FIAs 1, 2, 3 and 4 and the distribution of isograds
The samples which contain FIA 1, 2, 3 and 4 preserved in staurolite porphyroblasts are shown in Fig. 18. The total distribution of FIAs measured from this mineral (Fig. 12c) resulted from four periods of staurolite growth that show a consistent succession where relative timing criteria are available (Table 2). Staurolite isograds can be defined for these FIAs as shown in Fig. 18 a-d. A slightly clockwise and eastward shift in the staurolite isograd from FIA 1 to FIA 4 is apparent. Andalusite and cordierite porphyroblasts only contain FIA sets 3 and 4 with FIA set 4 mainly present in the latter phase. Foliations defining FIA set 4 in both of these mineral phases were always continuous with those in the matrix and deflections of these trails from porphyroblast to matrix are very slight (e.g. Fig. 10).
Figure 18. The staurolite isograds plotted on a geological map
The staurolite isograds plotted on a geological map. It shows all sample locations and their FIA and p-FIA trends measured in staurolite porphyroblast. (a), FIA set 1 (b), FIA set 2 (c), FIA set 3 and (d), FIA set 4. A slightly clockwise shift in these isograds has been observed from FIA 1 (oldest FIA) to FIA 4 (youngest FIA). The rose plot shows average FIA trends.
Figure 19. The andalusite and cordierite FIA generated isograds plotted on a geological map
The andalusite and cordierite FIA generated isograds plotted on a geological map. It also shows rare samples of andalusite, cordierite and sillimanite of post-FIA set 4 formation. (a) Shows all sample locations and their FIA and p-FIA trends measured in andalusite porphyroblasts. This mineral phase preserves inclusion trails of FIA set 3 and 4. In (b), all sample locations and their FIA and p-FIA trends measured in cordierite porphyroblast are shown.This mineral phase preserves inclusion trails of FIA set 3 and 4. A slightly clockwise shift in these isograds has been observed from FIA 3 to FIA 4. The rose plot shows the average FIA trends. (c), Shows all those rare sample locations in which the cordierite and andalusite porphyroblast just overgrew the matrix foliation without any deflection of inclusion trails associated with deformation during their growth. (d) Shows all those rare sample locations which contain fibrolitic sillimanite either within the matrix (e.g. Fig. 17e) or incorporated within cordierite (e.g. Fig. 17c) or andalusite (e.g. Fig. 17d). The fibrolitic sillimanite in all of these samples appears to postdate FIA set 4 and overprints the matrix foliation.
The andalusite and cordierite isograds for FIAs 3 and 4 are shown in Fig. 19 (a, b). A slightly clockwise shift with time in both of their isograds, similar to that for the staurolite isograds, is apparent. Inclusions of andalusite were found within the porphyroblasts of cordierite in sample C77 (Fig. 16a). Cordierite in this sample contains FIA 4 suggesting that the andalusite formed before or during this period of FIA development. Remnants of inclusions, which are mainly composed of quartz, muscovite, minor opaques and ilmenite, are geometrically and texturally very distinct from those found within the matrix and the cordierite remains. Traces of staurolite (e.g. C52) included in some big cordierite poikiloblasts contain inclusions trails texturally and geometrically distinct from those within their hosts (Fig. 15d); cordierite preserves FIA set 4 and its inclusion trails completely truncate those of staurolite (Fig. 15d). FIA measurements were impossible for staurolite in this sample, as very few thin sections contain this mineral.