Geoscience Reference
In-Depth Information
Overall grain size tends to increase with increasing
metamorphic grade, and is a good general guide for this (e.g.
gneisses are typically coarser than schists; Figures 8.16-8.20).
However, high strain can commonly
reduce
grain size (e.g. in
mylonites). Moreover, some grains (typically of one particular
mineral) may grow much larger than others, becoming
porphyroblasts in a fi ne matrix (e.g. Figure 9.1). To determine
metamorphic grade in the fi eld, therefore, you need to consider
mineral assemblages (Section 9.3.2) as well as grain size.
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9.2.3 Reaction textures
Textures recording reactions are valuable for piecing together
the metamorphic history of a rock; some common examples are
outlined below.
Pseudomorphs
Pseudomorphs are aggregates of new mineral grains that have
formed by alteration or replacement of a pre-existing
porphyroblast. In many cases, the aggregate retains the form of
the original crystal precursor, which may be identifi ed as a
result (Figure 9.6a). New minerals within the aggregate may be
identifi able using the hand lens. If either the original mineral
or its replacement can be identifi ed, the changes in pressure (P)
and/or temperature (T) conditions may be inferred.
Coronas
Coronas may indicate partial replacement of a mineral by
another, or represent a rim of new mineral formed at the
interface between two others (Figure 9.6b).
Figure 9.6
Reaction textures in
the fi eld. (a) Pseudomorphs of
fi ne-grained white mica and
clinozoisite (Ca,Al silicate) after
lawsonite in a retrogressed
blueschist, Switzerland. The
lawsonite is inferred partly from
crystal shape (rhombs). Largest
rhombs are 5 mm across. (b)
Corona texture in an eclogite,
Switzerland. Thin red coronas of
garnet separate white
pseudomorphs of talc (after olivine)
from pale bluish zoisite (after
plagioclase) and green
clinopyroxene. (a and b: Tom W.
Argles, The Open University, UK.)
(a)
(b)
