Jökull - 01.12.1982, Page 74

TABLE 1. Modal analyses of some Öræfajökull
rocks.
TAFLA 1. Niburstöbur steindagreiningar á nokkrum
sýnum.
Samplt no. plagio- clase olivine clino- pyroxene oxides matrix
0 166 tr tr tr 100.0
0 184 2.6 — 1.1 — 96.3
0 67 3.1 1.6 0.5 — 94.8
0 228 3.6 — 2.6 0.2 93.6
0 248 0.8 tr tr 0.3 98.9
0 267 2.1 — tr — 97.9
0 136 tr tr — — 100.0
0 98 tr — tr — 100.0
0 200 2.9 0.8 0.9 0.3 95.1
0 135 5.3 0.1 0.4 0.4 93.8
0 50 1.3 — — tr 98.7
0 261 5.1 tr tr — 94.9
— = absent as pheno- or microphenocryst
tr = trace amount (< 0.1%)
0 166 Tholeiite, Slaga
0 184 Tholeiite, Svinafell
0 67 Tholeiite, Kvisker
0 228 Tholeiite, Skaftafellsheidi
0 248 Tholeiitic icelandite, Hvannadalskambur
0 267 Tholeiitic icelandite, Breidamerkuríjall
0 136 Icelandite, Svinafellsfjall
0 98 Icelandite, Sandfellsheidi
0 200 Dacite, Hafrafell
0 135 Dacite, Svinafellsfjall
0 50 Rhyolite, Godafjall
0 261 Rhyolite, Breidamerkurfjall
phyritic varieties seem to be the most abundant
ones (Table 1). The variation in the texture - at
least partly - reflects the type of occurrence. Sub-
aerial flows and some dikes are commonly holo-
crystalline with an intergranular, subophitic or pil-
otaxitic groundmass texture, whereas most memb-
ers of the hyaloclastite units display hyalopilitic and
intersertal textures. In some places xenoliths of ba-
salt and gabbro are found.
Plagioclase (of varying size; up to 1 cm) is the
most common phenocryst mineral. The most calcic
plagioclases (accumulated crystals?) are found in
some plagioclase-rich subaerial basalts (Prestvik
1979, p. 19). These crystals are usually zoned, the
cores are commonly over Ana| in composition,
whereas the rims are labradorites (An.. ). Inother
tholeiites there is a wide range of phenocryst com-
position (Any3 ) within each sample. Reverse zon-
ing was found in one of the dikes where a core of
labradorite (An.8_.(|) is rimmed by bytow-
nite (An7Q). Microphenocrysts vary from Ang0 to
An?6, but they cluster around An63 g5 The matrix
plagioclase is usually a sodic labradorite (An__52).
In most samples subordinate amounts (< 5%) of
olivine and/or clinopyroxene phenocrysts occur to-
gether with the plagioclase. The composition of
olivine phenocrysts range from Fo86 to FoB9, where-
as microphenocrysts are considerably more fayalitic
(Fo4(_56). Phenocrysts (~ 1 mm in size) and micro-
phenocrysts (0.1-0.4 mm) ofclinopyroxene are aug-
ites varying in composition from Wo^En^5Fsg5 to
Wo+1En3gFs20 (Fig. 2). Microphenocrysts and rims
of phenocrysts are generally the less calcic and more
ferrous varieties. The Öræfajökull clinopyroxenes
are generally more calcic than pyroxenes from
Skaergaard (Fig. 2) and rather similar to those of
the transitional Bouvetoya series (Prestvik, in prep.).
This feature might indicate a transitional affmity of
the Öræfajökull series. However, the significance of
clinopyroxene composition as indication of magma
type has been questioned (Barberi et al. 1971).
Oxide minerals are scarce as real phenocrysts in
the tholeiites. Only in one sample have abundant
oxide crystals of about 1 mm in size been detected.
Microphenocrysts (0.1-0.3 mm) and matrix oxides
are found in many samples (Table 1). The oxide
composition of the tholeiites varies only slightly. All
analysed grains are titanomagnetites with a TiO,
content in the range 16-26%.
Fig. 2. Öræfajökull pyroxenes piotted in the pyr-
oxene trapezium. Lines indicate: B: Trend ofpyrox-
enes from the transitional Bouvetoya suite (Imsland
etal. 1977; Prestvik in prep.). S: Trend of Skaergaard
calcic pyroxenes (Deeretal. 1963).
Alynd 2. Niðurstöbur ejnagreininga á pyroxeniJrá Ortefa-
jökli.
70 JÖKULL 32. ÁR