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Symplectitic cordierite-orthopyroxene-garnet assemblages as products of contact metamorphism of pre-existing basement granulites in the Vredefort Structure, South Africa, and their relations to pseudotachylite by W. Schreyer; K. Abraham is a Earth and Planetary Sciences article available to read on EtoBox.

What is Symplectitic cordierite-orthopyroxene-garnet assemblages as products of contact metamorphism of pre-existing basement granulites in the Vredefort Structure, South Africa, and their relations to pseudotachylite about?

Former idioblastic garnet crystals of an Archaean granulite with Mg/(Mg + Fe + Mn)-ratios (= M) near 0.40 were converted to symplectitic pseudomorphs consisting mainly of cordierite (M= 0.6l), orthopyroxene (M = 0.40), and a relic garnet with a new composition (M=0.18), during a static metamorphism which is related in time to the Vredefort event. On the basis of experimental data for the continuous reaction garnet + quartz = orthopyroxene § cordierite the conditions of metamorphism were near 5 kb, 700 ~ C. Orthopyroxenes crystallized initially as minute, myrmekitic grains with metastable excess A1 contents up to 13 tool % A1203 and, through grain growth and A1 exsolution, transformed into dense aggregates of coarser crystals with equilibrated compositions near 3 tool % A1203. In the absence of free silica hercynitic spinel appears as an additional phase that coexists with cordierite, orthopyroxene, and garnet of more magnesian compositions than in the case of silica saturation. Pseudotachylite veins crosscutting the hornfelsed granulite consist of pyroxene of variable A1 contents, two feldspars, opaques, and quartz, and are finely recrystallized. There is textural and mineral chemi

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Author

W. Schreyer; K. Abraham

Publisher

Springer; Springer-Verlag; Springer Verlag; Springer Science and Business Media LLC; Society for Mining, Metallurgy and Exploration Inc. (ISSN 0010-7999)

Published

1978

Language

EN

Field

Earth and Planetary Sciences (Physical Sciences)