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Robertson (1999) defines the boundary between diagenesis and metamorphism in sedimentary
rocks as follows: “…the boundary between diagenesis and metamorphism is somewhat arbitrary
and strongly dependent on the lithologies involved. For example changes take place
in organic materials at lower temperatures than in rocks dominated by silicate minerals.
In mudrocks, a white mica (illite) crystallinity value of < 0.42D.2U obtained by
X-ray diffraction analysis, is used to define the onset of metamorphism (Kisch, 1991).
In this scheme, the first appearance of glaucophane, lawsonite, paragonite, prehnite,
pumpellyite or stilpnomelane is taken to indicate the lower limit of metamorphism
(Frey and Kisch, 1987; Bucher and Frey, 1994; Frey and Robinson, 1998). Most workers
agree that such mineral growth starts at 150 ± 50° C in silicate rocks. Many lithologies
may show no change in mineralogy under these conditions and hence the recognition
of the onset of metamorphism will vary with bulk composition.”
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