Petrogenesis and Geological Implications of Sodium-Rich Anorthosite from the Kempirsai Ophiolite in the Ural Orogenic Belt, Kazakhstan

The Kempirsai ophiolite is one of the largest and well-preserved ophiolitic bodies with a relatively complete succession in the southern Ural Orogen, Kazakhstan. This paper presents systematic petrological, zircon U-Pb geochronological, and major and trace element geochemical studies on the newly identified sodium-rich anorthosite, aiming to precisely constrain the formation age of the Kempirsai ophiolite and elucidate the petrogenesis and tectonic setting of its leucocratic cumulates. The results demonstrate that the sodium-rich anorthosite yields a weighted mean zircon U-Pb age of （373.9±1.4） Ma, indicating an Early Late Devonian magmatic event, which provides high-precision chronological constraints on the Kempirsai ophiolite. Geochemically, the anorthosite is characterized by high contents of SiO₂, Al₂O₃, Na₂O and Sr, high Sr/Y ratios and Mg# values, low contents of MgO, Y and Yb, as well as a distinct Na-rich and K-poor geochemical signature. It is also enriched in light rare earth elements and large-ion lithophile elements, with pronounced positive Eu anomalies, indicative of typical adakitic rocks. The magma was generated by partial melting of the subducted oceanic slab under garnet-amphibolite facies conditions, and experienced significant melt-rock interaction with depleted mantle peridotite during magma ascent. Comprehensive interpretations suggest that the sodium-rich anorthosite formed in an intra-oceanic subduction setting, indicating that the Paleo-Ural Ocean was still undergoing subduction and consumption during the Late Devonian. Its formation age is coupled with the main ore-forming period of regional podiform chromitite, implying that chromitite mineralization is closely related to adakitic melts and fluids activities derived from dehydration melting of the subducted oceanic slab. This study provides key evidence for the Late Paleozoic tectonic evolution of the Southern Ural Orogen and the metallogenic dynamics of ophiolite-hosted chromitite.