Abstract
LONDON. Geological Society, Mar. 7.—Hilda K. Cargill, L. Hawkes, and J ulia A. Ledeboer: The major intrusions of south-east Iceland. The main plutonic intrusions into the Tertiary plateau-basalts of Iceland were discovered some forty-five years ago in the south-east of the island, but little is known of their field-relations or petrology; the present paper summarises the results obtained on summer visits to the more accessible localities. The outcrops are scattered; the largest one (the Slaufrudal Stock) is elongate in plan, and its area covers 1 × 4f miles. The relationship of intrusives to country-rocks is a discordant one, the intrusions being stocks with steepsided walls and domed roofs (not laccoliths as formerly suggested). The elongation of the stocks is parallel to the strike of the regional dykes, and intrusion clearly took place under, and was facilitated by, crustal tension. All intrusions are multiple: the common association is that of gabbro and granophyre. A horizontal layered structure of granite and granophyre is visible in the Slaufrudal stock, which seems to have grown by the injection of successive sills with intermittent subsidence of the replaced block. The rocks belong to the calc-alkaline suite, and comprise in the order of differentiation gabbro-peridotite, gabbro, diorite, granodiorite, granophyre, granite, quartz-vein. Eight new analyses are given. The suite, with the addition of granodiorite, is similar to that of the main plutonic intrusions of Tertiary age in Scotland. Comment is made on the absence of alkaline types in a region of ‘Atlantic’ tectonics. In the whole Icelandic area intermediate rocks are relatively unimportant in bulk, the extrusives are dominantly basic, and the intrusives dominantly acid: this may be related to the superior mobility of the basic magma. The absence of a sedimentary ‘floor’ in Iceland is noted, and it is suggested that the preservation of the Iceland-Faeroes remnant is due to the intrusion beneath it of an acid magma.
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Societies and Academies. Nature 121, 521–523 (1928). https://doi.org/10.1038/121521a0
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DOI: https://doi.org/10.1038/121521a0