Migmatites and Ptygmatic Folding

         Let's examine ptygmatic folding in migmatites (huh?). Take a look first:








                                                         Migmatites are a mixture of metamorphic and igneous rock. The pink plagioclase feldspar in the rock below formed a ptygmatic fold after remelting of the lighter colored minerals:

         
           Thus, this remelting creates a mixture of the unmelted metamorphic part with the recrystallized igneous part. Migmatites tend to occur at very high-pressure and high-temperature zones and in very old rocks (such as Precambrian).

           The word ptygmatic was introduced into the geological literature by Jakob J. Sederholm in 1907 and originates from the ancient Greek word for “fold” or “anything folded.” The term “ptygmatic fold” is somewhat redundant, like saying “a folded fold.” However, the term “ptygmatic” in the geologic literature generally refers to tight folds that form when the folded material has a greater viscosity than the surrounding medium. In migmatites, ptygmatic folds generally form in the more viscous lighter layers.   

      A look at Bowen's Reaction Series:

              

          shows why the darker minerals such as olivine, pyroxene, amphibole, and biotite tend not to remelt as it takes higher temperatures for this to happen. The lighter-colored K-spar, muscovite and quartz melt at much lower temperatures forming those ptygmatic folds of newly igneous material.

      Sometimes, differentiating between migmatites and metamorphic gneisses can be difficult--but we'll leave that for a later, gneisser time.

Migmatitely,

Steph