A re-examination of the typology of peraluminous granite types in intracontinental orogenic belts

Conventional rock classification diagrams do not distinguish the variety of peraluminous rock series. Moreover, peraluminous granite types have not be en clearly discriminated in recent revisions. The study of several peralumi nous series in different intracontinental orogenic belts reveals that four distinct groups can be defined. Using an A-B diagram, these four groups are : (I)highly peraluminous granitoids (hP) characterised by high A values and typified by an increase in peraluminosity toward the most mafic varieties; (2) moderately peraluminous granitoids (mP) which occupy the intermediate field and generally show increasing peraluminosity towards the most felsic varieties; (3) low peraluminous granitoids (lP) which plot in the lowest pa rt of the peraluminous field defining negative slope trends; (4) highly fel sic peraluminous granites (fP) with poorly defined variation trends. In intracontinental orogenic belts, the genesis of peraluminous granitic se ries is favoured by the abundance of fertile crustal protoliths, mainly met apelites, metaigneous rocks and metagreywackes. The difficulty of attaining temperatures in excess of 950 degrees C at lower crustal levels during the tectonothermal evolution of thickened crust, inhibits the partial melting of more basic sources. Although the physical parameters of the melting proc ess influence their chemical and mineralogical characteristics, source rock composition ultimately determines the degree of peraluminosity of the gran itic series.