Ultramafic to mafic layered intrusions: Encyclopedia II - Ultramafic to mafic layered intrusions - Intrusive Mechanisms

Ultramafic to mafic layered intrusions - Intrusive Mechanisms

It is difficult to precisely determine what causes large ultramaficmafic intrusives to be emplaced within the crust, but there are two main hypotheses: plume magmatism and rift upwelling.

Ultramafic to mafic layered intrusions - Plume Magmatism

The plume magmatism theory is based on observations that most large igneous provinces include both hypabyssal and surficial manifestations of voluminous mafic magmatism within the same temporal period. For instance, in most Archaean cratons, greenstone belts correlate with voluminous dyke injection as well as usually some form of larger intrusive episodes into the crust. This is particularly true of a series of ultramafic-mafic layered intrusions in the Yilgarn Craton of ~2.8 Ga and associated komatiite volcanism and widespread tholeiitic volcanism.

Plume magmatism is an effective mechanism for explaining the large volumes of magmatism required to inflate an intrusion to several kilometres thickness (up to and greater than 13 kilometres). Plumes also tend to create warping of the crust, weaken it thermally so that it is easier to intrude magma and create space to host the intrusions.

Plume magmatism is supported in some intrusions by geochemistry. In particular the Noril's-Talnakh intrusions are considered to be created by plume magmatism and other large intrusions have been suggested as created by mantle plumes.

However, the story is not so simple, because most ultramafic-mafic layered intrusions also correlate with craton margins, perhaps because of the fact that they are exhumed more efficiently in cratonic margins because of faulting and subsequent orogeny.

Ultramafic to mafic layered intrusions - Rift magmatism

The presence of large layered complexes in Greenland such as the Skaergaard Intrusion which are not related to mantle plumes indicated other processes can form these intrusions. Here, the large magma volumes which are created by mid-ocean ridge spreading allow the accumlation of large volumes of cumulate rocks. Also, the problem of creating space for the intrusion is easily explained by the extensional tectonics in operation.

Adapted from the Wikipedia article "Intrusive Mechanisms", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki