Thirteen million years of silicic magma production in Iceland: Links between petrogenesis and tectonic settings

Abstract

The origin of the Quaternary silicic rocks in Iceland is thought to be linked to the thermal state of the crust, which in turn depends on the regional tectonic settings. This simple model is tested here on rocks from the Miocene to present, both to suggest an internally consistent model for silicic magma formation in Iceland and to constrain the link between tectonic settings and silicic magma petrogenesis. New major and trace-element compositions together with O-, Sr- and Nd-isotope ratios have been obtained on silicic rocks from 19 volcanic systems ranging in age from 13 Ma to present. This allows us to trace the spatial and temporal evolution of both magma generation and the corresponding sources. Low δ 18O (< 5‰ SMOW) in most silicic rocks results from partial melting of hydrothermally altered metabasaltic crust, whereas low Ba and Sr concentrations (down to 9 and 1 ppm, respectively), in addition to Th concentrations higher than 9 ppm in the silicic rocks, indicate an important role of fractional crystallisation during the final stage of magma formation. Trace-element ratios, such as Th/U, Th/Zr and Th/La, record important role of accessory minerals during final differentiation. The 143Nd/ 144Nd proves to be an excellent marker of the silicic magma source: high 143Nd/ 144Nd (0.51303 to 0.51296) characterizes a “rift-zone source”, from which the silicic magmas are generated by crustal anatexis, whereas low 143Nd/ 144Nd (0.51290 to 0.51297) is typical of an “off-rift source”, where rhyolites formed by fractional crystallisation of mantle-derived basaltic magma in a cooler environment far from the rift zone. The spatial and temporal distribution of samples having a “rift-zone” Nd-isotope signature allows inferences to be drawn about the past tectonic regime. At Snæfellsnes Peninsula, silicic magmas younger than 5.5 Ma have an “off-rift” Nd-isotope signature, whereas those older than 6.8 Ma have typical “rift-zone” characteristics. This suggests that before 6.8 Ma silicic rocks were generated in a rift zone by crustal melting caused by high geothermal gradient. But later than 5.5 Ma they were produced in a flank zone environment by fractional crystallisation alone, probably due to decreasing geothermal gradient, of basalts derived from a mantle source with lower 143Nd/ 144Nd. This is in agreement with an eastwards rift-jump, from Snæfellsnes towards the present Reykjanes Rift Zone, between 7 and 5.5 Ma. In the South Iceland Volcanic Zone (SIVZ), the intermediate Nd-signature observed in silicic rocks from the Torfajökull central volcano reflects the transitional character of the basalts erupted at this propagating rift segment. Therefore, the abundant evolved rocks at this major silicic complex result from partial melting of the transitional alkaline basaltic crust (< 3 Ma) generated in this propagating rift segment, suggesting rapid crustal recycling there. Improved understanding of silicic magmatism in Iceland can, therefore, be used for deciphering past geodynamic settings characterized by rift- and off-rift zones resulting from interaction of a mantle plume and divergent plate boundaries.