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