Protolith

A protolith (from Ancient Greek πρωτο- (prōto-), first and λίθος líthos, stone) is the original, unmetamorphosed rock from which a given metamorphic rock is formed .

For example, the protolith of a slate is a shale or mudstone. Metamorphic rocks can be derived from any other kind of non-metamorphic rock and thus there is a wide variety of protoliths. Identifying a protolith is a major aim of metamorphic geology.

Protoliths are non-metamorphic rocks and have no protoliths themselves. The non-metamorphic rocks fall into two classes: sedimentary rocks, formed from sediment, and igneous rocks, formed from magma. The source of the sediment of a sedimentary rock is termed its provenance. 

Protolith

Source :polarpedia.eu

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.

This article uses material from the Wikipedia article  Metasyntactic variable, which is released under the  Creative Commons Attribution-ShareAlike 3.0 Unported License.