Graphitic andesite tuffs resulting from high-Mg tholeiite and sediment interaction; Nfigssuaq, West Greenland

Abstract

Tuffs from the Agatdal Formation of Nilgssuaq show a dominance of olivine-graphite-andesites and orthopyroxe'ne-graphite-andesites while rock types similar to the overlying tholeiite basalts of the Vaigat Formation are subordinate. The tuffs were formed when highly magnesian tholeiitic picrite melts were emplaced into high-level magma chambers within carbon-rich sedimentary strata. Explosive reactions were triggered after extensive reactions with- the sediments. Glass grains are well preserved and some tuffs still contain quenched high-temperature sulphide blebs with metallic iron. Due to the high magnesium content the graphite-andesites are very unusual magma types and can be compared to some native iron bearing intermediate lavas from the Vaigat Formation of northern Disko thought to have erupted largely contemporaneously after having evolved through similar processes.