PETROLOGICAL STUDIES of the GEITAFELL CENTRAL VOLCANO, S.E. ICELAND JOFIANNA M THORLACIUS Thesis Submitted for the Degree Of
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PETROLOGICAL STUDIES OF THE GEITAFELL CENTRAL VOLCANO, S.E. ICELAND JOFIANNA M THORLACIUS Thesis submitted for the degree of Master of Philosophy University of Edinburgh 1991 DECLARATION I declare that this thesis is my own work except where otherwise stated. Jóhanna 11 Thorlacius [Ii ABSTRACT The Geitafell central volcano was active 6-5 my ago and involved a tholeiitic suite comparable to those of other Tertiary volcanic centres in Iceland. Geochemical evidence suggests that the volcano was located in an axial rift-zone, supplied by magmas derived from large degrees of melting in a relatively garnet-rich mantle source. The magmas are inferred to have collected and fractionated in a magma-chamber at slightly deeper crustal levels than those of the present axial rift-zone. Magmatic activity was for a long period characterised by the extrusion of relatively evolved, small volume, phenocryst-poor basaltic lavas, grading into basaltic andesites; however, early events included the mixing of basic and acid magma resulting in the production of composite units. A shorter period of intense intrusive activity followed, which saw the emplacement of less evolved, more porphyritic basalts as sheets and dykes with intermittent intrusive phases producing diorites and quartz-diorites. The extrusive rocks that may have been fed by the sheets and dykes have been removed by erosion, except for late-stage rhyolite lavas fed by relatively thick micro-granitic dykes. A phase involving the intrusion and extrusion of highly porphyritic magma was pene- contemporaneous with the rhyolitic phase; collectively these represent the last magmatic activity related to the Geitafell complex. Gabbroic intrusions of irregular (non-tabular) shape are related to the suite of lavas, sheets and dykes; they represent, to varying degrees, cumulates of plagioclase, pyroxene and FeTi-oxides ± divine. However, there is also clear evidence for the relative enrichment of plagioclase only in some extrusive and intrusive units, including gabbros. Thick doleritic dykes, which cut all other formations, are probably not cogenetic with the remainder of Geitafell suite. A sequence of medium- grained basaltic lavas, which partly bury the late-stage rhyolites, may or may not have been derived from the Geitafell centre. AKCNOWLEDGEMENTS First of all I would like to thank my supervisor, Brian Upton, for guidance, patience and flexibility during the course of this work. Gudmundur Omar Fridieifsson is sincerely thanked for forwarding the idea of this project to me originally, for invaluable introduction to the field area and for giving access to his samples and thin sections. Sveinn P Jakobsson is thanked for a stimulating discussion. I gratefully acknowledge the receipt of a FOC-award in my first year, made available through the British Embassy in Iceland. Dodie James, Godfrey Fitton, Peter Hill, Stuart Kearns, Peder Aspen and Diana Baty are thanked for their practical help at Edinburgh; as are Jan Barker, Alison Warren and Claire at the Royal Holloway and Bedford New College (London). I am especially indebted to Nick Walsh at the RHBNC for making the REE analysis of representative samples possible at short notice. Sue Wallis, Claire Linklater and Edward Follows are thanked for their gentle, good humoured support and for just being there. Everyone on the Mull 1987 excursion, especially Karen, Susan, Bev, Jean and Marie, are thanked for sharing a very good field trip. Sue Wallis cannot be adequately thanked for her help in the field and for an introduction to the EMAS data processing; access to programs written by Wallis and Winterburn and by Andy Walker is acknowledged. I am very grateful for the support and encouragement shown by my Icelandic friends in Edinburgh, with special reference to Sigga and Patrik whose intervention saved this project from extinction. Björn S Hardarson receives my sincere thanks for his contribution during the latter stages of this work and concern throughout. My family and my parents-in-law are deeply thanked for their unwavering support. Hildur and Hallfridur are thanked for their help and companionship in the field, the families at Smyrlabjärg, Hoffell and Svinafell for their assistance and hospitality, and the owners of the Krâksgil- and Vidbordsfjall huts for their generosity. Finally - I would like to thank my husband, Loftur Reimar, who alone made it all possible and worthwhile through constant love and tolerance; and who after years of support did a lot more than his fair. share of housework and babysitting while this thesis was written. And last but not least I would like to thank our little son, Loftur Sigurctur, for sharing all those sleepless nights with me. iv CONTENTS CHAPTER 1 INTRODUCTION page 1.1. The general geology of Iceland ................................ 