The parent magma for the calc-alkaline series is a high alumina basalt, a type of basalt that is largely restricted to the subduction zone environment, and the origin of which is controversial
{"Image DocumentName": "The parent magma for the calc-alkaline series is a high alumina basalt, a type of basalt that is largely restricted to the subduction zone environment, and the origin of which is controversial", "Image ImageDescription": "Some high-Mg (>8wt% MgO) high alumina basalts may be primary, as may some andesites, but most surface lavas have compositions too evolved to be primary. Perhaps the more common low-Mg (< 6 wt. % MgO), high-Al (>17wt% Al2O3) types are the result of somewhat deeper fractionation of the primary tholeiitic magma which ponds at a density equilibrium position at the base of the arc crust in more mature arcs. Here fractional crystallization of olivine and augite in the presence of water can produce the low-Mg high-alumina basalts and basaltic andesites observed. Further rise of these hydrous basalts \uf0ae volatile loss & partial crystallization. Magmas that do reach the surface are therefore highly phyric basaltic andesites and andesites. More evolved liquids are probably the products of fractional crystallization in shallow chambers.", "Image XResolution": "37", "Image YResolution": "37", "Image ResolutionUnit": "Pixels/Centimeter", "Image Software": "SlidePlayer", "Image YCbCrPositioning": "Centered"}