US EPA

7475546 

Journal Article 

Mineral chemistry and geothermometry of plutonic charnockitic rocks of northern and south-western Nigeria 

Olarewaju, VO 

1998 

34 

2 

157-170 

English 

Charnockitic rocks of igneous origin from the Precambrain Basement Complex of northern and south-western parts of Nigeria have been investigated in this work. The major constituents are quartz, alkali feldspar, plagioclase feldspar, orthopyroxene, clinopyroxene, amphibole, biotite and olivine. The accessory minerals include zircon, apatite and opaque ores. Electron microprobe studies show the composition range of the alkali feldspar as Or(95.37) Ab(4.63) An(0.00) to Or(84.50) Ab(15.05) An(0.6') plagioclase between An(42.63) and An(17.42) which is andesine to oligoclase. The amphiboles vary from pargasitic hornblende to edenite while the clinopyroxene compositions range from augite through salite to ferro-augite and orthopyroxene from hypersthene through ferrohypersthene to eulite. the biotites have low Al(vi), a typical igneous characteristic. Olivine, which is fayalitic in composition, is presnet only in the coarse-grained variety of the charnockitic rocks. Geothermobarometric calculations indicate that the crystallisation temperatures of the igneous charnockitites range from 718°C to 950°C with pressure estimate of 4 - 6 kbars suggesting that these rocks represent the low-medium pressure types of charnockitic rocks. Post-crystallisation deformation texture is shown by the preferred alignment of the large microperthitic K-feldspar observed in some outcrops of the igneous charnockites. 

charnockite; crystal chemistry; crystallization; geochemistry; geothermometry; mineralogy