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Bimodal volcanism at the Katla subglacial caldera, Iceland: insight into the geochemistry and petrogenesis of rhyolitic magmas

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Abstract

The Katla subglacial caldera is one of the most active and hazardous volcanic centres in Iceland as revealed by its historical volcanic activity and recent seismic unrest and magma accumulation. A petrologic and geochemical study was carried out on a suite of mid-Pleistocene to Recent lavas and pyroclastic rocks originated from the caldera. The whole series is characterised by a bimodal composition, including Fe-Ti transitional alkali basalts and mildly alkalic rhyolites. Variations in trace-element composition amongst the basalts and rhyolites show that their chemical differentiation was mainly controlled by fractional crystallisation and possible assimilation. The petrology and chemistry of the few intermediate extrusive rocks show that they were derived from magma mingling or hybridisation. The absence of extrusive rocks of true intermediate magmatic composition and the occurrence of amphibole-bearing felsic xenoliths support the hypothesis of partial melting of the hydrated basalt crust as the main process leading to the generation of rhyolites. The 143Nd/144Nd and 87Sr/86Sr values of Katla volcanic rocks fit the general isotopic array defined by late Quaternary to Recent lavas from Iceland. A few rock specimens are distinguished by low 143Nd/144Nd values suggesting assimilation and mixing of much older crustal material. Despite their similar whole-rock chemical compositions, the postglacial rhyolitic extrusives differ from the felsic xenoliths by their glass composition and the absence of amphibole. This, together with the general chemical trend of volcanic glasses, indicates that the postglacial rhyolitic extrusives were probably derived by a process involving late reheating and partial melting of crustal material by intrusion of basaltic magmas.

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Acknowledgements

The authors would like to thank M.T. Gudmundsson, G. Larsen, H. Björnsson, E. Sturkell and H. Soosalu for fruitful discussions during this research. Lacasse is very grateful to B. Bragason (“Benni”) for guiding us on snowmobiles over the Mýrdalsjökull ice cap. J. Watson, A. Tindle, P. van Calsteren, M. Gilmour (Open University), R. Gertisser (Keele University), and J. Devine (Brown University) are acknowledged for their analytical assistance. This research was partly supported by internal funds from the Open University. Sigurdsson and Carey wish to acknowledge support from the US National Science Foundation, grant OCE-9402296, for the conduction of this research in Iceland. O. Bachmann and an anonymous reviewer are acknowledged for thorough reviews of the manuscript.

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Authors and Affiliations

  1. 5, allée des Flandres, F-54390, Frouard, France

    C. Lacasse

  2. Graduate School of Oceanography, University of Rhode Island, Narragansett Bay Campus, Narragansett, RI, 02882-1197, USA

    H. Sigurdsson & S. N. Carey

  3. ISOR, Iceland GeoSurvey, Grensásvegur 9, IS-108, Reykjavík, Iceland

    H. Jóhannesson

  4. Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, Milton Keynes, UK

    L. E. Thomas & N. W. Rogers

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Lacasse, C., Sigurdsson, H., Carey, S.N. et al. Bimodal volcanism at the Katla subglacial caldera, Iceland: insight into the geochemistry and petrogenesis of rhyolitic magmas. Bull Volcanol 69, 373–399 (2007). https://doi.org/10.1007/s00445-006-0082-5

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