Rongcheng Ultra-Calcic Garnet Pyroxenite: Its Geochemistry, Origin and Implications for Pre-UHP Tectonics in the Sulu UHP Metamorphic Belt

We report an unusual garnet pyroxenite, as a nodule enclosed within kyanite eclogites from Rongcheng, northeastern Sulu UHP metamorphic belt. Apart from its unusual texture, high degrees of various minerals (garnet, titanite, and Fe-Ti oxides) and solid exsolution within garnet or clinopyroxene, this nodule has unusual major and trace element geochemistry as shown by (1) low SiO₂ (42.5%-43.1%), extremely high CaO (21.4%-21.9%), and high CaO/Al₂O₃ ratio (1.46-1.64); (2) high TiO₂ (1.77%-1.89%), Zr (to 150 μg/g), V (359-419 μg/g), Nb (to 8 μg/g), and Y (17.7-23.1 μg/g) contents; (3) LREE-enriched, and nepherine normative; and (4) low Cr, Ni, and Co. Previous study on the mineral chemistry and texture relationships between different generations of minerals has suggested that it was formed as a cumulative clinopyroxene magacryst with high Ca-tschermakite component (31%-34%) from a gabbroic magma at p= (15-22) ×10⁵ kPa, and T > 1 100 ℃. Major and trace element geochemistry on this nodule and its host kyanite eclogite suggest that the precursor of the Rongcheng pyroxenite was derived either from an ultramafic magma highly fractionated by accumulation of olivines or from an ultra-calcic basic magma that had lost some K₂O and Na₂O. No matter which was responsible for the formation of its precursor, both require a melting event and magma differentiation to occur at a pressure and temperature greater than 15×10^-5 kPa and 1 300 ℃, respectively. This contribution suggests that protoliths for some of the eclogites within the Sulu UHP metamorphic belt formed in a tectonic environment distinct from those of Neoproterozoic age.