Micro-Textural Evidence for Magma Mixing in the Petrogenesis of Melilite-Bearing Nephelinite from Southeast China

Abstract Magma mixing is widely recognized as a prevalent magmatic mechanism, yet its significance in the formation of coexisting nephelinite and melilitite has frequently been overlooked. This oversight stems from the conventional assumption that nephelinite forms either through fractional crystallization from melilititic melt or by direct derivation from the mantle with rapid crust transit. To bridge this knowledge gap, we provide a comprehensive petrological analysis of melilite-bearing nephelinite from Yongquan in Southeast China, which serves as a compositionally transitional facies between nephelinite and melilitite. In addition to the typical nephelinite assemblage of olivine, nepheline, clinopyroxene, and titanomagnetite, this lava also contains melilite, monticellite, sodalite, cancrinite and perovskite. Such a mineral association has not previously been documented in nephelinitic lavas. Crystals within this assemblage commonly display disequilibrium textures and intricate zoning patterns. Olivine microphenocrysts exhibit systematic textural and P compositional zonation. P-rich skeletal cores grade outward into P-rich polyhedral mantles and terminate in P-poor rims, while maintaining nearly uniform Fo (~77-81) values Fo=100 × molar Mg/(Mg+FeT). The mantle zones contain patchy domains of high-CaO olivine (Fo ~65–76), and the outermost rims are resorbed and commonly enveloped by monticellite. Clinopyroxene microlites preserve sector zoning, overprinted by pronounced oscillatory chemical bands (in Mg#, Cr, Ni, and Mn) and resorption surfaces at core–mantle transitions, frequently accompanied by reverse zoning. The phosphorus zoning in olivine and sector zoning in clinopyroxene collectively suggest a progressive decrease in undercooling. Based on mineral textures and coexisting mineral phases, we infer that the unique mineral assemblage in the Yongquan melilite-bearing nephelinite resulted from mixing between a pre-existing nephelinitic magma and a recharged melilititic magma. These observations demonstrate that magma mixing is the primary mechanism producing coexisting melilitite and nephelinite in Yongquan melilite-bearing nephelinite, rather than simple differentiation from a melilititic parent. The microphenocryst and groundmass microlites characteristics in these nephelinite magmas reveal short-timescale dynamics, highlighting the necessity of integrating textural data with bulk-rock compositions for precise interpretation of source signatures.