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Geochemistry and tectonic setting of two contrasting Archaean felsic volcanic associations in the Eastern Goldfields, Western Australia

Precambrian Research
Publication Date
DOI: 10.1016/s0301-9268(97)00006-5
  • Yilgarn Craton
  • Late Archaean
  • Greenstones
  • Felsic Volcanics
  • Geochemistry
  • Chemistry
  • Earth Science


Abstract Archaean felsic volcanic rocks from two areas in the eastern Yilgarn Craton of Western Australia comprise dacite and rhyolite with subordinate basalt and andesite. One suite from one of the areas (Black Flag Group) is dominated by subaqueous dacite lava lobes and their reworked products. The other suite from the other area (Melita and Jeedamya volcanics) consists of abundant rhyolitic and rhyodacitic pyroclastic deposits, with less abundant dacitic lava flows. Both suites are intercalated with volumetrically subordinate basic and intermediate volcanic rocks. Black Flag Group dacites and rhyolites have steep REE patterns ( ( La Yb ) cn about 40), and no Eu anomaly, high Sr and Al 2O 3, low Nb and Y, hence high Sr Y . Melita and Jeedamya dacites and rhyolites have flatter REE patterns ( ( La Yb ) cn between 2 and 9), negative Eu anomalies ( Eu Eu ∗ 0.4–0.6), and higher Zr, Nb, Y, and lower Sr (hence lower Sr Y ). On chemical grounds, mafic rocks in both areas are genetically unrelated to felsic volcanic rocks. Black Flag Group andesites, dacites and rhyolites are probably unrelated by fractional crystallization, but resulted from 22–28% partial melting of amphibolite, leaving a residual mineralogy of amphibole, plagioclase, garnet, ilmenite±clinopyroxene. Alternatively, andesites may have been derived by partial melting of a metasomatised mantle source. Jeedamya rhyodacite could have been produced by ∼35% vapour-absent melting of tonalite leaving a residual mineralogy of plagioclase, orthopyroxene±amphibole±ilmenite. The source would have had a slightly LREE enriched pattern with a positive Eu anomaly. Residual garnet in the source of Black Flag Group rocks requires melting at depths greater than 35 km, which is below the base of the present (tectonically) thinned crust. The most likely source is subducted oceanic crust, with Black Flag Group rocks erupted in a back arc basin. Melita and Jeedamya felsic volcanic rocks resulted from melting of thicker (continental) crust adjacent to the back arc.

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