Page 239 - Microtectonics

7.8 · Reaction Rims 229

Fig. 7.57. Double symplectitic corona between hornblende (left) and garnet (right). The left-hand, coarse corona consists of plagioclase and
orthopyroxene. The narrow corona at right consists of plagioclase, orthopyroxene and spinel. Notice that the narrow orthopyroxene-pla-
gioclase-spinel corona occurs only along the garnet and along cracks in the garnet, but not near hornblende. Therefore, the coarse corona is
thought to reflect a first reaction hornblende + garnet → orthopyroxene + plagioclase and the narrow corona represents a second reaction
garnet → orthopyroxene + plagioclase + spinel. In this way, the sequence of reactions can be established in a thin section. Bolingen Island,
Prydz Bay, Antarctica. Width of view 7 mm. PPL. (Photograph courtesy P. Dirks)

Fig. 7.58. Zoned corona between garnet (top) and orthopyroxene (bottom). The upper symplectitic corona adjacent to garnet consists of
cordierite and orthopyroxene. The lower corona consists mainly of sapphirine. Central Sri Lanka. Width of view 7 mm. PPL. (Photograph
courtesy P. Dirks. Thin section provided by L. Kriegsman)