| Summary: | The Breiðdalur volcanic system, eastern Iceland, comprises of a ~600 km3 central volcano, a 30 - 40 km long dyke swarm and plateau basalts erupted from this dyke swarm. The entire volcanic system was active from 10.1 to 7.8 Ma, the central volcano was only active from 10.1 to 9.1 Ma and the formation of the dyke swarm appears to be initiated at 9.9 Ma and continued for a further 1.3 Ma. The central volcano is predominantly basaltic (80-85 %) and basaltic lava formed the broad flanks of the volcano between 10.1 Ma and about 9.5/9.6 Ma. More evolved lavas of basaltic icelandite to icelandite composition also erupted onto the flanks during this time, a 400 m thick sequence of icelandites formed on the south east flank of Breiðdalur from 9.9 to around 9.7 Ma. These early-formed intermediate lavas appear to have formed through fractional crystallisation of basaltic magma, resulting the formation of a crustal magma storage zone/chamber below the volcano. At around 9.6 to 9.5 Ma silicic magmatism began at Breiðdalur volcano, one of the earliest silicic events was a caldera forming event. This created the depression of Breiðdalur caldera, which was infilled by silicic pumice agglomerate, and further deformed by subsequent eruptions. This early silicic magma was most likely formed via partial melting of hydrated basaltic crust and drove the explosive volcanism. The Breiðdalur caldera was 8 to 10 km wide and around 500 m deep, it is infilled with the products of silicic explosive eruptions, silicic effusive eruptions and mafic to intermediate late-stage intrusions. The unconsolidated deposits of the caldera infill were, at the time, extensively reworked by water and it featured caldera lakes, as evident from sequences of lake sediments containing some trace fossils. The caldera in-fill sequence is overlain by the Summit Group, a sequence of horizontal to sub-horizontal lavas, ignimbrite, tuffs and sediments that was erupted at around 9.3 Ma. At this time silicic lavas were erupted on the flanks of the volcano, most ...
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