Transcript Precambrian - Cal State LA

Precambrian
Geology
Precambrian
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Comprises 88%
of geologic time
Precambrian has 2
Eons
Geology hard to
Study . . .
 Preserved rocks
are
metamorphosed
 Very few fossils
present
 Relative timing
and correlation
impossible
Earth - 4.6 billion years ago
Pre-Archean Crustal Evolution
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Interior cooled from molten magma
 Early crust = mafic
– Form thin oceanic crust
 Recycled oceanic crust led to continental crust
– Released felsic components through partial melting
– Subduction formed andesitic island arcs
Continental Crust
Greenland, 3.8 b.y Itsaq gneiss
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Oldest crust about 4 b.y.
 Rocks in Greenland  3.8 b.y.
– Metamorphosed – so older
– Rocks in S. Africa, Minnesota – same age
 Zircon grains in sed rocks  crust about
4.1-4.2 b.y.
– Oldest evidence for liquid water on Earth
Zircon
Shields and Cratons
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Each present-day continent has Precambrian shield
 Shield  exposed Precambrian rock
 Platform  buried Precambrian rock outward from shield
 Shield + Platform = “Craton”
North American Canadian Shield
Precambrian Rocks, Ontario, Canada
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Cratons are relatively stable, immobile
parts of continent
In N. America, includes Canada,
Greenland, & Lake Superior
Archean Rocks
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Two main types
 Greenstone belts
– Metamorphosed
volcanic and
sedimentary rocks
– Green color due to
chlorite
 Granite-gneiss
complexes
Greenstone Belt
– Metamorphosed
granites and gabbros
Granite-Gneiss
Greenstone Stratigraphic Column
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3 Major Rock Units
Upper unit is sedimentary
rocks
– Mostly graywackes and
conglomerates
– Shallow marine deposits
 Middle units dominated
mafic volcanics
– Pillow lavas common
– Indicate underwater eruption
 Lower units are
ultramafic volcanics
– Surface temperature 1,600ºC
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Shows sequential
transition from
ultramafic to felsic
volcanics at top
Greenstones-Pillow Basalt
Greenstone Belt Structure
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Belts have synclinal form
Greenstone belts found between granite-gneiss
complexes
Tectonic Evolution of Greenstone Belt
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Backarc Basin Model
Magma intrudes
continents, formed by
subduction processes
 Convection beneath
backarc causes
extension and forms
basin
 Volcanics and sediments
collect in basin
 Accretion results in
metamorphism
 Formation of syncline
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Belts found between
protocontinents
Age of Archean Rocks
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Most greenstones
 2.5 b.y. old
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Australia Belt
 3.0 b.y. old
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Pongolo Supergroup
 3.0 b.y. old, SE
Africa
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Precambrian Rocks, Canada
Witwaterrand overlie
Pongolo, SE Africa
 2.5-2.8 b.y. old
 Non-marine
Archean Greenstone Belts
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Canadian Shield
Slave and Superior Craton
Superior Craton
Formation of Superior Craton
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Successive collision of arc with craton
produced greenstone belts
Proterozoic Eon
Proterozoic Eon
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42% of geologic time is Proterozoic
Archean vs. Proterozoic
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Most Archean rocks have been metamorphosed
and deformed
– Mostly metamorphosed greenstone belts and
granite-gneiss complexes
Proterozoic rocks have changed little
– Widespread sedimentary rocks on passive
margins
Continents were larger
– Due to accretion onto ancient Archean craton
Plate Tectonics similar to today
– Ophiolites preserved
– Quartzite-carbonate-shale assemblage
– Widespread glaciation
Proterozoic Eon
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Early Proterozoic (2.5-1.6 Ga)
– Deposition of most banded iron formation
(BIF)
– Oldest well-preserved complete ophiolite
– Amalgamation of Laurentia
– Oldest known red beds
– Origin of Central Plains, Yavapai & Mazantal
Province
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Middle Proterozoic (1.6 Ga-900 Ma)
– Igneous activity
– Midcontinent rifting
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Late Proterozoic (900-570 Ma)
– Widespread glaciation
Early Proterozoic
Banded Iron Formation (BIF)
 Red Bands  silica-rich = chert
 Black Bands  Iron oxide
– Deposited 2.0-2.5 Ga
– Record major oxygenation event
Early Proterozoic
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Oldest known complete
ophiolite (slice of oceanic
crust plastered to
continent) sequence
– Plate tectonics similar to
present day
– Shows areas of ancient
subduction zone
Jormua Complex, Finland
Early Proterozoic
Amalgamation of
Laurentia
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Laurentia – consisted of
North America,
Greenland, parts of NW
Scotland, and Baltic
Shield
– Archean rocks formed
nuclei around which
Proterozoic crust accreted
– Therefore, much larger
landmasses
– Archean and Proterozoic
cratons collided and
sutured producing
deformation belts – orogen
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Many orogens followed
Early Proterozoic
Amalgamation of Laurentia
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Trans-Hudson Orogen
– Record initial rifting
– Development of
ocean basin
– Led to origin of
subduction zone and
island arc
 Wopmay Orogen
– Oldest known
mountain-building
event
– Central Plains,
Yavapai, & Mazantal
Orogen
– Accretion along
southern border
Early Proterozoic
Oldest Known Red Bed (1.5 Ga)
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Oceanic oxygen saturated, so free oxygen building up in
atmosphere
Mid Proterozoic
Igneous Activity
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No major growth of
Laurentia
Extensive igneous activity
– Mostly granitic plutons
Precambrian granite
Mid Proterozoic
Midcontinent Orogeny &
Rifting
 Grenville Orogeny
– Final episode of Proterozoic
accretion
– Collision of 2 cratons
– Formation of
supercontinent
– Crystalline rocks in New York
and Texas
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Midcontinent Rifting
– Rift filled with thick basaltic
lava and quartzitecarbonate-shale
assemblage
Grenville Rocks, NY
Late Proterozoic
Widespread Glaciation
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Poorly-sorted, unstratified sediment
Proterozoic Supercontinent
Rodinia
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Possible configuration of
Late Proterozoic
North American and
Green-land part of
supercontinent
Proterozoic-Phanerozoic
transition
– Laurentia, Basaltica
separated from super
continent (800 Ma)
– Failed rifts (aulacogen) led
to N. America
development
§ Developed passive
margin
§ Shallow H2O LS, SS, & MS