Petrology of the Paleocene Picrites and Flood Basalts on Disko and Nuussuaq, West Greenland

Abstract

The c. 62^60 Ma rift-related Paleocene volcanic succession in West Greenland comprises voluminous picrites (the Vaigat Formation) overlain by flood basalts (the Maliga“ t Formation). A detailed stratigraphy for the c. 3 km combined succession on Disko and Nuussuaq has been established that provides evidence of the evolution of the volcanic systems, processes and sources through time. Picrites constitute about one-third of the total erupted volume. The primary magmas for the whole succession were highly magnesian; the weighted average of 16� 6 wt % MgO for the uncontaminated Vaigat Formation is a lower limit for the primary magmas. During the emplacement of the Vaigat Formation, the magmas fractionated olivine in the feeder channels before eruption but generally did not stop in magma chambers. Only minor magma batches stopped in high-level magma chambers, where many became crustally contaminated. At the transition to the Maliga“ t Formation magma chambers developed, presumably in the lower crust. From then on, the main eruption products were basalts with compositions buffered by refilling^fractionation^tapping processes. Large-scale cyclicity can be seen, with more and less fractionated steady-state compositions. The upper part of the Maliga“ t Formation is contaminated throughout, except for the three youngest flows. Repeated pulses of new picritic magmas can be seen in both formations. The asthenospheric mantle source was heterogeneous and the dominant component was depleted, mid-ocean ridge basalt (MORB)-like mantle with Nb/ La51. An additional, less depleted mantle component akin to the Iceland source is evident in the upper Vaigat Formation but disappeared again at the transition to the Maliga“ t Formation. A third mantle component similar to the source for the E-type lavas on Baffin Island may be present in the earliest rocks. Melting took place in the garnet peridotite facies because the melting column was curtailed by the 100 km thick lithosphere. The picrites show coupled isotopic and elemental variations even within single units, indicating that the accumulated primary melts were not homogenized when they left the melting column. An incompatible-element-rich lithospheric mantle component is seen in the ManaŒ tdlat Member of alkaline picrites and basalts. Single scattered tholeiitic lavas somewhat enriched in the same trace elements are considered to be slightly contaminated with this component.The volume of alkaline and enriched rocks is very small. The crustal contaminants are the sediments in the Nuussuaq Basin. The contamination processes included bulk assimilation, inmixing of partial melts of the sediments, assimilation^fractional crystallization processes, and exchange reactions. When carbon-rich shales were assimilated severe reduction led to formation of graphite or metallic iron. All rocks more evolved than basalt arose by contamination processes. The units of contaminated rocks constitute 5^9% of the Vaigat Formation and around 30% of the Maliga“ t Formation.