Precambrian Earth – 4.6 billion years ago Pre-Archean Crustal Evolution Continental Crust Shields in addition to Cratons

Precambrian Earth - 4.6 billion years ago Pre-Archean Crustal Evolution Continental Crust Shields in addition to Cratons

Precambrian Earth – 4.6 billion years ago Pre-Archean Crustal Evolution Continental Crust Shields in addition to Cratons

Kandas, Wayne, Host/Producer has reference to this Academic Journal, PHwiki organized this Journal Precambrian Geology Precambrian Comprises 88% of geologic time Precambrian has 2 Eons Geology hard to Study Preserved rocks are metamorphosed Very few fossils present Relative timing in addition to correlation impossible Earth – 4.6 billion years ago

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Pre-Archean Crustal Evolution Interior cooled from molten magma Early crust = mafic Form thin oceanic crust Recycled oceanic crust led to continental crust Released felsic components through partial melting Subduction as long as med in addition to esitic isl in addition to arcs Continental Crust Oldest crust about 4 b.y. Rocks in Greenl in addition to 3.8 b.y. Metamorphosed – so older Rocks in S. Africa, Minnesota – same age Zircon grains in sed rocks crust about 4.1-4.2 b.y. Oldest evidence as long as liquid water on Earth Shields in addition to Cratons Each present-day continent has Precambrian shield Shield exposed Precambrian rock Plat as long as m buried Precambrian rock outward from shield Shield + Plat as long as m = “Craton”

North American Canadian Shield Cratons are relatively stable, immobile parts of continent In N. America, includes Canada, Greenl in addition to , & Lake Superior Precambrian Rocks, Ontario, Canada Archean Rocks Two main types Greenstone belts Metamorphosed volcanic in addition to sedimentary rocks Green color due to chlorite Granite-gneiss complexes Metamorphosed granites in addition to gabbros Greenstone Stratigraphic Column 3 Major Rock Units Upper unit is sedimentary rocks Mostly graywackes in addition to conglomerates Shallow marine deposits Middle units dominated mafic volcanics Pillow lavas common Indicate underwater eruption Lower units are ultramafic volcanics Surface temperature 1,600ºC Shows sequential transition from ultramafic to felsic volcanics at top Precambrian Pillow Basalts, Michigan

Greenstones-Pillow Basalt Greenstone Belt Structure Belts have synclinal as long as m Greenstone belts found between granite-gneiss complexes Tectonic Evolution of Greenstone Belt Backarc Basin Model Magma intrudes continents, as long as med by subduction processes Convection beneath backarc causes extension in addition to as long as ms basin Volcanics in addition to sediments collect in basin Accretion results in metamorphism Formation of syncline Belts found between protocontinents

Age of Archean Rocks Most greenstones 2.5 b.y. old Australia Belt 3.0 b.y. old Pongolo Supergroup 3.0 b.y. old, SE Africa Witwaterr in addition to overlie Pongolo, SE Africa 2.5-2.8 b.y. old Non-marine Precambrian Rocks, Canada Archean Greenstone Belts Canadian Shield Slave in addition to Superior Craton Superior Craton Formation of Superior Craton Successive collision of arc with craton produced greenstone belts

Proterozoic Eon Proterozoic Eon 42% of geologic time is Proterozoic Archean vs. Proterozoic Most Archean rocks have been metamorphosed in addition to de as long as med Mostly metamorphosed greenstone belts in addition to granite-gneiss complexes Proterozoic rocks have changed little Widespread sedimentary rocks on passive margins Continents were larger Due to accretion onto ancient Archean craton Plate Tectonics similar to today Ophiolites preserved Quartzite-carbonate-shale assemblage Widespread glaciation

Proterozoic Eon Early Proterozoic (2.5-1.6 Ga) Deposition of most b in addition to ed iron as long as mation (BIF) Oldest well-preserved complete ophiolite Amalgamation of Laurentia Oldest known red beds Origin of Central Plains, Yavapai & Mazantal Province Middle Proterozoic (1.6 Ga-900 Ma) Igneous activity Midcontinent rifting Late Proterozoic (900-570 Ma) Widespread glaciation Early Proterozoic B in addition to ed Iron Formation (BIF) Red B in addition to s silica-rich = chert Black B in addition to s Iron oxide Deposited 2.0-2.5 Ga Record major oxygenation event Early Proterozoic Oldest known complete ophiolite (slice of oceanic crust plastered to continent) sequence Plate tectonics similar to present day Shows areas of ancient subduction zone

Early Proterozoic Amalgamation of Laurentia Laurentia – consisted of North America, Greenl in addition to , parts of NW Scotl in addition to , in addition to Baltic Shield Archean rocks as long as med nuclei around which Proterozoic crust accreted There as long as e, much larger l in addition to masses Archean in addition to Proterozoic cratons collided in addition to sutured producing de as long as mation belts – orogen Many orogens followed Early Proterozoic Amalgamation of Laurentia Trans-Hudson Orogen Record initial rifting Development of ocean basin Led to origin of subduction zone in addition to isl in addition to arc Wopmay Orogen Oldest known mountain-building event Central Plains, Yavapai, & Mazantal Orogen Accretion along southern border Early Proterozoic Oldest Known Red Bed (1.5 Ga) Oceanic oxygen saturated, so free oxygen building up in atmosphere

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Mid Proterozoic Igneous Activity No major growth of Laurentia Extensive igneous activity Mostly granitic plutons Mid Proterozoic Midcontinent Orogeny & Rifting Grenville Orogeny Final episode of Proterozoic accretion Collision of 2 cratons Formation of supercontinent Crystalline rocks in New York in addition to Texas Midcontinent Rifting Rift filled with thick basaltic lava in addition to quartzite-carbonate-shale assemblage Grenville Rocks, NY Late Proterozoic Widespread Glaciation Poorly-sorted, unstratified sediment

Proterozoic Supercontinent Rodinia Possible configuration of Late Proterozoic North American in addition to Green-l in addition to part of supercontinent Proterozoic-Phanerozoic transition Laurentia, Basaltica separated from super continent (800 Ma) Failed rifts (aulacogen) led to N. America development Developed passive margin Shallow H2O LS, SS, & MS

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