The petrogenesis of Þingmúli volcano, East Fjords; Iceland.

In this work we revisit Þingmúli volcano (Þ = Th), a classic locality known as an example of a complete tholeiitic differentiation. Þingmúli is a ~9.5 Ma extinct central volcano located in the East Fjords of Iceland, in which the whole compositional spectrum from basalt to rhyolites have erupted. These volcanic products have been
previously considered as petrogenetically related by an ideal fractionation trend,
regardless any temporal relationship or volumetric considerations.
Here we report new whole-rock geochemistry, mineral chemistry, isotope analyses,
estimation of residence times of the different eruptive deposits, and an update of the
original petrogenetic model. Our results highlight that an enriched source, likely spinel lherzolites, generated transitional-alkaline basaltic melts after 15-20% of partial melting at depths of 40-45 km. Many of these basaltic melts erupted at various stages of the volcano’s history, while others remained longer in the volcanic plumbing system. These evolved by fractional crystallisation into basaltic andesite magmas with a residence time of ~5 years based on the crystal size distribution of the plagioclase population.
Isotopic differences between the basalts/basaltic andesites (87Sr/86Sr ~ 0.7034;
143Nd/144Nd ~0.51315) and the erupted rhyolites (87Sr/86Sr ~ 0.7037; 143Nd/144Nd
~0.51304) indicate that the latter are not petrogenetically related to the former.
Therefore, instead of a fractional crystallisation mechanism to generate the rhyolites,
we propose the partial melting of ignimbrite layers located beneath the volcano. The
broad range of trace element concentrations in andesites and dacites and their
different isotopic values compared to the basalts strongly suggest that these magmas
have been generated by magma mixing between basaltic and rhyolitic melts, similar to modern day Icelandic volcanoes such as Hekla. These results highlight the need to
revisit previously studied Icelandic classic localities and reassess their traditionally
proposed petrogenetic models.