Archaean Komatiite Suite

 
Archaean Komatiite Suite
 
The Archean is the oldest phase of earth history.  Rocks produced during this time, especially oceanic lithospheric rocks,  are rare today since most have been destroyed through subduction. Samples that have been preserved consist of interlayered amphibolites (medium grade metamorphosed ultramafic igneous rocks) (often pillowed) and anorthosites (Ca rich plagioclases), sometimes with nickel and chromium rich layers (Windley, 1984).
 
The close spatial and chemical association of these rocks points to a common origin of fractional crystallization of an ultramafic (komatiite) parent at an oceanic rift system. During fractionation highly refractory components such as nickel, chromium and olivine crystallize out and settle to the bottom of the magma chamber forming the bottom of the Archean oceanic lithosphere.
 
 
Plagioclase and Pyroxene crystallize out along the walls of the chamber forming layered igneous rocks of the crustal portion of the lithosphere (these will be later metamorphosed to amphibolites). Magmas spill onto the ocean floor forming pillow basalts. Although some of these are komatiitic in composition, some have fractionated to tholeiite.

To understand these rocks we must go back to the earth's origins. In the beginning the earth was a red-hot molten ball, rich in ultramafic materials (the parent material), including large quantities of silica, iron, and calcium. High density materials sink toward the center, and the surface is a magma ocean cooling and solidifying to a lithosphere of Gabbros, basalts and anorthosites. 
    
In the mantle, turbulent convection cells form and convect heat to the surface via rising plumes of plastic rock.  Reduced pressure tens of kilometers beneath leads to melting, and subcrustal ponding of ultramafic magmas. Some of the magma escapes to the surface as volcanoes and the magma chamber begins a cooling phase. During the cooling, fractional crystallization occurs as geologic processes physically separate the various fractions into layered igneous rocks of ultramafics, anorthosites, and mafic volcanics.
 
Contributed by Lynn Fichter 
Tuesday, August 19, 2014
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