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

JOHN D. LANDOLL and KENNETH A. FOLAND
Department of Geological Sciences, Ohio State University, Columbus, Ohio 43210, U.S.A.


Abstract

The Mount Shefford alkaline complex of southern Quebec contains mafic rocks and both silica oversaturated 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 lithologies from a common mafic parent, some of which evolved to a syenitic stage by fractional crystallization without significant contamination. Initial 87Sr/86Sr and 143Nd/144Nd values display distinct and systematic variations, with overall ranges of 0.70343 to 0.7099 and 0.51256 to 0.51269, respectively. These isotope ratios are negatively correlated and indicate evolution via combined assimilation and fractional crystallization (AFC), with the quartz syenite being the most strongly 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 saturated and oversaturated 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 Petrogeny'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-earth-element-depleted mantle sources. At each location, there are negatively correlated variations in 87Sr/86Sr and 143Nd/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 via AFC seems to be a common and important process in epizonal alkaline intrusive complexes.