Phase equilibrium studies bearing on genetic links between alkaline and subalkaline magmas, with special reference to the limestone assimilation hypothesis

The limestone assimilation hypothesis for forming alkaline, magmas from subakaline magmas is based on circumstantial petrological evidence. It requires (1) desilication, (2) alkali transfer by CO_2 and H_2O. Experimental results indicate that limestone assimilation causes crystallization, and that the thermal barrier between granitic and feldspathoidal liquids persists in the presence of H_2O and CO_2 to high pressures. Desilication of SiO_2-oversaturated magmas remains an unlikely process. Results in silicate-salt-volatile systems suggest that alkali transfer is only effective from liquids already enriched in alkalis. In general, experimental studies have failed to validate proposed genetic links between alkaline and subalkaline magmas through assimilation.