how and where would the rock might have been formed that has large crystals of feldspar mica and quartz?

due to the large crystal grains of the rock and existence of feldspar and mica the rock could either be of igneous or metamorphic origin.incase it is of igneous origin the rock is intrusive igneous rock that form deep in the earth crust and cool at a relatively slow rate developing large crystals.in case of metamorphic origin the rock is high grade metamorphic rock that have experienced high metamorphic condition (high pressure and temperature);that is it is a gneiss that has characteristic coarse grains crystals.