Chemical subdivision of the A-type granitoids:Petrogenetic and tectonic implications

G. Nelson Eby

doi:10.1130/0091-7613(1992)020<0641:CSOTAT>2.3.CO;2

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Research Article|July 01, 1992

Chemical subdivision of the A-type granitoids:Petrogenetic and tectonic implications

G. Nelson Eby

Department of Earth Sciences, University of Massachusetts, Lowell, Massachusetts 01854

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Geology (1992) 20 (7): 641-644.

https://doi.org/10.1130/0091-7613(1992)020<0641:CSOTAT>2.3.CO;2

Abstract

The A-type granitoids can be divided into two chemical groups. The first group (A₁) is characterized by element ratios similar to those observed for oceanic-island basalts. The second group (A₂) is characterized by ratios that vary from those observed for continental crust to those observed for island-arc basalts. It is proposed that these two types have very different sources and tectonic settings. The A₁ group represents differentiates of magmas derived from sources like those of oceanic-island basalts but emplaced in continental rifts or during intraplate magmatism. The A₂ group represents magmas derived from continental crust or underplated crust that has been through a cycle of continent-continent collision or island-arc magmatism.