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Geochemical and Lithostratigraphic Constraints on the Formation of Pillow-Dominated Tindars from Undirhlíðar Quarry, Reykjanes Peninsula, Southwest Iceland

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Geochemical and Lithostratigraphic Constraints on the Formation of Pillow-Dominated Tindars from Undirhlíðar Quarry, Reykjanes Peninsula, Southwest Iceland Lithos 200–201 (2014) 317–333 Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos Geochemical and lithostratigraphic constraints on the formation of pillow-dominated tindars from Undirhlíðar quarry, Reykjanes Peninsula, southwest Iceland Meagen Pollock a,⁎, Benjamin Edwards b,SteinunnHauksdóttirc, Rebecca Alcorn a, Lindsey Bowman a a Department of Geology, The College of Wooster, 944 College Mall, Scovel Hall, Wooster, OH 44691, USA b Department of Earth Sciences, Dickinson College, Carlisle, PA 17013, USA c ÍSOR, Iceland GeoSurvey, Grensávegur 9, 108 Reykjavík, Iceland article info abstract Article history: Undirhlíðar quarry on the Reykjanes Peninsula in southwest Iceland exposes an almost complete cross-section of Received 1 August 2013 a pillow-dominated tindar. Detailed mapping and geochemical analyses of the quarry walls show that Accepted 23 April 2014 glaciovolcanic lithologies are controlled not only by ice conditions, but also by complex changes in magmatic Available online 14 May 2014 conditions. Undirhlíðar's glaciovolcanic deposits are dominated by pillow lavas (Lp1–3) but also include dikes (Ld1–3) and shallow intrusions, and interbedded tuff (T), lapilli tuff (LT) and tuff-breccia (TB). Lithostratigraphic Keywords: variations record shifts in eruptive vents and styles, including evidence for explosive subaqueous activity. Petro- Iceland Glaciovolcanic graphically, the units can be subdivided into plagioclase-phyric and olivine-phyric, while compositionally the fi Tindar units de ne two trace element populations: (1) incompatible element-enriched (LaN/SmN ~1.6; Nb/Zr ~0.15) Dike rocks comprising the lower (older) pillow units (Lp1–2), and (2) less-enriched (LaN/SmN ~1.3; Nb/Zr ~0.125) Pillow units including dikes (Ld1–3), west wall pillows (LpW), and the upper (younger) pillow units (Lp3). The Hyaloclastite combined lithostratigraphic, petrographic, and geochemical relationships suggest a four-stage model for the formation of Undirhlíðar: (1) an initial effusive phase (Lp1–2) that built the bulk of the ridge, which erupted olivine-free, incompatible element-enriched lava, (2) an explosive phase, which generated lenses of TB and LT, now exposed on the eastern side of the ridge, (3) a second effusive phase (Ld1–2, LpW) exposed in the west side of the quarry, which intruded the initial effusive deposits and erupted pillow lavas that drape over the west- ern edge of the existing ridge; this effusive phase is distinguished from the first as a less-enriched magma bearing large olivine phenocrysts, and (4) a final effusion on the east side of the quarry, which intruded (Ld3) units LT and TB, and erupted a capping layer of pillow lavas (Lp3). The products of this effusive event are genetically related to the previous effusive phase (LpW) but do not contain large olivine phenocrysts. The specificstratigraphic sequence of vitric fragmental units (TB2/LT) cut by dikes (Ld3) that feed pillow lava flows (Lp3) emplaced imme- diately above the fragmental units (TDP lithofacies association) is consistent with an explosive initiation to the final phase of volcanism evident in the quarry. The explosive unit (TB2/LT) also coincides with the transition from olivine-free, incompatible element-enriched lavas to olivine-phyric, less-enriched lavas. To explain the shift in trace elements between the early and late pillow lavas, we propose a model based on tapping of separate, small magma reservoirs at different depths. © 2014 Elsevier B.V. All rights reserved. 1. Introduction compared to subaerial lava flows, the emplacement dynamics of pillow lavas and the construction of pillow-dominated volcanoes are not as Pillow lavas are one of the most common types of lava morphologies well understood simply because of their relative inaccessibility; on Earth, forming in a wide variety of water-dominated environments the vast majority of present day pillow-dominated volcanoes form in including submarine (e.g., Batiza and White, 2000; Furnes et al., 2007; submarine environments along mid-ocean ridges (MOR) at depths of Johns et al., 2006; Walker, 1992), lacustrine (e.g. McClintock et al., at least 2.5 km. Although a wealth of information about submarine 2008), fluvial (Farrell et al., 2007), and glaciovolcanic (e.g., Edwards pillow lavas has been gleaned from high resolution mapping and sam- et al., 2009; Höskuldsson et al., 2006; Jones, 1969) settings. Yet pling of seafloor exposures (e.g., Ferrini et al., 2008; Soule et al., 2007), deep-sea drill cores (e.g., Tominaga et al., 2009), and analog experi- ments (Gregg and Fink, 1995, 2000), significant questions remain ⁎ Corresponding author. Tel.: +1 330 263 2202 (office); fax: +1 330 263 2249. about the eruptive processes that control the development of pillow- E-mail address: [email protected] (M. Pollock). dominated volcanic edifices. http://dx.doi.org/10.1016/j.lithos.2014.04.023 0024-4937/© 2014 Elsevier B.V. All rights reserved. 