Regional very low-grade metamorphism of basaltic lavas, Disko–Nuussuaq region, West Greenland

Abstract

The thick Paleocene basaltic lava succession exposed on Disko Island and Nuussuaq Peninsula, central West Greenland, experienced regional metamorphism shortly after volcanism followed by several events of localized hydrothermal alteration. Regional metamorphism resulted in well-developed lower zeolite facies mineral assemblages consisting primarily of mafic phyllosilicates and zeolites. In the upper Paleocene lava formation, the Maligât Formation, alteration is dominated by early mixed dioctahedral–trioctahedral smectite followed by chabazite and thomsonite. This same assemblage persists into the upper portions of the underlying Vaigat Formation. Within the Vaigat Formation, the chabazite–thomsonite assemblage is replaced at depth by an assemblage dominated by mafic phyllosilicates followed by thomsonite + chabazite + analcime + natrolite + gonnardite. Mafic phyllosilicate compositions within the latter zone become systematically more rich in the trioctahedral component with depth; the highest-grade mafic phyllosilicates consist of purely trioctahedral, mixed-layer chlorite–smectite. The mineral assemblage developed in the Vaigat Formation is distinct from similar grade assemblages formed in most large igneous provinces as a consequence of the unusually olivine-rich lava compositions that dominate this formation. The zeolites developed in the Vaigat Formation tend to be more Na-rich and somewhat Si-depleted relative to those that form in feldspar-phyric lavas such as the Maligât Formation. Late hydrothermal alteration led to a wide variety of low-grade metamorphic mineral assemblages largely confined to mineralized veins and their immediate surroundings. The distribution of regional alteration assemblages on Disko and Nuussuaq suggests relatively low geothermal gradients (< 45 °C/km) during metamorphism. The highest regional metamorphic grades occur at the base of the exposed succession on northern Disko, suggesting uplift of this area relative to the rest of the island, consistent with recent seismic data.