THE PRECAMBRIAN

HADEANARCHEAN

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

• After 1 million years matter began to cool enough to form atoms- Hydrogen- the building block of stars

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)

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

Formation of aProtostar

Center contracts

Center continues to heat up

Protostar radiates more heat

Fusion begins in the stars core

Shockwaves radiate outward releasing material

Material coalesces into planets, moons or comets

Other material is ejected to the periphery

Our Solar System

4 inner planets (terrestrial)

4 outer planets (gaseous)

Solar nebula photographed by Hubble

Structure of the Earth

• Solid Inner Core• Outer Liquid Core• Lower Mantle• Upper Mantle

– Asthenosphere– Brittle Mantle

• Lithosphere

• Crust

Structure of the Earth

• Refraction of Seismic Waves

• Changes in Velocity due to density

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

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

Solidifying Basalt- Hawaii

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

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

Hotspots and Flood Basalts

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

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

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

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

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

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

Early Continental Crust

• Amitsoq Gneiss

• Isua Greenland

• 4.0-3.8 by

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

Mechanisms Continental Growth

• Magma addition in arcs

• Terrane accretion

• Continental collision

• Welding of marginal sediments

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

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

time

vol

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

Precambrian Provinces of N. Am.

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)

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)

North American Craton-

shield, and platform

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

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

The Archean&

The Proterozoic

• 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

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

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

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

Precambrian Provinces of N. Am.

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

GREENSTONE BELTS

Greenstone Showing Well Developed Pillow Structures

Greenstone Belts of the Superior Province

Greenstone Belt: Barberton Mountain Land, South Africa

• Riches of Greenstone Belts

• Copper, Zinc, Silver and Gold

• Witswatersrand: Placer Gold Deposit

Plate Tectonic Model for the Development of Greenstone Belts and Growth of Continental Crust

Origin of Atmosphere

• Atmosphere evolved in 4 steps: – primordial gases, later lost from sun's

radiation – exhalations from the molten surface (volcanic

venting); bombardment from icy comets – steady additions of carbon dioxide, water

vapor, carbon monoxide, nitrogen, hydrogen, hydrogen chloride, ammonia, and methane from volcanic activity

– addition of oxygen by plant/bacterial life

ATMOSPHERE• Present Composition

– 78% Nitrogen; 21% Oxygen; trace amounts of CO2, Argon, ect.

• Atmosphere Unique Among Other Planets– Venus & Mars CO2

Gaseous planets H, He, CH4

– Pressure in Venus 100x Earth on Mars 1/100

– Surface Temperature 450-500oC Venus; -130-25oC Mars

• Atmospheric Gases Controlled by volcanoes and interactions between gases and the solid Earth & Oceans as well as biotic component

• Ozone (O3): produced by photochemical Rx absorbs harmful UV radiation

The Origin of the Atmosphere

• Primary Gases from Accretion– rich in H, He, CH4

• Secondary Atmosphere– Degassing of Earth by volcanic activity

• large number of volcanoes/volcanic rks• Rich in Argon-40 (99.6%) as compared to Sun

(0.01%)– 40Ar product of radioactive decay of 40K

Was there a Primary Atmosphere?

• No evidence that one existed or if it did it was gone soon after planetary accretion

• Primary Atmosphere then disappeared early as a consequence of:– Solar wind– Formation of moon

Secondary Atmosphere

• Degassing- liberates CO2 and H2O vapor– Outgassing of water occurred within first 1by– volcanism

• Gases in near surface reservoirs are identical to volcanic gases

– weathering

• Terrestrial atmosphere rich in CO2 and H2O by 4by

N2 retained in atmosphere, H2O vapor lost by condensation to ocean; CO2 combined with Ca &

Mg to form carbonate Rks; H2 lost to space

Oxygen in the Atmosphere

• Earth only planet in solar system with oxygen thus only planet able to sustain higher forms of life

• Oxygen produced by– Photosynthesis- algae and plants– Photolysis-fragmentation of water molecules into

Hydrogen and Oxygen

• Oxygen consumed by– Respiration– Decay– Weathering (chemical oxidation)

Oxygen in the Primitive Atmosphere

• Photosynthesis NOT important prior to advent of microorganisms; only after 3.5 by

• Controlled by rate of Photolysis which was controlled by the outgassing of water from volcanoes, the rate of hydrogen escaping to space and the losses from weathering

Geologic Indicators of Atmospheric Oxygen Levels

• Banded Iron Formations (BIFs)

• Redbeds, Sulfates and Uraninite

• Paleosols

• Biological Indicators

Banded Iron Formations• Sedimentary Rocks >15% iron• Hematite interbedded with chert• Chemically precipitated on the seafloor• Present from 3.8 to 0.8 by• During Late Archean and Early Proterozoic

dissolved iron entered the ocean from weathering and submarine volcanic activity and reacted with dissolved oxygen to form hematite and magnetite

• Only after most BIFs precipitated did oxygen escape from ocean to atmosphere ~1.7 by

BIFs

Redbeds, Sulfates and Detrital Uraninite & Pyrite

• Redbeds- sandstones and shales w/ iron oxides require enough oxygen to oxidized.– Absent from geologic record until 2.4 by and

only abundant after 1.5 by

• Sulfates (gypsum and anhydrite) require free oxygen; not present in geologic record until 2 by

• Uraninite (uranium mineral) & Pyrite unstable under oxidizing conditions; present in rocks 2.3 to 2.8 by, none younger

PALEOSOLS

• Ancient weathering profiles or soils

• Contain information about atmosphere

• Iron is not fully oxidized in weathering deposits >2 by

• Only oxidized in granites but not in basalts since basalts contain more Fe

Biological Indicators

• Archean and Early Proterozoic cells were primitive (prokaryotic) unicellular organisms

• Developed in oxygen-free environments

• Advanced cells requiring free-oxygen do not appear in the record until 1.5by at the time oxygen levels had to be 1% of present levels

• Appearance of simple multicellular organisms at about 700 my required 7% O2

Formation of the Oceans:Prevailing Theory

• The major trapped volatile was water (H2O). Others included nitrogen (N2), the most abundant gas in the atmosphere, carbon dioxide (CO2), and hydrochloric acid (HCl), which was the source of the chloride in sea salt (mostly NaCl).

