Suppose you are on a hike when you find a dark-colored igneous rock (pictured below). the rock has very small crystals that can only be seen under magnification. you bring the rock back to the lab and make a thin section of it so that you can examine its mineral composition using a petrographic microscope. in thin section, you are able to determine that the rock consists of plagioclase feldspar (mostly ca-rich) and several ferromagnesian minerals. what does this tell you about the rock's origin?
