Metamorphism: mineralogical and/or textural change brought about in a rock in the solid state as a result of increase in temperature and pressure.
 

• need to consider the initial and final states of equilibrium - the phase rule can be applied to assess the approach of the mineral assemblage to equilibrium
• need to consider which chemical and/or physical variables must change, and how, to provide a driving force for reequilbration of the mineral assemblage.
• Pressure
• Temperature
• P and T are interdependent and define a "geothermal gradient"

 

• anomalously high temperatures at shallow depths through heat input accompanying intrusion of magma.

• anomalously low temperatures at deep crustal levels associated with subduction of cold dense oceanic lithosphere.

• Partial pressure of H₂O and CO₂
• Internally controlled by reactions involving the thermal decomposition of hydrous and/or carbonate minerals.
• externally controlled by movement of fluid from a reservoir outside the system through the rock in large quantities.

• What is the significance of these models?

Regional Metamorphism

• covers 100's to 1000's of km²
• involves temperatures above normal  geothermal gradient (High P - Low T metamorphism is considered separately).

"Process" oriented petrologists recognize considerable overlap between contact and regional metamorphic processes, and recognize that elevated geothermal gradients indicated by regional metamorphism commonly require heat input from igneous intrusions and associated magmatic fluids.

This really shouldn't be so surprising as physical chemical thermodynamic principles predict rocks of similar composition metamorphosed to the same P-T conditions should show similar mineralogical changes regardless of the source of heat.

Metamorphic Textures / Structures

Any rock can undergo metamorphism. Thus, metamorphic rocks are compositionally, mineralogical, and texturally diverse.

Metamorphism and deformation commonly occur synchronously. Many combinations of heating rates and strain rates are possible.

Static metamorphism (increasing T) produces coarser grained minerals whereas deformation processes serve to reduce mineral size and impart a fabric (i.e.,  foliation and/or lineation) to the rock. The integrated effects of these two processes dictate the final appearance of the metamorphic rock.

• Textures of metamorphic rocks range from massive to foliated.
• Grain Sizes range from extremely fine to very coarse.

• metamorphic minerals are called "blasts".
• Porphyroblast: metamorphic minerals that are much larger than the surrounding matrix (e.g., garnet porphyroblasts).
• Granoblastic: a metamorphic rock comprised of equigranular crystals.
• Poikiloblast: porphyroblasts which contain numerous small inclusions of matrix minerals.
• Pseudemorph: replacement of a previous mineral by new, smaller metamorphic minerals whic retain the crystal form of the previous mineral.
• Idioblastic: well formed crystals.
• hypidoblastic: medium crystal development.
• xenoblastic: anhedral crystals.

• Increasing T cause an increase in grain size (in the absence of shear stress)
• annealing: increase in average grain size and formation of a polygonal texture (5-6 sided grains sharing 120^o "triple junction" grain boundaries.
• energy reduction process which serves to minimize the energy of the system and achieve textural equilibrium
• Final texture will be related to the types and proportions of minerals and the rates at which they grow (surface area).
• one large crystal has less surface area than a bunch of smaller crystals of equivalent mass. Thus a rock a constant P-T conditions can lower its total energy by simply growing coarser crystals!

• Grain size reduction (communition)
• dynamic recrystallization
• dislocation creep
• grain boundary migration
• fracturing
• Development of planar structures
• slaty cleavage
• phyllitic cleavage
• crenulation cleavage
• schistosity
• gneissic fabric
• mylonites
• tectonites
• Development of  linear structures
• mineral lineation
• rodding
• crenulation lineation
• intersection lineation

Based on mineralogy, texture, and structure. Widely used rock names (e.g., schist, gneiss) modified by a textural term (e.g., augen, mylonitic) or the presence of key minerals (e.g., garnet, biotite). Rocks that are predominantly monomineralic are named for the dominant mineral (e.g., quartzite, serpentinite).

The majority of metamorphic rocks fall into three categories (broadly speaking) based on protolith composition.

• Aluminous clastic sedimentary rocks (mudrocks, pelites, greywackes, dirty sandstones etc.)
• see the handouts
• Calcareous rocks (limestones, dolomites, marls)
• typically granoblastic marbles dominated by calcite and dolomite
• non-carbonate minerals include
• low metamorphic grade: phlogopite and tremolite
• higher metamorphic grade: diopside, wollastonite, calcic plagioclase, grossular, forsterite
• Calcsilicate rocks (dirty carbonates (marls) containing abundant detrital silicate minerals)
• follow a similar textural development to mud rocks (see handout)
• low grade: chlorite, epidote, actinolite
• higher grade: augite, hornblende, calci plagioclase, garnet.
• at very high grades can resemble amphibolites (metamorphosed basalts).
• Mafic to intermediate volcanic rocks (basalts, andesites, dacites, flows and pyroclastics)
• low grade "greenstones" greenschist facies due to the inherently greenish color of the metamorphic minerals which develop (chlorite, actinolite, epidote, Na-plagioclase)
• higher grade "amphibolites" dominated by hornblende and plagioclase
• highest grade "eclogite" dominated by opx, cpx, garnet, calcic plagioclase, and olivine.