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
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
Archean rocks
• Gneiss belts– Granite gneisses– Granite– quartzites
Gneiss belt
http://www.geosci.ipfw.edu/Geopics/Framesrc/Faults/quartzitefolds.html
http://picasaweb.google.com/lh/photo/Sifz5y7ygBA1hpZjsZ3xow
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
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
BIFs
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
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.
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.
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
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
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