Rit (Vísindafélag Íslendinga) - 01.06.1984, Side 65

Rit (Vísindafélag Íslendinga) - 01.06.1984, Side 65
PETROGRAPHY 61 only slightly palagonitized and the lithification process is relatively poorly developed. The rocks are therefore loose and friable. C. XENOLITHS In the chapter on classification the xenoliths were divided into three major groups: glassy xenoliths, hne-grained xenoliths and coarse-grained xenoliths. The hne-grained types are rocks belonging to the rock suite and are described in the relevant sections. The other two groups are described below in a similar manner to the rock groups. I. The glassy xenoliths Whole rock analyses: Jan 5, 142 & 149 Mineral analyses from: Jan 142 & 149 Chemically these are trachytic rocks and in one case rhyolitic. The trachytic rocks have a SÍO2 content around 63 wt. per cent, while the rhyolitic one contains 76 wt. per cent SiOz. These xenoliths occur in basaltic rocks all over the island, but in very small amounts at each place. They range from about 50 to almost 100 per cent colourless, occasionally highly vesicular glass. The great amount of vesicles gives the rock a frothy or spongy texture and they are accordingly extremely friable. Kirk (1953) concluded that these rocks were remelted earlier solid rock trachytes and not differentiation products of the basic magma they occur in. Jan 5, trachytic in composition, occurs in the lowest horizon of the tephra fallen from the 1970 eruption cloud. It is composed of almost 100 per cent pumiceous clear glass. The only minerals are microlites of feldspar. Mixing of this clear glass and the dark glass of the basaltic tephra is extremely limited but a very narrow zone of light brown glass exists between the two glasses at their contacts. Jan 142, trachytic in composition, is found in Fugleberget, the same locality as the glass inclusion analysed and studied by Kirk (op.cit.). It is much more crystalline than Jan 5 and consists of about equal amounts of feldspar and clear vesicular glass (Fig. 46). The feldspar is mostly an alkali feldspar, Or59_54 (An2). An oligoclase feldspar (An28-ig) occurs as well but in much smaller amounts. Small amounts of iron-titanium oxides occur and occasional small zircon grains. The feldspar appearance and compositions are reminiscent of the syenitic xenoliths described below. The glass is homogeneous in composition. Jan 149, from Turvassdalen, is rhyolitic in composition and is composed of about equal amounts of slightly vesicular clear glass and granular quartz grains. Small amounts of iron-titanium oxides occur as in the previous sample. Occasional grains of zircon also occur. The quartz grains are
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Rit (Vísindafélag Íslendinga)

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