GEOCHEMICAL AND FIELD RELATIONSHIPS OF PILLOW AND DIKE UNITS IN A SUBGLACIAL PILLOW RIDGE, UNDIRHLITHAR QUARRY, SOUTHWEST ICELAND

Undirhlíðar quarry is located on the Sveifluhals ridge, a Pleistocene subglacial pillow ridge within the NE-SW trending Krysuvik fissure swarm on the Reykjanes peninsula in Southwest Iceland. The walls of Undirhlíðar quarry provide excellent exposures of the internal architecture of the pillow ridge, yielding insights into the sequence of eruptive and intrusive events that construct subglacial ridges. In particular, the South and East walls expose at least 4 different pillow units and 3 dikes. The basal pillow unit (Unit A) is overlain in sequence by pillow units B, C, and D, separated by lappili tuffs that indicate pauses between eruptions. All of the pillows contain fine (<2 mm) plagioclase phenocrysts, but Unit C is also olivine bearing. Unit C is laterally discontinuous, appearing on the East wall and pinching out toward the south, where it contacts a glassy, black breccia wedge. The breccia wedge is intruded by dike 3, which feeds pillow Unit D. Dike 3 is an olivine-free intrusion, in contrast to the olivine-phyric dikes 1 and 2, which cut through pillow Units A and B on the western portion of the South wall. Previous geochemical analyses (Alcorn et al., 2010; 2011) show that the quarry contains two distinct chemical groups that cannot be related by shallow-level fractional crystallization: a high MgO-low (La/Sm)_N group (MgO ~9.5; (La/Sm)_N ~1.2) consisting of all of the dikes and pillow Unit D; and a low MgO-high (La/Sm)_N group (MgO ~8.0; (La/Sm)_N ~1.6) consisting of pillow units A and B. Pillow Unit C, which is newly described in the 2011 field season and has yet to be chemically characterized, could belong to either chemical group. Preliminary investigations of the West wall reveal another new pillow unit, which is mineralogically distinct from pillow Units A-D. In hand sample, the West wall pillows contain large (1-4mm) and abundant olivine phenocrysts. From field relationships alone, it is unclear how the West wall pillows correlate across the quarry, however, compositional analyses may reveal a geochemical relationship between the West wall pillows and the East and South wall units. Clearly, the construction of this subglacial pillow ridge is complex, involving multiple intrusive and eruptive events and the interaction of chemically diverse batches of magma.