Geoscience Reference
In-Depth Information
Figure 9.3
Interlayered
metapelites and metapsammites,
Spain. Metamorphic minerals have
grown much coarser in the pelites
(right half of view) than in the
psammites (under lens cap), where
quartz grains may even have been
reduced in size due to
recrystallization under strain. The
relative grain sizes of the original
sediments are thus reversed, and
their colour and inferred
compositions are much better
guides to their protoliths than grain
size. (Tom W. Argles, The Open
University, UK.)
9
(
(
Figure 9.4
(a) Gneissic banding defi ned by alternating felsic (pale) and mafi c (dark) bands on a centimetre
scale, northwest India. Subtler colour variation on a larger scale (5-10 bands width) suggests that the original
precursor was compositionally layered (sedimentary or volcanic). Field of view is about 5 cm across. (b) Cross-
stratifi cation in metaquartzite picked out by colour bands caused by concentrations of heavy minerals;
original younging direction is therefore to the right. Penknife at base for scale. (a and b: Tom W. Argles, The
Open University, UK.)
Distinguishing metamorphic banding from inherited
sedimentary layering may be almost impossible in some
gneisses where all sedimentary evidence has been erased. Look
for evidence of compositional variation that is distinct from the
dominant alternation of mafi c and felsic gneissic bands - for
example, in a series of pelitic or psammitic gneisses, layers
