Calculated relationships between activity of alumina and phase assemblages of silica-saturated igneous rocks: Petrogenetic implications of magmatic cordierite, garnet and aluminosilicate

摘要: Abstract Many peraluminous igneous rocks are characterized by the presence of Al-rich minerals such as cordierite, garnet and aluminosilicate polymorphs. Clearly, one important thermodynamic intensive variables controlling stability these minerals, relative to Al-poor phases ortho- clinopyroxene, must be activity Al 2 O 3 ( a ) in melt. Calculated phase relationships anhydrous mineral assemblages equilibrium with quartz-saturated liquids show that, at crustal pressures (less than about 10 kbar), garnet, only likely crystallize from melts which is up order magnitude higher that required for orthopyroxene+clinopyroxene crystallization. In spite this, granitoid saturated orthopyroxene+garnet or orthopyroxene+cordierite do not ordinarily contain significantly larger amounts excess (= form feldspars) orthopyroxene+clinopyroxene. These observations indicate there profound compositional controls on coefficient silicic melts, strongest appears total alkali content The varies directly oxide content, so alkali-poor can become markedly without ever becoming aluminous phases, whereas alkali-rich similar normative corundum cordierite even aluminosilicate. Because alumina melt simple function classification schemes based model mineralogical assemblages, reflect value effective concentration during crystallization, convey more petrologic information those chemical parameters saturation index.