Does flow banding only occur in silicic rocks?
Flow banding can occur in all igneous rocks, regardless of their composition. Any kind of molten rock that flows may eventually cause some flow banding. Sometimes the flow banding is not even obvious to the eye and can only be detected using geophysical methods such as magnetic anisotropy.

Here's an example of flow banding in a mafic rocks from Skaergaard intrusion in Greenland:

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This looks like a sedimentary rock, but it's in fact igneous. Those are layers of minerals such as olivine, pyroxene and plagioclase that settled from a melted magma chamber.

It doesn't even have to be a silicate rock, here's a picture of natrocarbonatite lava from Ol Doinyo Lengai volcano in Tanzania:

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Obviously, this rock exhibits flow banding and it hardly has any silica in it.