Precambrian Geology. Comprises 88% of geologic time Precambrian has 2 Eons Geology hard to...
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Transcript of Precambrian Geology. Comprises 88% of geologic time Precambrian has 2 Eons Geology hard to...
PrecambrianGeologyGeology
Precambrian
Comprises Comprises 88% of 88% of geologic timegeologic time
Precambrian has Precambrian has 2 Eons2 Eons
Geology hard to Geology hard to Study . . .Study . . . Preserved Preserved
rocks are rocks are metamorphosemetamorphosedd
Very few fossils Very few fossils presentpresent
Relative timing Relative timing and correlation and correlation impossibleimpossible
Earth - 4.6 billion years ago
Pre-Archean Crustal Evolution
Interior cooled from molten magmaInterior cooled from molten magma Early crust = maficEarly crust = mafic
– Form thin oceanic crust Recycled oceanic crust led to continental crustRecycled oceanic crust led to continental crust
– Released felsic components through partial melting– Subduction formed andesitic island arcs
Continental Crust
Oldest crust about 4 b.y.Oldest crust about 4 b.y. Rocks in Greenland Rocks in Greenland 3.8 b.y. 3.8 b.y.
– Metamorphosed – so older– Rocks in S. Africa, Minnesota – same age
Zircon grains in sed rocks Zircon grains in sed rocks crust crust about 4.1-4.2 b.y.about 4.1-4.2 b.y.
– Oldest evidence for liquid water on Earth
Greenland, 3.8 b.y Itsaq gneiss Zircon
Shields and Cratons
Each present-day continent has Precambrian shieldEach present-day continent has Precambrian shield Shield Shield exposed Precambrian rock exposed Precambrian rock Platform Platform buried Precambrian rock outward from shield buried Precambrian rock outward from shield Shield + Platform = “Craton”Shield + Platform = “Craton”
North American Canadian Shield
Cratons are relatively stable, Cratons are relatively stable, immobile parts of continentimmobile parts of continent
In N. America, includes Canada, In N. America, includes Canada, Greenland, & Lake SuperiorGreenland, & Lake Superior
Precambrian Rocks, Ontario, Canada
Archean Rocks Two main typesTwo main types
Greenstone Greenstone beltsbelts
– Metamorphosed volcanic and sedimentary rocks
– Green color due to chlorite
Granite-gneiss Granite-gneiss complexescomplexes
– Metamorphosed granites and gabbros
Granite-Gneiss
Greenstone Belt
Greenstone Stratigraphic Column 3 Major Rock Units3 Major Rock Units Upper unit is Upper unit is
sedimentary rockssedimentary rocks– Mostly graywackes and
conglomerates– Shallow marine deposits
Middle units Middle units dominated mafic dominated mafic volcanicsvolcanics
– Pillow lavas common– Indicate underwater eruption
Lower units are Lower units are ultramafic volcanicsultramafic volcanics
– Surface temperature 1,600ºC
Shows sequential Shows sequential transition from transition from ultramafic to felsic ultramafic to felsic volcanics at topvolcanics at top
Greenstones-Pillow Basalt
Greenstone Belt Structure
Belts have synclinal formBelts have synclinal form Greenstone belts found between Greenstone belts found between
granite-gneiss complexesgranite-gneiss complexes
Tectonic Evolution of Greenstone Belt Backarc Basin ModelBackarc Basin Model Magma intrudes Magma intrudes
continents, formed by continents, formed by subduction processessubduction processes Convection beneath Convection beneath
backarc causes backarc causes extension and forms extension and forms basinbasin
Volcanics and Volcanics and sediments collect in sediments collect in basinbasin
Accretion results in Accretion results in metamorphismmetamorphism
Formation of synclineFormation of syncline Belts found between Belts found between
protocontinentsprotocontinents
Age of Archean Rocks Most greenstonesMost greenstones
2.5 b.y. old2.5 b.y. old Australia BeltAustralia Belt
3.0 b.y. old3.0 b.y. old Pongolo Pongolo
SupergroupSupergroup 3.0 b.y. old, SE 3.0 b.y. old, SE
AfricaAfrica Witwaterrand Witwaterrand
overlie Pongolo, SE overlie Pongolo, SE AfricaAfrica 2.5-2.8 b.y. old2.5-2.8 b.y. old Non-marineNon-marine
Precambrian Rocks, Canada
Archean Greenstone Belts Canadian ShieldCanadian Shield
Slave and Superior CratonSlave and Superior Craton Superior CratonSuperior Craton
Formation of Superior Craton
Successive collision of arc with Successive collision of arc with craton produced greenstone beltscraton produced greenstone belts
Proterozoic EonProterozoic Eon
Proterozoic Eon 42% of geologic time is Proterozoic42% of geologic time is Proterozoic
Archean vs. Proterozoic Most Archean rocks have been metamorphosed Most Archean rocks have been metamorphosed
and deformedand deformed– Mostly metamorphosed greenstone belts and Mostly metamorphosed greenstone belts and
granite-gneiss complexesgranite-gneiss complexes Proterozoic rocks have changed littleProterozoic rocks have changed little
