The characteristics you've described—a light color and extremely coarse grains—indicate that the rock is likely an intrusive igneous rock, which forms from magma that cools slowly beneath the Earth's surface. The large grain size suggests that the cooling process was gradual, allowing crystals to grow large.
Given this information, the most likely origin of the rock would be:
a magma chamber.
Magma chambers are where magma collects and cools slowly, leading to the formation of coarse-grained igneous rocks like granite.
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