Detrital zircon resolve longevity and evolution of silicic magmatism in extinct volcanic centers: A case study from the East Fjords of Iceland
Résumé
Breiðuvík and Kækjuskörð are two neighboring extinct eruptive centers in
the East Fjords of Iceland. Together, they compose the second-largest volume of
silicic rock in the country (after Torfajökull, an active volcanic system in southern
Iceland). We use ages and compositions of detrital zircon collected from two riv-
ers, the Stóraá and Krossá-Kækjudalsá, to investigate the origins and longevity
of silicic magmatism at Breiðuvík-Kækjuskörð. Zircon populations from the two
catchments have identical median U-Pb dates (12.9 Ma), O isotopes (d
18
O Vienna
standard mean ocean water = 3.1‰ versus 3.3‰), and Hf isotopes (
İ
Hf
= 14.7). We
interpret coherence of zircon elemental and isotopic compositions to indicate
that a signifi
cant volume of relatively uniform silicic material was produced in
close temporal and spatial proximity between 11.2 ± 0.7 Ma and 15.0 ± 0.9 Ma (all
errors are 1
ı
), dominated by assimilation–fractional crystallization processes. To
test the robustness of this longevity estimate, we applied Monte Carlo modeling
to the Breiðuvík-Kækjuskörð detrital zircon results and found the age span to be
statistically resolvable at ³2.8 m.y. While this lifespan is comparable to those of
large mafi
c-silicic volcanic systems that have described in other settings glob-
ally, it is the longest reported estimate for any Icelandic volcano, where typical
longevity is thought to be ~0.5–1.5 m.y. The ³2.8 m.y. lifespan we present for
Breiðuvík-Kækjuskörð is a conservative assessment, because the dates used in
this study only represent the zircon-saturated period of magmatic activity. This
study demonstrates that detrital zircon analysis of volcanigenic sediment pro-
vides an effi
cient and powerful tool that can illuminate histories of zircon-satu-
rated magmatism at targeted volcanic centers and systems. This approach can be particularly valuable in dominantly mafi
c provinces where silicic material is
subordinate (e.g., ocean islands, fl
ood basalt provinces), where glaciation, ero-
sion, or alteration has transformed the landscape, or in areas that are inaccessi-
ble (e.g., obscured by glacial ice)