The Cretaceous Mont Saint-Hilaire complex (Quebec, Canada) comprises
three major rock units that were emplaced in the following sequence: (I)
gabbros; (II) diorites; (III) diverse partly agpaitic foid syenites. The
major element compositions of the rock-forming minerals, age-corrected
Nd and oxygen isotope data for mineral separates and trace element data
of Fe-Mg silicates from the various lithologies imply a common source
for all units. The distribution of the rare earth elements in
clinopyroxene from the gabbros indicates an ocean island basalt type
composition for the parental magma. Gabbros record temperatures of 1200
to 800 degrees C, variable silica activities between 0 center dot 7 and
0 center dot 3, and f(O2) values between -0 center dot 5 and +0 center
dot 7 (log delta FMQ, where FMQ is fayalite-magnetite-quartz). The
diorites crystallized under uniform a(SiO2) (a(SiO2) = 0 center dot 4-0
center dot 5) and more reduced f(O2) conditions (log delta FMQ similar
to-1) between similar to 1100 and similar to 800 degrees C. Phase
equilibria in various foid syenites indicate that silica activities
decrease from 0 center dot 6-0 center dot 3 at similar to 1000 degrees C
to < 0 center dot 3 at similar to 550 degrees C. Release of an aqueous
fluid during the transition to the hydrothermal stage caused a(SiO2) to
drop to very low values, which results from reduced SiO(2) solubilities
in aqueous fluids compared with silicate melts. During the hydrothermal
stage, high water activities stabilized zeolite-group minerals. Fluid
inclusions record a complex post-magmatic history, which includes
trapping of an aqueous fluid that unmixed from the restitic foid
syenitic magma. Cogenetic aqueous and carbonic fluid inclusions reflect
heterogeneous trapping of coexisting immiscible external fluids in the
latest evolutionary stage. The O and C isotope characteristics of
fluid-inclusion hosted CO(2) and late-stage carbonates imply that the
surrounding limestones were the source of the external fluids. The
mineral-rich syenitic rocks at Mont Saint-Hilaire evolved as follows:
first, alkalis, high field strength and large ion lithophile elements
were pre-enriched in the (late) magmatic and subsequent hydrothermal
stages; second, percolation of external fluids in equilibrium with the
carbonate host-rocks and mixing processes with internal fluids as well
as fluid-rock interaction governed dissolution of pre-existing minerals,
element transport and precipitation of mineral assemblages determined by
locally variable parameters. It is this hydrothermal interplay between
internal and external fluids that is responsible for the mineral wealth
found at Mont Saint-Hilaire.