The bimodal, Late Cretaceous–Palaeocene (71–61 Ma) Kap Washington Group volcanic sequence on the north coast of Greenland was erupted in a continental rift setting during the opening of the Arctic Ocean. On Kap Kane ca. 70 Ma silicic lavas and ignimbrites dominate over mildly alkaline basalts. Intermediate rocks are scarce (SiO2=57–62 wt.%) and cognate mafic inclusions, resorbed crystals, and highly variable clinopyroxene compositions indicate that they formed by magma mixing. Silicic lavas (66–74 wt.% SiO2) on Kap Kane show geochemical features typical of A-type granitoids and form two chemically and mineralogically distinct suites: (i) one-feldspar, peralkaline trachytes and rhyolites with elevated contents of high field strength elements (HFSE); and (ii) two-feldspar, weakly peraluminous, HFSE-poor trachytes and rhyolites. The peralkaline lavas
have Sr–Nd–Pb isotopic compositions which overlap with the compositions of mildly alkaline Kap Kane basalts. The peralkaline lavas are inferred to have originated from an evolved basaltic parent by ca. 90% fractional crystallisation of an assemblage consisting of plagioclase, alkali feldspar, clinopyroxene, Fe-Ti oxide, olivine, kaersutite and apatite. The peraluminous lavas have mixed mantle–crust Sr–Nd–Pb isotopic signatures and it is proposed that they were formed by partial melting of hybridised mafic crust with N70% contribution from juvenile, mantle-derived material in the form of lower crustal gabbros. Feldspar
phenocrysts have near pure end-member compositions and many crystals display a patch antiperthite texture characterised by patches of Or98-99 in a host of Ab99, reflecting low-temperature dissolution–reprecipitation reactions during hydrothermal alteration. Leaching experiments, moreover, indicate that the
Sr isotopic system of Sr-poor peralkaline lavas has been disturbed by interaction with a fluid with relatively low Rb/Sr and 87Sr/86Sr. The Nd and Pb isotopic systems do not appear significantly affected by this process.