Petrogenesis of Silicic Rocks from the Króksfjördur Central Volcano, NW Iceland

• Published in: Journal of Petrology
• Main Authors: JÓNASSON, K., HOLM, P. M., PEDERSEN, A. K.
• Format: Text
• Language: English
• Published: Oxford University Press 1992
• Subjects: Articles; Iceland
• Online Access: http://petrology.oxfordjournals.org/cgi/content/short/33/6/1345; https://doi.org/10.1093/petrology/33.6.1345

Calc-alkaline dacites are found in the Tertiary Kroksfjordur central volcano besides the more typical tholeiitic dacites and rhyolites characteristic of Icelandic rift zones. It is clear that these calc-alkaline dacites are not subduction related, but their chemistry and petrography are definitely calc-alkaline. They are much lower in Fe and higher in Ca and Al than other rocks from Iceland with comparable silica percentages. They are significantly depleted in the high field strength elements (HFSE, e.g., Nb, Zr, Y, and heavy rare earth elements), and some of them contain hydrous phases or their relics. The anhydrous phase assemblage and relatively high Fe content of the tholeiitic silicic rocks indicate generation at shallow depths (P H2o <1 kb). The high Ca and Al contents and the depletion of HFSE in the calc-alkaline dacites indicate generation by partial melting of amphibolite facies rocks at ∼ 5–6 km depth. The generation of the tholeiitic silicic rocks requires a shallow magma chamber, where they could be formed by fractionation of basaltic magma, or by partial melting of country rock heated by basaltic magma. The calc-alkaline dacites require a different volcanotectonic environment for their generation. The geothermal gradient required is comparable with that of regional geothermal gradients close to the active rift zones. They were probably formed when the central volcano was drifting away from the rift axis and its activity was waning. Intrusion of basaltic magma, probably related to another volcanic center, mobilized the dacite magma.