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.