Petrogenesis of Silicic Rocks from the Krksfjrdur Central Volcano, NW Iceland |
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| Authors: | JNASSON K; HOLM P M; PEDERSEN A K |
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| Institution: | 1Institut for Geologi, Copenhagen University, ster Voldgada 10, DK-1350 Copenhagen Denmark
2Geologisk Museum, Copenhagen University, ster Voldgade5–7, DK-1350 Copenhagen Denmark |
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| Abstract: | Calc-alkaline dacites are found in the Tertiary Kroksfjordurcentral volcano besides the more typical tholeiitic dacitesand rhyolites characteristic of Icelandic rift zones. It isclear 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 otherrocks from Iceland with comparable silica percentages. Theyare significantly depleted in the high field strength elements(HFSE, e.g., Nb, Zr, Y, and heavy rare earth elements), andsome of them contain hydrous phases or their relics. The anhydrousphase assemblage and relatively high Fe content of the tholeiiticsilicic rocks indicate generation at shallow depths (PH2o <1kb). The high Ca and Al contents and the depletion of HFSE inthe calc-alkaline dacites indicate generation by partial meltingof amphibolite facies rocks at  5–6 km depth. The generationof 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 volcanotectonicenvironment for their generation. The geothermal gradient requiredis comparable with that of regional geothermal gradients closeto the active rift zones. They were probably formed when thecentral volcano was drifting away from the rift axis and itsactivity was waning. Intrusion of basaltic magma, probably relatedto another volcanic center, mobilized the dacite magma. |
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