>>2350977Metamorphic rock generally requires heat as well as pressure. There are some exceptions but this is with a metric fuck ton of pressure such as near where plates collide so I doubt the weight of a glacier would cause them to change. Granite is an igneous rock that forms inside of magma chambers so maybe the magma present just didn't have much pyrite in it. I don't know much about the formation of Pyrite in general but Quartz forms last as the magma chamber cools resulting in more granitic rocks. (see pic related for the order in which minerals form as magma cools)
>>2350981Glaciers are technically considered metamorphic rocks but the pressure generated isn't enough to make the rock under it become metamorphic. They make some interesting sediment deposits and pretty valleys though.
