Microstructural evidence of orders of crystallization in granitoid rocks

Experimental melting studies of granitoid rocks have documented variations in the order of crystallization of minerals, depending on the melt composition, total pressure, and the activity of water and other volatiles in the melt. Microstructural criteria for independent determination of the order of crystallization are needed to permit application of the experimental data to the evaluation of the conditions of crystallization. Unfortunately, few criteria can be reliably applied to infer either the order of initial crystallization on the order in which minerals cease to crystallize in granitoid rocks. Most microstructures of granitoid rocks record simultaneous rather than sequential crystallization of minerals.

Grain-size criteria (e.g., phenocrysts vs. groundmass) can be used to infer a partial order of crystallization in many volcanic rocks, but the presence of extremely large phenocrysts of K-feldspar (that from experimental data must be the last mineral to crystallize) highlights the dangers inherent in the use of grain-size criteria.

Providing it can be determined that included minerals did not form subsequently along cracks or from entrapped melt inclusions, those inclusions that are demonstrably and consistently only near the centre of large grains must have ceased to crystallize before the host mineral. Where the host mineral occurs only as rims on the early formed mineral, it is possible to infer both the order of cessation and initiation of crystallization. Common examples are provided by the rimming produced by a discontinuous reaction relationship. Most other examples of inclusion relationships could result from simultaneous crystallization.

Microstructures involving moulding and impingement relationships are unreliable, as the microstructure is produced only after the minerals have begun to crystallize in the melt or on the melt-solid interface. As most minerals continue to crystallize right to the solidus, they cease to crystallize simultaneously.

New criteria, perhaps involving detailed chemical zoning patterns, need to be developed before the orders of crystallization can be reliably determined for granitoid rocks.