Origin of megacrysts in granitoids by textural coarsening: A Crystal Size Distribution (CSD) Study of Microcline in the Cathedral Peak Granodiorite, Sierra Nevada, California.

Stained slab

Microcline megacrysts in the Cathedral Peak granidiorite. In this slab 100 mm square the microcline has been stained orange.
Microcline megacrysts in the Cathedral Peak Granodiorite and other parts of the Tuolumne Intrusive Suite were formed by textural coarsening (Ostwald Ripening) of earlier formed crystals. The early-formed crystals nucleated and grew in an environment of increasing undercooling, probably during the ascent of the magma. Emplacement of the magma into warm host-rocks promoted textural coarsening. Crystals smaller than a certain size (the critical size) dissolved in the interstitial melt whilst larger crystals grew. Microcline was most sensitive to this effect as the magma temperature was buffered close to its liquidus for a long period by the release of latent heat of crystallisation. Positive feedback between textural coarsening and magma permeability channeled the flow of interstitial melt to produce a heterogeneous distribution of megacrysts. Megacryst growth was halted when cooling resumed at the end of the intrusive cycle. K-feldspar nucleation was then renewed and K-feldspar crystals grew to form part of the groundmass. It was the particular thermal history of this pluton that promoted textural coarsening – chemically similar plutons that lack megacrysts probably did not have the pause during cooling that was necessary for the development of this texture.
 
Higgins, M.D., 1999, Origin of megacrysts in granitoids by textural coarsening: A Crystal Size Distribution (CSD) Study of Microcline in the Cathedral Peak Granodiorite, Sierra Nevada, California., in Fernandez, C., and Castro, A., eds., Understanding Granites: Integrating Modern and Classical Techniques. Special Publication 158: London, Geological Society of London, p. 207-219.