Final answer:
Metamorphic rocks are commonly found in the crystalline basement rocks of shields, forming under significant pressure and heat. Metamorphic grades vary based on these conditions, with high-grade metamorphic rocks containing minerals indicative of these extreme conditions. Certain mineral assemblages can inform us about the pressure-temperature conditions where the rocks were formed.
Step-by-step explanation:
Metamorphic rocks, such as schist, slate, and marble, are indeed very common in the crystalline basement rocks of geological shields. These rocks are formed under conditions of high pressure and high temperature, often related to the deep burial and mountain-building processes associated with tectonic activities. The geological shields, being ancient and stable parts of Earth's crust, have been subjected to multiple episodes of metamorphism over their long history.
The grade of metamorphism, which can be high, medium, or low, is dependent on the specific conditions of temperature and pressure to which the protolith (parent rock) was subjected. High-grade metamorphic rocks typically contain minerals like sillimanite, kyanite, and garnet, which form under conditions of high temperature and pressure. For instance, shale as a protolith will transform into different types of rocks, like slate or schist, depending on the metamorphic conditions it endures.
The regions with specific types of minerals, such as areas with kyanite and sillimanite, point to medium-pressure terranes often associated with continent-continent collisions. In contrast, regions with andalusite and sillimanite typically reflect elevated geothermal gradients and are considered low-pressure terranes.