THE FORMATION OF QUARTZ SYENITE BY CRUSTAL CONTAMINATION AT MONT-SHEFFORD AND OTHER MONTEREGIAN COMPLEXES, QUEBEC

The Mont Shefford alkaline complex of southern Quebec contains mafic r ocks and both silica-oversaturated and -undersaturated syenites. Chemi cal and isotopic studies examine the relationship of the various litho logies. Isotopic, major, and trace-element compositions are consistent with derivation of all lithologies from a common mafic parent, some o f which evolved to a syenitic stage by fractional crystallization with out significant contamination. Initial Sr-87/Sr-86 and Nd-143/Nd-144 v alues display distinct and systematic variations, with overall ranges of 0.70343 to 0.7099 and 0.51256 to 0.51269, respectively. These isoto pe ratios are negatively correlated and indicate evolution via combine d assimilation and fractional crystallization (AFC), with the quartz s yenite bring the most strongly contaminated. The contamination probabl y occurred at a high level, perhaps near the level of intrusion. There is a clear relationship between the degree of silica saturation and t he amount of contamination; this indicates that saturated and oversatu rated rocks formed by AFC from a felsic undersaturated magma, whereas nepheline syenite formed in the absence of appreciable contamination. Such a process is shown to be consistent with the phase relations of P etrogeny's Residua System. Isotopic compositions for other Monteregian complexes containing oversaturated rocks are similar to those of Shef ford and indicate derivation from similar time-integrated, mildly ligh t-rare-earth-element-depleted mantle sources. At each location, there are negatively correlated variations in Sr-87/Sr-86 and Nd-143/Nd-144 that are consistent with production via AFC processes. Similar relatio nships have been documented at many other localities. Thus, production of oversaturated syenitic compositions from an undersaturated syeniti c magma via AFC seems to be a common and important process in epizonal alkaline intrusive complexes.