CRYSTAL SIZE DISTRIBUTIONS (CSD) OF FELDSPARS IN GRANITOIDS FROM TURKEY AND CALIFORNIA

Plutonic rocks solidify not only by nucleation and growth of crystals, but also by solution of crystals. The latter is commonly overlooked and can occur by pressure solution, changes in volatiles, P, T or by textural coarsening (Ostwald ripening, annealing). Solidification is the development of texture; hence by crystal size distribution analysis (CSD) of undeformed granitoids from Turkey and California (Tuolumne IS) we hope to gain insight into the solidification process.

1) Many granitoids have a continuous distribution of crystals down to nuclei. That is, the lower size limit of the CSD is only limited by the measurement technique. These rocks commonly have a straight CSD for small sizes, which is consistent with nucleation and growth in an environment of exponentially increasing undercooling. This could be produced by loss of volatiles, uplift or rapid cooling caused by hydrothermal circulation.

2) Some porphyritic rocks completely lack small crystals and have a concave down CSD. This can be produced from a straight CSD by extensive textural coarsening. This process involves the solution of small crystals and the simultaneous growth of large crystals. It is driven by minimization of surface energy and occurs when the magma is maintained close to the liquidus for a long time. This situation can occur when the magma chamber is kept warm by injection of mafic magma, or when the felsic magma is emplaced into very warm rocks. The same pattern is also seen in megacrysts from some granitoids.

3) Many felsic plutonic rocks have a continuous concave up CSD. This can be generated by physical mixing of two magmas with straight CSDs like those produced by process 1, but this seems physically unlikely in granitoids. Or it can also be produced by sequential solidification under two different cooling regimes (for instance processes 1 and 2).

4) The products of processes 1 and 3 can be further modified by textural coarsening. This will rotate the CSDs counter-clockwise as small crystals are dissolved and large crystals grow.