GLACIOVOLCANIC MEGAPILLOWS OF UNDIRHLITHAR, REYKJANES PENINSULA, SOUTHWESTERN ICELAND

At Undirhlithar, a glaciovolcanic ridge (tindar) on the Reykjanes Peninsula of southwestern Iceland, megapillows are among the features formed during a series of ridge-building eruptions. Mapping of the northeastern 3 km of the ridge and petrographic and geochemical analysis of the megapillow outcrops occurring throughout this area demonstrate their role in the multi-stage construction of the ridge modeled by Pollock et al. (2014). The outcrops exhibit radial jointing, bands of vesicles and glassy rims; they occur in high relief surrounded by basalt breccia resembling pillow rubble, and are composed of plagioclase-phyric olivine basalt with plagioclase-rich groundmass. They occur in multiple pillow lava units formed from two distinct magma batches. Two groups of outcrops are represented that are petrographically, geochemically and geographically distinct; the first group is near to and consistent with the pillow units of Undirhlithar quarry described by Pollock et al. (2014), and the second group, located near the tephra cone, is derived from a more evolved unit of the same magma. Megapillows show significant plagioclase accumulation with variable phenocryst zoning, indicating the movement of multiple pulses of magma through the megapillows. Megapillows at Undirhlithar may represent a significant mechanism, demonstrated elsewhere at a marine megapillow by Goto and McPhie (2004), for magmatic distribution: feeding and then overrunning pillows which propagate and are fed from their basal margins at the eruptive front.