Crustal thermal state and origin of silicic magma in Iceland: the case of Torfajokull, Ljosufjoll and Snaefellsjokull volcanoes

摘要

Pleistocene and Holocene peralkaline rhyolites from Torfajokull (South Iceland Volcanic Zone) and Ljosufjoll central volcanoes and trachytes from Snaefellsjokull (Snaefellsnes Volcanic Zone) allow the assessment of the mechanism for silicic magma genesis as a function of geographical location and crustal geothermal gradient. The low delta~(18)O (2.4 percent) and low Sr concentration (12.2 ppm) measured in Torfajokull rhyolites are best explained by partial melting of hydrated metabasaltic crust followed by major fractionation of feldspar. In contrast, very high ~(87)Sr/~(86)Sr (0.70473) and low Ba (8.7 ppm) and Sr (1.2 ppm) concentrations measured in Ljosufjoll silicic lavas are best explained by fractional crystallisation and subsequent ~(87)Rb decay. Snaefellsjokull trachytes are also generated by fractional crystallisation, with less than 10 percent crustal assimilation, as inferred from their delta~(18)O. The fact that silicic magmas within, or close to, the rift zone are principally generated by crustal melting whereas those from off-rift zones are better explained by fractional crystallisation clearly illustrates the controlling influence of the thermal state of the crust on silicic magma genesis in Iceland.