Modern crust formation - models and mechanisms

Mantle peridotite which is hydrated and so contains amphibole in addition to, or instead of, clinopyroxene. Will melt at a lower temperature than dry mantle The most abundant rock-type in a well developed volcanic arc. Andesites can be defined in terms of their silica and alkali contents (see Fig. 4.10). 

An important rock-type in a volcanic arc. They are more siliceous than basalts but less so than andesites (see Fig. 4.10) 

The composition of feldspar in a particular rock as calculated from the major element chemistry of the rock using a specific set of rules - the (CIPW) normative calculation. The feldspar content is expressed in terms of the weight % proportion of the Na-, K- and Ca-feldspars (Albite (Ab), Orthoclase (Or) and Anorthite (An)) Basalt from an arc environment which typically has about 20 wt% Al203, rather than the ca. 15 wt% found in oceanic basalts 

Trace elements which have a small ionic radius but which carry a high ionic charge. 

These tend to be the immobile elements during fluid alteration 

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