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

Abstract

Conventional rock c1assifieation diagrams do not distinguish the variety of peraluminous rock series. Moreover, peraluminous granite types have not becn clearly discriminated in recent revisions. The study of several peraluminous series in different intraeontinental orogenie belts reveals thal four distinct groups can be defined. Using an A- S diagram, these fout groups are: (1) 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 mafic varieties; (3) low peraluminous granitoids (IP) which plot in the lowest parl of the peraluminous field defin ing negative slope trends; (4) highly felsic peraluminous granites U P) with oorly defined variation trends. In intracontinental orogenic belts, the genesis of peraluminous granitie series is favoured by the abundante of fertile crustal protoliths, mainly metapelites, metaigneous rocks and metagreywackes. The difficulty of attaining temperatures in excess of 950°C al lower crustal levels during the tectonothermal evolution of thickened crust, inhibits the partial metting of more basic sources. Although the physical parameters of the melting process influence their chemical and mineralogical characteristics, source rock composition ultimately determines the degree of peraluminosity of the granitic series.