Li Miarolitic Pegmatite

Abbreviation

MI-Li

Classification

Li Miarolitic subclass < (LCT family) < Miarolitic class < Granitic pegmatite

Required Geological Setting

Plutonic rocks

Occurrence

Pegmatites with abundant miarolitic cavities (vugs) formed by gas exsolved as the result of pressure reduction from S-type granite magma associated with upper Greenschist facies to Amphibolite facies in Barrovian-type middle-P metamorphism. The Li miarolitic pegmatites may be associated with the lithium subclass pegmatites such as the elbaite subtype pegmatites of the rare-element class. Pegmatites are emplaced at shallow depth less than 0.15 GPa and less than 200 degree. Most of pegmatites are reported from active continental margin granites in compressional crustal settings such as the Pacific Ocean. Textures of host rocks are clearly cut by pegmatites, and host rocks are subjected to contact metamorphism by granites or pegmatites. Miarolitic cavities are considered to be formed by H₂O gas which exsolved from the low H₂O solubility melt due to deposition of B- and Li-bearing silicates during pressure reduction process. Pegmatites are rich in pockets. Although mineral assemblages in each miarolitic cabity are variable even in a granitic body and subdivision is difficult, the Li miarolitic subclass is subdivided into 4 types; Beryl-Topaz type, MI-spodumene type, MI-petalite, and MI-lepidolite. The Li miarolitic subclass is characterized by abundant Li, Be, B, and F.

Mineral Assemblages (Major minerals)

Quartz, Orthoclase, Microcline, Albite, Muscovite, (Spessartine)

Mineral Assemblages

Elbaite, Beryl, Topaz, Lepidolite, (Spodumene, Petalite, Pollucite, Microlite)