  A p-divisible group, or more generally an F-crystal, is said to be
Hodge-Newton reducible if its Hodge polygon passes through a break point of its
Newton polygon. Katz proved that Hodge-Newton reducible F-crystals admit a
canonical filtration called the Hodge-Newton filtration. The notion of
Hodge-Newton reducibility plays an important role in the deformation theory of
p-divisible groups; the key property is that the Hodge-Newton filtration of a
p-divisible group over a field of characteristic p can be uniquely lifted to a
filtration of its deformation.
  We generalize Katz's result to F-crystals that arise from an unramified local
Shimura datum of Hodge type. As an application, we give a generalization of
Serre-Tate deformation theory for local Shimura data of Hodge type. We also
apply our deformation theory to study some congruence relations on Shimura
varieties of Hodge type.
