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

Xenoliths in kimberlites are the most promising material for studying the composition and structure of the lower levels of the continental crust. This study is aimed at the estimation of P–T parameters and fluid regime of metamorphism for garnet–biotite–feldspar and orthopyroxene–garnet–biotite–feldspar rocks found as xenoliths in kimberlites of the Yubileynaya and Sytykanskaya pipes, Yakutian kimberlite province. The 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. Evidence of this reaction is specific reaction textures in the rocks, negative correlations between the contents of the minerals, and petrochemical characteristics of the rocks. Modeling of the mineral assemblages of the xenoliths using the pseudosection approach (PERPLE_X) revealed two groups of rocks corresponding to different depth levels of the Siberian cratonic crust. For rocks without orthopyroxene or with this mineral as single relics, the pressure was estimated at 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 had initially occurred. This indicates that the metamorphic evolution of the rocks during their exhumation was 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 strongly metasomatized rocks were located closest to the place of accumulation of crystallizing magmas.