 |
|
|
Petrologic Classification of Ophiolites
Ophiolites may have formed either at divergent plate boundaries (mid-oceanic ridges) or convergent plate boundaries (supra-subduction zones; i.e. island arcs and marginal basins). They are called MOR and SSZ types, respectively. These types are identified by chemical composition of the rocks and minerals in comparison with those from various tectonic settings on the earth at present.
Ophiolites have been divided into two groups:
- high-Ti group;
- low-Ti group.
These groups can be discriminated by studying the fractionation trends of both gabbroic complexes and lavas and dykes.
The high-Ti type typically shows MORB-like (Mid-Ocean Ridge Basalts) magmas. High-Ti ophiolites are petrologically and geochemically similar to major oceanic and ensialic back-arc basin crusts as well as oceanic crust generated during the intermediate and late-stage opening of intraoceanic back-arc basins.
The low-Ti magma types have a spectrum of composition from MORB to island arc tholeiites and boninite-like magmas often occur. Parental magmas and fractionation processes of low-Ti ophiolites fit with an hypothesis of their formation in the early stage of opening of intraoceanic back-arc basins.
|
|
|