Metamorphosed Dolerites in the Gneiss Area of Sunnmøre on the West Coast of Southern Norway By
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NORSK GEOLOGISK TIDSSKRIFT 30. METAMORPHOSED DOLERITES IN THE GNEISS AREA OF SUNNMØRE ON THE WEST COAST OF SOUTHERN NORWAY BY TORE GJELSVIK Contents. Abstract .... ..................................... ................ 33 Introduction .. .. .. ... ...... ... ........... ... 34 Acknowledgement .............................................. 34 Geogrnrphioal re}ations .......................................... 34 Earlier investigations in the area ................................ 35 General field relations .............................................. 36 Geological settin,g .............................................. 36 Distribution of doleri:te,s ........................................ 39 Fiel d rel ations of dolerites ...................................... 40 Petrography ...................................................... 41 The doleribes .................................................. 41 Mineral constituents ........................................ 41 Common features 42 Dolerite varieties .. 42 Met3Jmorphism . 48 Dolerite dikes . 58 Granitic dikes and pegmatite in dolerites . 59 Leuco-granodioritic dikes . 59 Gr anite pegmatite . 59 Dolerites of uncertain or igin . 61 Additional remarks on the mineralogy of dolerites . 63 On the methods of mineral identification of extrusive and hypa- byssa! rocks 63 Methods used in mineral identification in this paper .......... 69 Primary minerals .......................................... 70 Secondary minerals ........................................ 76 Chemical composition .............................................. 79 Bulk analyses 79 Trace elements . 84 Petrological interpretation . 86 Igneous phase . 86 Age relations of miner3Jls . 86 Mechanism of intrusion . 87 Temperature of cr ystallization . 83 Reaction between magma and wallrock . 89 Differentiation of magma . 90 Genesis of the titaniferous iron or es of dolerites 96 Norsk geo l. tidsskr. 30. 3 TORE GJELSVIK Metamor.phic .phase . 98 Exsolution 98 The coronas . 100 Other metamor.phic phenomena . 103 Metasomatism of dolerites . 104 Metamor.phicmineral facies . 105 Comparison with other dolerite provinces . 107 Geochemtcal aspect ............................................ 107 Comparison with the hyperites of SE Norw ay . 112 ClassificaJtion and nomenclature . 115 Age of dolerite intrusion . 118 The consanguinity of the Sunnmøre dolerites and eclogites . 123 Sum11nary of conclusions 125 Text figures. lllustrations. Fig. l. Location and ,geological setting of the investigated area . 37 » 2. Dis,tribution of dolerites within Sunnmøre . 39 » 3. Replacement and coronas of olivine . 50 >> 1. Amphibolitization of augite edges . 51 » 5. Coronas and concentric alteration of hypersthene . 52 » 6. Coronas of augite and hyper,sthene . 53 » 7. Coronas in olivine-free do1erites . 54 » 8. Advanced metamorphism of olivine dolerite . 56 » 9. End stage of metamor.phi,sm of dolerites . 57 » 10. Alteration of dolerite along granite pegmatite . 61 » 11. Optical propcrties of olivines . 73 » 12. Variation diagram of dolerites . 8:l » 13. Ore concentrations of dolerites 97 Plate figures. Fig. l. Dolerite di!k e, Ellingsøy » 2. Texture of olivine dolerite » 3. Coarsegrained dolerite » 4. Weathered surface of olivine dolerite » 5. Idiomor.phic olivine crystals >> 6. Pseudoporphyric hypersthene crystal » 7. Intergrowths of augite and hyperstene )) 8. -»- --»- » 9. Dolerite peg>matite » 10. Olivine coronas » 11. Secondary minerals in .plagioclase » 12. Replacement of olivine, early stage » 13. -»- later stage >' 14. Amphibolitization of augite edges » 15. Coronas and concentric alteration of hypersthene » 16. Coronas of augite and hypersthene » 17. Advanced metamorphism of dolerite » 18. Intergrowth of plagtoclase and diopside » 19. Replacement of iron ores METAMORPHOSED DOLERITES Fig-.20. End stage of metamorphism of dolerites » 21. Finegrained dike in dolerite, Haramsgrend » 22. Granite pegmatite in dolertte, Ellingsøy » 23. -»-- -»- » 24. -»- -),)- » 25. Porphyric olivine dolerite. Uncertain locality l) 26. Clouded plag-ioclase » 27. "Reaotion" rims of orthoclase » 28. Olivine cr ystals imbedded in augite Tables. Table l. Optical pr operties of olivines .. {2 » 2. Analy;sis of spinel . 76 » 3. Chemical analyses of Sunnmøre dolerites . 79 » 4. Nor·mes and modes -»- . 80 » 5. Modes of dolerites not ohemically analyzed . 81 » 6. Tr ace elements of Sunnmøre and Gudibrandsdaldole rites . 85 » 7. Calculation of the differentiation of dolerites . 92 » 8. Average chemical composition of various basaltic pr ovinces .. 108 » 9. Averag-e chemi·cal composition of basaltic provinces of Norway and Sweden . 