What is an isolated land area of one type of rock surrounded by land of a differing rock type called?
You might want to look at these topics, depends on the size of features that you want to look at:


Batholith -- s a large mass of intrusive igneous rock (also called plutonic rock), larger than 100 square kilometres (40 sq mi) in area,[1] that forms from cooled magma deep in the Earth's crust. Batholiths are almost always made mostly of felsic or intermediate rock types, such as granite, quartz monzonite, or diorite -- https://en.wikipedia.org/wiki/Batholith
Stock (geology) ==  is an igneous intrusion that has a surface exposure of less than 100 square kilometres (40 sq mi),[differing from batholiths only in being smaller. A stock has a discordant relationship with the rocks that it intrudes. The term stock usually refers to individual, relatively small plutons, less than 20 km (12 mi) diameter, usually in relative isolation.Many stocks are cupolas of hidden batholiths. Circular or elliptical stocks may have been vents feeding former volcanoes -- https://en.wikipedia.org/wiki/Stock_(geology)
Dike (geology) --  is a sheet of rock that is formed in a fracture in a pre-existing rock body. Dikes can be either magmatic or sedimentary in origin. Magmatic dikes form when magma flows into a crack then solidifies as a sheet intrusion, either cutting across layers of rock or through a contiguous mass of rock. Clastic dikes are formed when sediment fills a pre-existing cracks -- https://en.wikipedia.org/wiki/Dike_(geology)