THE CRYSTAL-CHEMISTRY OF AEGIRINE FROM MONT SAINT-HILAIRE, QUEBEC

Aegirine from five different microenvironments within the peralkaline East Hill Suite (EHS) at Mont Saint-Hilaire, Quebec, were studied by S EM-EDS, Mossbauer spectroscopy, wet-chemical and ICP-MS methods. Pyrox ene compositions range from aegirine-augite (Ae(38)Di(39)Hd(23)) to en d-member aegirine (Ae(97)Di(1.5)Hd(1.5)); the crystals exhibit strong zoning, with a core enriched in Ca + Zr, and a rim enriched in Na + Ti . More than 85% of the Fe is present as octahedrally coordinated Fe3+, suggesting conditions of extreme oxidation Drier to aegirine crystall ization. All samples of aegirine are enriched in the REE relative to c hondrite and display a strong negative Eu anomaly. A concave pattern, with enrichments in both heavy and light REE, is found in all microenv ironments except the one that led to fibrous aegirine. Such fibrous sp rays show a steep, negative slope and are strongly enriched in the lig ht REE. The aegirine is the product of fractionated batches of late-st age melt enriched in incompatible elements (e.g. Zr, Ti and REE) and N a. The extremely low Fe2+/Fe3+ values restrict pyroxene fractionation trends to the Di-Ae tie-line, unlike those noted elsewhere. The Ae and trace-element contents of the pyroxenes were used to develop an evolu tionary scheme for the EHS. Fractional crystallization of a parental m afic magma, possibly accompanied by liquid immiscibility, resulted in the formation of nepheline and sodalite syenites. Further fractionatio n of the melt that gave sodalite syenite, enriched in volatiles and in compatible elements, led to crystallization of the aplites and pegmati te dikes, units which display identical aegirine, and to later igneous breccias. A secondary fluid-rich phase is considered responsible for very late-stage metasomatic overgrowths of fibrous aegirine in the peg matite dikes.