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KREEP volcanism The term for possible pre-mare volcanic activity in the lunar crust, producing ‘KREEP basalts’ with high concentrations of Th, U, K, REE, P, and other incompatible elements. They are elusive as a rock type and only a few lunar specimens have been identified so far. The Apennine Bench formation, in the Imbrium Basin, has been proposed to be composed of such material. An alternative explanation is that both it and KREEP basalts are derived from impact-generated melts in basin-forming collisions.