318 M. Pollock et al. / Lithos 200–201 (2014) 317–333 A B C M. Pollock et al. / Lithos 200–201 (2014) 317–333 319 Fig. 2. Geological sketch map, Undirhlíðar quarry. A. Sketch map showing the locations of lithofacies described in the text and informal location names. Boxes show approximate locations of sketches in Fig. 4. B. Sketch map showing locations of samples analyzed for major and trace element geochemistry. For example, most models for the formation of basaltic glacio- Schmincke, 1999). These models are largely derived from fieldwork at volcanoes show a single period of effusive pillow formation, followed Pleistocene glaciovolcanic deposits, which are now exposed due to by a transition to entirely explosive eruptions as water depth shallows Holocene ice retreat in western Canada (Allen et al., 1982; Edwards (e.g., Allen et al., 1982; Hickson, 2000; Jakobsson and Johnson, 2012; et al., 2009, 2010; Hickson et al., 1995; Hickson, 2000; Mathews, 1947; Jones, 1970; Skilling, 1994, 2009; Smellie, 2001, 2008; Werner and Moore et al., 1995), Antarctica (e.g., Smellie and Skilling, 1994; Smellie Fig. 1. A. Map showing the active volcanic centers in Iceland (after Fig. 1, Edwards et al., 2012). B. Detailed map showing the location of Undirhlíðar (outlined black box) at the northern end of the Sveifluhals ridge (after Saemundsson et al., 2010), in the central part of the Reykjanes Peninsula. C. Aerial overview and general distribution of units in the vicinity of Undirhlíðar quarry. 320 M. Pollock et al. / Lithos 200–201 (2014) 317–333 et al., 2008), and especially in Iceland (Allen, 1980; Bennett et al., 2009; dike-fed pillow units. Based on his field observations, he proposed what Höskuldsson et al., 2006; Jakobsson and Johnson, 2012; Jarosch et al., has become a standard model for many basaltic glaciovolcanic eruptions: 2008; Jones, 1969, 1970; Loughlin, 2002; Moore and Calk, 1991; (1) initiation with effusion of pillow lava at water depths of greater than Saemundsson, 1967; Sigvaldason, 1968; Smellie, 2008; van Bemmelen 500 m, switching to (2) explosive eruptions and production of tephra at and Rutten, 1955; Werner and Schmincke, 1999; Werner et al., 1996). shallow water depths (~100–200 m), leading to (3) possible subaerial However, recent observations of glaciovolcanic pillow exposures in effusion. He noted that, in general, vesicularity of pillows seemed to Iceland (Alcorn et al., 2010; Bennett et al., 2009; Bowman et al., 2011) increase from the base to the top of the pillow pile, which is consistent and western Canada (Edwards et al., 2009) show packages of pillow with a growing edifice in a steady-state englacial lake. lavas separated by deposits of fragmental volcanic material, suggesting Höskuldsson et al. (2006) studied a series of pillow ridges in the that pillow ridges may be produced by a complicated sequence of erup- Kverkfjöll area of north-central Iceland. Their main focus was the varia- tive events (e.g., Johnson and Jakobsson, 1985). tion in vesicularity of pillows within the ridges, and they postulated that The most detailed studies of basaltic tindars, which are volcanic ridges those variations could be explained by sudden pressure releases caused formed by eruptions beneath ice (e.g., Edwards et al., 2009; Jakobsson by floods from the eruption site. Based on measured values of H2Ocon- and Gudmundsson, 2008; Jones, 1969), include the work of Jones centrations in pillow rim glasses, they estimated that the pillows formed (1969, 1970), Höskuldsson et al. (2006), and Edwards et al. (2009). beneath relatively thick ice (N1000 m), and that sudden flood events Jones (1969, 1970) described lithofacies variations at Kalfstindar, dropped the level of the englacial lake at the eruption site by as much Iceland, where he recognized a fundamental separation of basal, pillow- as 400 m. dominated material from an upper sequence of fragmental material. Edwards et al. (2009) described the lithostratigraphy at Pillow Ridge He noted small, intra-pillow zones of volcanic breccia, and possible in northern British Columbia, which is the only detailed description of a A D Pillow Lava (Lp1) (TB1) Pillow Lava (LpW) B Intrusion E (Li) (LT) ~10 m Pillow Lava (LpW) C F Pillow lava Pillow Lava (Lp1) (Lp3) Dike (Ld3) (TB2/LT) Dike (Ld2) (T) Fig. 3. Field images of lithofacies described in the text. A. Pillow lavas (Lp1), south-central quarry with person for scale. B. Intrusion, northwest quarry wall. C. Dike (Ld3) intruded through tuff-breccia (TB1) feeding pillow lavas (Lp3), southeastern quarry wall with person for scale. D. Tuff-breccia (TB1) comprising dominantly angular lithic fragments derived from Lp units; hammer for scale is 45 cm long. E. Vitric tuff-breccia/lapilli tuff (TB2/LT) comprising vitric clasts; horizontal ruler segment is 42 cm long. F. Tuff (T) interbedded with Lp1 and Lp2, intruded by a small dike (Ld2); lens cap for scale is 6 cm in diameter. M. Pollock et al. / Lithos 200–201 (2014) 317–333 321 large tindar outside of Iceland. They demonstrated that long-lived suggests that glaciovolcanic lithologies are controlled by the complex eruptions could produce significant lithostratigraphic complexity in a interaction between ice/water and magmatic conditions.
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