THE PROTOLITHES OF MANTLE LITHOSPHERE IN THE CENTRE AND NORTH-EAST OF SIBERIAN CRATON

Solov’eva L.V.*, Kalashnikova T.V.**, Kostrovitsky S.I.**

*Institute of Earth Crust SB RAS, Irkutsk, Russia; Solv777@crust.irk.ru

**Vinogradov Institute of Geochemistry SB RAS, Irkutsk, Russia; Kalashnikova@igc.irk.ru

 

The different mechanisms of lithosphere mantle formation and followed evolution are considered by the example of REE chemistry and Sr, Nd, O, S, C isotopy of deep-seated xenoliths from the Udachnaya  kimberlite pipe (central part of Siberian craton) and the Obnajennaya and Sludyanka pipes (in the north-east of craton). In the craton centre the mantle lithosphere is argued to originate as high-temperature light cumulates of high-magnesium olivine and orthopyroxene in ancient magmatic ocean. The cratonisation mechanism is considered as undertrusting and briquetting of little ultramafic plates with basalt cover under the largest plates. This mechanism is agreed with orthopyroxene enrichment of mantle lithosphere and presence of mafic matter (eclogites). It is also discussed the restite and subduction hypotheses. Afterwards the mantle lithosphere was treated by two large metasomatism events influenced by carbonatite fluids-melts during general metamorphism and early stage of kimberlite-forming cycle. The latter event was connected with the thermo-chemical plume uplifting to lithosphere base, the development of oil parental beds and biota mass extinction in Upper Devonian.

Geochemical and isotopic data for deep-seated xenoliths from Upper Jurassic Kuoika kimberlite field are evidence of lithosphere mantle composition beneath north-east geoblock. It is believed that no less than 50% substance was a result of basite melt crystallisation in supersubduction zone. Possibly, the layered series of Sp, Sp-Grt, Grt pyroxenites – Ol-websterites – lherzolites was formed as cumulates of basite melts in spinel facies. The influence of fluid or melt from mafic-ultramafic slab is supposed on the geochemical date. The residues from MORB-type mantle melting are believed to present in mantle lithosphere. Furthermore, the metasomatites connected with Perm-Triassic magmatic cycle (Phl-Ilm, Phl, Amph-Phl metasomatites) are developed. The Birekte terrain dating confirms the Lower Proterozoic formation time and later followed the accretion to Siberian craton.