The Archean Proterozoic 4 0 Ga to 543

The Archean & Proterozoic 4. 0 Ga to 543 Ma

Growth of Continental Crust • There is some considerable debate regarding the rate at which continental crust has been added to the earth. Slow-growth models Rapid growth models Pulse & Intermediate growth models

Onset of Plate Tectonics? There is actually considerable debate about when modern-style plate tectonics began on the planet. Through secular cooling and decay of long and short lived radioisotopes, the Earth’s thermal regime should be decreasing through time. The early hot earth may have inhibited subduction of oceanic crust (i. e. the convection and heat in the mantle would have stopped subduction. Average of subducted crust today is ~125 Ma. Contrary View: The early oceanic crust was of a different (denser) composition and therefore able to subduct because of its negative buoyancy.

In terms of thermal budget for the Earth, the overall heat budget from the decay of radioactive elements in the Late Neoproterozoic is not significantly different than today.

Komatiites • Komatiites are very high temperature lavas with Mg. O contents up to 33%. Uprise from great depths is the easiest way to achieve such high liquid temperatures. Now it used to be thought that such high temperatures (>1600°C) could only be achieved in the mantle of the early Earth (Archaean).

“Spinnefex” fabric in Komatiite lavas-Africa

Cretaceous Komatiites • It was thought that komatiites could not form in younger times, but they were discovered on the island of Gorgona (near Colombia). • Some newer ideas are that komatiites are formed from deep mantle melts (where temps are higher). • The Archean still had high temps and it is possible that a Komatiitic ocean crust could subduct.

Crustal Motion • There is clear evidence that continental drift did occur in the Archean. • Paleomagnetic studies on Archean-age rocks show that the crustal blocks did move, but continental drift is not the same thing as plate tectonics.

Ophiolites • Ophiolites-Pieces of oceanic plate that have been thrusted (obducted) onto the edge of continental plates.

How do we know? Pillow Basalts & Marine Sediments Feeder Dikes Massive Gabbro Layered Gabbro. Chemical equivalent of basalt. Metamorphic sole (base) and peridotite (Mg-silicate rock. Fault block melange Marine Sediments on Continental block

Franciscan melange

Peridotite

Layered Gabbro in Oman

Massive Gabbro

Sheeted Dyke Complex

Pillow Basalts

Ocean Floor Sediments

Ophiolites indicate Subduction of Oceanic Crust. • Eldredge Moores (1994) suggested that ophiolites became common around 1000 Ma and this marked the onset of modern-style plate tectonics. • A dismembered ophiolite was found in Dongwanzi China by Tim Kusky (2002). • The ophiolite is 2. 5 Ga and provides evidence that modern-style plate tectonics was already underway at 2. 5 Ga. • Others argue that the time interval from ~1. 9 -2. 1 Ga marked the onset of modern-style plate tectonics.

Are we arguing about silly things? • Possibly. However, we use modern plate tectonic models to help us identify regions of mineral wealth. • What is it that we know positively? – – Continental crust had formed in the Archean Continental crust was moving in the Archean Mafic crust in the Archean was formed in a hotter environment Most of that mafic crust is gone (subduction).

Types of Archean Continental Crust • Granite-Greenstone Belts • Tonalite-Trondhjemite Gneisses (TTG’s)

Tonalites-Trondhjemites • Tonalite is an igneous, plutonic (intrusive) rock, of felsic composition, with phaneritic texture. Mineral assembly is composed typically of plagioclase, more than 20% of quartz and rare alkali feldspar. Amphiboles and pyroxenes are common accessory minerals. Trondhjemite is a variety of tonalite where biotite is the only mafic mineral. • Both are associated with subduction and the formation of island arcs.

Granite-Greenstone Belts Granites are an intrusive felsic rock that forms at temperatures around 800 C. Greenstones are low-grade metamorphosed basalts. Their association in the Archean is quite strange. They are likely tectonic slices (faultbounded)