Explosive silicic volcanism in Iceland and the Jan Mayen area during the last 6 Ma:: sources and timing of major eruptions

Fifty-three major explosive eruptions on Iceland and Jan Mayen island were identified in 0-6-Ma-old sediments of the North Atlantic and Arctic oceans by the age and the chemical composition of silicic tephra. The depositional age of the tephra was estimated using the continuous record in sediment of paleomagnetic reversals for the last 6 Ma and paleoclimatic proxies (delta O-18, ice-rafted debris) for the last 1 Ma. Major element and normative compositions of glasses were used to assign the sources of the tephra to the rift and off-rift volcanic zones in Iceland, and to the Jan Mayen volcanic system. The tholeiitic central volcanoes along the Iceland rift zones were steadily active with the longest interruption in activity recorded between 4 and 4.9 Ma. They were the source of at least 26 eruptions of dominant rhyolitic magma composition, including the late Pleistocene explosive eruption of Krafla Volcano of the Eastern Rift Zone at about 201 ka. The central volcanoes along the off-rift volcanic zones in Iceland were the source of at least 19 eruptions of dominant alkali rhyolitic composition, with three distinct episodes recorded at 4.6-5.3, 3.5-3.6, and 0-1.8 Ma. The longest and last episode recorded 11 Pleistocene major events including the two explosive eruptions of Tindfjallajokull volcano (Thorsmork, ca. 54.5 ka) and Katla volcano (Solheimar, ca. 11.9 ka) of the Southeastern Transgressive Zone. Eight major explosive eruptions from the Jan Mayen volcanic system are recorded in terms of the distinctive grain-size, mineralogy and chemistry of the tephra. The tephra contain K-rich glasses (K2O/SiO2 > 0.06) ranging from trachytic to alkali rhyolitic composition. Their normative trends (Ab-Q-Or) and their depleted concentrations of Ba, Eu and heavy-REE reflect fractional crystallisation of K-feldspar, biotite and hornblende. In contrast, their enrichment in highly incompatible and water-mobile trace elements such as Rb, Th, Nh and Ta most likely reflect crustal contamination. One late Pleistocene tephra from Jan Mayen was recorded in the marine sequence. Its age, estimated between 617 and 620 ka, and its composition support a common source with the Borga pumice formation at Sor Jan in the south of the island. (C) 2001 Elsevier Science B.V. All rights reserved.