Minnesota's Rocks and Waters
Lecture Outline Set 7
Archean Geologic History of Minnesota
Day 12
Summary of Precambrian geologic
events in Minnesota
This summary will be useful throughout days 12-15. You should refer
to it frequently to keep track of the sequence of Precambrian geologic events
Introduction
- oldest rocks of Minnesota may give clues to the ancient crust.
- three main groups of Archean rocks
include, in order from oldest to youngest
- earliest earth history not revealed in these rocks
Gneisses as original granitic crust of the continents?
- Morton gneiss in Minnesota River Valley near Morton is 3.6 Ga
- not original continental crust
- could the original granitic crust have been destroyed by recycling
of geologic materials? meteorite bombardment?
- 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 "greenstone"
belts of ancient gneiss alternating with belts of metamorphosed volcanic
and sedimentary rocks.
- 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.
- 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.
- Model for deposition of volcanic
and sedimentary rocks
- initial formation of more than15000 feet basalt pillow lavas
- explosive eruption of more sialic volcanic blocks, cinder and ash
(pyroclastic debris) from submarine volcanoes which eventually built
above sea level. Similar to western margin of Pacific plate?
- 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).
- thickness of the interbedded volcanic and reworked volcanic material
is greater than 30,000 feet
- 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.
- originated more than 15 miles beneath the surface
- the granites solidified more than a mile beneath the surface
- 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
- 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
