Anorthosites

Anorthosites are defined as plutonic rocks with over 90% plagioclase. They are highly felsic, felsic minerals, however is a Calcic plagioclase, which along with subordinate associated high temperature mafic minerals. The anorthosites can be classified as Archaean and Proterozoic –

Archaean Anorthosites :-

Most Archaean anorthosites cluster in the range 3.2 to 2.8 Ga. They are kilometer scale in metamorphic terrains. The extensive deformation of Archaean terranes makes the original thickness of the anorthosites difficult to determine.

Archaean anorthosites are associated with gabbroic rocks and tends to be internally layered and are similar to layered mafic intrusions, but plagioclase is much more prominent. Many Archaean anorthosite bodies are emplaced as shallow sills into “Supracrustal” rocks.

• The original shallow-water supracrustal sediments are now pelitic schists, quartzites, and marbles. These may be associated with basaltic volcanic. This package is a common constituent of Archaean greenstone belts.
• The REE concentrations of Archaean anorthosite are primitive with flat HREE near chondritic values and slightly enriched LREE.
• Because of the high accumulative plagioclase concentrations, all anorthosite REE diagrams show a pronounced Positive EU anomaly.
• The initial Sr and Nd isotopic rations indicate a range of depleted to enriched contributions (depleted dominating)

Proterozoic Anorthosites :-

Proterozoic anorthosites are generally referred to as Massif-type (large size). Proterozoic anorthosites differ from their counterparts in several ways. They are larger and less sill like, the plagioclase crystals are shaped in the common tabular form and are less anorthic, they contain less mafic matrix or mafic cumulates, and they are associated with more granitoids and not greenstone belts/supracrustal.

• Proterozoic anorthosites are almost always associated with nearly anhydrous pyroxene-bearing granitoid rocks (charnokites), as well as with Fe-rich and K-rich diorites, monzonites and other K-rich granitoids.
• Scientists have referred to the association as AMCG complexes (for anorthosite-mangerite-charnokite-granite).
• The tectonic setting is characteristically anorogenic and the massifs are intruded into thick, stable cratonic crust.
• Proterozoic anorthosites defined two belts in reconstructed Pangea. A northern hemisphere belt (Laurasia) extends from the Ukraine, through Fennoscandia and Greenland into North America. A southern hemisphere belt (in Gondwanaland) extends from India through Madagascar into Africa.