The petrology and chemistry of the Setberg volcanic region and of the intermediate and acid rocks of Iceland

Abstract

The tertiary to Mid-Quaternary Setberg volcano in western Iceland consists of two centres. The northern, Centre 1 is tholeiitic and defined by cone sheets, a caldera and a gabbro intrusion. The southern, Centre 2 is chemically transitional between a tholeiitic and an alkalic basalt series, and contains a cone sheet swarm, gabbroic ring features and a major granophyric cone sheet. Each centre can be related to a minor magmachamber at a depth of 2-3 km. General hydrothermal alteration around both centres has resulted in aureoles of epidotization and the development of greenstones, containing garnet in the central areas. Late-Quaternary volcanism in the Setberg area has produced an alkalic basalt succession, ranging from alkalic olivine baalts to benmoreites. This activity is related to the E-W Snaefellsnes volcanic zone and its causes are explained in terms of Icelandic plate tectonics. The tholeiitic seies of centre 1, form a differentiated sequence from olivine tholeiites to rhyolites, contaning a Ca-poor pyroxene and augite in the basic and intermediate rocks, and a large compositional interval devoid of olivine. Plagioclase is the sole feldspar phase. The transistional series of Centre 2 is olivine-bearing throughout, with augite and rare phenocrysts of orthopyroxene in the basalts. Two lineages of differentiated rocks of this series are typified either by iron-enrichment (to icelandites) or by alkali enrichment (to rhyodacites and alkalic-rhyolites). The latter contains anorthoclase and potassic oligoclase or sanidine, as well as sodic pyroxene. Rocks of the alkalic series are olivine and pyroxene-bearing, with anorthoclase appearing in the benmoreites, while phlogopite and hornblende have been found in some of the lavas. Apatite occurs as a phenocryst in members of both the transitional and alkalic series. The origin of the transitional basalt magma is discussed according to two hypotheses. Its formation is accounted for by solidification from above in a magma reservoir of great vertical dimensions, leading, to confinement of the magma to depths where orthopyroxene replaces olivine on the liquidus, resulting in a trend towards undersaturation. The chemistry of the Late-Quaternary alkalic basalts of Setberg and Snaefeilsnes is related to magma generation at greater depths than in the case of the tholeiitic basalts of the Icelandic central volcanic zone. The volume, bimodal distribution and diversity of acid and intermediate magmas in Iceland are discussed. The high proportion of acid to basic volcanic rocks in eastern Iceland is contrasted with 3-4% in the rest of the country. Two types of acid centres are identified: centres of Thingmuli type, where tholeiitic basalts are associated with low-alkalic rhyolites; and alkalic centres, containing comendites, quartz- trachytes, alkalic-rhyolites and associated alkalic basalts or transitional basalts. Peralkaline acid rocks are identified and described from Iceland for the first time. Their genesis is discussed and related to fractionation from alkalic-rhyolites and trachytes by the "plagioclase effect". No evidence has been unearthed in this study which contravenes the hypothesis that the Icelandic acid rocks are a product of fractionation from basic magma.