Petrology of early Tertiary nephelinites off mid-Norway. Additional evidence for an enriched endmember of the ancestral Iceland plume

Abstract

Plugs of alkaline volcanic rocks (≈56 Ma) penetrating Mesozoic sediments crop out at 63°25′N on the continental shelf off Norway. The rocks are highly silica-undersaturated, comprising both nephelinites and basanite. The nephelinitic rocks are porphyritic, with abundant phenocrysts of Mg-rich olivine and some clinopyroxene. Many olivine crystals are altered along grain margins and fractures. The fine-grained matrix consists of fresh augite, biotite, rhönite, chlorite, oxides, and altered nepheline. Apatite and calcite are accessories. The basanite has much less olivine and abundant matrix plagioclase. The occurrence of rhönite indicates crystallization in the presence of a fluid phase at P<60 MPa and temperatures between 840 and 1200°C. The rocks are characterized by relatively high contents of MgO, CaO, P 2O 5, Ni, Cr, Sr, and incompatible elements. The REE are highly fractionated with LREE up to 500× chondrite. Initial strontium isotope ratios ( 87 Sr/ 86 Sr ) i range from 0.703685 to 0.704025 and ϵ Nd from +3.13 to +3.31 in the nephelinites. In the basanite ( 87 Sr/ 86 Sr ) i is 0.704447 and ϵ Nd+0.01. The nephelinites are enriched in 208 Pb/ 204 Pb . We suggest that the basanite formed from enriched lithospheric mantle and that the nephelinites were low melt fractions of a sublithospheric (asthenospheric) source. This source probably represents an enriched, low-temperature melting end member component of the North Atlantic (Icelandic) plume.