Trace elements in continental-margin magmatism: Part III. Alkali granites and their relationship to cratonization: Summary

Abstract

Alkali granites form as the final tectonic-magmatic event in shield areas and in many Phanerozoic orogenic belts. They occur mostly as discordant, post-tectonic plutons. Plutons of various ages and geographic locations are lithologically extremely similar, with an average composition of 72.5% SiO 2 , 0.3% TiO 2 , 14.1% A1 2 O 3 , 2.2% total Fe as Fe 2 O 3 , 0.4% MgO, 1.3% CaO, 3.6% Na 2 O, and 4.8% K 2 O. The alkali granite suites differ from the felsic rocks of most calcalkalic batholiths in having higher K 2 O/MgO ratios and few rocks with less than 65% SiO 2 . Alkali granite magmas may form by normal crystallization differentiation from calc-alkalic adamellites (quartz monzonites), although not all alkali granite magmas have originated by that process. Most alkali granites do not show evidence of having been derived from a crustal source. In some occurrences, pre-existing sial apparently was not present in the area at the time that the alkali granite magmas evolved. Thus, alkali granites may be part of the process that causes development of stable cratons by addition of new sialic material to thicken the continental crust. The higher concentration of K 2 O in younger shield areas and compensatory changes in abundances of other elements are caused by the higher abundance of alkali granites in younger cratons.