caa a22 4500 386401535 CHVBK 20180307112200.0 cr unu---uuuuu 161130e198911 xx s 000 0 eng 10.1017/S0016756800006956 doi S0016756800006956 pii (NATIONALLICENCE)cambridge-10.1017/S0016756800006956 Structure, petrochemistry and evolution of a sheet swarm in an Icelandic central volcano [Elektronische Daten] Strike, dip, and thickness were measured for 504 sheets (inclined sheets and dykes) in the 4-6 Ma old Hafnarfjall central volcano in southwest Iceland. The average dip of sheets is 65°, 80% are less than 1.2 m thick, and the thickness tends to decrease with decreasing dip. In 0.5 km long traverses perpendicular to the average strike of sheets, the percentage of sheets ranges from about 6 to 11. Of 140 chemically analysed sheets most are quartz-tholeiites; a few are intermediate or acid. The sheets are chemically more evolved than the host rock and were generated by a shallow crustal magma chamber at a mature stage of the central volcano, whereas the host rock was generated earlier before the chamber was established. Trace element results suggest that the sheet magmas evolved by low-pressure fractional crystallization as well as by mixing of primitive magmas and crustal melts. A model is proposed where most of the sheets are generated by a growing shallow magma chamber. As the chamber grows its shape changes, and so does the local stress field associated with it. Because the sheets follow the stress trajectories of the local stress field, the potential pathways of the sheets change with the growth of the chamber, which may explain the common occurrence of cross-cutting sheets. From the evolved chemistry of the sheets, as well as from the pattern of the stress trajectories, it is concluded that the bulk of the sheets were injected from the upper part of the shallow magma chamber. Copyright © Cambridge University Press 1989 Gautneb Håvard Nordic Volcanological Institute, University of Iceland, 101 Reykjavik, Iceland Gudmundsson Agust Nordic Volcanological Institute, University of Iceland, 101 Reykjavik, Iceland Oskarsson Niels Nordic Volcanological Institute, University of Iceland, 101 Reykjavik, Iceland Geological Magazine Cambridge University Press 126/6(1989-11), 659-673 0016-7568 126:6<659 1989 126 GEO https://doi.org/10.1017/S0016756800006956 text/html Onlinezugriff via DOI 1 research-article jats NATIONALLICENCE 856 40 https://doi.org/10.1017/S0016756800006956 text/html Onlinezugriff via DOI NATIONALLICENCE 700 1- Gautneb Håvard Nordic Volcanological Institute, University of Iceland, 101 Reykjavik, Iceland NATIONALLICENCE 700 1- Gudmundsson Agust Nordic Volcanological Institute, University of Iceland, 101 Reykjavik, Iceland NATIONALLICENCE 700 1- Oskarsson Niels Nordic Volcanological Institute, University of Iceland, 101 Reykjavik, Iceland NATIONALLICENCE 773 0- Geological Magazine Cambridge University Press 126/6(1989-11), 659-673 0016-7568 126:6<659 1989 126 GEO CC0 http://creativecommons.org/publicdomain/zero/1.0 nationallicence BK010053 XK010053 XK010000 NATIONALLICENCE NATIONALLICENCE NL-cambridge