Archean History of Minnesota 3'652'5 Ga

1
Archean History of Minnesota (3.65-2.5 Ga)
2
Distribution of Archean Bedrock in Minnesota
3

• First continental crust forms 3.6 Ga
• Minnesota River Valley Gneisses

Montevideo Gneiss
4
Formation of Metamorphic Rocks

• Pressure/Temperature conditions of metamorphism
• Contact and regional metamorphism
• Plate tectonic setting of regional metamorphism
• Contact metamorphism
• Metamorphic intensity and metamorphic rock types
• Shields of continents

5
P/T Conditions of Metamorphism Contact and
Regional
6
Contact Metamorphism
7
Plate Tectonic Setting of Regional Metamorphism
8
Foliation (layering in regional metamorphic rocks)

• Requires pressure for formation
• Obscures original structures in the parent rock,
  such as bedding
• Forms when new minerals crystallize perpendicular
  to the directed pressure

9
Metamorphic Intensity and Metamorphic Rock Types
10
Shields of Continents
11
Geologic Cross Section of Arrowhead Region, NE MN
12
Volcanic Island Arcs form on floor of sea in
northern Minnesota 2.7 Ga
Pillow Lavas
13
Greenstone Belts of Minnesota
14
Formation of Granite-Greenstone Terranes
15
Pillow Lavas

• Indicators of eruption of lava in water

16
Sediment Deposited by Turbidity Currents on
flanks of Volcanoes and atop unconformity on
Saganaga Granite 2.7 Ga (including Knife River
Group)
Turbidity Current Animation
17
Turbidity Current Deposits
Sand/mud couplets (sand is light-colored)
Pike River Dam, MN
Jay Cook State Park, MN
18
Graded Bedding and Turbidity Currents
19
Graded Bedding
This graded bed does not consist of sand mud as
in most classic turbidity current deposits.
Instead there are small pebbles at the base,
grading upward to coarse, then fine sand
20
Soudan Banded Iron Formation (BIF)

• Near Soudan Mine at Tower, Minnesota
• Folds were formed when sediment was soft and
  under pressure of burial
• Light bands are more silica-rich, dark bands are
  more iron-rich

21
Age of Globally Abundant BIFs

• Most BIFs are globally abundant in Early
  Proterozoic time (2.5-2.2Ga)
• The Soudan BIF, shown in the previous slide, is
  an exception. Its age is 2.7 Ga

22
Intrusion of Saganaga Granite contemporaneous
with Sedimentary and Volcanic Rocks
Saganaga Granite
23
Granite Hand Specimen
24
Granite Composition and Texture

• Coarsely crystalline - crystals can be seen with
  the naked eye
• Indicates slow cooling at great depth within the
  crust
• Light-colored - composed mainly of feldspar and
  quartz, minerals high in silica and typical of
  continental crust
• Low density when compared to ocean crust

25
Intrusive Igneous RocksGeometry of the rock
bodies - Plutons
Most granite plutons are batholiths, which form
deep in the core of mountain ranges. They form
at subduction zones where plates of the Earths
crust collide with one another.
26
Plate Tectonic Setting of Granite Magmas and
Intrusion of Granite Plutons
Continental crust forms at subduction zones (see
right side of cross section)
27
Map of PlatesSubduction zones shown as heavy
black lines with teeth
28
Vermillion Complex, Including Giants Range
Batholith 2.67 Ga
29
Confusion Hill, Giants Range Batholith

• Study the cross
• cutting relationships between the light-colored
  granitic rock and the dark colored diorite
• One interpretation is that the light-colored
  granite cuts across the darker igneous rock and
  has inclusion of darker rock within it
• These observations make the light-colored
  granitic rock the younger of the two.

30
Collision of Minnesota Valley Gneiss with
volcanic island arcs to create Kenoraland, the
nucleus of the North American Continent2.7-2.5
Ga