Geological Survey of Finland Bulletin 289 - arkisto.gsf.fi

22 Geological Survey of Finland, Bulletin 289
Fig. 22. Small prisms of hornblende forming a dense network between
plagioclase borders. Note how the hornblende has grown into the pigmented
and brown-coloured plagioclase (no. 136).
sm all grains of quarz lie interspersed in the fibrous hornblende. The fibrous
hornblende often forms lighter parts in the middle with opaque pigments densely
intersperced in it. The lighter parts (Fig. 20) are usually surrounded by a dark
green rim of coarser-grained hornblende. The same appearance of hornblende is
reported by Windly (1970) in "C-type dykes" among the so-called ferro-dolerites
of western Greenland. The lightcoloured fibrous central parts seem to be alternation
products from pyroxene. In some of the broadest dykes small residues of
pyroxene can be found in the hornblende, or even whole grains of pyroxene
(Fig.21).
In general the fibrous or fine-grained hornblende extends over the rims of
the plagioclase borders (Fig. 22) and in many places there are signs of resorption
of plagioclase. The plagioclase is replaced by hornblende.
In the fine-grained chilled margins the proportions are somewhat different.
In the chilled margin against the surrounding rock the plagioclase laths He parallel
to the contacts in a flow structure (Fig. 19). The plagioclase occurs partly as
isolated grains several millimetres in length in a very fine-grained matrix of
hornblende and plagioclase. The hornblende and plagioclase in the matrix have in
places merged in a subophitic texture. The hornblende is fine-grained and consists
of large numbers of small prisms. In the chilled margin there is generally no
trace of pyroxene or of pseudomorphs. Nor do the fields of pale fibrous hornblende
with green margins described earlier exist here. Instead the majority of the available
evidence suggests that the hornblende in the chilled margins, at least for