Why would the presence of mica and garnet signify about the grade (or intensity) of metamorphism?

Garnet mica schists are typical products of the metamorphism of mudstones, siltstones and muddy sandstones. Most commonly they form under uppermost greenschist facies to amphiobolite facies regional orogenic (moderate grade in simple terms) metamorphism. With decreasing grade of metamorphism, garnet disappears (though this is also strongly dependent on the rock composition too) giving rise to things such as chlorite-mica schist.

With increasing grade muscovite mica is typically replaced by sillimanite and K-feldspar, and at higher temperature still, biotite mica is typically replaced by garnet, K-feldspar and orthopyroxene, marking the onset of granulite facies (high grade) metamorphism. Garnet mica schists are typically very sensitive to metamorphic conditions (more so than many other metamorphic rocks) and are thus extremely useful for reconstructing past pressure-temperature conditions. They also often grow metamorphic zircon and monazite which are datable minerals allowing the timing of metamorphism to be dated.

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