Figure 6.20. Sillimanite Pseudomorph of Kyanite Prograde Ky --> Sil reaction is indicated by a Ky core to a Sil porphyroblast. Click on the image to see a larger version with more information.
Figure 6.21. Mn-Zoned Garnets. Mn map of calcareous schist showing strong Mn-zoning. Click on the image to see a larger version with more information. Use the radio buttons there to choose the element map(s) shown. Then use the "back button" to return here.
Many approaches have been used by petrologists to determine the P-T paths of metamorphic rocks. Most of them depend on identifying prograde features of the rock that have been preserved in some way and not erased by achieving whole rock equilibrium at the maximum P-T path temperature. Some examine retrograde features, typically involving hydration reactions that are only partially completed due to limited availability of water.
Pseudomorphs of one or more minerals after another mineral are textural features that may give P-T path information. For example, the sillimanite (Sil) porphyroblast in Figure 20 is not only in the shape of a kyanite (Ky) crystal, but it also has a core of kyanite remaining. This texture is evidence that the rock's P-T path crossed the Ky=Sil reaction with increasing temperature. Similarly, this outcrop photo from SE Vermont shows garnet porphyroblasts that are partially replaced by chlorite, a retrograde hydration reaction.
Another important P-T path constraining textural feature consists of bands of one or more product minerals separating reactant minerals. The sillimanite (Sil) grains in Figure 6.20 are surrounded by a band of anothite (An) plus spinel (Spl) and then a band of cordierite (Crd), both bands separating the reactant Sil from reactant garnet (Grt). Similarly, bands of products garnet and clinopyroxene (Cpx) separate reactants plagioclase (Pl) and hornblende (Hbl) in the cover photo for this website. The Grt+Aug bands are evidence for the reaction Hbl + Pl + Qz = Grt + Cpx + H2O.
Some metamorphic minerals, notably garnet, may be compositionally zoned. See for example the Mn chemical map of a schist in Figure 6.21. Because the equilibrium composition of solid-solution minerals changes with temperature and mineral assemblage, mineral zoning is a partial record of the rock's P-T path. The bright purple (higher Mn) cores of the euhedral garnet porphyroblasts, are evidence for different, cooler conditions when the garnets began their growth. The preservation of compositional zoning in garnet is a result of the comparatively slow rate of diffusion of many elements in garnet. Click on Figure 6.21 and use the selector buttons to view the zoning of Ca, Mg, and Fe as well as Mn in these garnets. The use of compositional zoning of garnet to deduce information about P-T paths was pioneered by Frank Spear and his students (see Spear et al., 1984). You can listen to Frank Spear discuss P-T Paths in this recording of the 2012 Murchison Medal Lecture.
PT paths provide important constraints on the tectonic events that produced the metamorphic rock. The contraints are even more valuable if age information can be added to make a Pressure-Temperature-time (P-T-t) path.
