The Geochemistry of Archean shales derived from a mafic volcanic sequence, Belingwe greenstone belt, Zimbabwe: Provenance, source area unroofing and submarine versus subaerial weathering

Axel Hofmann, Robert Bolhar, Paul Dirks, Hielke Jelsma

Research output: Contribution to journalArticlepeer-review

59 Citations (Scopus)

Abstract

Shales of Ο2.7 Ga Zeederbergs Formation, Belingwe greenstone belt, Zimbabwe, form thin (0.2-2 m) horizons intercalated with submarine lava plain basalts. Shales of the overlying Cheshire Formation, a foreland basin sedimentary sequence, form 1-100 m thick units intercalated with shallow-water carbonates and deep-water, resedimented basalt pebble conglomerates. Zeederbergs shale is characterized by high contents of MgO and transition metals and low concentrations of K2O and LILE as compared to average Phanerozoic shale, indicative of an ultramafic to mafic source terrain. Cheshire shales have similar major and trace element contents, but MgO and transition metals are less enriched and the LILE are less depleted. Zeederbergs shales have smoothly fractionated REE patterns (LaN/YbN = 2.84-4.45) and no significant Eu anomaly (Eu/Eu*= 0.93- 0.96). REE patterns are identical to those of the surrounding basaltic rocks, indicating local derivation from submarine reworking. Cheshire shales have rather flat REE patterns (LaN/YbN = 0.69 -2.19) and a small, negative Eu anomaly (average Eu/Eu*= 0.85), indicative of a mafic provenance with minor contributions of felsic detritus. A systematic change in REE patterns and concentrations of transition metals and HFSE upwards in the sedimentary succession indicates erosion of progressively more LREE-depleted basalts and ultramafic volcanic rocks, followed by unroofing of granitoid crust. Weathering indices confirm the submarine nature of Zeederbergs shale, whereas Cheshire shale was derived from a source terrain subjected to intense chemical weathering.

Original languageEnglish
Pages (from-to)421-440
Number of pages20
JournalGeochimica et Cosmochimica Acta
Volume67
Issue number3
DOIs
Publication statusPublished - 1 Feb 2003
Externally publishedYes

ASJC Scopus subject areas

  • Geochemistry and Petrology

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