cryptozoic rocks. archean rocks greenstones: – meta-volcanic rocks, including basalt (with...

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