A protolith is the original rock that is subject to metamorphism and hence is subject to changes. If the protolith is a basalt rock the metamorphosis will convert it to the following rocks with increasing temperature and pressure in a setting of continental crust, following the normal geothermal gradient (the olive green arrow): Greenschist, Amphibolite, Mafic Granulite (Figure 1) going through the corresponding facies in the diagram, where greenshist is the shallowest facies and mafic granulite is the most deep facies.

Figure 1: Pressure-Temperature diagram (P-T diagram) showing various paths that rocks may undergo during metamorphism. The figure is modified from Understanding Earth Copyright 1994 W. H. Freeman and Company.

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