"A majority" is fine, though you could use "the majority" as well.
If you got all the works together that were written by Page, either alone or with Robertson, they would constitute a majority of the works.
That's not the only majority of those works you could have though. Another majority of the band's works could have been released by Sony.
In that case, the majority would be by Page and Robertson, and the majority released by Sony, but they would not necessarily be the same works in both cases.
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