Abyssal Pegmatite

Abbreviation

AB

Classification

Abyssial class < Granitic pegmatite

Required Geological Setting

Middle-pressure metamorphic rocks

Occurrence

Pegmatite associated with low-pressure granulite facies in Barrovian-type middle-P metamorphism. Formation condition is 700-800 degree and 0.4-0.9 GPa (12-27 km depth). Pegmatites are observed as a part of S-type granites formed by partial melting or re-crystallization of sedimentary rocks. Pegmatites are usually concordant with schistose textures of host metamorphic rocks subjected to low-pressure granulite facies metamorphism. Pegmatites are massive and pockets are very rare. Mineral assemblages are peraluminous, and this class is classified into 4 subclasses based on its mineral assemblages; AB-HREE, AB-LREE, AB-U, and AB-BBe.

Mineral Assemblages (Major minerals)

Quartz, Microcline, Albite, Muscovite, Biotite, Andalusite, Almandine, (Corundum)

Mineral Assemblages (AB-HREE subclass)

Euxenite-(Y), Samarskite-(Y), Fergusonite-(Y), Pyrochlore, Uraninite, Zircon, (Allanite-(Ce))

Mineral Assemblages (AB-LREE subclass)

Allanite-(Ce), Monazite-(Ce), (Uraninite, Thorite)

Mineral Assemblages (AB-U subclass)

Uraninite, Thorite, Zircon, (Allanite-(Ce))

Mineral Assemblages (AB-BBe subclass)

Dumortierite, Grandidicrite, Kornerupine, Werdingite, Chrysoberyl, Sapphirine, Surinamite