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 at_is_a_greenstone_belt.php worlds-oldest-rocks-challenged-.html

Archean rocks Gneiss belts – Granite gneisses – Granite – quartzites

Gneiss belt u/Geopics/Framesrc/Faults /quartzitefolds.html hoto/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 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 O 2 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 O 2 Free O2 set stage for evolution of more heterotrophs – organisms that use more O 2 to find food, rather than more CO 2 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