Program, Abstracts, and Guidebooks - University of Minnesota Duluth

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K—AR AGES
OF btkFIC DIKES IN NORTHEASTERN MINNESOTA
C. N. HANSON and R. MALMOTRA
Department of Earth and Space Sciences
State University of New York
Stony Brook, New York
ABSTRACT
Sixteen mafic dikes in the Vermilion District, Minnesota
and in the Saganaga—Northern Light Lakes area, Minnesota—Ontario
botder, give K—Ar whole—rock and mineral ages of 2600 m.y.., 1900-
2000 m.y., 1500—1600 m.y., 1400 n.y., and 100—1100 n.y. One sample
of a Logan Sill near Suomi, Ontario gives a whole—rock K—Ar age of
.1380 m.y. The dikes range in composition from hornblende andesite
with modal quartz and microcline to tholeiitic basalt. There does not
appear to be a clear—cut difference in composition as a function of
age nor. a difference in strike. Most dikes have a north—northwest
strike in the Vermilion district and a northerly strike in the Saganaga—
Nor them Light Lakes area.
Dikes with ages greater tjtan 1500 m.y. have a characteristic
alteration, possibly due to burial metamorphism, as shown by highly
sericitized plagioclase and the development of actinolite, chlorite,
epidote, sphene, prehnite, and calcite. The younger dikes do not show
this same style of alteration nor are they as highly altered. The
dikes which give 1500—1600 m.y. whole rock K—Ar ages are extensively
altered, and these ages may indicate the time of recrystallization
rather than the time of intrusion.