Contrasting TiO2 compositions in early cenozoic mafic sills of the Faroe Islands : an example of basalt formation from distinct melting regimes

Hansen, Jógvan; Davidson, Jon; Jerram, Dougal; Ottley, Christopher; Widdowson, Mike

Abstract

The Paleocene lava succession of the Faroe Islands Basalt Group (FIBG), which is a part of the North Atlantic Igneous Province (NAIP), is intruded by numerous basaltic sills. These can be grouped into three main categories according to their geochemical characteristics: A low-TiO2 sill category (TiO2 = 0.7-0.9), a relatively high-TiO2 sill category (TiO2 = 1.95-2.6) and an intermediate-TiO2 sill that displays major element compositions lying between the other two categories. Mantle normalised plots for the high-TiO2 and low-TiO2 sills display relatively uniform flat LREE trends and slightly steeper HREE slopes for high-TiO2 relative to low-TiO2 sills. The intermediate-TiO2 Morskranes Sill is LREE depleted. Mantle normalised trace elements of low-TiO2 sill samples define positive Eu and Sr anomalies, whereas high-TiO2 sill samples display negative anomalies for these same lements. Different Nb and Ta anomalies (positive versus negative) in many high-TiO2 versus low-TiO2 sill samples suggest various metasomatism of their sources prior to partial melting. The intermediate-TiO2 sill displays noticeably lower 87Sr/86Sr, 206Pb/204Pb and 208Pb/204Pb ratios relative to both the high-TiO2 and the low-TiO2 sill samples. Pb isotope compositions displayed by local contaminated basaltic lavas imply that some of these assimilated distinct crustal material from E Greenland or basement from NW Britain, while others probably assimilated only distinct E Greenland type of crustal material. A third crustal source of E Greenland or Rockall-type basement could be required in order to explain some of the range in lead isotopes displayed by the intermediate-TiO2 Morskranes Sill. Geochemical modelling suggest that Faroese high-TiO2 sills, could have formed by ~4 to 7.5% batch melting of moderately fertile lherzolites, while 16 to 21% batch melting fertile mantle sources could explain geochemical compositions of Faroese low-TiO2 sills. The intermediate-TiO2 sill samples could have formed by a range of 6 to 7% batch melting of a depleted mantle source, probably with a composition comparable to sources that gave rise to local low-TiO2 and intermediate-TiO2 host-rocks. Most Faroese sill samples probably developed outside the garnet stabilitry field and probably formed by batch melting of mantle materials comparable in composition to those reported for the sub-continental lithospheric mantle (SCLM) previously at depths of ≤ 85 km. Relative enrichments in LREE (and LILE in general), and their varying Nb and Ta anomalies point to sources affected by metasomatism.

Citation

Hansen, J., Davidson, J., Jerram, D., Ottley, C., & Widdowson, M. (2019). Contrasting TiO2 compositions in early cenozoic mafic sills of the Faroe Islands : an example of basalt formation from distinct melting regimes. Earth sciences (New York, N.Y. Print), 8(5), 235-267. https://doi.org/10.11648/j.earth.20190805.11

• Journal Article Type: Article
• Acceptance Date: Sep 23, 2019
• Online Publication Date: Oct 9, 2019
• Publication Date: Oct 31, 2019
• Deposit Date: Oct 22, 2019
• Publicly Available Date: Nov 12, 2019
• Journal: Earth sciences
• Print ISSN: 2328-5974
• Electronic ISSN: 2328-5982
• Publisher: Science Publications
• Peer Reviewed: Peer Reviewed
• Volume: 8
• Issue: 5
• Pages: 235-267
• DOI: https://doi.org/10.11648/j.earth.20190805.11
• Public URL: https://durham-repository.worktribe.com/output/1287252