Jökull


Jökull - 01.01.2017, Page 24

Jökull - 01.01.2017, Page 24
Holocene lavas in Bárðardalur, NE-Iceland ratios of a sample erupted through a young basaltic crust, free from the influence of seawater-alteration, can be expected to be similar to the ratios of its man- tle source (e.g., Sigmarsson et al., 1992; Furman et al., 1995; Sigmarsson and Halldórsson, 2015). Fur- thermore, as at many oceanic islands, it is well estab- lished that the Icelandic mantle displays notable varia- tions in radiogenic isotopes which are almost entirely related to primary variability of the underlying man- tle (e.g., Stracke et al., 2003; Thirlwall et al., 2004). Therefore, when such tracers are coupled to geochem- ically well-characterized eruptive units, they probably represent the most powerful tool available to assign volcanic units to their mantle source, and therefore a likely volcano of origin. Here, we present a geochemical study of early- Holocene large lava flows in the Bárðardalur region. The primary aim is to determine the eruptive site of these lavas and elucidate their petrological context within the NRZ. In order to do this, we compare the chemical and isotopic characteristics of the lavas from Bárðardalur to previously published data from Bárð- arbunga and other volcanic systems of the NRZ. Ad- ditionally, we report chemical and isotopic data for several eruptive units from the region north of Vatna- jökull and from the Bárðarbunga central volcano. The notion that Bárðarbunga has produced large-volume lava flows that travelled vast distances, not only to- wards the south coast of Iceland but also towards the north (Figure 1), needs further testing, not least because of its significance for improving our under- standing of the magmatic history of the NRZ. GEOLOGICAL SETTING AND SAMPLING Northern segment of the Bárðarbunga fissure swarm The eruptive record of the Bárðarbunga-Veiðivötn system indicates that similar to the southern part, the northern part of the fissure swarm has also produced several lava flows of significant volume during the Holocene time period (e.g., Hartley and Thordarson, 2013; Sigmarsson and Halldórsson, 2015). Holocene lava fields in the Dyngjuháls region cover vast areas north of Vatnajökull, reaching from Gæsavötn in the west to Urðarháls in the east (Figure 2) (Sigurgeirsson et al., 2015). The Dyngjuháls lavas are partly cov- ered by younger lavas of the nearby shield volcano Trölladyngja (<4500 years BP; Sigurgeirsson et al., 2015). Owing to the ruggedness of the region, it has not been studied to the same extent as the Veiðivötn region, despite a large number of Holocene eruptions there. The nearby volcanic unit Urðarháls, located to the east of Dyngjuháls (Figure 2), shows evidence of glacier erosion (Sigbjarnarson, 1988), suggesting a pre-Holocene age. The largest and most promi- nent crater row in the region is Gígöldur, situated to the northeast of Urðarháls (Figure 2). The Gígöldur crater field represents a large eruptive unit, made up by tens of fissures, craters and crater rows that are connected in a complex way (Sigbjarnarson, 1988). The craters show signs of explosive eruptions towards the end of the last glacial period, suggesting high rates of volcanic activity around 10 kyr BP. More- over, these craters are the likely source of numerous prehistoric lava flows that disappear under younger lava units (e.g., Trölladyngja) (Sigbjarnarson, 1988; Hansen and Grönvold, 2000). The Krepputunga lava, which covers a substantial area to the E and NE of the crater row, has been suggested to originate from Gígöldur (Sigbjarnarson, 1988; Vilmundardóttir and Kaldal, 1991; Hjartarson and Kaldal, 2004; Hjartar- son, 2004). Hrímalda, situated to the west of Gígöldur (Figure 2), also formed from many eruptions occur- ring towards the end of the last ice age (Sigbjarnarson, 1988). The Holocene lavas of the Bárðardalur region A prominent prehistoric lava field of the NRZ occurs in the narrow Bárðardalur valley (Figure 1), where Holocene lava units can be traced northwards, almost reaching the north coast of Iceland. Earlier studies of these lava units in Bárðardalur are generally de- scriptive geological and petrographic studies (Tómas- son, 1967; Sigbjarnarson, 1988; Vilmundardóttir and Kaldal, 1991; Hjartarson and Kaldal, 2004; Hjart- arson, 2004). Based on field observations and pet- rographic characteristics, Sigbjarnarson (1988) sug- gested the Gígöldur crater row as the eruptive cen- tre of the majority of plagioclase-rich lavas found in Bárðardalur valley. More recent studies conducted JÖKULL No. 67, 2017 19

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