Variations in Trace Element and Isotope Composition of Neoarchean Mafic Granulites of the Southwest Siberian Craton: a Consequence of Various Mantle Sources or Crustal Contamination

Abstract

The paper presents geochemical and isotopic characteristics of Neoarchean (2.7–2.66 Ga) mafic granulites of the Sharyzhalgay uplift in the southwestern Siberian craton. Mafic and predominant felsic granulites compose fragments of the metamorphic complex among the Neoarchean and Paleoproterozoic granitoids. The mafic granulites are characterized by the mineral association Cpx + Pl ± Hbl ± Opx ± Qz and include two types with different major and immobile trace element contents. The dominant rocks of the first type have a wide range of Mg# and concentrations of TiO₂ and immobile trace elements (REE, Zr, Nb), and mainly positive ε_Nd(Т) values. The first type of mafic granulites show elevated (La/Sm)_n and enrichment in Th and LREE relative to Nb, which is typical of subduction-related or crustally contaminated basalts. The absence of negative correlation between (La/Sm)_n and ε_Nd(Т) and a clear positive correlation of TiO₂ with Nb testify against the effect of crustal contamination on the composition of the mafic granulites. The magmatic protoliths of the first type of mafic granulites are suggested to form by the melting of depleted peridotites of the subcontinental lithospheric mantle modified by melts derived from basalts or terrigenous sediments of the subducting plate. Mafic granulites of the second type have a narrower range of Mg#, TiO₂ content, positive ε_Nd(Т), flat rare earth patterns and no subduction signatures, which indicates an asthenospheric depleted mantle source. Mafic granulites contaminated by the Paleoarchean crust are characterized by increased (La/Sm)_n, depletion in Nb relative to Th and LREE, and negative ε_Nd(Т) values. Post-magmatic influence of granitoids leads to the enrichment of mafic granulites in biotite and apatite, an increase in concentrations of K₂O, P₂O₅, a significant enrichment in Zr, Nb, Th, LREE, and negative ε_Nd(Т) values. The difference between mafic granulites of the first and second types is not related to crustal contamination, but is caused by melting of two types of sources: asthenospheric and subcontinental lithospheric mantle. The subcontinental lithospheric mantle of the Irkut block was isotopically depleted at the Neoarchean time (∼2.7 Ga), and its enrichment in incompatible trace elements was likely caused by felsic melts generated from the rocks of subducting plate immediately prior to mafic magmatism.