Summary of Precambrian geologic events in Minnesota

This summary will be useful for our discussions of Precambrian geology of Minnesota (both Archean and Proterozoic). You should refer to it frequently to keep track of the sequence of Precambrian geologic events

Summary of signigicant world-wide geologic events in the Archean

Introduction

• oldest rocks of Minnesota may give clues to the ancient crust. Note that the most ancient crust of the continent is at the core of the Canadian Shield. This suggests that the continent has grown from this nucleus by the adding of continental crust at the margins by subduction during plate tectonic events
• three main groups of Archean rocks include, in order from oldest to youngest
  • gneisses - 3.0-3.6 Ga
  • interlayered volcanic rocks and sedimentary rocks older than 2.7 Ga
  • granite intrusives2.7-2.5 Ga
  • Archean rock on the bedrock geologic map of Minnesota
  • Precambrian geologic terranes in Minnesota
  • Precambrian geologic terranes in Minnesota including ages
  • geologic relationships shown in this cross section
• 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.
• Pillows in the greenstones of the Gilbert area of northern Minnesota have been exposed by glacial scour during the Pleistocene
• 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, (more pillows), mpeg clip of pillows forming off Hawaii
  • explosive eruption of more sialic volcanic blocks, cinder and ash (pyroclastic debris) from submarine volcanoes which eventually built above sea level. Similar to the stratovolcanoes or composite cones of the 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
  • example of folded turbidity current deposits in the Archean of Minnesota
  • 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 greenstones and 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