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THE PRECAMBRIAN

THE PRECAMBRIAN. HADEAN ARCHEAN PROTEROZOIC. The Big Bang. Event that occurred approximately 13.7 BILLION years ago All the mass and energy concentrated at a point The universe began expanding and continues to expand

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THE PRECAMBRIAN

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  1. THE PRECAMBRIAN HADEANARCHEANPROTEROZOIC

  2. The Big Bang • Event that occurred approximately 13.7 BILLION years ago • All the mass and energy concentrated at a point • The universe began expanding and continues to expand • After 1 million years matter began to cool enough to form atoms- Hydrogen- the building block of stars

  3. Galaxies and Stars • Galaxy- huge rotating aggregation of stars, dust, gas held together by gravity • Earth, the sun and our solar system is part of the Milky Way • Stars are massive spheres of incandescent gases (hydrogen and helium)

  4. The Solar System • Our solar system is located away from the galaxy’s center • Our sun and the planets originated from a solar nebula that had been enriched with heavy elements from nearby super novae (Stellar Synthesis) • Solar system is approximately 5 Billion years old • Composition is 75% hydrogen, 23% helium and 2% other materials

  5. Formation of a Protostar

  6. Center contracts Center continues to heat up Protostar radiates more heat

  7. Fusion begins in the stars core

  8. Shockwaves radiate outward releasing material Material coalesces into planets, moons or comets Other material is ejected to the periphery

  9. Our Solar System 4 inner planets (terrestrial) 4 outer planets (gaseous)

  10. Solar nebula photographed by Hubble

  11. Structure of the Earth • Solid Inner Core • Outer Liquid Core • Lower Mantle • Upper Mantle • Asthenosphere • Brittle Mantle • Lithosphere • Crust

  12. Structure of the Earth • Refraction of Seismic Waves • Changes in Velocity due to density

  13. A Magma Ocean • Lunar evidence • Textures, Uniform Composition, Age • Crystallization of well-mixed magma ocean produces uniform layered crust • Terrestrial Magma Ocean • Existence of large amount initial heat • Outer part of Earth melt during accretion • Depth estimates 100 to >1000 Km • Ultramafic (high Fe & Mg) • Crystallization complete in 100 my

  14. Composition of the Early Crust • Ultramafic • High Fe & Mg • Komatiites: volcanic, extrusive rocks • Rapid break-up and recycling of crust • Due to vigorous convection • Impacts • Existence of Plate Tectonics

  15. Solidifying Basalt- Hawaii

  16. The Earth • Core is composed of mixtures or alloys of iron (pressure is more than a million times that at the surface and temperature is estimated to be at 4000°C); has a solid inner core and a liquid outer core (earth's magnetic field may be produced by the motion of the liquid material in the iron-rich outer core) • Layer outside the earth's core is the mantle; it is solid but very hot, near the melting point of rocks, so it flows almost like a liquid, though much slower; it is 70% of the earth's volume

  17. The Earth • Outermost layer is the crust; it is extremely thin (is thinner under the oceans than under the continents) • Oceanic crust is made of basalt (low in silica and high in iron and magnesium) and has a higher density compared to continental crust, which is made of granite (high content of aluminum and magnesium silicate with quartz and feldspar) and has a lower density • Thus, continents lie above sea level and oceanic crust lies below sea level because of density differences

  18. Hotspots and Flood Basalts

  19. The Lithosphere and Mantle Reservoirs • Archean • hotter Earth >> thinner lithosphere • steeper geothermal gradient >> Komatiites (require higher temperatures) • lithospheric plates smaller and less stable • Mantle Reservoirs • Between 4 by and 2 by mantle separated into reservoirs which have remained homogeneous since formation

  20. The Origin of the Crust • Age based on lunar rocks and meteorites 4.4 to 4.5 by • Archean rocks of Canada’s Slave Province 3962 +/- 3 my based on zircon mineral crystals • Hadean rocks of Australia’s Pilbara region 4400 my based on zircon • Recycled due to rapid convection

  21. Composition of the Early Crust • Composition largely Speculative • Oldest lunar crustal rocks- representation of early earth • Granitic? • Too buoyant, resists subduction, no evidence • Lunar Highlands (4.4 bybp) • Fractional crystallization of basaltic magma • Gabbros and anorthosites, rich in mafics minerals • Komatiites and Basalts

  22. Anorthosite vs Komatiite • Anorthosite • dry magma forms crust (moon) • wet magma plagioclase sinks and does not form crust (earth) • Komatiite or Basalt • abundant in Archean terranes • high density and convective drag forces makes for easy recycling • formed as localized islands

  23. Lower Crust • Metamorphism • low grade for at shallow depths (<15 km) • medium grade (15-25 km) • high grade (25-80 km) • Granulites • Rapid tectonic uplift <50 my • Continuous convergence leads to crustal thickening, erosion and uplift will eventually expose high grade rocks at the surface

