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Generation of Icelandic rhyolites: silicic lavas from the Torfajökull central volcano

Gunnarsson, Björn and Marsh, Bruce D. and Taylor, Hugh P., Jr. (1998) Generation of Icelandic rhyolites: silicic lavas from the Torfajökull central volcano. Journal of Volcanology and Geothermal Research, 83 (1-2). pp. 1-45. ISSN 0377-0273. http://resolver.caltech.edu/CaltechAUTHORS:20130508-142342503

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Abstract

The Torfajökull central volcano in south-central Iceland contains the largest volume of exposed silicic extrusives in Iceland (∼225 km^3). Within SW-Torfajökull, postglacial mildly alkalic to peralkalic silicic lavas and lava domes (67–74 wt.% SiO_2) have erupted from a family of fissures 1–2.5 km apart within or just outside a large caldera (12×18 km). The silicic lavas show a fissure-dependent variation in composition, and form five chemically distinct units. The lavas are of low crystallinity (0–7 vol.%) and contain phenocrysts in the following order of decreasing abundance: plagioclase (An_(10-40)), Na-rich anorthoclase (<Or_(23)), clinopyroxene (Fs_(37-20)), FeTi oxides (Usp_(32-60); Ilm_(93-88)), hornblende (edenitic–ferroedenitic) and olivine (Fo_(22-37)), with apatite, pyrrhotite and zircon as accessory phases. The phenocryst assemblage (0.2–4.0 mm) consistently exhibits pervasive disequilibrium with the host melt (glass). Xenoliths include sparse, disaggregated, and partially fused leucocratic fragments as well as amphibole-bearing rocks of broadly intermediate composition. The δ^(18)O values of the silicic lavas are in the range 3.6–4.4, and these are lower than the δ^(18)O values of comagmatic, contemporaneous basaltic extrusives within SW-Torfajökull, implying that the former can not be derived from the latter by simple fractional crystallization. FeTi-oxide geothermometry reveals temperatures as low as 750–800°C. To explain the fissure-dependent chemical variations, ^(18)O depletions, low FeTi-oxide temperatures and pervasive crystal-melt disequilibrium, we propose the extraction and collection of small parcels of silicic melt from originally heterogeneous basaltic crustal rock through heterogeneous melting and wall rock collapse (solidification front instability, SFI). The original compositional heterogeneity of the source rock is due to (1) silicic segregations, in the form of pods and lenses characteristically formed in the upper parts of gabbroic intrusives, and (2) extreme isostatic subsidence of the earlier, less differentiated lavas of the Torfajökull central volcano. Ridge migration into older crustal terranes, coupled with establishment of concentrated volcanism at central volcanoes like Torfajökull due to propagating regional fissure swarms, supplies the heat source for this overall process. Continued magmatism in these fissures promotes extensive prograde heating of older crust and the progressive vitality and rise of the central volcano magmatic system that leads to, respectively, SFI and subsidence melting. The ensuing silicic melts (with relict crystals) are extracted, collected and extruded before reaching complete internal equilibrium. Chemically, this appears as a two-stage process of crystal fractionation. In general, the accumulation of high-temperature basaltic magmas at shallow depths beneath the Icelandic rift zones and major central volcanoes, coupled with unique tectonic conditions, allows large-scale reprocessing and recycling of the low-δ^(18)O, hydrothermally altered Icelandic crust. The end result is a compositionally bimodal proto-continental crust.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1016/S0377-0273(98)00017-1DOIUNSPECIFIED
http://www.sciencedirect.com/science/article/pii/S0377027398000171PublisherUNSPECIFIED
Additional Information:© 1998 Elsevier Science B.V. Received 17 April 1997; accepted 5 January 1998. We are grateful to Ole Stecher and Richard Carlson at the Department of Terrestrial Magnetism, Carnegie Institute for Sr and Nd-isotope analysis on the Torfajökull samples. We much appreciate insightful reviews by Tanya Furman, Sigurdur Steinthorsson, and Fergus Gibb. B.G. would like to thank Maria-Victoria Gunnarsson for her encouragement and valuable help in writing the initial draft of this paper as well as the initial drafting of many of the figures. Funding for this project came from the American Scandinavian Foundation to Bjorn Gunnarsson and the National Science Foundation grants EAR 86-18202, EAR 88-1734, and OPP 9418513 to Bruce Marsh and grants EAR 90-19190 and EAR 93-17036 to Hugh Taylor.
Funders:
Funding AgencyGrant Number
American Scandinavian FoundationUNSPECIFIED
NSFEAR 86-18202
NSFEAR 88-1734
NSFOPP 9418513
NSFEAR 90-19190
NSFEAR 93-17036
Subject Keywords:silicic lavas; partial melting; rift zones; crystal fractionation; magmatic system; volcano
Record Number:CaltechAUTHORS:20130508-142342503
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20130508-142342503
Official Citation:Björn Gunnarsson, Bruce D. Marsh, Hugh P. Taylor Jr., Generation of Icelandic rhyolites: silicic lavas from the Torfajökull central volcano, Journal of Volcanology and Geothermal Research, Volume 83, Issues 1–2, July 1998, Pages 1-45, ISSN 0377-0273, 10.1016/S0377-0273(98)00017-1. (http://www.sciencedirect.com/science/article/pii/S0377027398000171)
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:38364
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:08 May 2013 22:39
Last Modified:08 May 2013 22:39

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