The Precambrian Record

Key Events of Precambrian time Acasta Gneiss is dated at 3.96 bya. It is near Yellowknife Lake, NWT Canada Zircons possibly a bit older in Australia

Precambrian 4.6 billion years to, say, 548 or 544 million years (depending on method). Represents 88% of all of the history of the earth. Referred to as the Cryptozoic Eon. –“hidden life” (prokaryotes) (no more BIFs)

Early Hadean Highlights 1 Earth formed about 4.6 billion years ago from coalescing interstellar dust.Earth formed about 4.6 billion years ago from coalescing interstellar dust. Earth was bombarded by large planetesimals adding to earth’s mass (adds heat)Earth was bombarded by large planetesimals adding to earth’s mass (adds heat) Hot spinning pre-earth mass melted, caused differentiation of materials according to density.Hot spinning pre-earth mass melted, caused differentiation of materials according to density. Distinct earth layers begin to formDistinct earth layers begin to form –Dense iron and nickel migrate to center (core) –silicate material moves out to mantle

Early Hadean Highlights 2 Huge impact from a Mars-sized planetessimal created the moon.Huge impact from a Mars-sized planetessimal created the moon. –Caused earth to spin faster. –Possible Tilt change –Moon controls earth’s spin and creates tidal forces. –Moon’s orbit at an angle to planets around Sun –Earth got most of the core – outer part molten. Earth rotates. We have magnetic field and, therefore, an atmosphere

Moon Origin hypotheses -1 Speed and approach angle unlikely

Moon Origin hypotheses - 2 Chemical composition of the Moon suggests that it could not have co-formed with the earth.

Moon Origin hypotheses - 3

Precambrian Early Atmosphere First earth atmosphere H He lost to solar wind. No magnetic field Early permanent earth atmosphere mostly Nitrogen (inert) and CO 2 Post-differentiation start of liquid core dynamo Liquid water is required to remove CO 2 from atmosphere. –Mars is too cold to have liquid water. –Venus is too hot to have liquid water. –So both have CO 2 atmospheres. On Earth, most of the world’s CO 2 was converted to O 2 by photosynthesis. Enough by 2.0 bya to sustain life. CO 2 is locked up in life, limestones, dolomites. MarsEarthVenus

Gasses from cooling magmas formed early atmosphere mostly N 2, CO 2, with CH 4, H 2 O Early earth not conducive to modern oxygen breathing organisms: too much solar UV gets through atmosphere. Little oxygen occurred in the atmosphere until the evolution of photosynthetic organisms (Eubacteria) 3.5 billion years ago. Fully oxygenated about 1.9 billion years ago. Early Permanent Atmosphere

Precambrian Early Oceans from 4 bya Much water vapor from volcanic degassing. Salt in oceans is derived from weathering and carried to the oceans by rivers. Blood of most animals has chemistry of seawater. Part of the earth’s water probably came from comets. –Comets are literally large dirty snowballs. –Provide fresh water. OCEANS

Archean To Proterozoic Sedimentary Rocks Archean To Proterozoic Sedimentary Rocks Archean 4 bya: mostly deep water clastic deposits such as mudstones and muddy sandstones. –high concentration of eroded volcanic minerals (Sandstones called Graywackes). 3 bya: absence of shallow water shelf carbonates. –increasing chert. – low oxygen levels, free iron was much more common in the Archean. –Iron formed “chemical sinks” that consumed much of the early planetary oxygen. –Formed banded ironstones, commonly with interbedded chert. Proterozoic – 2 bya Carbonates* become important * Franklin Marble Field Trip 1 - Non-marine sediments turn red – iron is oxidized by the oxygen in AIR

Precambrian Hadean Formation of Continents Early earth surface was magma sea, gradually cooled to form the crust. Continents did not always exist but grew from the chemical differentiation of early, mafic magmas in the young hot earth. Floating “Volcanic Islands” of less dense higher silica magmas.

Precambrian Hadean and Archean Formation of Felsic Islands Convection was fast due high temperatures – ultramafic magma Partial Melting of base of Ultramafic Islands, OR Partial Melting of base of Ultramafic Islands, OR Fractional Crystallization of Mafic Magmas THEN Fractional Crystallization of Mafic Magmas THEN Once both mafic and felsic rocks (with different densities) exist, subduction under protocontinents possible. Once both mafic and felsic rocks (with different densities) exist, subduction under protocontinents possible. Water squeezed from subducted ocean materials partially melts mantle Water squeezed from subducted ocean materials partially melts mantle Volcanic arcs add land to protocontinents. Volcanic arcs add land to protocontinents. Increasing amounts of Felsic continental materialIncreasing amounts of Felsic continental material

First continental crust Density differences allow subduction of mafic rocks. Further partial melting and fractionation makes higher silica melt that won’t subduct Water out Komatiite partially melts, Basalt gets to surface, piles up. The stack sinks, partially melts when pressure high enough. Fractionation makes increasingly silica-rich magmas First Then: At high temperatures, only Olivine and Ca-Plagioclase crystallize “Komatiite”

Archean: Growth of the early continents Magmatism from Subduction Zones causes thickening

Growth of the early continents Island Arcs and other terranes accrete as intervening ocean crust is subducted Little Archean ocean crust survives: most subducted But silica-rich continental crust too buoyant to subduct.

