geochemical features of high-alumina fenites of the Mont Saint-Hilaire alkaline
Yakovleva O.S.*, Pekov I.V.*,**, Bryzgalov I.A.*, Horváth L.***, Yapaskurt V.O.*, Guseva E.V.*
2Institute of Geochemistry and Analytical
Chemistry of RAS,
Saint-Hilaire complex, part of the Monteregian Hills alkaline province, is
the contact metamorphism (Currie et
al., 1986). High-alumina rocks found at Mont Saint-Hilaire have not been studied
before, only the occurrence of corundum, columbite and hercynite were noted.
This paper is the first systematic characterization of these rocks, considered by us as specific
High-alumina rocks, uncovered in the Poudrette quarry, form small bodies at the contact of alkaline pegmatites with hornfels and granites. Also they replace granite veins. These fenites are fine- to medium-grained rocks consisting substantially of feldspars (albite, K,Na feldspar and potassic feldspars are abundant, albite-oligoclase is subordinate). Hercynite (in all the rocks discussed below) and biotite (in almost all rocks) are the other rock-forming minerals. Typically biotite is high-alumina annite close to siderophyllite. Sporadically nepheline becomes a rock-forming mineral.
We have subdivided the high-alumina oxide-silicate rocks of Mont Saint-Hilaire to four types:
(1) A rock with significant biotite content (to 15%). Isolated
spessartine crystals (up to
(2) A rock with corundum-hercynite intergrowths surrounded by annite rims (sometimes with muscovite). Siderite is typically observed in cores of hercynite grains. The rock contains rare impregnation of pyrrhotite and manganocolumbite.
(3) A rock with Zn-rich hercynite grains with annite rims and intergrowths of biotite and muscovite. A lot of zoned well-shaped and skeletal zircon crystals occur there. The rock is strongly altered and contains different carbonates: sideriteŃmagnesite series members, ankerite, dolomite; calcite, mckelveyite, Ca,REE fluorocarbonates are subordinate; late natrolite and fluorite are also present. Accessory minerals are members of the ferrocolumbiteŃmanganocolumbite series, ilmenite, Nb-bearing rutile (var. ilmenorutile), uedaite-(Ce), pyrrhotite, monazite-(Â╚), xenotime-(Y), and a Ti-free REE niobate, probably fergusonite-(Ce). Isolated inclusions consisting of quartz (60%) and biotite (typically annite) (40%) are observed in a rock. Some biotite crystals contain inclusions of metacrystals of Ti-bearing spinelides of the magnetite-chromite series.
(4) A rock with
equal impregnation of Zn-rich hercynite (up
to 20%), accessory zircon and ilmenite (with ferrocolumbite microinclusions).
Sporadically, hercynite forms aggregations containing clusters of zircon with
thorite inclusions. There are big (up to
Similar rocks are
known in large fenitized xenoliths in the Khibiny massif (
┴Besides the common mineralogical features, we note some differences. Majority of Khibiny high-alumina fenites contain nepheline that is not typical for the high-alumina Mont Saint-Hilaire rocks. Carbonates are widespread at Mont Saint-Hilaire unlike at Khibiny.
The oxide-silicate rocks of Mont Saint-Hilaire and Khibiny are characterized by high contents of Al, Fe, Na, and K while concentrations of Ca and Mg are low. Similar rare-element speciality of high-alumina rocks of both massifs should be noted: the majority of accessory minerals contain Nb (+Ta), Zr, Ce (+ other LREE), and Th, less Y (+HREE), Sr, Ba, and U.
When the geochemistry of high-alumina rocks of both alkaline massifs are compared, we noted the higher alkalinity (especially Na) for Khibiny and relative enrichment in CO2 and Mn for Mont Saint-Hilaire rocks. Content of S in high-alumina rocks of Mont Saint-Hilaire is lower compared to Khibiny, while contents of rare elements are close. The important common feature at both localities is the reducing conditions of mineral formation, that is shown by the abundance of hercynite (both massifs), and the presence of native iron (Khibiny) or Fe2+ carbonates (Mont Saint-Hilaire).
Geochemically, the high-alumina fenites contrast sharply with relatively Al-poor host agpaitic rocks. It can be explained by the nature of a protholite that coud be initially Al-rich. In Khibiny, the protholite is most likely metamorphic rocks (muscovite shales or gneisses) formed after clays, whereas in Mont Saint-Hilaire it was hornfels or granite. High-alumina rocks of Khibiny were altered by alkaline fluids more strongly compared to their Mont Saint-Hilaire analogues. In the Khibiny xenoliths, nepheline-bearing rocks are typical products of the fenitization unlike the majority of Mont Saint-Hilaire high-alumina rocks, in which almost all Na is concentrated in feldspars. Note that nepheline-free varieties of high-alumina fenites, very similar to Mont Saint-Hilaire ones, are common also at Khibiny.
This study was supported by grant of President of Russain Federation No. 863.2008.5, grant of RFBR No. 08-05-00077-a and grant of Russian Science Support Foundation (I.V.P.).
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