Metasomatism in the Precambrian Crust of the Siberian Craton: Results of a Study of Garnet(±orthopyroxene)-biotite-feldspar Xenolith Rocks From Yubileinaya and Sytykanskaya Kimberlite Pipes, Yakutia

Abstract

Xenoliths in kimberlites are the most perspective objects for studying the composition and structure of the lower levels of the continental crust. Present work is aimed at estimation of P-T fluid conditions of metamorphism for garnet-biotite-feldspar and orthopyroxene-garnet-biotite-feldspar rocks represented as xenoliths in kimberlites of the Yubileynaya and Sytykanskaya pipes, Yakutian kimberlite province. Seven studied samples show inverse dependences of relative contents of garnet and orthopyroxene, orthopyroxene and biotite, garnet and plagioclase, plagioclase and potassium feldspar. This indicates a consistent series of transformations of the assemblage garnet + plagioclase + orthopyroxene ± quartz to the assemblage garnet + biotite + potassium feldspar. In this process, the replacement of plagioclase by potassium feldspar was the leading reaction. Now it is represented by specific reaction textures in the rocks, negative correlations of the mineral contents, as well as in petrochemical characteristics of the rocks. Modeling of xenolith mineral assemblages using the pseudosection approach (PERPLE_X) revealed two groups of rocks corresponding to different depth levels of the Siberian cratonic crust. For rocks where orthopyroxene is absent or is present as single relics, pressure estimates are 9.5–10 kbar, and it is 6–7 kbar for orthopyroxene-bearing samples. The xenolith rocks have close metamorphic peak temperatures of 750–800°C. They experienced 200–250°C cooling and 3–4 kbar decompression, regardless of the level of the crust at which they were initially located. This points to the metamorphic evolution of the rocks during their exhumation, probably associated with collisional processes during the amalgamation of individual terrains of the Siberian craton. Xenoliths enriched in K-feldspar might have been products of metamorphic reactions with participation of aqueous-(carbonic)-salt fluids, which were sourced from basaltic magmas in the lower crust. The most metasomatized rocks were located closest to the place of accumulation of crystallizing magmas.