Centered around hudson bay in north america and extending to greenland, the shield is composed of precambrian rocks that cover 2.8 million square miles.

Canadian shield is composed of Precambrian rocks that cover 2.8 million square miles.

Magma forms by partial melting of upper mantle and crust. partial melt means that only a fraction of the available material forms a melt, and that the remainder stays solid. the partial melt rises because of its lower density and ascends through he crust. this is so conclusive mean the answer would be magma igneous rocks or balistic magma.