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Similar Archean intrusives called sanukitoids have been reported in the rocks of several early cratons.
Boninite is defined by
Boninite magma is formed by second stage melting in forearcs via hydration of previously depleted mantle within the mantle wedge above a subducted slab, causing further melting of the already depleted peridotite. The extremely low content of titanium, which is an incompatible element within melting of peridotite is the result of previous melting events that removed most of the incompatible elements from the residual mantle source. The first stage melting typically forms island arc basalt.
Boninite attains its high magnesium and very low titanium content via high degrees of partial melting within the convecting mantle wedge. The high degrees of partial melting are caused by the high water content of the mantle. With the addition of slab-derived volatiles, and incompatible elements derived from the release of low-volume partial melts of the subducted slab, the depleted mantle in the mantle wedge undergoes melting.
Evidence for variable enrichment or depletion of incompatible elements suggests that boninites are derived from refractory peridotite which has been metasomatically enriched in LREE, Sr, Ba and alkalis. Enrichment in Ba, Sr and alkalis may result from a component derived from subducted oceanic crust. This is envisaged as contamination from the underlying subducted slab, either as a sedimentary source or as melts derived from the dehydrating slab.
Boninites can be derived from the peridotite residue of earlier arc tholeiite generation which is metasomatically enriched in LREE before boninite volcanism, or arc tholeiites and boninites can be derived from a variably depleted peridotite source which has been variably metasomatised in LREE.
Areas of fertile peridotite would yield tholeiites while refractory areas would yield boninites.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Boninite". A list of authors is available in Wikipedia.|