Archean & Proterozoic Tectonics

REVIEW OF THE PRECAMBRIAN TIME SCALE

• ARCHEAN (3.8 TO 2.5 BY) & PROTEROZOIC (2.5 TO 0.55 BY) EONS
  • Proterozoic Eon subdivided into Early (2.5-1.6 by), Middle (1.6-0.9 by), & Late (0.9-0.55 by)
• RADIOMETRIC AGES ONLY - NO FOSSILS GENERALLY UNTIL THE VERY LATEST PRECAMBRIAN & EXTENSIVE METAMORPHIC & IGNEOUS TERRANES

PRE-ARCHEAN CRUSTAL EVOLUTION

• METEORITE BOMBARDMENT - May have formed EARLY CONTINENTAL CRUST
• EARLY PLATE TECTONICS - MAFIC & ULTRAMAFIC CRUST INITIALLY; LATER ANDESITIC & GRANITIC CRUST GENERATED BY PARTIAL MELTING OF MAFIC/ULTRAMAFIC PRECURSORS

PRECAMBRIAN SHIELDS

• CORES OF CONTINENTS
  • broadly upwarped; geologically stable; extend under Phanerozoic cover rocks
• CONSIST OF SMALLER ARCHEAN CRATONS WELDED TOGETHER & SURROUNDED BY PROTEROZOIC MOUNTAIN BELTS (OROGENS)
  • Canadian Shield - Wyoming, Superior, Nain, Hearne, Rae & Slave provinces (Archean cratons); Trans-Hudson & Wopmay provinces (Early Proterozoic orogens >1.8 by); Mazatzal province (Early Proterozoic orogen < 1.8 by); and Grenville province (Middle Proterozoic orogen)(see p. 171, Figure 8-11)

ARCHEAN CRATONS

• THESE CONTAIN THE OLDEST PRESERVED CRUST & ARE A RECORD OF THE EARLIEST TECTONIC PATTERN
• TWO TYPES OF ROCKS:
  • 1. Granite-Gneiss - represent the primitive continents & is both older & younger than the greenstone belt rocks
    • GRANITES ARE SODIUM-RICH, INDICATING A GREATER MANTLE SOURCE THAN LATER POTASSIUM-RICH GRANITES
  • 2. Greenstone Belts - represent the primitive oceans
    • GREENSTONE BELTS CONTAIN UNUSUAL ROCKS ->ULTRAMAFIC VOLCANICS
      • result from total melting of UPPER mantle [higher temperatures] & are found only in the Archean
      • the remainder of sequence resulted from partial melting of mantle + differentiation of magma + erosion of older crustal rocks
        
        
    • Quartzite & carbonate rocks are generally absent, as well
      • NO CONTINENTAL SHELVES - SEDIMENTS ARE GRAYWACKES DEPOSITED IN DEEP-MARINE ENVIRONMENT OF DEPOSITION (see next section)

         

• ARCHEAN CRATONS PROBABLY FORMED BY COLLISION OF GRANITE-GNEISS MICRO-CONTINENTS WITH PIECES OF PRIMITIVE GREENSTONE OCEANS WERE TRAPPED ALONG SUTURES
• ARCHEAN PLATE TECTONICS WAS CHARACTERIZED BY INTENSE MANTLE CONVECTION, RESULTING IN SMALLER CONVECTION CELLS & SMALLER "PLATES" OF THINNER LITHOSPHERE
• CULMINATES WITH CONTINENTAL CRUST BEING CONSOLIDATED INTO LARGE STABLE CRATONS
  • North America & Northern Europe = Laurentia; South America & Africa = West Gondwana; & Antarctica, Australia, India & parts of Asia = East Gondwana

PROTEROZOIC MOBILE BELTS

• THREE MAJOR TYPES OF ROCKS:
  • 1. Widespread Shallow-marine Quartzite-Carbonate-Shale Assemblages
    • REPRESENT DEPOSITION ON & AT PASSIVE CONTINENTAL MARGINS OF WIDESPREAD STABLE CRATONS
  • 2. Tillites/Diamictites (pebbly mudstones)
    • REPRESENT TWO (2) EPISODES OF CONTINENTAL GLACIATION
      • Late Archean/Early Proterozoic (2700-2300 my ago) - Bruce & Gowganda sequences in North America; numerous localities elsewhere
      • Late Proterozoic (900-600 my ago) [VARANGIAN] globally, even on continents along the equator!
        • Earth's surface almost froze solid
      • Continental glaciation also in the Ordovician-Silurian, Pennsylvanian-Permian, & Cenozoic
  • 3. Iron Ores
    • RECORD CHANGING OXYGEN CONTENT OF ATMOSPHERE
      • Atmospheric oxygen content is a balance between sources & sinks
        • main source of oxygen is photosynthetic organisms, which developed in the Archean [~3.5 by ago ]
        • main sink for oxygen during the Archean & Earliest Proterozoic was previously dissolved iron (Fe⁺² )
        • main sink for oxygen after the Latest Early Proterozoic (from ~2000 my ago to the present ) has been weathering of iron-rich rocks
      • Latest Archean/Earliest Proterozoic - Banded Iron Formations (BIFs) [iron oxide & chert] {mostly (92%) Earliest Proterozoic}
        • Archean & Earliest Proterozoic oxygen combined with previously dissolved iron (Fe⁺²) to form BIFs & did NOT accumulate in the atmosphere
        • By the end of Early Proterozoic time, the iron sink was used up & free oxygen began to accumulate in the atmosphere
      • Latest Early Proterozoic to the present (after 1800 my) - Redbeds
        • Redbeds result from hematite produced during weathering of iron-rich rocks in an oxidizing atmosphere
  • 4. Note: Proterozoic greenstone belts do occur, but without ultramafic volcanics

PROTEROZOIC CRUSTAL EVOLUTION

• DOMINATED BY WILSON CYCLES (rifting of "supercontinents" into smaller fragments followed by dispersal, & subsequent consolidation of continental crust into "supercontinents")
  • Represented by well preserved rift strata in fault basins, overlain by passive continental margin sediments, & culminating in active continental margin sediments, often containing slivers of ocean crust (ophiolites)
  • Wopmay Orogen (1.9-1.8 by [EARLY PROTEROZOIC]) = oldest completely preserved Wilson Cycle
    • WEST SIDE OF SLAVE PROVINCE RIFTED OFF, THEN SUTURED BACK
  • Other Early Proterozoic orogens >1.8 by also indicate rifting & subsequent suturing along the margins of the Archean provinces in the Canadian Shield
  • Early Proterozoic orogens <1.8 by = series of exotic terranes (island arcs) sutured to North America - probably developed in large ocean basin formed following the Early Proterozoic consolidation of Laurentia 1.8 by ago
• GRENVILLE OROGEN (1.3-1.0 BY [MIDDLE PROTEROZOIC]) = COLLISION OF LAURENTIA WITH WEST GONDWANA
  • Midcontinent rift contemporaneous
• LATE PROTEROZOIC (see Vendian-Paleozoic Tectonics notes)

SUMMARY OF PRECAMBRIAN CRUSTAL EVOLUTION & TECTONIC PATTERNS

• THE EARTH'S SURFACE IS EXTREMELY MOBILE = PLATE TECTONICS
• PLATE SIZE GETS LARGER THROUGH TIME -> CONVECTION INTENSITY DECREASES THROUGH TIME AS THE SUPPLY OF HEAT DECREASES & THE EARTH COOLS
  • ARCHEAN "continents" - 100-500 km in width
  • Proterozoic continents - 1000-2000 km in width
  • Phanerozoic continents - 5000-10000 km in width

Study Questions

1. What are the general characteristics of Precambrian shields?

2. What are the general characteristics of Archean cratons?

3. What is the significance of the 2 types of rocks occurring in Archean cratons?

4. What is the significance & uniqueness of the ultramafic volcanics in Archean greenstone belts?

5. What is the significance of the general absence of shallow-marine quartzite & carbonate rocks?

6. What are the general characteristics of Archean tectonics (the nature of plate tectonics in the Archean in terms of convection rates & thickness of lithosphere)?

7. What is the mechanism by which Archean cratons were formed?

8. What are the different types of rocks contained in Proterozoic Mobile Belts?

9. What is the significance of Proterozoic quartzite-carbonate-shale assemblages in terms of lithospheric thickness?

10. What is the significance of tillites in terms of changes in Earth's climate?

11. What is the significance of BIF's & redbeds in terms of changes in the composition of Earth's atmosphere?

12. What is the primary cause for changes in Earth's atmosphere during the Cryptozoic?

13. What are the differences between Proterozoic & Archean crustal evolution & tectonics?

14. When and what region were affected by North American Proterozoic Wilson Cycles & tectonic events?

15. What was the cause of the changing tectonic scale from the Archean to the Phanerozoic?

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