How schlieren form has been a geologic controversy for some time. Numerous suggestions have been made.
1) Rhythmic crystallization, with first mafic minerals crystallizing, then felsic minerals.
2) The bands are the smeared out remnant of any of a number of things, including enclaves, restite, and xenoliths.
3) Structures such as the one above, which resembles cross-bedding, have lead to the idea that schlieren are formed during flow of the granite, with crystals behaving similarly to sediment in a stream. This would account for the bedding and cross-bedding like shapes. However, the above structure could just as easily be a curved area of schlieren that has been cut by a latter pulse of granitic intrusion that only resembles cross-bedding.
(I think I'm forgetting one mechanism).
I favor mechanisms that relate to flow within the granite, as I have noticed that schlieren are more common near contacts between granite phases.
