Page 236 - Microtectonics

226 7 · Porphyroblasts and Reaction Rims

Fig. 7.53. Part of adjacent hornblende (at left) and garnet (at right) porphyroblasts in garnet-hornblende schist. The garnet has a sharply
defined inclusion free idiomorphic rim, indicating changing conditions during its growth. S is more fine-grained then the matrix. The
i
inclusions in the hornblende porphyroblast are similar in orientation and shape to those in the garnet, suggesting the contemporaneous
growth of both minerals. Note the presence of secondary white mica along the contact of garnet and hornblende, suggesting a retro-
grade reaction consuming hornblende. Kittelfjäll. Västerbotten, Sweden. Width of view 4 mm. Polars nearly crossed
7.8 7.8 Reaction rims form in several metamorphic settings
Reaction Rims in response to both retrograde and prograde metamor-
phic reactions. Substitution of andalusite by sillimanite
7.8.1 is obviously prograde, while most coronas and symplec-
Introduction tites are retrograde. Reaction rims form because progress
of reactions generally depends on the presence of a fluid
A change in metamorphic conditions can give rise to por- phase along grain boundaries for the transport of ions
phyroblast growth, or to partial replacement of some to and from the reaction site. Reaction rims are most
minerals by others. Such replacement usually occurs common in high-grade rocks, eclogites and ultramafic
along grain boundaries and causes development of reac- rocks. This is probably due to the limited availability of
tion rims (Fig. 7.54, ×Video 7.54ad). Reaction rims are aqueous fluids in these rocks, which inhibits reactions
invaluable tools in the reconstruction of a sequence of to reach equilibrium on the scale of a thin section.
metamorphic reactions. They can be monomineralic A special type of reaction rim is formed by chemical
or polymineralic and can be divided into several geo- zoning within a mineral. This may be visible by a gradual
metric types. If they form closed rings around grains or abrupt change of the extinction angle or of the colour
(shells in three dimensions), they are known as coronas of pleochroic minerals; in other minerals it may only be
(Fig. 7.54c–f, ×Videos 7.54ad, 7.54e,f). Monomineralic detectable with a microprobe. Zoning can form in at least
coronas are also known as moats (Figs. 7.54c, 7.55, two different ways: (1) as growth zoning reflecting chang-
×Video 7.54ad, ×Photos 7.54a,b, 7.55a,d); polymineralic ing P-T conditions during growth, and (2) as reaction
ones composed of an intergrowth of small elongate new zoning in a pre-existing crystal by an ion-exchange reac-
grains are known as symplectitic coronas (Figs. 7.54d, tion along the rim. Reaction zoning of an element A usu-
7.57–7.61, ×Video 7.54ad). The structure of lamellar or ally follows the outer rim of a zoned crystal and may
vermicular fine-grained intergrown material is known advance further into the crystal, where it is in contact
as symplectite (Figs. 7.54d, 7.57–7.61, ×Video 7.54ad). with another mineral rich in A, or along fractures. Growth