Page 219 - Microtectonics

7.6 · Problematic Porphyroblast Microstructures 209
7.6.3 anti-perthite. In K-feldspar, deformation of old crystals
False Inclusion Patterns may cause local enhancement of exsolution, especially
when high-grade feldspars are deformed at low grade
Some structures in porphyroblasts may resemble pat- (Sect. 3.12.4).
terns of passive inclusions, but form in another way In general, exsolution structures may be distinguished
and can be a source of error. Rutile needles in biotite or from passive inclusions by their composition and their
quartz could be misinterpreted as passive inclusions, control by crystallographic planes.
but usually lack a clear preferred orientation. Alteration
(e.g. of feldspar to sericite) or exsolution structures along 7.6.4
crystallographically controlled planes may also be dif- Mimetic Growth
ficult to distinguish from inclusion patterns. Minerals
that are solid solutions of two or more phases can Mimetic growth (Sect. 4.2.7.6) can be a problem in
show exsolution when metamorphic conditions change. porphyroblast analysis. In some cases porphyroblasts
This is especially common for minerals that crystal- of a mineral A may be mimetically replaced by a min-
ised at high temperature. During retrogression, small eral B that inherits the inclusion pattern of A and the
grains of the minor phase may form in the host crystal deflection pattern of the foliation in the matrix around
(Figs. 7.28, 7.29). The most common examples are found it. This is a potential source of error; however, in our
in feldspar (Fig. 7.28), amphiboles, pyroxenes (Fig. 7.29) experience this replacement is seldom complete. Gener-
and spinel. ally, it can be detected by the presence of relicts of
In the case of feldspars, the new phase may occur as mineral A, or by other crystals of mineral B that show
elongate grains or lamellae with a strong preferred ori- the correct microtectonic relation. This kind of (partial)
entation within the host crystal, parallel to crystallo- replacement structures can give important infor-
graphic directions. In K-feldspar, this exsolution struc- mation on the metamorphic evolution and is explained
ture is known as perthite (Fig. 7.28); in plagioclase as in Sect. 7.8.

Fig. 7.28. Ellipsoidal and flame-shaped albite lamellae in perthitic K-feldspar from a granodiorite. Two sets of ellipsoidal lamellae of differ-
ent size are present, possibly reflecting two stages of dissolution at different temperature (Sect. 9.9). The flame-shaped perthite-lamellae
result from unmixing during greenschist facies deformation. The preferred orientation (N-S) is controlled by crystallographic directions.
This microstructure should not be mistaken for passive inclusions. St. Barthélemy Massif, Pyrenees, France. Width of view 0.6 mm. CPL