ORIGIN OF THE IGNEOUS ZIRCONS: Constraints from Morphology and Trace Element Chemistry

Detailed study on the morphology and trace element chemistry of the Zijinshan alkaline complex reveals that there is no igneous zircon in the phonolitic, trachytic and volcanic breccia, with minor xenocryst zircons in the purple and white phonolite and trachyte porphyry. LA-ICP MS analysis results indicate that there are five stages for the formation of these zircons, i.e. Archean, Paleoproterozoic, Late Paleozoic, Early Mesozoic and Late Mesozoic. Most of the zircons present as xenocrysts, which show erosion textures in the CL images. Trace element chemistry suggests a complex crystallization process for the zircons in sub-alkaline rocks, with REE patterns characterized by multi-stage crystallization and alteration. It is concluded that the zirconium is unsaturation in the alkaline magma, with all the xenocrysts corroded. The zircon in the overflow facies magma just reaches the critical point of saturation, so there is no igneous zircon in these melts.