The Tertiary Sierra La Vasca intrusive complex of the Mexican Eastern Alkaline Province consists of diverse alkaline-to-peralkaline granitoids and syenites and is a rare example of silica oversaturated peralkaline magmatism characterized by eudialyte. The intrusion of these peralkaline rocks into Cretaceous carbonate country rocks resulted in the development of a unique cuspidine, Zr-bearing cuspidine, hiortdahlite and wollastonite exoskarn. This study is focussed on a eudialyte-bearing vein and accompanying banded exoskarn which illustrates the unusual skarn-forming metasomatic effects of Zr mobilization. The skarn consists of six mineralogically distinct zones: (1) a parental Nb-poor eudialyte-bearing quartz granitoid vein; (2) a region of eudialyte pseudomorphed by intergrown Zr-sorosilicates; (3) an andradite–cuspidine–hiortdahlite–wöhlerite zone; (4) a zone of skeletal-to-prismatic cuspidine plus wollastonite which is transitional to zone (5); a coarser grained and heterogeneous zone consisting of complex intergrowths of tabular and prismatic cuspidine–hiortdahlite solid solutions, wollastonite, fluorite, apatite and rare calcite; (6) a contact calcite marble lacking any metasomatic silicates, phosphates or fluorite. Skarn formation was the result of alteration of eudialyte and separation of Si-rich hydrothermal fluids with high F/H2O ratios from the parental Si-oversaturated peralkaline magma and subsequent infiltration of Si–Zr–REE–P-bearing fluids into the country rock carbonates. Zircon was probably transported as Zr-fluoride and chloride complexes and the acidic fluids reacted with calcite to form cuspidine–hiortdahlite solid solutions and wollastonite as the principal skarn minerals. All of the Si required to form this unique skarn assemblage was derived from the hydrothermal fluids as the country rocks do not contain Si-bearing minerals. Skarn formation is considered to have occurred at temperatures below 500°C.