Eclogite and garnet pyroxenite xenoliths from kimberlite pipes of north Siberian craton - evidences of subduction processes and cumulate origin

The lithospheric mantle structure and evolution is one of the fundamental problems of the Earth's history. Eclogites and clinopyroxenite xenoliths are characterized by a similar two-mineral composition (garnet and clinopyroxene), but differ in mineralogical and petrographic features (Gonzaga et al., 2010). Questions of their origin and relationship with peridotites remain controversial. There are several classifications of eclogites based on various attributes: structural and textural features (Mercier & Nicolas, 1975; MacGregor & Carter, 1970), chemical composition of garnet (Coleman, 1965), clinopyroxene (Taylor & Neal, 1989), as well as the whole rock composition (Aulbach et al., 2016 and other), the given classifications may not coincide. The geochemical properties of eclogite xenoliths from kimberlite pipes suggest two main points of view for genesis: implication of subduction processes or cumulates of high-pressure melting in lithosphere mantle (Condie, 1993; Jacob et al., 1994). The "classical" cratonic eclogites represent an ancient oceanic crust subsequently subducted and altered possible further metasomatic processes. These rocks are characterized by significant variations in the composition of minerals, a relatively high content of Al₂O₃ (14-20 wt%) and a low MgO content (10-15 wt%), depletion of elements of the LREE and an Eu anomaly (Gonzaga et al., 2010). In addition, eclogites have a wide range of oxygen isotopic composition in garnet δ¹⁸O 4.51 - 8.69 (much higher than mantle values ​​5.3 ± 0.3) (9). Garnet pyroxenites are characterized by a more magnesian garnet - pyrope and bulk composition (MgO - 15-20 wt.%). The oxygen isotope composition of Grt from clinopyroxenites is close to that of the mantle - δ¹⁸O 5.2 - 5.8. It is assumed that these rocks are a consequence of the polybaric partial melting at high temperatures and pressures (Gonzaga et al., 2010). The mantle xenoliths from upper-Jurassic Obnajennaya kimberlite pipe (Kuoika field, Yakutia) were studied. Eclogites and clinopyroxenites occupy about 10-15% population among xenoliths. Garnet in the eclogites differs from that in the clinopyroxenites by a higher content of CaO and FeO (Prp_55-62 Alm_22-30Grs_8-18 in clinopyroxenites and Prp_40-45Alm_13-29Grs_15-30 in eclogites). Clinopyroxenes are distinguished by reduced magnesia content (Mg# 91-84), as well as low calcium content (16-18 wt.%). The high contents of jadeite components in the clinopyroxene (NaAl[Si₂O₆] - 25-32%) classify this group of rocks as eclogites. The high δ¹⁸O varies in eclogite Cpx (more than 6.0), positive Eu anomaly is assumed that the formation of the protolith of the xenolith group occurred as melts in the subduction zone during accretion of the Birekte block to the Siberian craton (Rosen, 2003). However, the presence of garnet clinopyroxenites with narrow variations in mineral composition and relatively low δ¹⁸O suggests melting processes in the lithospheric mantle and the formation of megacrystalline pyroxene cumulates.

The research was supported by Russian Science Foundation grant №20-77-00074.