Selectively contaminated magmas of the Tertiary East Greenland macrodike complex

Chemical interaction between tholeiitic magmas of the East Greenland Tertiary macrodike complex and anatectic melts of the Precambrian basement produced a wide range of hybrid magmas. Field evidence indicates that, although coexisting magmas were stirred, mechanical mixing only occurred to a limited extent before segregation of magmas into a stratified system. The initial ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd isotope ratios for hybrid compositions fall between those of the mafic and felsic end-members. However, the covariation of these isotope ratios differs from that expected of bulk mixing. Major- and trace-element distributions in hybrid magmas are also inconsistent with simple mixing, as well as with fractional crystallization coupled with bulk assimilation (AFC) involving reasonable end-members of the macrodike-crust system. Rather, the chemical and isotopic modification of mafic and felsic magmas of the macrodike complex appears to have been controlled fundamentally by interdiffusion of silicate liquid species during mingling and buoyant roofward segregation of crust-derived granophyres. The relationships among juxtaposed hybrid magmas of the Miki Fjord macrodike are shown to be consistent with expectations of selective diffusional exchange based on available experimental interdiffusion data for silicate liquids. Comparison between these hybrid compositions and rocks from the felsic series of the Vandfaldsdalen macrodike suggest that the latter compositions were affected by a similar opensystem process operating presumably during the transient development of the felsic cap. Once hybrid magmas ponded at the roof of the intrusion they effectively were isolated from further exchange.