THE ARCHEAN WORLD: A Non-Analog Earth

• Facts About the PreCambrian
• Accounts for 8/9ths of Earth history
• Proterozoic Eon
• 2500-570 MY BP
• Archean Eon
• 3950-2500 MY BP
• Hadean Eon
• >3950 MY BP
• PreCambrian Shields
• SHIELDS are stable continental centers
• Surrounded by PLATFORMS
• CRATONS Form approx. 5% Earth's surface
• Defined as topographically, low-domal in features, and composed of iron ores, copper, nickel, gold, silver, uranium.
• Archean Provinces
• Archean provinces separated by collisional orogens
• Superior > 2500 MYBP
• Wyoming > 2500 MYBP
• Slave > 2500 MYBP
• Nain > 2500 MYBP
• Hearne > 2500 MYBP
• Rae > 2500 MYBP
• PreCambrian Crust
• Oceanic crust is mafic (dark iron & magnesium silicates)
• Magmatic cooling resulted in ultramafic rocks - komatiites
• >1100^oC reflecting a hotter mantle
• Magmatic crust believed affected by convection cells and recycling.
• Mantle cooling results in mafic magmas (basalts)
• Continental crust is felsic (feldspars, quartz & muscovites)
• Oldest Archean Rocks
• 3950 MY (Yellowknife) metamorphosed tonalites
• Variety of igneous diorite >10% quartz
• 4000-4300 MY zircons found in western Australia in Archean sedimentary rocks
• Believed to have formed at subduction zones either by:
• Compressive deformation and suturing;
• Island arc and microcontinent accretion to host continents along subduction zone;
• Welding of prisms of sediment onto continental margins
• Archean Rock Associations
• Granulites
• Gneisses from severely metamorphosed (strongly heated & deformed) tonalites, granodiorites, granites & layered intrusives (anorthosites).
• Greenstones
• Found in trough-like belts composed of basalt, andesite, and rhyolite volcanics along with metamorphosed volcanics
• High in chlorite minerals
• Pillow structures in lava
• Greenstone Sequences
• Vertical transition from mafic (komatiites) to felsic (volcanics, graywackes, shales, etc.).
• Conglomeratic rocks at top provide clues to nature of Archean terranes
• No terrestrial lowland or shelf sediments!
• Greenstone sequences can be surrounded by granitic intrusives.
• Archean Tectonics
• Granulite & greenstone associations in association with volcanic- island arcs near subjection zones
• Compression of lavas results in metamorphosed rocks to granulite terranes
• More Archean Tectonics
• Greenstone belts in back-arc sites
• Lavas in basal rift sequences followed by later felsic lavas
• Followed by sediments derived from the adjacent uplands
• Belt compression resulted in greenstone facies.
• Barbeton Mountains, South Africa
• Sandstones & Shales
• Mafic & Felsic volcanics (basalts with green minerals - chlorite & hornblende)
• Chert
• Ultramafic volcanics (komatiites)

GE 142