomatsm in Archaean times (with participation of mature continental sediments) activated by plumes at 1.8 Ga and earlier which produced pervasive focused melt flow with remelting of mica-amphibole metasomatites giving continuous REE and LILE enrichment in mantle lithologies from 5.8 to 2.5 GPa.