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Cordierite gneisses and associated lithologies of the Guri area, Northwest Guayana Shield, Venezuela

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Abstract

Gneisses in the Guri area of the Venezuelan Guayana Shield contain mineral assemblages with cordierite, garnet, sillimanite, hypersthene, biotite and Fe-Ti oxide intergrowths.

Analysis of mineral assemblages and compositional relationships in the light of experimental data indicate metamorphic conditions of 725–800° C, 5–6 kb P T , \(P_{{\text{H}}_{\text{2}} {\text{O}}}\) <P T for the highest grade rocks and 650–700° C, 5–7 kb P T , \(P_{{\text{H}}_{\text{2}} {\text{O}}}\) approximating P T for the lowest grade rocks. Oxygen fugacities in different lithologies ranged between those of the MH and QFM buffers.

The distribution coefficient K D (Mg-Fe) (gar-bio), decreases by 0.006 per atom percent increase in (Mn/Mn+Mg+Fe)gar, falls in the range of K D typical of the sillimanite-K feldspar zone and granulite facies, and is systematically lower in lower grade rocks-all in accord with observation in other localities. K D (Mg-Fe) (cord-bio) ranges from 3.0 in the highest grade rocks to 10.0 in the lowest grade rocks, appears independent of FeO/MgO of cordierite or biotite, and varies systematically with grade. In contrast with conclusions based on observation in other localities, data from the Guri area suggest -KD(cord-bio) may be a sensitive index of grade.

A number of mineralogic and geologic observations are difficultly reconciled with existing experimental data.

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  1. Allegheny College, Meadville, Pennsylvania

    Thomas W. Dougan

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Dougan, T.W. Cordierite gneisses and associated lithologies of the Guri area, Northwest Guayana Shield, Venezuela. Contr. Mineral. and Petrol. 46, 169–188 (1974). https://doi.org/10.1007/BF00487554

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