Precambrian: general term for all time before the Phanerozoic Eon.
Other names for Precambrian: Azoic (lifeless) or Cryptozoic (hidden life).

Special terminology for Precambrian rock exposures and distributions:

• Precambrian Shield: Exposed surface of Precambrian bedrock
• Platform: Shield covered by post-Precambrian rocks
• Craton: Precambrian Shield + Platform
• Orogen: Material accreted onto margins of craton by later orogenic episodes

Standard international stratigraphic nomenclature does not recognize a formal "Precambrian". Instead, uses three divisions: the Hadean (of Hades), Archean (ancient), and Proterozoic (earliest life) Eons.

Hadean Eon:

• That time period before the oldest known rocks on Earth
• A "zen" time period, in that it is defined by absence, and one which will always shrink as new discoveries of ever-more-ancient rocks are made.
• Based on estimates of the age of the Earth, the Hadean begins 4.56 Ga. It ends 4.03 Ga (in terms of whole rocks) or 4.40 Ga (in terms of oldest individual zircon crystals) as of February 2001
• Not yet a formal unit under the International Commission on Stratigraphy, because by definition cannot be represented by rocks!

Prior to the Hadean, the solar system (and constiuent bodies) coalesced out of matter from previous generations of supernovae. Proto-Earth (and most everything else) was a combination of bits of ice, rock, metal, and carbonaceous gunk.

During the Hadean, the proto-Earth differentiated into at least a metallic core and a rocky mantle. Heat for differentiation came from impacts, from gravitation, and from radioactive decay. Based on data from asteroids, this differentiation was finished within a few tens of millions of years.

Within the first 100 million years, proto-Earth collided with a Mars-sized body (called "Theia" by some). This blast ripped off part of the mantle into orbit. This orbiting mantle blob was deprived of water and other volatiles by this blast, and coalesced to form the Moon.

Archean Eon:

• 4.03 (or 4.40) - 2.5 Ga
• By definition, the oldest rocks known
• Recently subdivided into the Eoarchean (> 3.60 Ga), Paleoarchean (3.60 - 3.20 Ga), Mesoarchean (3.20 - 2.80 Ga), and Neoarchean (2.80 - 2.50 Ga). These boundaries are arbirtrarily defined, and do not correspond with particular correlative phenomena (although the upper boundary of the Neoarchean is close to the origin of modern plate tectonics).

During Archean:

• Atmosphere extremely different: little free oxygen
• Impacts common early on, drop off by around 4 Ga
• No large continental masses known (however, new very old zircons indicate continental-type crust present by at least 4.40 Ga!)
• Vastly more geothermal energy: radioactive materials had not decayed as much

Find unusual lithologies and suites of rocks in Archean:

• Komatiite: Ultramafic volcanics, very common in Archean, very rare afterwards. Require temperatures of greater than 1600 ºC (modern lavas max out at 1350 ºC). Hint at the extreme activity of Archean mantle.
• Banded Iron Formations (BIFs): Interbedded layers of chert and iron-rich minerals (iron sulfides, iron carbonates, etc.). Cannot form under modern levels of oxygen: indicate low levels in Precambrian. First appear 3.8 Ga; much more common in Proterozoic. Rare after 1.9 Ga, last appear c. 720 Ma. Major iron ore.
• Granitoid-Greenstone Complex:
  • THE typical Archean lithological suite.
  • Pods of metamorphosed granitic rock (now gneisses) separated by greenstone belts: bands of sequences of weakly metamorphosed komatiites -> basalts -> felsic volcanics -> marine sediments (turbidites, cherts, banded iron formations, etc.).
  • Oldest 4.03 Ga, disappear around 2.5 Ga.
• Cratonic Complexes: "Modern" style shallow marine and terrestrial siliciclastic deposition.
  • First appear c. 3.1 Ga; common 2.5 Ga.
  • Indicate presence of sizeable land surfaces produced by suturing together of island arcs
    • Some evidence (common age, shared position in younger land masses) that cratons in South Africa, Madagascar, Sri Lanka, India, western Australia, and parts of East Antarctica that all date from about 3.1 Ga formed the first continent: Ur
    • Additionally, some evidence that various Archean provinces had actually united to form a decent-sized supercontinent Kenorland (comprised of Ur, parts of what would become Baltica (northwestern Europe) and parts of what would become Laurentia (North America plus Greenland) by 2.7 Ga
  • During Archean and Paleoproterozoic, contain detrital uranite and pyrite (unstable under higher levels of oxygen).

Some famous localities:

• Acasta Gneiss:
  • Oldest known rocks in the world (4.03 Ga)
  • Granitoid-greenstone complex, NW Territories, Canada.
• Isua Formation: In Greenland, extremely ancient rocks containing oldest evidence of life.
• Pongola Supergroup:
  • Oldest well-known cratonic complex.
  • Sits conformably on top of greenstone belt, changes to shallow marine and non-marine sediments. (Non-marine include common gold deposits).
  • Southern Africa.
• Jack Hills
  • Metasediments containing oldest known terrestiral minerals (zircons dated at 4.40 Ga) indicating incredibly ancient continental rock
  • Western Australia

Where did granitic material come from, if midocean ridges only produce mafic materials?
A "hot topic" in historical igneous studies, but seems to be related to partial melting and recycling of materials near ancient subduction zones: as material is subducted and remelted in the presence of water, produces lots of felsic materials. These bubble up as a sort of "scum". Comparable granitic material is present in modern Iceland.

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Last modified: 2 January 2008