Program, Abstracts, and Guidebooks - University of Minnesota Duluth

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EARLY PRECAMBRIAN GEOLOGY OF THE SAGkNAGA-NO1HEBN
LIGHT LAKES AREA, MINNESOTA-ONTARIO
G. N. HANSON
State University of New York at Stony Brook
Stony Brook, N. Y. 11790
5. 5. GOLDICH
Northern Illinois University
DeKalb, Illinois 60115
ABSTRACT
The principal Early Precambrian rock units
in the Saganaga—Northern
Light Lakes area of Ontario and Minnesota, from oldest to youngest, include
the Keewatin volcanic and related rocks, the Northern Light Gneiss, the
Saganaga Tonalite, formerly called the Saganaga Granite, and the Knife
Lake Group. These units were intruded by numerous small plutons and dikes.
The Northern Light Gneiss, the Saganaga Tonalite, and syenodioritic
to granodioritic phases of a small pluton at Icarus Lake, from oldest
to youngest on the basis of field relationships, have been dated by the
Rb-Sr, whole-rock technique.
The isochron ages range from 2700 to
2750 m.y. and are indistinguishable but suggest that all these rocks formed
within a time span o' less than 100 m.y. and probably less than 50 m.y.
Modal and chemical analyses show that the greater part of the Northern
Light Gneiss is trondhjemitic. in composition. As suggested originally by
Frank Grout the gneiss nay have resulted from the lit—par—lit injection
of the Keewatin greenstones during a period of folding. The gneiss,
however, may have been formed by folding and metamorphism of a Keewatin
volcanic pile composed of basaltic, trondhjeniitic, and rhyolitic volcanic
rocks, and possibly some sediments.
The Saganaga Tonalite is a late or postkinematic intrusion emplaced
in the greenstones and the Northern Light Gneiss, and inclusions of both
rock types are found in the tonalite. The Icarus Lake pluton intrudes
both the Northern Light Gneiss and the Saganaga Tonalite. It wnsists of
an older western phase of syenodiorite and a younger eastern phase of
granodiorite. Both rocks are alkalic, containing aegerine-augite and
hastingsite.
Rb—Sr and K-Ar mineral ages from the principal rock units range from
2500 to 2700 n.y. and are difficult to interpret. In part these ages
may be related to faulting and alteration. Movements on the major fault
zones ceased before the deposition of the Anini.ikie sediments. Metamorphism
is low-grade, greenschist facies of the Abukuma type.