Cryptozoic Rocks. Archean rocks Greenstones: – Meta-volcanic rocks, including Basalt (with...
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Transcript of Cryptozoic Rocks. Archean rocks Greenstones: – Meta-volcanic rocks, including Basalt (with...
![Page 1: Cryptozoic Rocks. Archean rocks Greenstones: – Meta-volcanic rocks, including Basalt (with pillows) Komatiites Andesite/rhyolite (less common, toward.](https://reader037.fdocuments.us/reader037/viewer/2022103023/56649e3b5503460f94b2cdab/html5/thumbnails/1.jpg)
Cryptozoic Rocks
![Page 2: Cryptozoic Rocks. Archean rocks Greenstones: – Meta-volcanic rocks, including Basalt (with pillows) Komatiites Andesite/rhyolite (less common, toward.](https://reader037.fdocuments.us/reader037/viewer/2022103023/56649e3b5503460f94b2cdab/html5/thumbnails/2.jpg)
Archean rocks
• Greenstones:– Meta-volcanic rocks, including
• Basalt (with pillows)• Komatiites• Andesite/rhyolite (less common, toward the tops of
sequences)– Meta-greywackes
• Volcanic rock fragments• Feldspars• Poorly sorted and rounded• Graded bedding
![Page 3: Cryptozoic Rocks. Archean rocks Greenstones: – Meta-volcanic rocks, including Basalt (with pillows) Komatiites Andesite/rhyolite (less common, toward.](https://reader037.fdocuments.us/reader037/viewer/2022103023/56649e3b5503460f94b2cdab/html5/thumbnails/3.jpg)
Greenstones
http://scienceblogs.com/highlyallochthonous/2007/07/what_is_a_greenstone_belt.php
http://www.newscientist.com/article/dn14818-discovery-of-worlds-oldest-rocks-challenged-.html
![Page 4: Cryptozoic Rocks. Archean rocks Greenstones: – Meta-volcanic rocks, including Basalt (with pillows) Komatiites Andesite/rhyolite (less common, toward.](https://reader037.fdocuments.us/reader037/viewer/2022103023/56649e3b5503460f94b2cdab/html5/thumbnails/4.jpg)
Archean rocks
• Gneiss belts– Granite gneisses– Granite– quartzites
![Page 5: Cryptozoic Rocks. Archean rocks Greenstones: – Meta-volcanic rocks, including Basalt (with pillows) Komatiites Andesite/rhyolite (less common, toward.](https://reader037.fdocuments.us/reader037/viewer/2022103023/56649e3b5503460f94b2cdab/html5/thumbnails/5.jpg)
Gneiss belt
http://www.geosci.ipfw.edu/Geopics/Framesrc/Faults/quartzitefolds.html
http://picasaweb.google.com/lh/photo/Sifz5y7ygBA1hpZjsZ3xow
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Interpretation
• Greenstones = oceanic & subduction rock• Gneisses = teeny unstable continents• Many small, fast-moving thin plates with
many subduction zones and many collisions• Thin plates allowed intraplate activity – mini-
rifts and plate over-rides• Everything was much hotter, so faster rates
and more metamorphism
![Page 7: Cryptozoic Rocks. Archean rocks Greenstones: – Meta-volcanic rocks, including Basalt (with pillows) Komatiites Andesite/rhyolite (less common, toward.](https://reader037.fdocuments.us/reader037/viewer/2022103023/56649e3b5503460f94b2cdab/html5/thumbnails/7.jpg)
Proterozoic rocks
• Lower Proterozoic: 2 common rock suites in North America– Type 1:• Well-sorted quartz sandstones• Quartz-rich greywackes• Limestones with stromatolites
– Type 2:• Banded iron formations (BIFs)• Slates and dark greywackes
![Page 8: Cryptozoic Rocks. Archean rocks Greenstones: – Meta-volcanic rocks, including Basalt (with pillows) Komatiites Andesite/rhyolite (less common, toward.](https://reader037.fdocuments.us/reader037/viewer/2022103023/56649e3b5503460f94b2cdab/html5/thumbnails/8.jpg)
BIFs
![Page 9: Cryptozoic Rocks. Archean rocks Greenstones: – Meta-volcanic rocks, including Basalt (with pillows) Komatiites Andesite/rhyolite (less common, toward.](https://reader037.fdocuments.us/reader037/viewer/2022103023/56649e3b5503460f94b2cdab/html5/thumbnails/9.jpg)
BIFs are puzzling
• Age: from Archean through Middle Proterozoic, with a bit at end of Proterozoic; most date from about 3.0-1.5 GY.
• 90% of iron in rock is in the BIFs; they hold 20X more oxygen than currently in the atmosphere – yet deposited in an Fe-poor atmosphere
• Very thin banding that goes for hundreds of kilometers
![Page 10: Cryptozoic Rocks. Archean rocks Greenstones: – Meta-volcanic rocks, including Basalt (with pillows) Komatiites Andesite/rhyolite (less common, toward.](https://reader037.fdocuments.us/reader037/viewer/2022103023/56649e3b5503460f94b2cdab/html5/thumbnails/10.jpg)
BIFs are weird
• Fine layering:– Iron-rich minerals (oxides, carbonates, sulfides,
clays, amphiboles, micas)– Chert
• But no redbeds as we know them from Phanerozoic rocks – no red shales or sandstones. So there could not be much free oxygen in the atmosphere.
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Possible explanations for BIF’s
1. Why so much iron?– Iron from volcanic eruptions– Iron coming from hydrothermal vents– Early weathering conditions were highly acidic –
that would weather out and transport the iron. – So ocean was full of iron ions, and no oxygen
ions.
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Possible explanations for BIF’s
2. Why alternating iron-rich & iron poor?– Evolution of photosynthetic organisms: they
produce oxygen which immediately bonded with iron.
– In warm water silica stays in solution but bacteria would produce more O2 and iron precipitation. Summer = red iron bands
– In cold water silica is deposited, and bacteria become inactive. Winter = silica bands
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Why did BIF production stop?
• Eventually enough O2 was produced to oxidize available iron, and so it started to build in atmosphere.
• Development of ozone layer allowed organisms to invade surface waters: more efficient photosynthesis, much more rapid production of O2
• Free O2 set stage for evolution of more heterotrophs – organisms that use more O2 to find food, rather than more CO2 to make food
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Late Proterozoic
• Mid-Continent:– Keweenawan suite: basalt, gabbro, red
sandstones and shales– What’s the tectonic suite?
• Yes, rift valley – a very long failed rift.
• We will look at other regions in more depth