GEOCHEMISTRY AND TECTONIC SETTING OF 2 CONTRASTING ARCHEAN FELSIC VOLCANIC ASSOCIATIONS IN THE EASTERN GOLDFIELDS, WESTERN-AUSTRALIA

Archaean felsic volcanic rocks from two areas in the eastern Yilgarn C raton of Western Australia comprise dacite and rhyolite with subordina te basalt and andesite. One suite from one of the areas (Black Flag Gr oup) is dominated by subaqueous dacite lava lobes and their reworked p roducts. The other suite from the other area (Melita and Jeedamya volc anics) consists of abundant rhyolitic and rhyodacitic pyroclastic depo sits, with less abundant dacitic lava flows. Both suites are intercala ted with volumetrically subordinate basic and intermediate volcanic ro cks. Black Flag Group dacites and rhyolites have steep REE patterns (( La/Yb)(cm) about 40), and no Eu anomaly, high Sr and Al2O3, low Nb and Y, hence high Sr/Y. Melita and Jeedamya dacites and rhyolites have fl atter REE patterns ((La/Yb),, between 2 and 9), negative Eu anomalies (Eu/Eu 0.4-0.6), and higher Zr, Nh, Y, and lower Sr (hence lower Sr/Y ). On chemical grounds, mafic rocks in both areas are genetically unre lated to felsic volcanic rocks. Black Flag Group andesites, dacites an d rhyolites are probably unrelated by fractional crystallization, but resulted from 22-28% partial melting of amphibolite, leaving a residua l mineralogy of amphibole, plagioclase, garnet, ilmenite +/- clinopyro xene. Alternatively, andesites may have been derived by partial meltin g of a metasomatised mantle source. Jeedamya rhyodacite could have bee n produced by similar to 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 Fla g Group rocks requires melting at depths greater than 35 km, which is below the base of the present (tectonically) thinned crust. The most l ikely source is subducted oceanic crust, with Black Flag Group rocks e rupted in a back are basin. Melita and Jeedamya felsic volcanic rocks resulted from melting of thicker (continental) crust adjacent to the b ack are. (C) 1997 Elsevier Science B.V.