THE FORMAf,ION OF OUARTZ SYENITE BY CRUSTAL CONTAMINAf,ION AT MONT SHEFFORD AND OTHER MONTEREGIAN COMPLEXES, OUEBEC

The Mont Shefford alkaline complex of southern Quebec contains mafic rocks and both silica-oversatwated and -undersaturated syenites. Chemical and isotopic studies examine the relationship of the various lithologies. Isotopic, major, and trace-element compositions are consistent with derivation of all Iithologies from a common mafic paren! some of which evolved to a syenitic itage by fractional crystallization without sigtrificant contamination. Initial 87srr6sr and l43Nd/144Nd values display distinct and systematic variations, with overall ranges of 0.70343 to 0,7009 and 0.51256 to 0.51269, respectively. These isotope ratios are negatively correlated and indicate evolution yla combined assimilation and fractional crystallization (AFC), with the quartz syenite being the nost snongly contaminated. The contamination probably occurred at a high level, perhaps near the level of intrusion. There is a clear relationship between the degree of silica saturation and the amount of contamination; this indicates that saturatsd and oversaturated rocks formed by AFC from a felsic undersaturated magm4 whereas nepheline syenite forrned in the absence of appreciable contamination. Such a process is shown to be consistent with the phase relations of Peftogeny's Residua System. Isotopic compositions for other Monteregian complexes containing oversaturated rocks are similar to those of Shefford and indicate derivation from similar time-integrated" mildly light-rare-earthelement-depleted mantle sources. At each location, there are negatively correlated variations in 87SrF6Sr ard l4?Nd/144Nd that are consistent with production via AFC processes. Similar relationships have been documented at many other localities. Thus, production of oversaturated syenitic compositions from an undersaturated syenitic magma vla AFC seems to be a common and imFofiant process in epizonal alkaline infiusive complexes.