What is a mafic stratified complex in geology ?

Basic stratified complex : definition

A basic stratified complex is a magmatic set of laccolitic form, characterized by a stratified structure due to the alternation of ultra-mafic level (peridotites, etc...) and mafic rocks (gabbros, norites, anorthosites...) resulting from the fractional crystallization of a magmatic chamber. The magmatic segregation which accompanies this phenomenon can cause the formation of thick beds and cumulates of magnetite, chromite, ilmenite, platinum, etc... transforming these complexes into chromium, nickel, titanium or platinum deposits.

Laminate complexes can occupy large areas. The Bushveld (South Africa) is the largest in the world. It is a gigantic stratified complex of mafic and ultramafic rocks which extends over 400 km long and 300 km wide, and continuously outcrops over an area of approximately 65,000 km². It is the world's largest reserve of chromium, platinum and vanadium, metals mined in many mines.

The thickness of the complex reaches 9 km, with from bottom to top :

- a thin basal norite
- bronzitites and harzburgites (the powerful lower zone of 1500 m)
- a finely banded group of pyroxenite, anorthosite, chromitite and norite (the "critical" zone, powerful 1500 m, rich in chromite horizons and which carries at its summit the famous platiniferous horizon known under the name of Merensky Reef).
- a set of gabbronorite constituting the main zone 3700 m.
- an upper zone (gabbro, anorthosite and magnetite levels) of 2300 m
The Bushveld complex is also capped by the 3000 m powerful Lebowa granitoids (the "Lebowa granite suite").

Despite this cumulative thickness of 12 km, the Bushveld does indeed have a laccolite structure (an intrusion of magmatic rocks which is set up parallel to the host by deforming the upper layer.).