• The volatiles were probably released early in the Earth's history, when it melted and segregated into the core, mantle, and crust. This segregation occurred because of differences in density, the crust being the "lightest" material.

• Volcanoes have released additional volatiles throughout the Earth's history, but probably more during the early years when the Earth was hotter.

• Probably, the oceans formed as soon as the Earth cooled enough for water to become liquid, about 4 billion years ago. The oldest rocks on the earth's surface today are 3.8 billion years old.

Outgassing

Oceans are byproducts of heating and differentiation: as earth warmed and partially melted, water locked in the minerals as hydrogen and oxygen was released and carried to the surface by volcanic venting activity

Archean Life• Organic compounds and macrofossils or

microstructures• Carbon isotopes (C-13 to C-12) in kerogen similar

to modern organisms; 3.8 by– Isua, Greenland

• Rodshaped and filamentous structures, spheroidal bodies common in Archean cherts from 3.6 by– Warrawoona Group, Pilbara Region Australia

– Fig Tree Group, Barberton, South Africa• Stromatolites: laminated domed-shaped mounds

deposited by cyanobacteria 3.6 by, Pilbara region of Australia

Fig. 9.8f

Proterozoic• Rocks widespread on stable cratons covered by

younger sedimentary and volcanic rocks• 60% of present continental crust evolved into stable

craton by 2.4 by• Plate tectonics well established by 2 by• Canadian Shield: Huronian Supergroup (2.4-2.3 by)

– 12 km thick sequence of clastic sediments (sandstones)– Nearshore marine to fluvial origin– lower section >> detrital uraninite (anoxic)– upper section >> red beds (oxygenated)– Gowganda Fm. tillites & varves >> glaciation

Proterozoic Orogenies

• Wopmay 2.1 by- suture of Slave Plate and Bear Plates

• Accretion in southwestern US by a series of arcs between 1.8 and 1.2 by, starting with the Cheyene suture

• Mazatzal Orogeny 1.4-1.3 by

BIF upper peninsula MI

BIF Wadi Kareim, Egypt

• Animikie Group• western shores of Lake

Superior• BIFs record presence of free

oxygen• some deposits over 1000 m

thick and over 100 km in extent

• Gunflint Chert contains a series of interesting assemblage of cyanobacteria

• Keweenawan Sequence• clean quartz sandstone & conglomerates & basaltic

volcanics• lava flows are several km thick and contain copper• Rift zones associated with volcanism developed 1.2 to

1.0 by

The Grand Canyon• Older lower units Vishnu

Schist– metamorphosed sed rks

and gneisses

– intensely folded and intruded by granites emplaced during the Mazatzal Orogeny (1.4-1.3 by)

• Younger Precambrian rocks >> Grand Canyon Supergroup– clastic rks; sandstones,

conglomerates, shales

– accumulated in a trough

– Chuar Gr. Contains algal spheres

• Glaciations

Gowganda Fm

240 my periodof glaciation evidencein UT, NV, w. CanadaAK, Greenland, S. AmScandinavia, Africa

Major Glaciations

Witwatersrand Fm2.8 by

Precambrian Mineral Deposits• Magmatic Deposits

– Bushveld Complex; fractional crystallization of basalts and komatiites

• Platinum, chromium

• Massive Sulfide Deposits (Keweenawan)

• BIFs (Animikie)

• Placer Gold – Witwatersrand South Africa, ancient river

channels, conglomerates and sandstones

• Diamonds– kimberlites, alkali-rich ultramafic rocks formed in

the upper mantle, 200km deep, South Africa

ProtoPangea

• Grenville Orogeny ~1.0 by

• Consolidated continents to form the supercontinent RODINIA

• The great ocean that surrounded Rodinia was MIROVIA

• Break up between 700-600 my

The Neoproterozoic supercontinent

Rodinia as it began to break

apart. (After Hoffman, P. F. 1991. Science 252: 1409-1412.)

Proterozoic Life• By 2 by unicellular organisms

widespread

• 1.9 by Gunflint fauna– thread bacteria & cyanobacteria

• Abundant stromatolites– reached peak diversity 750 my

• By 1.8 by evolution of eukaryotic cells– Acritarchs, unicellular, spherical

microfossils, planktic, photosynthetic, common in rocks <1.5by

A: EoasterionB: EophaeraC: AnimikieaD: Kakabekia

Organisms from the GunflintChert

• Advent of the Metazoans• Metazoans

– multicellular, differentiated cells with tissues and organs

– established by 1by

• Ediacaran Fauna– Australia– Pound Quartzite– 31 species; soft bodied

• annelids, cnidarians, arthropods, echinoderms

– Late Proterozoic(550my)

Ediacaran Fauna

Spriggina floundersi

segmented worm

Kimberella- mollusc-like

Cloudina, the earliest known

calcium carbonate

shell-bearing fossils.