Outline of "Minnesota's Geology"
Pages 23-33
Archean Geologic History
Introduction
- oldest rocks of Minnesota may give clues to the ancient crust. Can
we identify the primeval crust? the original crust of the Earth?
- three main groups of Archean
rocks include, in order from oldest to youngest
- earliest earth history not revealed in these rocks
- age of Earth ~4.6 Ga - originated along with rest of solar system from
interstellar cloud of dust and gas which collapsed and contracted under
gravitational field and began to rotate
- layered structure of earth developed as densest material sank to center
and successively less dense material formed outward from center
- early hydrosphere and atmosphere formed as last step in this process
by outgassing from interior
- early atmosphere different from today - free oxygen was not present
but evolved later by breakdown of water vapor and addition by photosynthesizing
organisms
Gneisses as original granitic crust of the continents?
- Morton gneiss in Minnesota River Valley near Morton is 3.6 Ga
- not original continental crust, because the gneisses themselves
originated from regional metamorphism of sedimentary and volcanic rocks
which therefore must have been still older
- could the original granitic crust have been destroyed by recycling
of geologic materials?
- or was all the original crust oceanic basalt?
- on other planets or satellites (the Moon), meteorite bombardment has
been severe enough to cause extensive cratering and to destroy original
crust. Could this have happened on Earth? The answer is yes, but only prior
to development of the hydrosphere and atmosphere. Such crustal destruction
could have happened between 4.6 and 3.6 Ga.
- at any rate, the ancient gneisses of Minnesota and adjacent Canada
were the basement upon which the rest of the rocks were deposited,
and into which younger rocks were intruded
Volcanism and sedimentation
- Archean rocks of northern Minnesota and adjacent Canada are characterized
by northeast-trending belts of ancient
gneiss alternating with belts of metamorphosed volcanic and sedimentary
rocks. Because the metamorphosed rocks have abundant green minerals
in them, these belts are referred to as "greenstone belts".
- the existence of northeast-trending
belts is most likely the result of structural deformation
of the crust following the formation of the volcanic and sedimentary
rocks. The volcanic and sedimentary rocks laid down on top of the gneisses
were therefore preserved in downfolded areas and removed from upfolded
areas, exposing the underlying gneisses.
- Minnesota belts of greenstone and intervening gneisses are best seen
in the Vermillion district from Tower and Soudan northeast to Ely
and to Saganaga Lake. Two other belts are exposed along Rainy Lake and
50 miles to the west.
- Model for deposition of
volcanic and sedimentary rocks
- initial formation of more than15000 feet basalt pillow lavas
without small holes produced by escaping gas. Formed in water as much as
3000 feet deep, probably on top of gneissic basement but not certain, as
bottoms of lava flows are never exposed
- explosive eruption of more sialic volcanic blocks, cinder and ash
(pyroclastic debris) from volcanoes.
- Initially submarine, but then volcanoes built above the surface of
the sea to form great island chains like those along the western and northern
rim of the modern Pacific Ocean
- Was this area similar to a convergent plate margin and subduction zone?
Controversial, because many geologists think plate tectonic processes may
have been different during the Archean, with greater amounts of heat driving
earth's internal engine. More and smaller plates?
- when volcanoes built above sea level, landslides and erosion brought
large amounts of sediment to their shores. This sediment was then carried
down the submarine slopes as great clouds of swirling sediment moving
along the bottom (turbidity currents). Evidence for turbidity currents
is graded bedding in the sediment (larger particles at the base of a bed,
becoming finer toward the top).
- thickness of the interbedded volcanic and reworked volcanic material
is greater than 30,000 feet
- some of the sediment could also have come from granite batholiths which
were intruding the pile of material contemporaneously with volcanism and
sedimentation
- finding the volcanic centers by mapping is important, because they
concentrate metal deposits such as lead, zinc and copper, brought up from
deeper within the crust.
Intrusion of granite batholiths and mountain building
- The thick piles of volcanic and sedimentary rocks along with the
underlying gneiss basement were folded by intense compressional forces,
forming the northeast-trending belts
- Intrusion of granite batholiths was contemportaneous with deformation
and may have even caused some of the deformation.
- coarsely crystalline granites originated more than 15 miles beneath
the surface, probably near the base of the crust or top of the mantle,
as indicated by the low ratio of Sr 87/86
- the granites solidified more than a mile beneath the surface as indicated
by the coarsely crystalline textures which require slow cooling for their
development
- ages of the granite batholiths cluster around 2.7 Ga.
- granites cut across the older volcanic-sedimentary rocks and also
include fragments of these earlier rocks within the batholiths
- contact metamorphism of the older volcanic-sedimentary rocks occurred
next to the granites, and the squeezing and burial of the entire region
resulted in low-grade regional metamorphism, resulting in the growth
of green minerals in the pillow basalts (hence the term "greenstone").
- faulting affected the entire Archean sequence as folding and
intrusion occurred
- the folding, intrusion and faulting was all part of a great mountain-building
event at the end of the Archean which is sometimes referred to as the Algoman
or Kenoran Orogeny
- Erosion during Middle Precambrian time exposed the roots of
the Algoman mountains and developed
a major unconformity upon which Middle Precambrian rocks were formed
