Porphyroblast inclusion trails and their microstructural relationships
Garnet, staurolite, andalusite and cordierite porphyroblasts preserve foliations as inclusion trails. Garnet and staurolite porphyroblasts are most common (e.g. Figs. 7, 8 and 9) and contain a very well developed schistosity (e.g. Fig. 7) or a differentiated crenulation cleavage (e.g. Figs. 8 and 9) as inclusion trails. These foliations are most commonly straight with curvature near the porphyroblast rims (e.g. Fig. 7) and many are truncated (e.g. Fig. 7c) by the matrix foliations but some are not (e.g. Figs. 7 b, d and 10). Garnet porphyroblasts are generally euhedral and contain excellent inclusion trails (e.g. Fig. 7a). The internal foliations (Si) in most of these porphyroblasts are truncated by the external foliations (Se). Their size varies from greater than 2 mm to 0.5 mm. Ten or more are common per thin section. In a few samples garnet has been replaced by muscovite and biotite.
Figure 7. Representative photomicrographs and line diagrams of garnet and staurolite porphyroblasts.
Representative photomicrographs and line diagrams of vertical thin sections of different samples illustrating variation in inclusion trail geometry, truncation and continuity with the matrix foliation. (a) Garnet porphyroblast preserves an oblique foliation that curves CW to sub-vertical (Si). (b) Garnet porphyroblast preserves a sub-vertical foliation (Si) continuous with the matrix (Se). (c) Staurolite porphyroblast preserves a sub-horizontal foliation (Si) that is truncated with that in the matrix and has a slight anti-clockwise curvature. (d) Staurolite porphyroblast with straight inclusion trails that are continuous with the matrix foliation. A slightly anti-clockwise curvature was observed in the rims or from the porphyroblast into the matrix in some porphyroblasts. Sample numbers, strikes and way up of the vertical thin sections are shown in the upper left corner (thick singly barbed arrow). PPL = plane polarized light; XPL= cross polarized light, Se = external foliation, Si = Internal foliation, St = staurolite, Bt = biotite, Grt = garnet.
Figure 8. Photomicrograph and the line diagram of a garnet porphyroblast
(a) Photomicrograph and the line diagram (b) of a garnet porphyroblast containing a differentiated crenulation cleavage with relics of the crenulated cleavage in Q-domains. The pseudo FIA is located in 3D where the crenulated cleavage changes asymmetry. The FIA is located where the slightly clockwise curving crenulation cleavage changes from ACW as shown to CW. (c) Detail within the garnet core. Thick barbed arrow shows way up and strike. Plane polarized light. Se = external foliation and Sia is the crenulated cleavage, Sib is the crenulation cleavage. St = staurolite and Bt = Biotite.
Figure 9. Photomicrograph and the line diagram of a staurolite porphyroblast
(a) Photomicrograph and the line diagram (b) of a staurolite porphyroblast containing a differentiated crenulation cleavage with relicts of the crenulated cleavage in Q-domains. The pseudo FIA is located in 3D where the crenulated cleavage changes asymmetry. The FIA is located where the slightly clockwise curving crenulation cleavage changes from ACW as shown to CW. (c) Detail within the garnet core. Thick barbed arrow shows way up and strike. Plane polarized light. Se = external foliation and Sia is the crenulated cleavage, Sib is the crenulation cleavage. St = staurolite and Bt = Biotite.
Figure 10. Photomicrographs and line diagrams of Andalusite and cordierite porphyroblasts
Representative photomicrographs and line diagrams of vertical thin sections of samples illustrating variation in inclusion trail geometry, truncation and continuity with the matrix foliation. (a,b) andalusite porphyroblast preserves a foliation that is continuous with that in the matrix and has a slight anti-clockwise curvature. (c,d) cordierite porphyroblast with straight inclusion trails that are continuous with the matrix foliation. A slightly clockwise curvature was observed in the rims or from the porphyroblast into the matrix in some porphyroblasts. Sia is crenulated by Sib with an anticlockwise asymmetry. The same two cleavages are present in the matrix (Sea and Seb). Sample numbers, strikes and way up of the vertical thin sections are shown in the upper left corner (thick singly barbed arrow). PPL = plane polarized light; XPL = cross polarized light
Staurolite porphyroblasts range in shape from poikilitic euhedral, anhedral to subhedral and preserve excellent inclusion trails that are mainly sigmoidal (Fig. 7c). Si in most of these porphyroblasts is truncated by Se (Fig. 7c). They range in size from 7 to 10 mm. Five or more porphyroblasts are common per thin section. Staurolite porphyroblasts in a few samples have been completely replaced by muscovite and chlorite.
Andalusite porphyroblasts are generally poikiloblastic. Inclusion trails vary from poorly preserved to excellent (Fig. 10a). Si in some of porphyroblasts is truncated by Se. The size varies from as big as 7 to 20 mm and 2 to 5 are present in each thin section of samples containing this phase. Shapes of the grains are anhedral to subhedral and rarely euhedral. In a few samples andalusite has replaced the micaceous part of the matrix retaining only quartz, opaques, biotite and some graphite as inclusions.
Cordierite porphyroblasts are mostly poikiloblastic and contain well-preserved inclusion trails (Fig. 10b). Inclusion trails within these porphyroblasts are truncated by matrix foliations or continuous with them. They range in shape from euhedral to anhedral, vary in size from 8 to 15 mm and 5 or more are common in each thin section of samples containing this phase. Some of cordierite porphyroblasts are corroded and have changed primarily to coarse-grained muscovite. Examples of staurolite being altered to cordierite and coarse-grained muscovite are many (see below).