Continental basement under Iceland revealed by old zircons
Abstract
The Iceland mantle plume is likely to have migrated from beneath Greenland contemporaneous with the opening of the North-Atlantic and reached the spreading axis around 20 Ma ago. Only a few geochronological studies have been undertaken in Iceland and the oldest known volcanics are 13 Ma and 15 Ma at the eastern and western extremities, respectively. As a part of a new dating study, we present U-Pb results on zircons separated from the ignimbrite of Hvítserkur formation in NE-Iceland. Recent advances in LA-ICP-MS geochronology provide precise and accurate U-Pb ages. In-situ U-Pb analysis of c.a. 50 μm-wide selected zones was performed on a set of 16 zircon grains in using a 193 nm Geolas excimer laser coupled to a Thermo Finnigan Element high resolution ICP-MS. Cathode-luminescence images expose inherited and zoned cores surrounded by complex rims. Only a single grain displays a euhedral shape with a regular zoning similar to that observed in zircons from several Icelandic rhyolites. This grain yields a 206Pb/238U age of 12.5 +/- 0.8 Ma (± 2 sigma) that corresponds to the age of the surrounding volcanic formations. However, the 15 remaining zircon crystals give generally (13/15) concordant U-Pb ages in the range of 126 +/- 4 Ma to 242 +/- 7 Ma. These Mesozoic zircon crystals yield much older ages than the oldest Miocene rocks in Iceland. The magma producing the Hvítserkur ignimbrite therefore extracted from depth and exposed at the surface ancient rocks containing these old zircons. Consequently, a continental crust must exist beneath at least part of Iceland. This crust most likely formed during the crustal thinning phase of Pangaea, since the continental break-up and sea-floor spreading in the NE Atlantic only began at 55-60 Ma. It could be a part of the Jan Mayen ridge that is thought to extend south under the Iceland plateau. Our results thus support the hypothesis that the Jan Mayen microcontinent was captured by oceanic ridge jumping events, which separated it as a thinned continental margin from the Greenland crustal-shield. The presence of a light continental crust under Iceland provides straightforward explanations to numerous Icelandic anomalies, such as the inferred abnormally thick crust (up to 45 km), gravity and seismic characteristics and isotope composition in and off E-Iceland.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.V33A0642P
- Keywords:
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- 1115 Radioisotope geochronology;
- 8178 Tectonics and magmatism;
- 9325 Atlantic Ocean;
- 9609 Mesozoic