When did the major volcanic eruptions that flooded the impact basins and formed the maria primarily occur?

Answer

Primarily between about 3 and 4 billion years ago

The phase of eruption that filled the basins with lava took place primarily during or shortly following the period known as the late heavy bombardment, approximately 3 to 4 billion years ago.

When did the major volcanic eruptions that flooded the impact basins and formed the maria primarily occur?
SpaceGeologyastronomycompositionsurfaceMoonlunarMaria