Here's why:
* Mafic rocks are rich in dark silicate minerals like pyroxene and olivine, typically with low amounts of plagioclase feldspar.
* Felsic rocks are rich in light-colored minerals like quartz and alkali feldspar, with very little to no dark silicate minerals.
* Intermediate rocks fall between these two extremes, having a significant proportion of both light and dark minerals. This is the case with your described rock, with its high plagioclase content and substantial amount of dark silicate minerals.
Therefore, a rock with this composition would likely be classified as andesite if it is extrusive (volcanic), or diorite if it is intrusive (plutonic).
