I. Class discussion and illustration items:

• the basis of division of the Precambrian into the Archean and Proterozoic Eons.
• the global distribution of Precambrian shields
• examples (Africa and Australia) of Archean to Proterozoic continental accreation (we will discuss N. America in the next lecture).
• the nature and origin of gneiss and greenstone terraines

• 4.6-~3.8 b.y.
• no record
• entire earth's surface volcanic
• small, ever changing plates
• <heat after end of meteorite impact ~3.9 b.y.
• small blocks begin surviving

• ~3.8-2.5 b.y.
• two major rock suites; greenstones & gneiss belts
• 3x heat generation at 3.5 b.y. (radioactive decay)
• high temperature metamorphism
• a distribution and abundance of greenstone belts not seen in the younger geologic record.
• area of continental crust (10-40% of today - Archean)

 

 

A. Archean gneiss assemblage

1. the bulk of Archean regions
2. plutonic (granitic), volcanic and sedimentary parent rocks
3. metasediments include: metamorphosed quartzites, volcaniogenic sediments, iron formations and carbonate rocks
4. some deformed mafic and ultramafic complexes
5. mostly derived from metamorphism of felsic igneous rocks
6. evidence: concentration of light elements (K, Ca, Na, Al)
7. isotope ratios & RE elements - derived from more chemically primitive mantle
8. high-grade metamorphic gneissic regions exhibit amphibolite (hornblende and garnet; 500-650 C and 200-500 MPa) or granulite metamorphism
conclusions:
• remnants of incipient continents
• crust: thin, less rigid, widespread high temp. metamorphism
• unstable w/partial melting of metamorphic & sedimentary, rocks, sediments (or former sediments) schists, BIFs, quartzites, marbles

B. Greenstone Belt (3.8-3.0 B.y.)
1. giant pods of elongated downwarps (illustrated in class)
2. oldest in S. Africa (3.6 b.y.), those of Canadian shield (2.7-2.5 b.y.)
3. characteristics (illustrated in class):
• lower unit [ultramafic (e.g. peridotite) and mafic (e.g. basalt) ]
• middle unit dominated by volcanics (basalt)
• upper unit sedimentary (sandstones and shales)
• granite intrusives through middle and upper units
4. synclinal structure, intruded by granitic magmas, complex folds, cut by thrust faults.
5. volcanics w/green minerals: epidote, chlorite, serpentine formed by low grade metamorphism.... gives its dark green color.
6. Greenshist (chlorite and actinolite) or lower grades of metamorphism. Pressure and tenperature of greenshist is 400-500C and 200-500 MPa
7. Common pillow lavas indicating subaqueous eruption, also subaerial eruptions.
8. Presence of ultramafic lava flows (requires >1600C surface temperatures. Such crustal flows are rare today.)
9. richer in magnesium than younger rocks: higher melting temperature
10. some chromium & nickel rich: ultramafic crust
11. sediments: immature graywackes, conglomerates, argillites ( metamorphic shales) & sandstones- poorly sorted & rounded, volcanics & feldspars rich; eroded & deposited rapidly-"poured in or dumped".
12. graded bedding: deep water between volcanics

From the above class summary of greenstone environments
C. Some Models for origins of Greenstone belts:
1. Island-arc marginal basin (illustrated in class)
2. Intracontinental Rifts (illustrated in class)
3. Greenstone belts terrestrial equivalent of lunar maria (illustrated in class)
4. remnants of oceans and volcanic arcs between continents

 
D. Problems with models
1.  quartzite-carbonate-mafic-ultramafic assocation in gneissic terranes have no modern equivalent
2. ultramafic lavas of Archean almost lacking in modern marginal basins and island arcs
3. If greenstone belts are marginal basins, they lack accompanying mature arc
4. If greenstone belts are island arcs, they differ in composition from modern arcs and lack the zonal differences associated with polarity of the subducting slab.

E. Probable Archean conditions:
1. volcanism widespread, almost everywhere,
2. magma generated more rapidly
3. continents with continental shelves not present or greatly reduced in size
4. some lithospheric plates but smaller, thinner, less rigid, intense mantle convection
5. Finally (late Archean), major period of granite formation between 2.5-3.0 b.y. when much of present crustal volume accreted and modern style plate tectonics began