Continental geochemical signatures in dacites from Iceland and implications for models of early Archaean crust formation

Abstract

Whether early Archaean felsic crust was formed by processes related to plate subduction or melting of thick basaltic plateaus is vividly debated. Ultimately, the discussion hinges on the question of how Archaean felsic crust has obtained its distinct chemical traits. Here we report chemical and isotopic data for a suite of Cenozoic felsic volcanic rocks from Iceland. The samples exhibit the key-chemical characteristics of early Archaean felsic continental crust such as calc-alkaline composition, strong enrichment in Na relative to K, high Pb/Ce, La/Nb, and Ta/Nb ratios. Involvement of pre-existing continental lithosphere in the petrogenesis of the samples can be excluded, because their 207Pb/204Pb and 206Pb/204Pb ratios plot well within the range of Iceland basalts. Model calculations suggest that the chemical characteristics were produced by high-pressure partial melting of basaltic lower crust followed by fractional crystallisation of amphibole, plagioclase, and ilmenite. These findings demonstrate that plate subduction and melting of subduction-modified mantle or lithosphere are not necessarily required to produce the key-chemical signatures of continental crust. Hence, the calc-alkaline dacites provide intriguing support for early Archaean continental crust formation by melting of thick mafic plateaus.