Identifying Mantle Carbonatite Metasomatism through Os-Sr-Mg Isotopes in Tibetan Ultrapotassic Rocks

Abstract:

Mantle-derived magmas at convergent plate boundaries provide unique insights into the nature of materials subducted to and recycled from depths. Here we present a study of Os-Sr-Mg isotopes on the Oligocene-Miocene ultrapotassic rocks aimed at better understanding geochemical evolution of mantle lithosphere beneath southern Tibet. Isotopic data confirm that ultrapotassic rocks in southern Tibet are of mantle origin, but underwent crustal contamination as evidenced by variably high ¹⁸⁷Os/¹⁸⁸Os that obviously deviates from normal mantle reservoir. Still some samples with mantle-like ¹⁸⁷Os/¹⁸⁸Os exhibit variably lower δ²⁶Mg than mantle and crustal lithologies, suggesting that the isotopically light Mg may not be the result of crustal contamination but retain specific fingerprint of carbonate-related metasomatism in mantle sources. Mantle carbonatite metasomatism is also manifested by the inverse δ²⁶Mg-⁸⁷Sr/⁸⁶Sr correlation, as well as the relative depletion of high field strength elements and the enrichment of CaO in ultrapotassic rocks. The positive co-variation between δ²⁶Mg and Hf/Sm in low-¹⁸⁷Os/¹⁸⁸Os ultrapotassic rocks further highlights the potential of recycled dolomites to change mantle Mg isotopic compositions. The correlated spatial variations of δ²⁶Mg and Hf/Sm indicate that the carbonatitic metasomatism during the northward subduction of the Neo-Tethyan slab may have laid the framework for generating postcollisional ultrapotassic rocks in southern Tibet.