Tertiary picrites in West Greenland: melting at the periphery of a plume?

Abstract

The West Greenland/Baffin Island Tertiary volcanic province differs from other CFB provinces in containing an unusually high proportion (30-50% by volume) of magnesian picritic lavas and hyaloclastites. Olivine-liquid equilibrium considerations suggest the presence during the earlier stages of eruption of picritic melts with MgO contents as high as 20%. Calculations based on McKenzie-Bickle melting models point to high degrees of melting (24-30%) at depths of 60-90 km in the underlying mantle, and require potential temp-" eratures of 1540-1600°C. Such high potential temperatures are inconsistent with reconstructions that attribute the West Greenland volcanism to melting on the margins of the incipient Iceland plume-head. The distribution of Tertiary volcanic activity in Greenland, in particular its relation to Mesozoic-Tertiary extensional basins, indicates that lithospheric structure plays a part in determining where the plume-head can undergo melting. But to explain the restriction of high-temperature picrites to West Greenland, together with their distinctive trace element geochemistry, it is necessary to invoke (a) either an elongated Icelandic plume initially extending as far as West Greenland or a short-lived precursory plume head that developed directly beneath West Greenland, and (b) an active extensional regime that allowed rapid access of picritic melts to the surface.

Mantle melting systematics: transition from continental to oceanic volcanism on the southeast Greenland Margin

“…Pedersen and Skogseid (1989) provide an alternative hypothesis by suggesting that melts are pooled for a much wider region and focused under the zone of continental rupture. Explanations such as these are fundamentally different than the prevailing hypothesis in which large volumes of melt are produced by passive upwelling of anomalously hot mantle (e.g., White and McKenzie, 1989;Gill et al, 1992;White et al, 1987). Increased mantle temperature would cause melting to begin deeper and the resulting magmas to represent higher extents of melting and thus would be apparent in the geochemistry of the erupted lavas.…”

“…Recent workers (White and McKenzie, 1989;Campbell and Griffiths, 1990) have argued that, in the early Tertiary, the hot spot was a much larger feature, underlying the whole North Atlantic region that experienced volcanism, from West Greenland to the British Isles and along 2000 km of the East Greenland and conjugate European margins. White and McKenzie (1989), as well as others (Gill et al, 1992;Larsen et al, 1992), assume that the ancestral Iceland plume axis was situated beneath the Kangerlussuaq area in East Greenland, although a number of models of relative plate motions place the axis 300−500 km to the northwest of that site ( Fig. 1; Duncan and Richards, 1991;Lawver and Müller, 1994).…”

The North Atlantic Igneous Province: A review of models for its formation

“…After a significant Early Paleocene uplift, later volcanism was almost entirely sub-aerial (Dam et al, 1998). The volcanic rocks are divided into three formations (Gill et al, 1992): the lower formation mainly consists of picritic and other olivine-rich tholeiitic basalts, with local evidence of crustal contamination. The other two formations are composed of olivine and plagioclase phyric olivine tholeiitic basalts, basaltic tuffs, transitional basalts and acidic ignembrites.…”

“…15 Vol.% in E Greenland, Nielsen et al, 1981) (Gill et al, 1992;Larsen et al, 1992;Holm et al, 1993). Definitions of picrites are not always unambiguous (Le Bas, 2000); Gill et al (1992) defined all basaltic rocks with MgO concentrations higher than 10 wt.% as picrites. The MgO content of the rocks analyzed by Gill et al (1992) range between 15 to 30 wt.%.…”

“…Definitions of picrites are not always unambiguous (Le Bas, 2000); Gill et al (1992) defined all basaltic rocks with MgO concentrations higher than 10 wt.% as picrites. The MgO content of the rocks analyzed by Gill et al (1992) range between 15 to 30 wt.%. Petrologists debate the nature of these rocks: do the melts represent unmodified high-MgO mantle melts (e.g.…”