1 I-type Granites. 2  I-type granites are granites considered to have formed by melting of an original igneous type source. 3  These are generally metaluminous granites, expressed mineralogically by the absence of peraluminous minerals and the absence of peralkaline minerals, as discussed below. wide variety of igneous rocks occur in the continental lithosphere, a reflection of its heterogeneous nature compared to oceanic lithosphere. In addition, because the continents are not subducted and are subject to uplift and erosion, older plutonic rocks are both preserved and accessible to study.