109 » lO. Tr ace elements of various basaltic pr ovinces . lll » l l. Comparison betw een dolerites of Sunmøre and hyper ites of SE N or w ay ................. ............................. 113 Abs t r a c t. Dolerites o.f olivine basaltic composition occur in the northern gneis•s area of Sunnmøre (also farther N and NE ).Tw o main types ar e found; both rich in plagioclase, namely subophitic olivine dolerites, fine to medium grained, occasiona1ly with (optically anomalous) hypersthene. and subdoleritic, olivine-free dolerites, medium to coars e gr ained, usually carrying- small amounts of (optically normal) hypersthene. '!1heshape of the dolerite is invariably that of a symmetrical laccolith, concordantly enveloped by gneiss. Althoug-h a few minor, tr ansg-ressive structures ar e found, neither crosscutting feeding channels nor stoped block s of the wallrock ar e observed. Ghemically, the dolerites ar e characterized by high Al, relatively low Ca, and very low H20. Of the tr ace elements, V is abundant, while Cr is pr actically wbsent. Af ter consolidation, large•scaled reactions betw een thepyrogenic mine rals took place, and a .series of secondary minerals were form ed: br onzite, diopside, garnet, biotite, amphibole, etc., initially in a peculiar corona pattern. The final pr oducts of metamorphism ar e eclogite-like, amphibolitic rock s with cr Y"stalloblastic texture. The pr imary and secondary stag-es of the genetic history of the dolerites are discussed. A comparison with other dolerite provinces discloses a rather close similarity with the hyperites of SE Norw ay and SW Sw eden. Finally, the age pr oblem of the intrusion is considered, and the cons•ang-uiruity of the Sunnmøre dolerites and eclogites is shortly discussed. A summar y of the petrological discussions is pr esented on pp.125-131. 34 TORE GJELSV!K lntroduction. Acknowledgements. The author is greaHy indebted to Professor, Dr. T. F. W. Barth, Director of the Mineralogical-Geological Museum of the University of Oslo. Dr. Barth has taken a continuous interest in the investigations of Sunnmøre, and has made all facilities of the Museum available for the author. During the work, the author has profited from many discussions with Dr. Barth and Dr. H. Neumann, and from communication with Dr. E. S. Larsen jr., Professor Emeritus from Harvard University, Cambridge, Mass., U. S. A. The author's expression of sincere gra titude to them does not implicate that they share the responsibility for the views expressed in the paper. The author has received substantial financial support from "Width Endresens Fond til Fremme av Sunnmøres Næringsliv" and · Fondet "Nationalgaven til Chr. Michelsen", and the Roya,l Norwegian Council for Scientific and Industrial Research. The following persons are to be thanked for valuable assistance of different kinds: Professor, Dr. I. Oftedal, Mineralogical Institute of the University of Oslo. Civil engineers B. Bruun and S. Rutlin, Statens råstofflaboratorium, (chemical and spectrochemical analyses). Miss B. Mauritz (micro-photography). Miss B. Bjerknes, Mrs. H. Hjort, and Miss J. Spange (illustrations). Mr. A. Granli and Mr. E. Fjellet (preparation of thin sections). Mr. A. Iversen (preparation of polished sections). Mrs. Fredrik Barth ( correction of language). Geographical relations. Most of Sunnmøre consists of a rather rugged mountain region between 3000 and 6000 feet a.s. The highest peaks reach 6000 feet, and the mountain plateau gently slopes down to 1000-2000 feet towards the coast. A great number of narrow fjords and valleys are cut deeply and steeply into the mountain region. On the islands and along the coast a low plateau - the strandflat, is formed. The fiords and valleys owe their present shape to the erosion of the great inland METAMORPHOSED DOLERITES 35 ice wihich covered Scandinavia in Pleistocene time. The moving ice, no doulbt, mostly folilowed old river valleys, and many O'f the fiords resemble drowned valleys. These old vaNeys, again, follow the pattern determined by the main structures of the :gneisses: east-west strike and transverse fractures. The result of the work of the eroding agents and later organic life is a country of great lbeauty: blue fiords and green valleys surrounded by mgged mountains, the peaks of which are covered by snow a .great part of the year. Some of �he most famous fiords of Norway are situated here, the biggest being Nordfjord, Hjørundfjord, Storfjord and Romsdals[jord. Tthis beautitul country, however, also gives the scientist trying to investigate the geology of the area, a very exhausting and difficult job. Many o[ the mountainslopes are almost vertical, at !east from a practica1l poin1 of view. The high mountains also bring about rather heavy rainfalls of the humid winds of the Atlantic Ocean, even at summertime long spells of dry weather are rare. The plentiful rain, too, gives rise to a rather luxuriant