Structure and petrochemistry of the Reykjadalur central volcano and the surrounding areas, Midwest Iceland

The main geological feature of the research area is a Tertiary flood basalt pile, about 2.7 km thick, within which are three Tertiary central volcanoes. The "basement" consists of faulted and tilted basalts, probably about 12-13 m.y. old. These basalts were overlain by the Hredavatn sedimentary horizon, 7 m.y. ago, followed by lavas of the Hallarrauli central volcano which was active from 6.7 to 6.0 m.y, ago. The time sequence of the volcanism was of acid ignimbrites, then intermediate flows, and finally thin tholeiite flows. The volcano did not evolve the structural or geothermal characteristics usually associated with central volcanoes. The Reykjadalur central volcano became active about 5.8 m.y. ago and is situated on a series of thick tholeiite flows which overlie unconformably the Hallarrauli central volcano. Voluminous acidic and some intermediate rocks formed first and were succeeded by thin tholeiite flows. A collapse caldera, 10 km in diameter, then formed and was filled with volcanics. After the formation of the caldera, a basic to acid cone sheet swarm, about 20 km in diameter and concentric to the caldera, intruded the country rocks. About 4.3-4.4 m.y, ago, soon before the volcano became extinct, a massive icecap covered the volcano and its environs; the Holtavorduheidi sedimentary horizon covered the area east and southeast of the volcano at this time. The Laugardalur central volcano consists of a caldera and its volcanic filling; it may be parasitic to the Reykjadalur central volcano. Later, 3.4 m.y, ago, acid tephra was erupted from the Litla Baula vent and the Baula acid intrusion was emplacedat this time. Volcanic activity was renewed about 2 m.y. ago, represented by the Snjofjoll series and a few more recent volcanics. The Tertiary basalts are tholeiitic. The rocks from the central volcanoes show a trimodal volume distribution, with basalts and rhyolites more abundant than rocks of intermediate compositions. Three distinct breaks occur in the chemical properties : between the olivine tholeiites and the quartz tholeiites; between the quartz tholeiites and the basaltic icelandites; and within the dacites. The basalts were derived from the Upper Mantle, olivine tholeiites being produced by a greater degree of partial melting and/or at greater depths than the quartz tholeiites. On volume and time relations and on geocheraical grounds, the acid and intermediate volcanics are considered to be derived by two-stage melting of the Icelandic lower crust. The acid magmas were generated first, followed by intermediate magmas from partially melting the residuum at a higher temperature. Although this process is fuelled by basaltic intrusions within the lower crust, the late appearance of basalts in the central, volcanoes is attributed to the density-depth relations between the crust and the rising magma.