Orthopyroxene-Bearing Rocks of Charnockitic Affinities in the South Savanna--Kanuku Complex of British Guiana

Orthopyroxene-bearing rocks of the South Savanna—Kanukucomplex of British Guiana vary in composition from basic toacid types. They include basic granulites; acid, orthopyroxene-bearingbiotite gneisses; orthopyroxene-bearing acid granulites; orthopyroxene-bearinggranites, and orthopyroxene-bearing xenoliths contained in theSouth Savanna Granite. The field, petrographic, and chemicalcharacters of the rocks bear strong resemblances to charnockitesdescribed from other parts of the world, but the origins formany charnockites are inconsistent with evidence advanced forthe evolution of the South Savanna—Kanuku rocks. Fieldand petrographic criteria indicate that the basic granuliteswere originally intrusive basic rocks and have suffered regionalmetamorphism of Granulite Facies grade, which induced the conversionof clinopyroxene to orthopyroxene. The orthopyroxene-bearing,acid biotite gneisses and acid granulites have resulted fromcontamination by enclosed basic granulites which occur as boudins,bands, and lenses. The orthopyroxene-bearing granites and xenolithsare related to the assimilation of intruded orthopyroxene-bearingcountry rocks by the South Savanna Granite and its apophyses. The chemistry of the rocks appears to support the conclusionsreached from field and petrographic studies by indicating consistentenrichment of Ca, Mg, Fe, and Na in the orthopyroxene-bearingrocks compared with their non-orthopyroxene-bearing equivalents.Variation diagrams also indicate slightly anomalous plots forthe orthopyroxene-bearing rocks of the complex.