Geoscience Reference
In-Depth Information
isolated crystals and/or colours in the glass) become aligned
with the direction of fl ow. In the fi eld the clearest manifestation
of this may be simply in the way the rock fractures or weathers.
Look out for fi amme to distinguish pyroclastic rocks from fl ow-
banded lavas. You should be on the look out for fl ow banding
in plutons too, especially granite that may have been emplaced
as a crystal-rich mush, although it is surprisingly rare.
7
Cumulate layering
This was once thought simply to result from crystals settling to
the fl oor of a magma chamber, but it has been shown that
similar textures can result from crystal growth against magma
chamber walls. Occasionally, cross-stratifi cation is evident.
Cumulate layers occur on a centimetre scale (Figure 7.21) and
may be evidenced by variations in mineral abundances. For
example, in a gabbro alternating layers of 60% : 40% versus
40% : 60% plagioclase and pyroxene is called 'ratio layering'.
The appearance and disappearance of a particular mineral
phase is called 'phase layering'. An example of the latter would
be a gabbro having alternating layers with and without olivine.
Cumulate layering can be recorded by means of a graphic log
(Section 6.3), paying attention to whether changes are gradual
or sharp (e.g. Worked Example 7.3).
Other attributes of mineral texture can provide insights on the
sequence of crystallization. Isolated crystals larger than the
groundmass are probably phenocrysts that began to grow early
(perhaps during magma storage). When one crystal encloses
Figure 7.21
Cumulate layering
in a gabbro in the Oman
Ophiolite, Arabia. Dark olivine-rich
layers with sharp bases gradually
become more felsic upwards,
repeated in a scale of a few
centimetres. On this weathered
surface, the felsic layers are
reddened. As can be seen from the
orientated compass-clinometer, in
this example the layers strike
approximately 060° and dip
steeply northwards. (David A.
Rothery, The Open University, UK.)
