Náttúrufræðingurinn

Ukioqatigiit

Náttúrufræðingurinn - 1963, Qupperneq 27

Náttúrufræðingurinn - 1963, Qupperneq 27
NÁTTÚRUFRÆÐINGURINN 21 SUMMARY On inclusions in Icelandic Rocks (Preliminary Report) by Jón Jónsson State Electricity Authority, Reykjavík. The object o£ this report is to draw attention to the existence of coarse grained inclusions of foreign origin found in the fine grained Icelandic vol- canics. Xenoliths are previously known from several localities in Iceland. They liave mostly been found in pyroclastic rocks. Here the author gives a brief description of inclusions found in lavas in 18 different localities in Iceland. (For the principal localities see the map fig. 1). The inclusions are light coloured contrasting witli the usually dark or black basaltic material. They contain plagioclase, clinopyroxene and/or olivine. A number of inclusions liave been examined in thin sections but a detailed minerological study is in preparation. The inclusions are found in basaltic lavas of late Tertiary age, interglacial doleritic lavas, palagonite tuffs and in recent lavas. They liave, however, not been found so far in the older Tertiary flood basalts. Near Reykjavik there are at least 5 different lava flows named Hólms- hraun (hraun = lava flow), which are according to a C14 dating less tlian 5300 ± 340 years old. In one of these flows, Hólmshraun II the second oldest, inclusions are founcl in great number. Average diameter is 3—5 cm, but in- clusions up to 10 cm have been observed. Most of the inclusions in Hólmshraun II are vesicular just as the lava itself. Tlie largest inclusions show, however, a compact core witli gabbroic. appearance. Principal mineral constituents are plagioclase, clinopyroxene and olivine cf. Table I 4. For comparison the mineral composition of the Hólms- hraun II lava: Plagioclase 44,5, Pyroxene 43,2, Olivine 7,6 and opaqc material 4,7. To the unaided eye tlie contacts between dark lava and light inclusion mostly are sharply defined, under the microscope, however, it is more or less gradational. At the contact plagioclase crystals frequently are heavily corroded, indicating nonequilibrium with the surrounding melt. The transition zones between lavas and inclusion seern to indicate the start of thermal breakdown of the rock. In an inclusion specimen from Hólmshraun II, refractive indices of plagioclase cleavage fragments were determined in immersion liquids. nDZ ranges frorn 1,570 to 1,576, An65—An77 t.c. labradorite and bytonite. In phenocrysts of the lava itself nDZ ranges from 1,566 to 1,569. It is to bc stressed, however, that so far only few measurements liave been done. The plagioclase of the same inclusion is polysyntetically twinned, but zon- ing has not been observed. In the plagioclase phenocrysts o£ the lava zoning is frequently observed. Hour glass structure is common in the pyroxene pheno- crysts of the lava but has not been observed in the inclusions. All the inclusions hitherto found have nearly the same composition (Table I).

x

Náttúrufræðingurinn

Direct Links

Hvis du vil linke til denne avis/magasin, skal du bruge disse links:

Link til denne avis/magasin: Náttúrufræðingurinn
https://timarit.is/publication/337

Link til dette eksemplar:

Link til denne side:

Link til denne artikel:

Venligst ikke link direkte til billeder eller PDfs på Timarit.is, da sådanne webadresser kan ændres uden advarsel. Brug venligst de angivne webadresser for at linke til sitet.