The Mt. Coot- tha portion of the Enoggera granite consists mainly of pink phase, much of the mass containing xenoliths of the previously intruded grey phase. Hand specimen identification and thirteen modal analyses allow this mass to be divided into microadamellite, xenolithic microadamellite and porphyritic microadamellite.
Chemic al analyses of the potash feldspars used in conjunction with optical analyses of the plagioclase feldspars of several specimens show the temperature of crystallization of the mass to vary from 587°C to 650°C.
The effect of the granite on the contained xenoliths is localized, being restricted to the margins of the xenoliths. The contact of the granite with the country rock is sharp and many of the quartz and plagioclase phenocrysts in the mass sho w evidence of straining and breaking. These factors indicate that the temperature of intrusion was low, and consequently very close to the temperature of crystallization.
On a "Quartz - Potash feldspar - Oligoclase" phase diagram, the porphyritic microadamellite plots well above the low temperature boundary line, while the microadamellite plots almost on this line. This suggests that these two rock types are members of a differentiation series. The textural variations in the rocks are due to different percentages of constituent minerals, these differences being brought about by the differentiation of the magma.