Program, Abstracts, and Guidebooks - University of Minnesota Duluth

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GEOLOGY OF TEE ALKALIC ROCK — CARBONATITE
COMPLEX AT PRAIRIE LAKE, ONTARIO
DAVID H. WATKINSON
Department of Geology
University of Toronto
ABSTRACT
The Prairie Lake complex of ijolitic rocks and carbonatite (age:
1112 million years) is intrusive into granitic gneisses 25 miles
northwest of Marathon, Ontario. The complex has positive relief, is
somewhat circular in plan, and is composed of concentric arrangements
of carbonatites and rocks of the pyroxenite — melteigite — ijolite —
urtite series. The latter series has two culminations: nepheline—
rich rocks characterized by melanite, wollastonite and alkali feldspar
with interstitial calcite and nepheline —feldspar intergrowths; and
pyroxene—rich rocks characterized by magnetite and biotite. Pyroxenitic
rocks are often separated from carbonatite by micaceous zones. The
carbonatites are strongly banded with near—vertical dips; banding is
a consequence of biotite and olivine + magnetite concentrations. Most
carbonatites are calcite—rich, but some are dolomitic and breccias
with groundmass dolomite intrude the calcitic rocks. Pyrochlore is
common in the carbonatites and in calcite—rich interstices and lenses
in pyroxenites. In some zones pyrochlore contains as much as 30 weight
Z U3O. Some fenitized country—rock occurs at the contact with
carbonatite. The complex is interpreted to have formed by intrusions
of magmas generated by strong differentiation of a carbonated, neph—
elinitic parent.