Continental crust beneath southeast Iceland

The magmatic activity (0–16 Ma) in Iceland is linked to a deep mantle plume that has been active for the past 62 My. Icelandic and northeast Atlantic basalts contain variable proportions of two enriched components, interpreted as recycled oceanic crust supplied by the plume, and subcontinental lithospheric mantle derived from the nearby continental margins. A restricted area in southeast Iceland—and especially the Öræfajökull volcano—is characterized by a unique enriched-mantle component (EM2-like) with elevated ⁸⁷Sr/⁸⁶Sr and ²⁰⁷Pb/²⁰⁴Pb. Here, we demonstrate through modeling of Sr–Nd–Pb abundances and isotope ratios that the primitive Öræfajökull melts could have assimilated 2–6% of underlying continental crust before differentiating to more evolved melts. From inversion of gravity anomaly data (crustal thickness), analysis of regional magnetic data, and plate reconstructions, we propose that continental crust beneath southeast Iceland is part of ∼350-km-long and 70-km-wide extension of the Jan Mayen Microcontinent (JMM). The extended JMM was marginal to East Greenland but detached in the Early Eocene (between 52 and 47 Mya); by the Oligocene (27 Mya), all parts of the JMM permanently became part of the Eurasian plate following a westward ridge jump in the direction of the Iceland plume.The North Atlantic Igneous Province covers vast areas in Baffin Island, Greenland, United Kingdom, Ireland, the Faroe Islands, and offshore regions (Fig. 1). Volcanic activity commenced ∼62 Mya (1) and is attributed to the impingement of a mantle plume head on the lithosphere (2). Enriched and depleted geochemical signatures in the Paleogene to Recent basalts from Iceland reflect a combination of plume-derived and shallow asthenospheric material, representing a classic case of plume–ridge interaction. The volcanic products of the Iceland plume have signatures of recycled oceanic crust and primordial-like material with high ³He/⁴He ratios (up to 50 R_A, where R_A is the ³He/⁴He ratio of air), spanning a range of refractory to fertile compositions (3, 4). Additional shallow-level contributions include the depleted mid-ocean ridge basalt–type asthenosphere, variably mixed with subcontinental lithospheric mantle material (5–8). The proximity of Iceland to the Jan Mayen Microcontinent (JMM) (Fig. 1) raises the question of whether Iceland includes underlying fragments of continental crust (9, 10). There are unconfirmed reports of both Precambrian and Mesozoic zircons in young basalts from Iceland, but only in abstract form (9, 11). The recovery of a 1.8-Ga-old grain from the most primitive Öræfajökull basalts (9) in southeast Iceland (Figs. 1 and ​and2A)2A) is potentially very interesting, although two Jurassic zircon grains (160 Ma) uncovered in the same separation are now suspected to be due to laboratory contamination.Open in a separate windowFig. 1.Crustal thickness map based on gravity inversion and revised location of the Iceland plume (white star symbols in 10-My intervals) relative to Greenland back to 60 Ma (see Fig. 6A for details). Transition between continental and oceanic crust (COB, black lines), plate boundaries (blue lines), site locations for dated North Atlantic Igneous Province magmatism (yellow circles with black fill), and the estimated size of the “classic” Jan Mayen Microcontinent (JMM) and the extended JMM (JMM-E) are also shown. The classic JMM is ∼500 km long (200 km at its widest and shown with four continental basement ridges), and crustal thicknesses are about 18–20 km (scale at Upper Left), which suggests considerable continental stretching before drifting off Greenland. We extend JMM 350 km southwestward (∼70 km wide) beneath southeast Iceland where we calculate maximum crustal thicknesses of ∼32 km (Fig. 2B). The size of JMM-E is a conservative estimate and could be as large as the white-stippled area. Öræfajökull location is shown as a yellow star.Open in a separate windowFig. 2.(A) Simplified geological map of Iceland outlining the major rift zones (RZ), rift zone central volcanoes (black outline), flank zone central volcanoes (colored outlines), the location of the Öræfajökull central volcano, and the position of the current plume axis of Shorttle et al. (25). Yellow areas are fissure swarms. WRZ, ERZ, NRZ, and MIB: Western, Eastern, and Northern Rift Zone and Mid-Iceland Belt, respectively. The stippled west-northwest–trending line through the plume axis depicts the position of the cross sections in Fig. 9. (B) Enlarged crustal thickness map (Fig. 1) with superimposed earthquake locations (earthquake.usgs.gov/earthquakes) and contour intervals (in meters) showing that the anomalously thick crust under southeast Iceland extends offshore to the northeast suggesting that it is a southerly fragment of the Jan Mayen Microcontinent (JMM) rather than an extension of the southeast-northwest–orientated Iceland-Faroes Ridge.