  24. The First Continents • Continental crust resists recycling due to buoyancy • Produced by partial melting of oceanic crust in subduction zones • Tonalites- abundant plagioclase, quartz, high in Ca,Na, Al • Accretion of small islands into bigger continents • Oldest remnants 3.8 to 4.0 by • detrital zircons 4.2-4.4 by • < 500km diameter • Tonalites and granodiorites

  25. Early Continental Crust • Amitsoq Gneiss • Isua Greenland • 4.0-3.8 by

  26. Growth of Crust- Mechanism • Vertical growth- thickening • underplating (intrusion of magma to lower crust) • Lateral growth • rifting • High grade granulites (representing 35-40 km) on the surface underlain by normal 40km thick crust implies • underplating kept pace with uplift and erosion

  27. Mechanisms Continental Growth • Magma addition in arcs • Terrane accretion • Continental collision • Welding of marginal sediments

  28. Mechanism for Continental Growth • (a) Magma addition in arcs • (b) Seaward migration of ocean plate • (c) Terrane accretion through suturing • (d) Continental collision’ • (e) Welding of marginal sediments

  29. Continental Growth Rates • Rapid early growth • recycling not feasible • Linear growth • Episodic growth • 2.7by, 2.0 by, 1.0 by correspond to major orogenic episodes in North America vol time

  30. Crustal Provinces • Large segments >107 km2 • Provinces are identified by geologic history and isotopic dates • most gneisses and granites • Recognized several large Precambrian provinces in North America >2.5 by • Nain; Rae; Slave; Hearne; Wyoming: Superior • 5 provinces <2.5 by but >0.9 by • Wopmay; Yavapai-Mazatzal; Trans-Hudson; Mid-Continent; Grenville

  31. Precambrian Provinces of N. Am.

  32. The Assembling of North America • Collision and suturing of provinces to make a continent • Assembly of Archean plates took only 10 my • 50% Late Archean (2.5-3.0 by) • 30% Early Proterozoic (1.6-2.0 by) • <10% Mid to late Proterozoic (0.9-1.2 by) • <10% Phanerozoic (<544 my)

  33. Cratons • Stable part of continent • Oldest part of continents • Composed of Shield and Platform • All continents contain at least one cratonic mass • Small <500km across • Time of formation varies • 50-100 my after major orogenies • uplift 200-400 my and erosion • deposition of sediments (Platform)

  34. North American Craton- shield, and platform

  35. The Hadean Eon • No direct record of the first 800 my • Formation of core 4.4 to 4.5 by • Creation of Magma Ocean and cooling of a komatiite crust • Mosaic of small rapidly moving plates • Recycling of crust at subduction zones • Partial melting of crust gives rise to tonalite magmas

  36. Hadean Crust • (a) 4.6 to 4.3 by; rapid recycling of an unstable crust • (b) 4.3 to 3.8 by; the formation of continental islands

  37. The Archean&The Proterozoic

  38. Subdivisions of the Precambrian • Major Events • Origin of the Earth • Major outgassing development of internal structure • Origin of Life • BIFs • Kenoran Orogeny • Red Beds • Glaciations • Grenville Orogeny

  39. Precambrian Basement • Igneous & Metamorphic Rocks • Association of rocks based on Superposition and Cross-cutting relationships • Divided into ARCHEAN and PROTEROZOIC • Archean 3.96 b.y. to 2.5 b.y • Proterozoic 2.5 b.y to 0.544 b.y. • Hadean No record >3.96 b.y. • Differentiation of Earth • No free Oxygen • Rich in CO2 & H2O vapor • Meteoric impacts for 100 m.y. • No evidence on Earth, But evidence on Moon & Mars

  40. Shields • Geologic Stable Regions- Every continent has 1 or more • Canadian Shield, center of North America around Hudson Bay • Exposed by Pleistocene Glaciation • Surrounded by Platforms • Thin Blankets of Sedimentary Rocks • Shield + Platform = CRATON

  41. Canadian Shield • 11 Provinces • Superior, Wyoming, Slave, Nam, Hearne, Rae & Grenville, Wopmay, • Based on Faults and Folds • Based on Age of Rocks • Boundaries marked by • Truncations in Structural Lineations • Bands of severely deformed rocks • Suture zones consolidated by 1.9 b.y

  42. Precambrian Provinces of N. Am.

  43. Archean Rocks • The Granitoid- Greenstone Association • Broad basins, subsiding, subaqueous volcanics • Shallow water deposition > stromatolites • Greenstone Belts • folded and metamorphosed • linear to irregular-shaped successions • chlorite, amphiboles, pillow basalts • some chert, BIFs, komatiites, felsic and intermediate volcanics, greywackes • Granitoid Gneiss • intrusive granitic rocks > metamorphosed to gneiss

  44. GREENSTONE BELTS

  45. Greenstone Showing Well Developed Pillow Structures

  46. Greenstone Belts of the Superior Province

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