On Earth, granite is all over the place. The reason it's all over the place is that granite commonly forms in subduction zones, where it's created through tectonic activity. As far as we know, Mars doesn't have any tectonic activity, but it's thought that prolonged volcanism (like a billion years of it) might be able to form a sort of granite anyway.
Finding this diversity of rocks (even if it's not much of a diversity) is somewhat of a surprise, because we'd thought that Mars was almost entirely volcanic basalt. As we send more and more spacecraft and get back more and more data, Mars is turning out to be more interesting (and prettier) than we ever thought it would be.
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Via Georgia Tech