What sequence of events produces porphyritic texture in rocks

For small degrees of undercooling the nucleation rate will be low and the growth rate moderate.  A few crystals will form and grow at a moderate rate until they run into each other.  Because there are few nuclei, the crystals will be able to grow to relatively large size, and a coarse-grained texture will result.  This would be called a phaneritic texture.

The higher you go up the less oxygen there is