Volcanic record of continental thinning in Baffin Bay margins: Insights from Svartenhuk Halvø Peninsula basalts, West Greenland

Abstract

Abstract We report major, trace elements and Pb, Hf, Nd and Sr isotopes in 61–54\u202fMa old basalts from Svartenhuk Halvo Peninsula (Greenland). This area corresponds to the northernmost exposure of the West Greenland volcanic province, of which the emplacement marks the continental thinning and breakup process leading to the opening of Baffin Bay during the Eocene. Its wedge structure displays typical characteristics of inner seaward-dipping-reflectors (SDR) with an exposed volcanic sequence thicker than 7\u202fkm. Our results cover the entire volcanic sequence starting with an earliest V1 unit reflecting rather low degree partial melts (transitional basalts), which are geochemically imprinted by continental crust contamination. This unit is followed by much thicker and mainly tholeiitic V2 and V3 basaltic lava piles, and ends with a rather thin V4 unit, consisting of alkali basalts and associated trachytes. Pressures and temperatures of melt extraction were estimated based on major element concentrations and rare earth elements patterns. Our results suggest that melts were extracted from the garnet-spinel transition zone at greater depths for the initial V1 and the final V4 lavas (alkali to transitional basaltic compositions) than for the main V2 and V3 mostly tholeiitic lava piles (at 2\u202f±\u202f0.5\u202fGPa and 1350\u202f±\u202f100\u202f°C). Isotope signatures suggest that mantle sources of the melts were controlled by the mixing of ambient upper mantle and Icelandic plume-type materials. The proportion of ambient upper mantle involved in the lava source appears to increase together with melting rates and the upward propagation of melting zone during V3 emplacement, suggesting that melting in this area progessed as follows: 1. Initiation of partial melting (low degrees) of deep-seated Icelandic plume-type material (V1 and V2); 2. Upward propagation of the melting area within the shallower mixed upper mantle (V3); 3. Progressive decrease of “ambient upper mantle” involvement, deepening of magma extraction and decrease of partial melting associated with the emplacement of V4 lavas. We show that these changes are consistent with the tectonic stages of continental thinning that preceded the Eocene breakup in Baffin Bay.