Formation of cogenetic quartz and nepheline syenites

摘要: Abstract Understanding the processes involved in formation of cogenetic silica-undersaturated and silica-saturated felsic rocks alkaline igneous complexes has been a long-standing enigma because constraints imposed by phase equilibria relationships. This problem is discussed general drawing upon relationships at two magmatic centers: Marangudzi Zimbabwe Mt. Brome Quebec, Canada. At each locality, quartz nepheline syenites appear to be derived from common critically undersaturated magmas. Strontium neodymium isotope results indicate that bear signatures substantial amounts crustal assimilation whereas lack or display lesser effects. In model outlined, syenite melts develop silica magmas granitic crust coupled with fractional crystallization form without large contamination. compatible equilibria.