Preheat the oven to 180°C and grease a deep 20cm tin with margarine.
Purée the diced oranges in a food processor and set aside.
Cream the margarine and sugar in a bowl until light and fluffy.
Mix in the eggs, then add the baking powder and semolina.
Stir to combine and then fold the orange pulp through the mixture.
Pour the mixture into the prepared tin and place in the oven.
Reduce the heat to 170°C after 10 minutes and bake for 40 more minutes.
Cool the cake in the tin before releasing with a knife and turning out.