Geochemistry of high-Mg andesites from Cape Vogel, Papua New Guinea

Abstract

A detailed geochemical investigation of high-Mg andesites from Cape Vogel, Papua New Guinea, shows that these rocks are chemically as well as mineralogically unusual. They are nonetheless important to our understanding of the Earth's mantle since they represent primary magmas, possibly derived in an island-arc environment. Two groups of high-Mg andesites are recognized at Cape Vogel: type C — characterized by a concave REE pattern [ ( La Yb ) N < 2 ] and Zr Nb ≈ 35 , and type E — with ( La Yb ) N of 3–7 and Zr Nb ≈ 19 . The origin of the two groups and their unique trace-element patterns is attributed to a complex series of enrichment events superimposed on a previously depleted mantle source. In general, the enrichment events do not seem similar to those represented in island-arc volcanics while some similarities are noted for oceanic ridge and island settings. A marked depletion in Ti relative to Zr and Y, amongst high-Mg andesites, is a unique facet of their geochemistry. Trends in the major- and trace-element geochemistry of type-E high-Mg andesites were modelled. Orthopyroxene fractionation (and/or accumulation) can explain much of the variation although decoupling of major and selected trace elements is noted. Source heterogeneity may also be a factor. The origin of these rocks and their tectonic setting remains enigmatic, however, enough geochemical and petrographic evidence is available to suggest they are distinct from most low-Ti basalts associated with ophiolites.