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Cryptozoic Rocks

Cryptozoic Rocks. 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. Greenstones.

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Cryptozoic Rocks

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  1. Cryptozoic Rocks

  2. 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

  3. Greenstones http://www.newscientist.com/article/dn14818-discovery-of-worlds-oldest-rocks-challenged-.html http://scienceblogs.com/highlyallochthonous/2007/07/what_is_a_greenstone_belt.php

  4. Archean rocks • Gneiss belts • Granite gneisses • Granite • quartzites

  5. Gneiss belt http://picasaweb.google.com/lh/photo/Sifz5y7ygBA1hpZjsZ3xow http://www.geosci.ipfw.edu/Geopics/Framesrc/Faults/quartzitefolds.html

  6. 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

  7. 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

  8. BIFs

  9. 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

  10. 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.

  11. Possible explanations for BIF’s • 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.

  12. Possible explanations for BIF’s • 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

  13. 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

  14. 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

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