READING TEXTURE IN GRANITIC ROCKS

Igneous rocks that consist of alkali feldspars and quartz are granitic in composition, but four textural variants are common: granitic, porphyritic, aplitic, and pegmatitic. Each texture reveals fundamental differences in the conditions of crystallization.

Crystallization of liquids occurs only when they are undercooled below their liquidus temperature. The magnitude of liquidus undercooling, ΔT, when crystallization commences correlates with supersaturation of liquid in crystal-forming components and with ΔG, which becomes increasingly negative for the reaction liquid → crystals as ΔT increases.

Granitic texture results from crystallization close to the liquidus (ΔT £ ~ 50°C). For this to be true, the nucleation rates for quartz and feldspars must keep pace with cooling. Because nucleation rates are very low close to the liquidus, the key process of granitic texture appears to be very slow rates of cooling for the melt.

Porphyritic texture is commonly thought to represent igneous crystallization in two different and sequential P-T regimes. Fenn (1977, Can. Mineral. 15: 135-161) correctly observed, however, that porphyritic texture might result during isobaric cooling because nucleation rates for Kfs are lower than for Ab at most values of ΔT (≈ 50°-150°C).

Because aplitic and pegmatitic textures occur simultaneously together, their P-T regime for crystallization must be essentially the same. Aplitic texture results when both quartz and feldspars exhibit an extremely high rate of nucleation in relation to growth rates. For hydrous granitic melts, this maximum in nucleation occurs at ΔT ≈ 200°-250°C. The same is true for the defining texture of pegmatites: graphic granite or granophyre. These textures develop at ΔT ≈ 200°-250°C, which translates to crystallization temperatures of ~ 400°-450°C.

If granitic texture arises from slow cooling close to the liquidus, and aplitic-pegmatitic textures originate from crystallization at large ΔT, how do these two textures form sequentially in the same magma body, as for example the Reformatory Granite of the Wichitas Igneous Province? The apparent answer is that despite slow cooling, crystallization that commences close to the liquidus to make granitic and porphyritic textures stops, and when it resumes, ΔT has increased from ~ 50°C to ~ 200°-250°C.