Accretion.
When oceanic crust subducts, there is smear (not a geologic term). It's not a discrete boundary between the plates. The frosting gets scraped off the cake, so to speak, and as it does, it gets all f**ked up. And then, if this winds up getting smashed between two continental slabs, and spending some quality time at high P/T under a 20,000 foot mountain whose roots are deep and melting, it winds up as part of the continental crust.
Continental plates are not homogenous. They are demolition derby cars made of clay.