Contrasting TiO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; Compositions in Early Cenozoic Mafic Sills of the Faroe Islands: An Example of Basalt Formation from Distinct Melting Regimes

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

doi:10.11648/j.earth.20190805.11

Contrasting TiO&lt;sub&gt;2&lt;/sub&gt; Compositions in Early Cenozoic Mafic Sills of the Faroe Islands: An Example of Basalt Formation from Distinct Melting Regimes

Jógvan Hansen, Dougal Jerram + Show 3 more

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https://doi.org/10.11648/j.earth.20190805.11

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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.

Highlights

Numerous hypotheses on the petrogenetic evolution of large basaltic igneous provinces (LIPs) such as the North Atlantic Igneous Province (NAIP) have been proposed earlier [1,2]
Cenozoic high-TiO2 basaltic rocks from W Greenland are interpreted to have developed by periodical replenishmenttapping-fractionation (RTF) processes from local low-TiO2 basaltic melts, with plagioclase and clinopyroxene as the main fractionating assemblages and relatively high (16 to 20%) degrees of partial melting of a moderately depleted mantle have been invoked [15]
The petrogenetic interpretations presented for the Faroese sills in this study are considered in the context of late-stage magmatic processes during the formation of the NAIP, and in particular we look at the effects of partial melting at relatively shallow mantle levels during the waning stages of basaltic magmatism in this LIP

Summary

Introduction

Numerous hypotheses on the petrogenetic evolution of large basaltic igneous provinces (LIPs) such as the North Atlantic Igneous Province (NAIP) have been proposed earlier [1,2]. Formation of lowTiO2 basalts have at times been attributed to relatively largedegree melting of enriched material from the continental lithospheric mantle or else developed in response to melting of other mantle sources contaminated with subducted crust/sediments and/or associated expulsion of fluids [10,11,12] They could result from relatively largedegree melting (~20%) of depleted mantle sources [7,8]. Cenozoic basaltic rocks from E Greenland, which gave rise to high-TiO2 basalts, presumably resulted from ~4 to ~8% mantle melting [16,17] compared to the 19 to 20% mantle melting probably needed to generate primary magmas that produced the lowTiO2 basalts in parts of this region [17] Compositional differences between their respective mantle sources likely affected the ultimate geochemistry of low-TiO2 versus highTiO2 basaltic rocks of this region [17,18]. Heterogeneous mantle sources have been inferred for basaltic rocks from E Greenland [19,20,21,22]; W Greenland [15]; Iceland [21], NW

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