The Isaac Lake quadrangle lies between the Mesabi and Vermilion
ranges, and includes a segment of each. The Lower Precambrian rocks
that characterize the Vermilion range underlie the northern 2/3 of the
quadrangle, and include both the Ely Greenstone (with interbedded
iron-formation) and an overlying sequence of interlayered metasedimentary
and meta-volcanic rocks. These rocks strike northwestward
and dip steeply northeast or southwest; they were folded and intruded
by the Giants Range Granite during the Algoman orogeny (2.5 b. y.).
The grade of regional metamorphism increases southeastward along
strike from Tower into the Isaac Lake quadrangle; the Ely Greenstone
and meta-sedimentary and meta-volcanic rocks within the quadrangle
contain mineral assemblages characteristic of the upper amphibolite
The well known Middle Precambrian Animikie Group of the
Mesabi range uncomformably overlies the Giants Range Granite in the
southeastern corner of the quadrangle and these rocks have been
intruded by dikes of Keweenawan (?) age and metamorphosed by the
Middle Keweenawan Duluth Complex.
The basalts and tuffaceous sedimentary rocks of the Ely Greenstone
are metamorphosed to massive or schistose amphibolites, and
contain thin lenses of cherty iron-formation. The overlying highly
metamorphosed sedimentary and volcanic rocks have been informally
divided into two units, the layered gneiss and the (overlying ?) Argo
gneiss. The Argo gneiss consists of fine-grained weakly foliated biotite
gneisses with rare thin mafic layers; these gneisses are metamorphic
equivalents of greenschist facies slates and graywackes which lie along
strike to the northwest. The layered gneiss comprises amphibolite and
leucocratic biotite and hornblende gneisses, interlayered on all scales
and well-foliated parallel to the compositional layering. Thick layers of
very coarse-grained biotite gneiss within the layered gneiss apparently
were partially mobile during metamorphism. The layered gneisses are
correlative with less-metamorphosed volcanic and volcaniclastic rocks
to the northwest. The higher-grade parts of the layered gneiss are
migmatized by fme-grained leucotrondhjemites that were foliated by
the regional metamorphism, recrystallized to gneissic textures, and cut
by granitic dikes.
The Algoman Giants Range Granite intrudes the older rocks and
underlies the southern half and the northeastern corner of the
quadrangle. It is predominantly a coarse-grained hornblende granodiorite
or monzonite. The granite is strongly foliated parallel to its
contacts and is generally conformable to the structure of the country
rocks. The batholith is interpreted as a late-kinematic forceful
intrusion, emplaced at relatively shallow depths.
Two major transcurrent faults strike northeastward across the
quadrangle. The western one, here named the Waasa fault, offsets the
granite-gneiss contact three and three-fourth miles. The eastern, or
Camp Rivard fault offsets this contact about two miles. Contacts and
fold axes within the gneisses appear to be offset even further, and other
evidence also suggests that the faults were active both before and after
emplacement of the granite. Numerous smaller faults are sub-parallel
to the larger ones. Rocks north of the Waasa fault dip steeply northeast;
those between the two faults generally dip steeply south west. Statistical
analysis of foliations and lineations suggests non-cylindrical folding
about a northwest-trending axis. In the northwestern corner of the
quadrangle there has been minor later crossfolding about an axis
perpendicular to the axis of the major folding.
The Middle Precambrian Animikie Group, in the East Mesabi
district of the Mesabi range consists of three comformable sedimentary
formations, the Pokegama Quartzite at the base, the Biwabik Ironformation,
and the Virginia Formation at the top. They are exposed in
the southeastern part of the quadrangle where they lie unconformably
on the Giants Range Granite. The Pokegama Quartzite fills minor
topographic irregularities in the older granite surface; accordingly, it is
variable in thickness, ranging from near zero to approximately 30 feet.
The Biwabik Iron-formation is approximately 400 feet thick and is
subdivided into seven cartographic units on the basis of bedding
characteristics, texture, and gross mineralogy. These units are readily
correlated with those previously recognized by Wolff (1917) and
Gundersen (1960). The Virginia Formation is not exposed in the
quadrangle and is known only through diamond drilling.
The structure of the Animikie Group is relatively simple. The beds
dip 5° - 15° SE, but the lower part of the Biwabik Iron-formation is
warped by several small-scale folds whose axes trend north-northwest
and plunge south-southeast at low angles. Structural closure on each
fold seems to decrease upward so that near the top of the iron-formation
the small-scale folds are no longer apparent.
The Animikie Group was intruded by gabbro dikes of possible
Middle Keweenawan age, and metamorphosed by the Middle Keweenawan
Duluth Complex. The formation is characterized by abundant
prograde and retrograde cummingtonite with minor amounts of
prograde fayalite, orthopyroxene, hedenbergite, and diopside.
Griffin, W.L. and Morey, G.B., The Geology of the Isaac Lake Quadrangle, St. Louis County, Minnesota. Special Publication Series 8. 57 p.
57 p., 1 pl.
Griffin, W.L.; Morey, G.B..
SP-08 The Geology of the Isaac Lake Quadrangle, St. Louis County, Minnesota.
Minnesota Geological Survey.
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