1 1.1.1. Iceland and the Mid-Atlantic ridge..................... 1 1.1.2. The structure and stratigraphy of Iceland.............. 3 1 .1.2.1. Tertiary...................................... 5 1.1.2.2. Plio-Pleistocene.............................. 8 1.1.2.3. Upper Pleistocene ............................. 10 1.1.2.4. Postglacial (Recent) .......................... 11 1.1.3. The three rock series.................................. 12 1 .1.4. Lava types............................................. 13 1 .2. Central volcanoes ............................................. 14 1.3. The Geitafell central volcano................................. 17 1.3.1. Introduction........................................... 17 1.3.2. Stratigraphy........................................... 23 1 .3.3. Intrusive phases....................................... 29 1.3.4. Alteration............................................. 37 1.4. Recent work in Geitafell ...................................... 40 1 .4.1. Fieldwork ............................................. 40 1.4.2. Classification ......................................... 49 CHAPTER 2 PETROGRAPHY 2 .1. Introduction.................................................. 51 2 .2. Lavas ......................................................... 51 2 .2.1. Phenocrysts............................................ 51 2 .2.2. Groundmass............................................. 55 2 .3. Minor Intrusions.............................................. 57 2 .3.1. Basic ...... ............................................ 57 2.3.2. Intermediate ........................................... 64 2 .3.3. Acid................................................... 67 2 .4. Major intrusions.............................................. 68 2.4.1. Gabbros and associated rocks ......... .................. 68 2.4.1.1. Gabbros ........................................ 68 2.4.1.2. Associated leucocratic rocks.................. 81 2.4.2. Acid and intermediate.................................. 84 2.5. Pyroclastic and mixed-magma rocks............................. 87 2.5.1. Minor units............................................ 87 2 .5.2. Major units............................................ 93 CHAPTER 3 MINERAL CHEMISTRY 3.1. Feldspars ..................................................... 94 3.1.1. Lavas..................................................94 3.1.2. Minor intrusions.......................................96 3.1.3. Major intrusions.......................................97 3.1.4. Zonation 100 3.2. Pyroxene 102 3.2.1. Lavas..................................................104 3.2.2. Minor intrusions.......................................104 3.2.3. Major intrusions.......................................106 3.2.4. Al and Ti in the pyroxenes.............................108 3.3. Other phases..................................................110 3.3.1. Olivine ................................................110 3.3.2. Opaque oxides..........................................111 3.3.3. Biotite ................................................112 3.3.4. Glass...................................................112 CHAPTER 4 WHOLE ROCK CHEMISTRY 4.1. Introduction ..................................................116 4 .2. Major element variations......................................125 4 .3. Trace element variations......................................132 4.4. Inter-element variation diagrams..............................149 4.5. Chondrite normalised Incompatible element patterns............157 4.6. Rare earth element abundances (chondrite normalised) ..........160 CHAPTER 5 DISCUSSION..............................................169 REFERENCES .........................................................178 APPENDIX I Samples and locations............................... 187 APPENDIX II Modal analyses...................................... 195 APPENDIX III Mineral data ........................................ 197 APPENDIX IV Whole-rock data...................................... 208 APPENDIX V REE data............................................ 219 APPENDIX VI Analytical methods.................................. 223 VI LIST OF FIGURES, PLATES AND TABLES page Figure 1.1 Iceland and the ocean ridges......................... 2 U 1.2 The volcanic zones in Iceland........................ 2 1 .3 Tectonic map of Iceland .............................. 6 U 1.4 Geological map and profile from Geitafell ............ 21 U 1.5 Stratigraphic column ................................. 22 ii 1.6 Panoramic photograph ................................. 41 N 1.7 Field photographs.................................... 42 Plate 1 Petrography of some lavas............................ 69 2 Petrography of some lavas...........................