archean craton of North America I

Develop a deeper understanding of the Archean crust and how it was different from the modern continents

1.

Archean crust = thin, mafic, full of small evolved (=felsic) reservoirs

• similar to oceanic plateaus

Vertical tectonics from very high vertical mantle heat flow mixes rock densities + constant anatexis + dissipated strain (no fault/shear) →

density inversion/crustal overturns

• blobby appearance with greenstone belts (downwelling zones) dividing gneiss domes (core complexes)

  • deep crust to the surface often not observed due to reworking except Pilbara Australia

2.

great oxidation event = intermediate magma

early subduction

Introduce the Archean cratons of North America

µ

90% found in Canada

• took 2.5 billion years for the archean continents to develop into the known archean provinces

Gain insight into the components and assembly of the North American Craton

*What were the environment pre and during archean

1. Hadean Eon ——- 4.54-4Ga

   • no lithosphere, magma ocean, small mafic and primitive (chondritic) crust segments from impacts

2. Archean ——— 4-2.5Ga

   • harsh atmosphere (Co2, H2S + reducing gasses = N2, CH4), 60C

   • first prokaryotes (photosynthesis) emergence 3.7Ga

   • crustal evolution = felsic =

*What are the oldest known materials on earth

Oldest zircon population = 4.4Ga

• western australia - Yilgarn Craton

• formed directly from differentiated magma + contain lots of felsic mineral assemblages/inclusions

Oldest rock, gneiss = 4.2Ga

• yellowknife, NWT - slave craton

• rare fragment of the mafic crust representing the first continents on earth

Discover the resources that the Archean cratons provide and how these can help us develop a greener society

What did the Archean lithosphere look like and how did its tectonics differ from modern continental lithosphere

Achean lithosphere: mafic crust + mixed crust + mantle

tectonics:

Which tectonic provinces make up the Archean core of the North American continent?

1. Nain

2. Rae = Churchill

3. Hearne = Churchill

4. Slave

5. Superior

6. Wyoming

IGNORE REST

continental crust

• mafic basaltic → intermediate andesitic

  • due to O2 accumulation in ocean = great oxidation event 2.4Ga

    • removal of mafic crust = stoped oxidation chemical processes = O2 produced could accumulate

PALEO-ARCHEAN 3.6-3.2

• eventually due to density contrast one plate sinked below the other = primitive subduction 3.2Ga

MESOARCHEAN

• high geothermal gradient metamorphism

  • tectonic processes but also magmatic loading (batholiths emplacement)

• bimodal metamorphism with paired metamorphic belts

  • subduction fransiscan with Burrovian

TTG

• geochemical features of magmatic arcs magmas, like andesites but also of active margin intrusions, like granodiorite

  • TTG = intrusive rocks of the earliest active margins assuming very shallow flat-slab subduction

• using HFS (high field strength elements) we now know TTG = fractional crystallization from melting mafic plagioclase cumulate rock like gabbros (oceanic lithosphere)

  • protocontinents base (gabbro) buried by the mass added by erupting basalt = met melting conditions

Why are the Archean Provinces of North America economically important for Canada

Gold

Diamonds

REE

What is a kimberlite and why does it contain diamonds

kimberlite = around the margin and core of archean cratons, phanerozoic roots feeding on depleted mantle-subduction fluids zones which create diamonds

How can Archean rocks help us become greener

lots of copper

lots of REE (Nd, Sm, Dy) in highly evolved archean igneous rocks

• Geologists

  • find the rocks

  • develop feasibility, env & community impact of dev

  • establish clean & sustainable extraction