Program, Abstracts, and Guidebooks - University of Minnesota Duluth

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THE SOUTHERN
ASSOCIATED
PART OF THE DULUTH COMPLEX AND
KEWEENAWAN ROCKS, MINNESOTA
BILL BONNICHSEN
Cornell University,
Ithaca, N.Y. 14850
A B ST EtA CT
The southern part of the Duluth Complex was reported to consist
mainly of troctolitic rocks and older anorthositic rocks at the
15th Annual Institute on Lake Superior Geology and elsewhere (Bonni—
chsen, 1969). Geologic mapping in several 7½—minute quadrangles in
the Babbitt—Hoyt Lakes area (Bonnichsen, 1970) shows that troctolitic
rocks lie north and west of the principal occurrences of anorthositic
rocks, thus forming the footwall side of the complex in the same manner
as at Duluth (Taylor, 1964). Between Duluth and the Babbitt—Hoyt Lakes
area, troctolitic rocks predominate across the width of the complex;
exposures of anorthositic rocks are restricted to isolated occurrences
and inclusions within the troctolitic rocks, rather than large areas
with contiguous outcrops.
In 1969, the writer suggested that troctolitic magmas had
intruded along a contemporaneously widening fracture zone between the
previously—formed anorthositic rocks to the east and the Early and
Middle Precambrian basement to the west. Field work during the summer
of 1969 and examination of drill core in recent months tends to substantiate
this view. Recently obtained knowledge on the variety and
diversity of rock types within the southernpart of the complex indicates
the development of the troctolitic rocks was a complex event
involving multiple injections of magma, the incorporation of a great
amount of previously—formed Keweenawan igneous rocks as inclusions and
the development of relatively small quantities of Fe— and Ti—rich magma,
some of which was ultramafic, from nagmas which initially were troc—
tolitic.
Much of the 1969 field season was devoted to looking for
and examining outcrops along, and east of, the eastern or hanging
wall margin of the complex. This margin, for the first 30 miles north
of Duluth, is mainly a contact between troctolitic and locally anor—
thositic rocks to the west and gabbroic and dioritic intrusives to
the east. Exposures of mafic volcanics are uncommon east of the
southern part of the complex, except within one or two miles of Lake
Superior.