Jökull


Jökull - 01.12.1986, Síða 35

Jökull - 01.12.1986, Síða 35
and the eastern mountain complex which includes Hrómundartindur, Hagavíkurfjöll and Katlatjarnir. The Hengill area thus probably contains two active volcanic subsystems, that of Hengill and another centered on Hrómundartindur to the east (Saemunds- son, pers. comm. 1986). Numerous porphyritic high-MgO picritic basalts have been discovered in Iceland and the possibility that such lavas may represent high-MgO Iiquids is of crucial importance to an understanding of basalt petrogenesis. In this paper the main emphasis is on the Maelifell picritic basalt which belongs to the eastern Hrómund- artindur volcanic system. By combining data from microprobe analysis with detailed petrography, I have attempted (1) to test whether or not the olivine pheno- crysts were in equilibrium with the groundmass liquid and (2) to determine the origin of the picritic basalt. the picrite basalt The Maelifell picrite basalt is from the Weichselian period (Saemundsson 1967). The sample studied was taken from pillows west of Ölfusvatnsá and corre- sponds to material extruded late in the eruption. Maelifell is composed mostly of pillow Iava, but hyaloclastites are found in the northern part of the mountain. The pillow lava has been divided into older and younger units by Saemundsson (1967). The younger unit was not considered here owing to the extensive alteration (Saemundsson, pers. comm. 1981) it has suffered. Chemical composition. Glass in the picrite basalt occurs both residually in the matrix, in the pillow nnds (quenched glass), and as inclusions in pheno- crysts. Table 1 shows the quenched glass composition °f pillow rinds, and the whole-rock composition. The Quenched glass contains much Iower Mg than the whole rock; with consequently higher percentages of other major elements. Petrography. The samples of the older pillow lavas contain olivine phenocrysts (38.6 vol.%), with micro- phenocrysts of Cr-spinel and macrophenocrysts of clinopyroxene (8.9%) in a subophitic to spherulitic groundmass composed of: pyroxene (29%), plagioclase (16.5%), olivine (0.9%), opaque minerals (6.1%; in- cluding the Cr-spinel microphenocrysts) and glass. Rare plagioclase phenocrysts (Ango) have been found (Saemundsson 1967) and are interpreted to be xeno- crysts (Grönvold & Óskarsson, pers. comm. 1983). The samples are fresh except for a few olivine grains, which are oxidized along fractures. The interstitial Table 1. Composition of quenched glass* (micro- probe analyses) in pillow rinds, whole-rock compo- sition (XRF analyses) of the Maelifell picrite basalt (taken from Hardardóttir 1983) and the calculated high-MgO liquid composition. sample glass whole- liquid rock composition Si02 47.05 45.11 46.8 tío2 1.04 0.59 0.8 ai2o3 14.83 8.67 11.7 Cr203 0.07 0.23 MgO 8.78** 25.87 18.0 FeO 9.44 8.79 8.1 Fe203 not anal. 1.29 1.1 MnO 0.17 0.17 0.17 CaO 14.20 8.68 11.6 Na20 1.65 0.84 1.14 K20 0.03 0.02 0.03 p2o5 0.04 0.02 0.03 Total 97.30 100.28 99.47 Mg/(Mg+- Fe2+) 0.62 0.84 * average of six analysis ** values range from 8.60 to 8.97 glass is devitrified, as is in part the quenched glass found in pillow rinds. Two types of olivine phenocrysts are observed: those that exhibit reverse chemical zoning (group I) and those that are normally zoned or unzoned (group II). Both types are subhedral in shape and reach up to 1 cm across. Group-I olivine is, on average, 2.2 mm across in thin section. The larger grains are character- ized by large (>500 pm) crystallized silicate-melt in- clusions up to 1.8 mm across. Numerous small inclu- sions of glass (<50 pm) are also commonly included in the group-I olivine. Euhedral zoned plagioclase con- taining devitrified, irregularly shaped inclusions of glass is occasionally included in the reversely zoned olivine phenocrysts. Primary Cr-spinel has not been found enclosed in the olivine belonging to group I with two exceptions where they were located at the rim. In two cases, bright green anhedral clinopyroxene was found moulded on (or surrounding) olivine of group I. Group-II olivine averages 2.5 mm across in thin 33
Síða 1
Síða 2
Síða 3
Síða 4
Síða 5
Síða 6
Síða 7
Síða 8
Síða 9
Síða 10
Síða 11
Síða 12
Síða 13
Síða 14
Síða 15
Síða 16
Síða 17
Síða 18
Síða 19
Síða 20
Síða 21
Síða 22
Síða 23
Síða 24
Síða 25
Síða 26
Síða 27
Síða 28
Síða 29
Síða 30
Síða 31
Síða 32
Síða 33
Síða 34
Síða 35
Síða 36
Síða 37
Síða 38
Síða 39
Síða 40
Síða 41
Síða 42
Síða 43
Síða 44
Síða 45
Síða 46
Síða 47
Síða 48
Síða 49
Síða 50
Síða 51
Síða 52
Síða 53
Síða 54
Síða 55
Síða 56
Síða 57
Síða 58
Síða 59
Síða 60
Síða 61
Síða 62
Síða 63
Síða 64
Síða 65
Síða 66
Síða 67
Síða 68
Síða 69
Síða 70
Síða 71
Síða 72
Síða 73
Síða 74
Síða 75
Síða 76
Síða 77
Síða 78
Síða 79
Síða 80
Síða 81
Síða 82
Síða 83
Síða 84
Síða 85
Síða 86
Síða 87
Síða 88
Síða 89
Síða 90
Síða 91
Síða 92
Síða 93
Síða 94
Síða 95
Síða 96
Síða 97
Síða 98
Síða 99
Síða 100

x

Jökull

Beinleiðis leinki

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

Link til denne avis/magasin: Jökull
https://timarit.is/publication/1155

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.