Transcript Precambrian
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://www.newscientist.com/article/dn14818-discovery-ofworlds-oldest-rocks-challenged-.html http://scienceblogs.com/highlyallochthonous/2007/07/wh at_is_a_greenstone_belt.php Archean rocks • Gneiss belts – Granite gneisses – Granite – quartzites Gneiss belt http://picasaweb.google.com/lh/p hoto/Sifz5y7ygBA1hpZjsZ3xow http://www.geosci.ipfw.ed u/Geopics/Framesrc/Faults /quartzitefolds.html 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 – minirifts 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