A granite can be transformed from an igneous rock to a sedimentary rock and then to a metamorphic rock or the sedimentary rock can be transformed back into granite again by metamorphism. First of all, a granite often occurs in mountains and is susceptible to weathering and erosion.. Weathering such as argillic weathering can help break t he rock down to its constituent minerals which are mostly quartz, orthoclase feldspar and biotite mica. This then gets washed down into the rivers and thence to the coast and gets deposited in a marine environment as part of a delta or alluvial fan, in the form of sand, silt, clay and gravel. As this sediment becomes compacted, it turns into rocks like sandstone, siltstone etc. With deeper burial and the addition of heat and pressure these rocks get converted into such metamorphic rocks as quartzites, slates and phyllites, etc. I have seen a meta sandstone grade into a granite in the Canadian Shield so granite can then form from metamorphic processes as well as igneous processes.
