CONTROLS OF METAMORPHIC

CONTROLS OF METAMORPHIC

1. TEMPERATURE AND PRESSURE

   • Reflect the energy content of rocks, which in turn determines, through appropiate thermodynamic properties, which minerals appear in reactions, which remain stable over long intervals, and which minerals are possible under a given set of metamorphic conditions.

2. COMPOSITIONS OF METAMORPHIC FLUIDS

   • They are generally thought to be rich in water, although fluids may be carbon dioxide-rich as the result of metamorphism of calcareous rocks or methane-rich in low-to-medium-temperature-metamorphism of mudrocks that contain carbonaceous matter or graphite. Minor or trace constituents may include fluorine, chlorine, O2, H2, H2S, and dissolved mineral species.

3. FLUID INCLUSIONS

   • Are very small (10 micrometers or less) ovoid trapped bubbles of fluid in metamorphic minerals, usually quartz. Many fluid inclusions are multiphase, containing a bubble of gas in some liquid; and very small solid inclusions of halide, sulfide, or other minerals are sometimes present as well.

4. METAMORPHIC REACTIONS

   • Textural evidence for prograde mineralogic reactions is sometimes found in metamorphic rocks, even though the crystallization of new, higher grade minerals, along with deformation, tends to obliterate evidence for lower grade precursors. One indication of reaction is pseudomorphic replacement of lower grade minerals by higher grade ones.

5. REACTION MECHANIMS

   • It must be established whether all the atomic constituents for the reactions are present within the minerals themselves.

6. TYPES OF METAMORPHIC REACTIONS

   • Discontinuous (Univariant) Reactions
     • These reactions may or may not be devolatilization reactions but are notable because they are approximately univariant and they involve a shift in tie lines in the appropiate compositional diagram. The first of these reactions occurs at medium grades of metamorphism and involves the minerals quartz, muscovite, biotite, chlorite, staurolite, and garnet.
   • Continuous (Divariant) Reactions and Distribution Coefficients
     • When one or more minerals phases portrayed in a compositional diagram have varying compositions within the diagram, and thus varying plotting points, three phase triangles can gradually shift positions with changing pressure and temperature, rather than changing abruptly as in discontinuous reactions.