Lab 6: Metamorphic Rocks - Grades and Zones


Metamorphic rocks are unique in the way different minerals and foliations form within specific temperature and pressure ranges, leading to a straightforward association of the conditions of metamorphism with different metamorphic rocks. Characteristic metamorphic minerals and the grades of metamorphism in which they occur, are as follows:


    low grade (200 degrees C)           intermediate grade                  high grade (800 degrees C)


     -------------- chlorite ---------------

          ----------- muscovite --------------

                          -------------- biotite ---------------

                                         ----------------- garnet -----------

                                                ------- staurolite -------

                                                           -------------- sillimanite -------------

  -------------------------------------------- quartz ---------------------------------------------

  ------------------------------------------- feldspar --------------------------------------------


  not altered |||| slate |||| phyllite |||| schist ||||||||||||| gneiss |||||||||||||||||| melting


Those minerals are index minerals for the grades shown. That means if you find, say staurolite, you know the metamorphism was intermediate to high grade, whereas, if you found chlorite in a rock, you would know that metamorphism was only low grade. In mountain ranges resulting from plate collision (subduction or continent-to-continent collision), there are often mappable zones, with low grade zones flanking higher grade zones toward the center, as follows:




      \\\\\\\\\\\\\\\\\\\\\\\\ Chlorite Zone \\\\\\\\\\\\\\\\\\\\\\\\




  \\\\\\\\\\\\------------------ Garnet Zone ---------------\\\\\\\\\\





  \\\\\\\\\--------****** Sillimanite Zone *********----------\\\\\\\











The area of the sillimanite zone represents the "core" area of the mountain range, where erosion has stripped deeper to reveal rocks that were once deep down within the collision belt. The area of the chlorite zone, in contrast, represents the outer edges of the collision belt, where metamorphism at lower grades happened at shallower depths. So, you have to look at such a map as representing the stripped off (eroded off) top of a mountain belt. The metamorphism that happened during the plate collision happened millions of years ago, and since then, as erosion of the mountains happened, gradual uplift continued in response to the removal of material, exposing more and more of the originally deep-down rocks.