Late-Orogenic, Post-Orogenic, and Anorogenic Granites: Distinction by Major-Element and Trace-Element Chemistry and Possible Origins

Abstract

Granites classified into four categories based solely on tectonics of occurrence and associated rock types also have compositional characteristics that are consistent within groups and different among groups. Orogenically related granites include late-orogenic varieties (LO) associated with calc-alkaline batholiths, and post-orogenic varieties (PO), which occur in broad zones of isolated diapiric plutons in recently deformed orogenic belts. Inclined REE patterns, moderate Sr contents, and $$K_{2}O-SiO_{2}$$ relationships show that late-orogenic granites formed by fractionation of plagioclase, clinopyroxene, and amphibole from calc-alkaline magmas. Flatter REE patterns and $$K_{2}O$$ contents near 5%, plus the absence of associated magmatic rocks, indicate that the post-orogenic granites developed by partial melting of subduction-produced mafic/intermediate magmatic rocks. Both the late- and post-orogenic granites can be part of material newly added to continental crust as a result of orogeny. Anorogenic granites in anorthosite/rapakivi complexes (AR) or alkaline ring complexes (RC) have LIL contents too high to have been equilibrated with a mafic mineral assemblage. These anorogenic rocks probably formed by partial melting of preexisting sialic crust and do not represent new crustal increment.