Growth of the early continents Sediments extend continental materials seaward Quartz sand becomes SS or quartzite, too buoyant to subduct

Growth of the early continents Continent-Continent collisions result in larger continents Again, not very big in Archean; convection cells too small

Archean-Age Surface Rocks

Precambrian Early Continents (Cratons) Archean Archean cratons consist of regions of light-colored felsic rock (granulite gneisses) surrounded by pods of dark-colored greenstone (chlorite-rich metamorphic rocks). –Pilbara Shield, Australia –Canadian Shield –South African Shield. Mafic Greenstone Belts Felsic Islands 40km Chlorite is the Low T metamorphic product of clay muds.

Archean Crustal Provinces were once separated Canadian Shield assembled from small cratons Intensely folded rocks where cratons were later sutured together in Early Proterozoic Longest: Trans-Hudson Orogen Discussion: Where to look for diamonds

Granulite gneiss and greenstone Canadian Shield Exposed by Pleistocene glaciers

Stratigraphic Sequence of a Greenstone belt Younger lavas richer in silica Komatiites form at very high temps. They are absent later as Earth cooled Increasingly Silica-rich extrusives, some rhyolites with granites below them. Banded Iron Formations DEMO: Banded Iron Sample

Archean Formation of greenstone belts Early continents formed by collision of felsic proto-continents. Greenstone belts represent volcanic rocks and sediments that accumulated along and above subduction zones and then were sutured to the protocontinents during collisions. Protocontinents small, rapid convection breaks them up

Proterozoic Tectonics: The Wilson Cycle Proterozoic – Convection Slows Rift Phase –Coarse border, valley and lava rocks in normal faulted basins Drift Phase –Passive margin sediments Collision Phase –Subduction of ocean floor, island arcs form – Then collision

Crustal provinces: Proterozoic Tectonics Intensely folded rocks where cratons were sutured together in Early Proterozoic Slave Craton Rift and Drift Followed by Wopmay Orogen: remnants of old collisional mountains

Wilson Cycle 1&2 Rift & Drift Coronation Supergroup Proterozoic 2 bya as Slave craton pulled apart 1. Rift 2. Passive Margin sediments 1. Rift Valley Much later stuff

Near-collision phase of the Wilson Cycle in the Wopmay Orogen

3. End of Wilson cycle in the Wopmay Orogeny Coronation Supergroup thrust faulted eastward over Slave Craton Note the vertical exaggeration

Key Events of Precambrian time

Proterozoic Assembly of Laurentia Trans-Hudson Orogen mostly bya –Superior, Wyoming, Hearne plates sutured –Mountain range now eroded away Greenland, N. Gr. Brit., Scandinavia by 1.8 bya Continued accretion bya of island arcs. Most of S. US “Mazatzal Province” Last piece “Grenville Orogeny” bya Assembly of Rodinia by about 750 mya

Proterozoic Oxygen - Rich Atmosphere Eubacteria are photosynthetic 2 bya formed stromatolites along shores Free oxygen in atmosphere Band Iron Formations (common 3.8 – 2 bya) become rare, probably depended on disappearing conditions 2 bya Redbeds begin forming when iron in freshwater sediment is exposed to abundant atmosphere oxygen Oxygen in atmosphere irradiated - Ozone layer forms, protecting shallow water and land life forms from UV

First Redbeds

Key Events of Precambrian time

Final Assembly of Rodinia Grenville Orogeny 1.3 – 1.0 BYA Eastern US Grenville collided with west coast of S.America (maybe) We saw this Field Trip 1, Volcanic Arc at Park n Ride Southwest US collided w/ Antarctica –Grenville Orogeny continues in Antarctica South collided with Africa Rifted apart by about 600 mya, after time of “Snowball Earth”

Growth of Laurentia Grenville: Shallow Water sandstones (lots of graywacke), mudstones and carbonates subjected to high-grade metamorphism and igneous intrusion

Some workers think our local Grenville Collider was NW S. America

Proterozoic Rifting Grenville Time Rifting 1.3 – 1 bya Kansas to Ontario to Ohio Rift Valley sediments and lavas 15 km (9 miles) thick! Rich in Copper, as are the rift valley sediments here. Why?

Midcontinent rift 1500 km long, exposed near L. Superior

Key Events of Precambrian time

Grenville Orogen Plenty of highlands, equator to poles What Plate Tectonic conditions favor glaciation?

Snowball Earth Rodinia: abundant basalts with easily weathered Ca feldspars. Ocean gets Ca + +. CO 2 tied up in extensive limestones. Less greenhouse effect. Atmosphere can’t trap heat – Earth gets colder Grenville Orogeny left extensive highlands –From high latitudes to equator About 635 mya glacial deposits found in low latitudes and elevations Huge Ice sheet reflects solar radiation “Albedo” Some workers believe oceans froze

 13C and  18O :3 - 4 Proterozoic Glaciations Earth surface became cold enough to produce glaciations and ice ages G - Glaciation BIF - Banded Iron Formation Snow-ball Earth Cambrian Stable isotopes of C and O

Break up of Rodinia Hypothesis: Ice an insulator, heat builds up Heavy volcanic activity poured CO 2 into atmosphere – greenhouse effect Warming melted snowball earth