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Book Chapter

Discovery of the oldest oxic granitoids in the Kaapvaal Craton and its implications for the redox evolution of early Earth

By
Shunso Ishihara
Shunso Ishihara
Geological Survey of Japan, Tsukuba, 305-8567, Japan
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Hiroshi Ohmoto
Hiroshi Ohmoto
Penn State Astrobiology Research Center of the NASA Astrobiology Institute and the Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA
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Carl R. Anhaeusser
Carl R. Anhaeusser
Economic Geology Research Institute, University of the Witwatersrand, Johannesburg, South Africa
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Akira Imai
Akira Imai
Kyushu University, Fukuoka, 812-8581, Japan
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Laurence J. Robb
Laurence J. Robb
Economic Geology Research Institute, University of the Witwatersrand, Johannesburg, South Africa
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Published:
January 01, 2006

Phanerozoic granitoids have been classified into magnetite and ilmenite series based on the abundance of magnetite, which is related to the Fe2O3/FeO ratio of the rock and the oxygen fugacity (f O2 ) of its parent magma. We have examined the temporal and spatial distributions of both series in Archean granitoids from the Barberton region and the Johannesburg Dome of the Kaapvaal Craton, South Africa. The oldest syntectonic TTG (tonalite-trondhjemite-granodiorite) granitoids (ca. 3450 Ma in age) were found to be ilmenite series, whereas some intermediate-series granitoids occurred locally. Younger and larger syntectonic TTGs (e.g., the 3230 Ma Kaap Valley plutons) comprise nearly equal quantities of magnetite and ilmenite series. The major 3105 Ma calc-alkaline batholiths (e.g., Nelspruit batholith), emplaced during the late-tectonic stage, comprise mostly magnetite-series granitoids, suggesting that an oxidized continental crust already existed by this time.

The rare earth element ratios and δ18O values, as well as the Fe2O3/FeO ratios, of the Archean magnetite-series granitoids suggest that their magmas were generated from the partial melting of subducted oceanic basalts that had been oxidized by interaction with seawater on mid-oceanic ridges; the processes of magma generation were much like those for Phanerozoic magnetite-series granitoids. This further suggests that the concentrations of oxidants (O2 and/or SO4 2−) in the Archean oceans were similar to those in Phanerozoic oceans.

Low concentrations of chlorine in the magmas, as well as deep levels of granite erosion, appear to explain the absence of major mineral deposits associated with the Kaapvaal granitoids.

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GSA Memoirs

Evolution of Early Earth's Atmosphere, Hydrosphere, and Biosphere - Constraints from Ore Deposits

Stephen E. Kesler
Stephen E. Kesler
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Hiroshi Ohmoto
Hiroshi Ohmoto
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Geological Society of America
Volume
198
ISBN print:
9780813711980
Publication date:
January 01, 2006

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