Jökull


Jökull - 01.01.2017, Side 39

Jökull - 01.01.2017, Side 39
Svavarsdóttir et al. Bárðardalur – Southern segment of the Bárðarbunga fissure swarm (the Veiðivötn region) In Figures 8e and f, whole rock major and trace el- ement analyses of samples from the Veiðivötn re- gion (Jakobsson, 1979; Manning and Thirlwall, 2014) are plotted together with whole rock and groundmass analyses of the lavas of Bárðardalur valley. Due to the plagioclase-rich character of lavas in this region, these whole rock analyses are likely skewed towards lower K2O and MgO, relative to the compositional field de- fined by Bárðarbunga tephra. These whole rock anal- yses are therefore less likely to define a representa- tive liquid line of descent for Bárðarbunga magmas. Although the samples from Bárðardalur valley are generally more primitive than the Veiðivötn products, they follow a nearly identical evolutionary trend, both with respect to major and trace elements. Also seen by comparison with Bárðarbunga tephra, this suggests that Bár-1 and Kinnarhraun samples represent the primitive endmember of the Bárðarbunga products. This endmember is chemically similar to glass com- positions of the Kistufell table-mountain (9.27–12.56 wt% MgO / 0.05-0.09 wt% K2O; Breddam, 2002), the most primitive magma identified in this region, which has ratios of radiogenic isotopes (87Sr/86Sr = 0.70304–0.70308; 143Nd/144Nd= 0.51306-0.51310; 208Pb/204Pb = 37.935-37.964) that are similar to the lavas found in Bárðardalur valley. The Þjórsárhraun lava, being the largest lava flow from the Veiðivötn region, shares many characteris- tics with the Kinnarhraun and Bárðardalshraun lavas. These are all large lava flows that flowed from the Ice- landic highlands towards the lowlands during an ac- tive period following deglaciation in the region (Vil- mundardóttir, 1977; Hjartarson, 2011). Their petro- logical characteristics are also nearly identical, all be- ing highly porphyritic lavas with variable amounts of plagioclase macrocrysts, reaching up to ∼30 vol% in some samples (Halldórsson et al., 2008). This raises the question whether these large lava flows were emplaced during the same eruption. To inves- tigate this idea, groundmass and whole rock analyses from the lavas of the Bárðardalur region were plot- ted with groundmass analyses from Þjórsárhraun lava from Halldórsson et al. (2008) (Figure 8e and f). The Þjórsárhraun lava groundmass is more evolved, as it displays generally higher K2O, Ba, Y and lower MgO contents relative to groundmass of the Kinnarhraun and Bárðardalshraun lavas. Significantly, the lack of intermediate chemical composition between Þjórsár- hraun lava, on one hand, and the Kinnarhraun and Bárðardalshraun lavas, on the other, supports the no- tion that these lavas were emplaced in separate erup- tive events. However, these lavas fall on the same geochemical trend with respect to major and trace ele- ments, possibly implying that they originate from the same volcanic system. Bárðardalur – Bárðarbunga central volcano and northern segment of the Bárðarbunga fissure swarm Hansen and Grönvold (2000) published major and trace element analyses of glass samples from Hrímalda and Gígöldur, which confirm their some- what primitive nature (Hrímalda: 7.9–8.8 wt% MgO; Gígöldur crater row: 7.7–9.3 wt% MgO). Thus, their analyses suggest that Hrímalda and Gígöldur resem- ble the lavas in Bárðardalur valley somewhat more than Bárðarbunga products from the Veiðivötn region. In the following text, new analyses of Bárðarbunga basement rocks, Dyngjuháls, Urðarháls, Hrímalda, and Gígöldur will be systematically evaluated and compared to the lava flows of Bárðardalur valley. The analyses of the basement rocks of the Bárðar- bunga central volcano are much more scattered than analyses of the lavas of Bárðardalur (Figures 9a and b). Significantly, two Bárðarbunga samples (SAL– 307 and SAL–312) plot well outside the representa- tive chemical range of Bárðarbunga basalts, with high TiO2, high Zr and Y but low MgO contents, signifying highly evolved basalts, commonly observed within mature caldera systems in Iceland (e.g., Nicholson et al., 1991; Hartley, 2012). Two other Bárðarbunga samples plot also outside the chemical trend of the Bárðarbunga basement rocks (SAL–308, SAL–314) with low TiO2 and MgO contents relative to other samples. These are samples with large amounts of plagioclase macrocrysts, similar to what has been de- scribed for Kinnarhraun and Bárðardalshraun lavas (Figure 4b). If these four anomalous samples are ex- cluded, the chemical composition of Bárðarbunga and the lava flows of Bárðardalur are clearly within the 34 JÖKULL No. 67, 2017

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