Generation of metaluminous A-type granites by low-pressure melting of calc-alkaline granitoids

Melting experiments on two calc-alkaline rocks, a tonalite and a granodiorite, demonstrate that shallow dehydration melting of hornblende- and biotite-bearing granitoids generates metaluminous A-type granitic melts. Profuse crystallization of calcic plagioclase + orthopyroxene during low pressure (P ≤ 4 kbar) incongruent melting explains all of the major and trace element characteristics of A-type granites (e.g., their low Al, Ca, Mg, Sr, and Eu contents, and high Ga/Al and K/Na ratios), as well as their common association with anorthositic and mangeritic rocks. The residual assemblage produced during melting of the same rocks in the deep crust (P ≥ 8 kbar) is dominated by clinopyroxene, and, as a consequence, most of the chemical features diagnostic of A-type granites are lost. The high temperatures (>900 °C) and low H₂O contents (≤ 4 wt%) that are characteristic of A-type magmas are adequately explained by the H₂O-poor nature of the granitoid source materials, and require neither a granulitic nor a F-rich source. Previous attempts to explain the origin of A-type granites have focused on the nature of the source material. These experiments show that pressure of melting is at least as important as source composition in engendering A-type melts.