The continental crust is commonly described as having a granitic composition. This emphasises the contrast with the mafic rocks of the oceanic crust and ultramafic nature of the mantle, from which both the continental crust ultimately derives. However, there are important subtleties in the mineralogy and composition of silica rich rocks that have potential implications for their derivation and I will therefore start with the classification of silica-rich, ‘granitic’ (sensu lato) rocks, as more properly termed, granitoids.

The term granitoid applies to the suite of felsic plutonic rocks whose compositions are summarised in Figure 2.1. The dominant...

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