Tracking carbonatitic melt infiltration in the Miaoya carbonatite-syenite complex (central China) from zircon textures and geochemistry

The Miaoya carbonatite-syenite complex, located at the Qinling Orogenic Belt (central China), which hosts both Nb and REE mineralization. We studied the relationship of zircon grains within the carbonatitic and syenitic matrix by scanning electron microscope and optical microscopy combined with cathodoluminescence (SEM- and OM-CL) to identify primary versus secondary features. The primary zircon grain zones with magmatic oscillatory zoning and dull CL luminance crystallized at a relatively early stage of magma differentiation. In contrast, the secondary zones of zircon grains showing bright CL luminance were formed by recrystallization and contained numerous fractures with micro-inclusions of calcite and apatite, etc. The complicated texture of zircons strongly implies that zircons in both syenite and carbonatite had experienced solid-state recrystallization and dissolution-reprecipitation processes, respectively. Temperatures calculated using the Ti-in-zircon thermometer are similar to 600 degree celsius for the secondary zircons both in syenite and carbonatite. Hence, recrystallization processes were probably caused by varying degrees of metasomatic reactions involving late-stage residual carbonatitic melts, which were initially generated by silicate-carbonate liquid immiscibility at an earlier stage of magma evolution. The metasomatic reactions led to distinct geochemical and isotopic compositions of the secondary zircon. For instance, the secondary zircon is characterized by relatively lower concentrations of trace elements (e.g., REE, Th, U and Pb), LREE/HREE ratios, disturbances of the U-Pb ages and heterogeneous epsilon Hf(t) values (0.41 to 10.2). Our results imply that metasomatism due to infiltration of late-stage residual carbonatitic melts could play an important role in the evolution and of carbonatite-syenite system and related mineralization.