Cordierite gneisses and associated lithologies of the Guri area,Northwest Guayana Shield,Venezuela |
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| Authors: | Thomas W Dougan |
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| Institution: | (1) Allegheny College, Meadville, Pennsylvania |
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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
T
for the highest grade rocks and 650–700° C, 5–7 kb P
T
,
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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