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Journal of Petrology Advance Access published online on July 8, 2005
Journal of Petrology, doi:10.1093/petrology/egi059
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© The Author 2005. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oupjournals.org
Received April 5, 2004
Accepted May 31, 2005

Article

The Geochemistry of Ultramafic to Mafic Volcanics from the Belingwe Greenstone Belt, Zimbabwe: Magmatism in an Archean Continental Large Igneous Province

KENJI SHIMIZU 1*, EIZO NAKAMURA 1, and SHIGENORI MARUYAMA 2

1 THE PHEASANT MEMORIAL LABORATORY FOR GEOCHEMISTRY AND COSMOCHEMISTRY, INSTITUTE FOR STUDY OF THE EARTH'S INTERIOR, OKAYAMA UNIVERSITY, MISASA, TOTTORI 682-0193, JAPAN
2 DEPARTMENT OF EARTH AND PLANETARY SCIENCES, TOKYO INSTITUTE OF TECHNOLOGY, TOKYO, 152-8551, JAPAN

* To whom correspondence should be addressed.
KENJI SHIMIZU, E-mail: shimmy{at}misasa.okayama-u.ac.jp


  Abstract

The evolution of the late Archean Belingwe greenstone belt, Zimbabwe, is discussed in relation to the geochemistry of the ultramafic to mafic volcanic rocks. Four volcanic types (komatiite, komatiitic basalt, D-basalt and E-basalt) are distinguished in the 2·7 Ga Ngezi volcanic sequence using a combination of petrography and geochemistry. The komatiites and D-basalts are rocks in which isotopic systems and trace elements are depleted. Chemical variations in komatiites and D-basalts can be explained by fractional crystallization from the parental komatiite. In contrast, komatiitic basalts and E-basalts are siliceous and display enriched isotopic and trace element compositions. Their chemical trends are best explained by assimilation with fractional crystallization (AFC) from the primary komatiite. AFC calculations indicate that the komatiitic basalts and E-basalts are derived from komatiites contaminated with ~20% and ~30% crustal material, respectively. The volcanic stratigraphy of the Ngezi sequence, which is based on field relationships and the trace element compositions of relict clinopyroxenes, shows that the least contaminated komatiite lies between highly contaminated komatiitic basalt flows, and has limited exposure near the base of the succession. Above these flows, D- and E-basalts alternate. The komatiite appears to have erupted on the surface only in the early stages, when plume activity was high. As activity decreased with time, komatiite magmas may have stagnated to form magma chambers within the continental crust. Subsequent komatiitic magmas underwent fractional crystallization and were contaminated with crust to form D-basalts or E-basalts.

Keywords: komatiite; crustal assimilation; Belingwe greenstone belt; continental flood basalt; plume magmatism.
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