K-Ar ages have been determined for hornblende concentrates
from granitic intrusive and metamorphic rocks
from the Minnesota River valley, northeastern Minnesota,
and an adjacent area in Ontario, which were involved in
the Algoman orogeny. Nine determinations giving ages
ranging from 2570 to 2770 m. y. have an average of 2670
+ 30 m.y. This is significantly older than the age of about
2500 m.y. indicated by the Rb-Sr and K-Ar methods for
biotite from these rocks. The age, however, agrees
with the U -Pb ages of about 2700 m. y. for coexisting zircons.
Mineral and whole-rock K-Ar ages for basic rocks
that invaded the granitic and metamorphic rocks show two
periods of intrusion in Minnesota--at 1600-1800 m. y. and
at or before 2100 m. y. --in addition to the late Keweenawan
igneous activity at about 1100 m. y.
RI-08 K-Ar Ages for Hornblende from Granites and Gneisses and for Basaltic Intrusives in Minnesota.
Minnesota Geological Survey.
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