Land's End Granites and Their Relation to the Experimental Granite System

Abstract

RECENT work on the experimental granite system^1,2 has established a series of quaternary minima shifting in the direction of the NaAlSi₃O₈ apex with increasing water vapour pressure, and becoming eutectic points at pressures in excess of 3.6 kbars. The “ternary” diagram (Fig. 1) shows the composition of six points at which components simultaneously melt under given isobaric conditions, each of these points representing minimum temperatures on the experimental granite solidus^1,2. It is thought that crystallization in the natural granite system, in the presence of volatiles the vapour pressure of which was decreasing, produced trends very similar to those of the experimental system. Field evidence and chemical data³ support this view, and the widespread occurrence of tourmaline, fluorite, and hydrothermally altered granites in Cornubia^3,4 suggest that boron and fluorine, together with water vapour, fulfilled the role of volatiles. Because of the higher fluxing action of boron and fluorine compared with water, the shift of the quartz-feldspar boundary towards the albite–orthoclase sideline would be accentuated.