BEDROCK GEOLOGY OF THE
TRANSITION ZONE OF GRANOPHYRE
BODIES INTO THE NORTH SHORE
VOLCANIC GROUP
Meredith Kraner, Sarah Sauer, Sarah
Gordee, Terrence Boerboom
Sarah
S.
Terry
TEAM TERRY
Meredith
Sarah G.
OBJECTIVES



Produce a detailed map of the southern margins of the
Lima and Pine Mountain quadrangles, Cook County,
Minnesota which was previously only mapped on a
regional scale.
Past work has shown that current reconnaissance geologic
maps left out a great deal of detail.
Delineate volcanic stratigraphy in areas currently defined
as ‘North Shore Volcanic Group, undivided.’ Secondly,
define the intrusive (and possibly faulted) relationships
with the Brule-Hovland diabase/gabbro and Eagle
Mountain granophyre bodies.

The volcanic rocks have been metamorphosed by the intrusions,
and the felsic volcanics have likely been remelted by the mafic
intrusions, forming small marginal hybrid bodies.
BASECAMP
c
MAPPING AREA
Regional Geology
Regional Geology
•
•
•
Volcanic rocks likely to include everything
from rhyolite to basalt. They are also contact
metamorphosed and expect to see hybrid
border phases adjacent to mafic intrusions.
Gabbroid to granophyric intrusions,
intermediate border phases.
Probably many smaller dikes/sills within
volcanic pile.
Area previously mapped by
Davidson in 1977
Field Logistics
 Field
area was accessed by vehicle or ATV.
 Wide
range of field conditions varying from dense, impenetrable
bush to swamps, fields and steep cliffs.
 Used
LiDar to help locate feasible outcrop
FIELD OPERATIONS
c
Rock Types
Mafic to
felsic
volcanics
• Rhyolite, Andesite, Basalt
Intrusive
• Granophyre, FerromonzoniteFerromonzodiorite, diabase
Sedimentary
• Sandstone
Felsic to Intermediate Intrusive Rocks

Leucogranite:
 Pink,
micrographic to
intergranular
Felsic to Intermediate Intrusive Rocks

Ferrogranite to
ferromonzodiorite:
 fine- to medium-grained,
locally coarse grained,
weakly porphyritic, with
white-bleached, feltytextured plagioclase
surrounded by pink alkali
feldspar and granophyre
 Locally contains sparse
glassy quartz grains, and
commonly contains finegrained granophyric
quartz and alkali feldspar
intergrowths.
Felsic to Intermediate Intrusive Rocks


Unit also includes
oxide-rich,
medium- to
coarse-grained,
equigranular to
ophitic, augite
troctolite to
olivine gabbro
These units exhibit
complex internal
contact
relationships.
Olivine gabbro
Augite troctolite
Felsic Volcanics

Rhyolite:
 Plagioclase-
and
pyroxene-porphyritic
 Very fine-grained to
aphanitic, locally
conchoidally fracturing
groundmass
 Incipient columnar
jointing is observed
locally
Rhyolite

Interpreted to be a lava,
this unit locally appears to
be a fine-grained
marginal phase of
ferrogranite to
ferromonzodiorite units
and also exhibits intrusive
contacts with adjacent
fine-grained basalt and
andesite lava units.
Intermediate Volcanics

Andesite:
 Coherent
 fine-grained,
feltyand intergranulartextured, with minor
proportions of
small, pink-altered,
rectangular
plagioclase
phenocrysts
Andesite
 Flow-top


breccia
monomictic, chaotic, clast-supported, angular, amygdaloidal andesite
breccia
very fine-grained, siliceous matrix that is interpreted to be silt or
sandstone
Mafic Volcanics

Basalt:

Felty-textured

medium-grained, subophitic to subprismatic, and locally amygdaloidal. Contains
sparse, small blocky, epidote-altered plagioclase phenocrysts, variably altered
poikilitic to subprismatic augite, minor Fe-Ti oxides, chlorite and apatite. Amygdules
are locally filled with epidote, chlorite and quartz.
Basalt
 Ophitic
 Brown
to mottled
maroon and gray
 Fine- to mediumgrained
 Oikocrysts up to 7
millimeters
 Locally amygdaloidal
Interflow sedimentary rocks

Sandstone:
fine-grained, well-sorted
litharenite to litharkose
 thin beds are
distinguished via
variations in grain size.
 Contains abundant clear,
glassy quartz grains in
relatively more coarsegrained beds

Miscellaneous diabase intrusions

Ophitic Diabase
fine- to medium-grained,
inequigranular, ophitic
 constituent minerals
include plagioclase,
augite, olivine and Fe-Ti
oxides
 relatively resistant and
can be traced along
strike as narrow, lowlying topographic ridges

Structures & Textures
Glacial Striations
Ophitic Texture

S
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RED ROCK CONFUSION
GRANOPHYRE
MONZONITE
Geologic findings

Mapped half a quadrangle in area



Determined which units were ridge forming units and
which were low forming units
Observed 2 or 3 actual contacts in outcrop



Mapped a total of 347 outcrops
Contacts were also inferred based on the observed relative
resistance of the units along with the lateral continuity along
strike using LiDar.
The presence of the fault is inferred based on the
truncation of laterally continuous units along strike.
Based on ongoing mapping, we were able to extend
rock units from the adjacent Grand Marais
quadrangle
ACKNOWLEDGMENTS
WITH THANKS TO TERRY BOERBOOM, DIRECTORS OF
THE PRECAMBRIAN RESEARCH CENTER; JIM MILLER, DEAN
PETERSON, GEORGE HUDAK. ADDITIONAL THANKS TO
THE PRIVATE AND CORPORATE CONTRIBUTORS WHO
MADE THIS PROJECT POSSIBLE
References cited
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

Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson,
D.M., 2001, Geologic map of the Duluth Complex and related rocks,
northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map
M-119, pl. 1, scale 1: 200,000.
Boerboom, T.J., and Green, J.C., 2010, Bedrock Geology of the Grand
Marais Quadrangle, Cook County, Minnesota: Minnesota Geological Survey
Miscellaneous Map M-189, scale 1: 24,000.
Vervoort, J.D., Wirth, K., and Kennedy, B., 2007, The magmatic evolution of
the Midcontinent rift: New geochronologic and geochemical evidence from
felsic magmatism: Precambrian Research, v. 157, nos. 1-4, p. 235-268.
Davidson, D.M., Jr., 1977, Reconnaissance geologic map of the Pine
Mountain quadrangle, Cook County, Minnesota: Minnesota Geological
Survey Miscellaneous Map M-31, scale 1:24,000.