GRANITOIDS OF RAPAKIVI GRANITE-ANORTHOSITE AND RELATED ASSOCIATIONS

Mid-Proterozoic anorthosite-mangerite-charnockite-rapakivi granite (AMCG) magmatic suites have commonly intruded Proterozoic orogenic belts within 100 Ma or less to as much as 600 Ma after cessation of calc-alkalic crustal generation processes. High FeO/ (FeO + MgO), high Na2O + K2O, low Al2O3, and reduced redox condition of the granitoids coupled with the bimodal character of AMCG magmatism are dissimilar to calc-alkalic associations. Although included in the broad classification of A-type granites, the granitoid rocks of AMCG suites have mineral and chemical characteristics that permit them to be more precisely defined as a distinctive subset of that category. Comparison with other mid-Proterozoic late orogenic, post-orogenic, and (LPA) granites illustrates the extended compositional range and evidence for higher temperature earlier magmas exhibited by AMCG granitoid suites. Many AMCG granitoid assemblages contain clear evidence of extensive fractionation that led in some cases to extremely evolved products. Common abundance of the earlier crystallized fractions supports the view that AMCG granitoid suites developed by way of extended crystal-liquid fractionation from voluminous parent melts and not by small degrees of partial melting of relatively depleted granulitic sources.