– Widespread sedimentary rocks on passive Widespread sedimentary rocks on passive marginsmargins
Continents were largerContinents were larger– Due to accretion onto ancient Archean cratonDue to accretion onto ancient Archean craton
Plate Tectonics similar to todayPlate Tectonics similar to today– Ophiolites preservedOphiolites preserved– Quartzite-carbonate-shale assemblageQuartzite-carbonate-shale assemblage– Widespread glaciationWidespread glaciation
Proterozoic Eon Early Proterozoic (2.5-1.6 Ga)Early Proterozoic (2.5-1.6 Ga)
– Deposition of most banded iron formation Deposition of most banded iron formation (BIF)(BIF)
– Oldest well-preserved complete ophioliteOldest well-preserved complete ophiolite– Amalgamation of LaurentiaAmalgamation of Laurentia– Oldest known red bedsOldest known red beds– Origin of Central Plains, Yavapai & Mazantal Origin of Central Plains, Yavapai & Mazantal
ProvinceProvince Middle Proterozoic (1.6 Ga-900 Ma)Middle Proterozoic (1.6 Ga-900 Ma)
– Igneous activityIgneous activity– Midcontinent riftingMidcontinent rifting
Late Proterozoic (900-570 Ma)Late Proterozoic (900-570 Ma)– Widespread glaciationWidespread glaciation
Early Proterozoic
Banded Iron Formation (BIF)Banded Iron Formation (BIF) Red Bands Red Bands silica-rich = chert silica-rich = chert Black Bands Black Bands Iron oxide Iron oxide
– Deposited 2.0-2.5 Ga– Record major oxygenation event
Early Proterozoic Oldest known Oldest known
complete ophiolite complete ophiolite (slice of oceanic crust (slice of oceanic crust plastered to plastered to continent) sequencecontinent) sequence– Plate tectonics similar to Plate tectonics similar to
present daypresent day– Shows areas of ancient Shows areas of ancient
subduction zonesubduction zone
Jormua Complex, Finland
Early ProterozoicAmalgamation of Amalgamation of
LaurentiaLaurentia Laurentia – consisted of Laurentia – consisted of
North America, North America, Greenland, parts of NW Greenland, parts of NW Scotland, and Baltic Scotland, and Baltic ShieldShield – Archean rocks formed Archean rocks formed
nuclei around which nuclei around which Proterozoic crust accreted Proterozoic crust accreted
– Therefore, much larger Therefore, much larger landmasseslandmasses
– Archean and Proterozoic Archean and Proterozoic cratons collided and cratons collided and sutured producing sutured producing deformation belts – deformation belts – orogenorogen
Many orogens followedMany orogens followed
Early ProterozoicAmalgamation of LaurentiaAmalgamation of Laurentia
Trans-Hudson Trans-Hudson OrogenOrogen– Record initial riftingRecord initial rifting– Development of Development of
ocean basinocean basin– Led to origin of Led to origin of
subduction zone and subduction zone and
island arcisland arc Wopmay OrogenWopmay Orogen
– Oldest known Oldest known mountain-building mountain-building event event
– Central Plains, Central Plains, Yavapai, & Mazantal Yavapai, & Mazantal OrogenOrogen
– Accretion along Accretion along southern border southern border
Early Proterozoic
Oldest Known Red Bed (1.5 Ga)Oldest Known Red Bed (1.5 Ga) Oceanic oxygen saturated, so free oxygen building up Oceanic oxygen saturated, so free oxygen building up
in atmospherein atmosphere
Mid ProterozoicIgneous ActivityIgneous Activity No major growth of No major growth of
LaurentiaLaurentia Extensive igneous Extensive igneous
activityactivity– Mostly granitic plutonsMostly granitic plutons
Precambrian granite
Mid ProterozoicMidcontinent Orogeny Midcontinent Orogeny
& Rifting& Rifting Grenville OrogenyGrenville Orogeny
– Final episode of Final episode of Proterozoic accretionProterozoic accretion
– Collision of 2 cratonsCollision of 2 cratons– Formation of Formation of
supercontinentsupercontinent– Crystalline rocks in New Crystalline rocks in New
York and TexasYork and Texas Midcontinent RiftingMidcontinent Rifting
– Rift filled with thick Rift filled with thick basaltic lava and basaltic lava and quartzite-carbonate-shale quartzite-carbonate-shale assemblageassemblage
Grenville Rocks, NY
Late ProterozoicWidespread GlaciationWidespread Glaciation
Poorly-sorted, unstratified sedimentPoorly-sorted, unstratified sediment
Proterozoic SupercontinentRodiniaRodinia Possible configuration of Possible configuration of
Late ProterozoicLate Proterozoic North American and North American and
Green-land part of Green-land part of supercontinentsupercontinent
Proterozoic-Phanerozoic Proterozoic-Phanerozoic transitiontransition– Laurentia, Basaltica Laurentia, Basaltica
separated from super separated from super continent (800 Ma)continent (800 Ma)
– Failed rifts (aulacogen) Failed rifts (aulacogen) led to N. America led to N. America developmentdevelopment
§ Developed passive margin
§ Shallow H2O LS, SS, & MS