Bimodal volcanism at the Katla subglacial caldera,Iceland: insight into the geochemistry and petrogenesis of rhyolitic magmas |
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Authors: | C Lacasse H Sigurdsson S N Carey H Jóhannesson L E Thomas N W Rogers |
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Institution: | (1) 5, allée des Flandres, F-54390 Frouard, France;(2) Graduate School of Oceanography, University of Rhode Island, Narragansett Bay Campus, Narragansett, RI 02882-1197, USA;(3) ISOR, Iceland GeoSurvey, Grensásvegur 9, IS-108 Reykjavík, Iceland;(4) Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA Milton Keynes, UK |
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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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Keywords: | Katla Iceland Subglacial caldera Bimodal volcanism Rhyolite Basalt Magma mixing |
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