The igneous rock with the same chemical composition as basalt but a different grain size is gabbro.

Here's why:

* Basalt is an extrusive igneous rock, meaning it cools and solidifies quickly on the Earth's surface. This rapid cooling results in small, fine-grained crystals.

* Gabbro is an intrusive igneous rock, meaning it cools and solidifies slowly underground. The slow cooling allows larger crystals to form.

Both basalt and gabbro have the same mafic composition, meaning they are rich in magnesium and iron.