Mobilization and enrichment of high-field strength elements during late- and post-magmatic processes in the Shuiquangou syenitic complex, Northern China

The Shuiquangou syenitic complex consists of melasyenite and leucosyenite. In the northeastern part, there is a small area where the leucosyenite was affected by alteration. The altered leucosyenite contains much higher concentrations of high-field strength elements (HFSE), especially Nb, Zr, REE and Y compared to fresh leucosyenite. REE patterns of the altered leucosyenite show moderate negative Eu anomaly, in contrast to the slightly positive to non-anomalous Eu signature of the fresh syenitic rocks. Allanite from the hydrothermal mineral assemblage in the altered leucosyenite also contains much higher concentrations of REE and Ti than the euhedral magmatic allanites in unaltered leucosyenite. The enrichment of HFSE in altered leucosyenite was interpreted to be caused by the hydrothermal fluids based on mineral chemistry and whole-rock data. Within the fresh leucosyenite, allanite occurs in two habits, euhedral ones indicating a primary magmatic origin and anhedral ones suggesting a late-magmatic origin. The latter has higher concentrations of REE and Ti than the former. The anhedral allanite was presumed to be formed coeval with the alteration of the euhedral allanite as a result of residual magmatic fluid derived directly from the syenitic magma. The alteration of magmatic euhedral allanite and the enrichment of REE and Ti in the anhedral late-magmatic allanite suggest that these elements were mobilized by the residual magmatic fluid. Both residual late-magmatic and hydrothermal fluids have an inferred composition characterized by high alkalic and F contents, suggesting that F may be the most ligand that complexed the high-valence cations.