Lecture 8: Metamorphism

Metamorphism

• Transformation of the mineralogy, texture, and sometimes chemical composition of a parent rock (protolith)

• Process of change undergone by an existing rock (protolith) in solid-state (<5% melting or anatexis) to another rock

  • Lower limit: Diagenesis — sedimentary process

  • Upper limit: Melting — igneous process

  

• Types of Metamorphism:

  1. Zone of burial metamorphism

  2. Blueschist and eclogite metamorphism

  3. Regional metamorphism

  4. Zone where wet fractional melting starts

  5. Granite magma rises and causes contact metamorphism

     

Protolith

• Parent rock, represents the rock from which metamorphic rock was derived

• May be igneous, sedimentary, or metamorphic in origin

• The chemical composition of the protolith determines the degree of change (isochemical vs. allochemical) imposed by a metamorphic agent

• Examples:

  • Shale —> Slate

  • Granodiorite —> Folded gneiss

Metamorphism Agents

• Heat

  • Provides energy that drives chemical reactions to proceed and result to recrystallization of existing minerals

    • Recrystallization - formation of new or enlarged crystals at the expense of pre-existing mineral grains

    • Example: Limestone —> Marble

    • Sources of heat:

      • Geothermal gradient

      • Magmatic bodies

      • Radioactive decay

• Pressure

  • provides energy that compacts and drives chemical reactions to proceed and result to recrystallization of existing minerals and may also result to the reorientation of the linear to platy minerals to a preferred direction

    • Lithostatic / Confining Pressure

      • pressure from overburden with equal stress applied from all directions causing further compaction and phase change producing denser polymorphs of pre-existing minerals

      • Example of polymorphs:

        • Kyanite - high pressure variety of aluminosilicate subgroup

        • Sillimanite - high temp variety of aluminosilicate subgroup

        • Andalusite - low temp and pressure variety

    • Compressional / Differential Pressure

      • unequal pressure in different directions wherein rocks subjected to compressional stress are shortened parallel to the direction of maximum stress and elongated in the perpendicular direction

• Chemically Active fluids

  • dissolve and transport ions within the rock facilitating recrystallization and sometimes, when source from a different material, exchange ions between the hydrothermal fluids and the rock

    • Hydrothermal Solution

      • hot and ion-rich fluids that are expelled from a magmatic body as it cools and solidifies

    • Metasomatism

      • exchange of ions between the invading hydrothermal fluid and the compositionally different host rocks changing the overall chemical composition of the rocks

      • The seafloor is highly metamorphosed

    • Fluid types:

      • Connate water - fluids in pore spaces of original rock / sediments

      • Liberated water from hydroxides - fluids from dehydrated mineral (phlogopite, amphibole)

      • Juvenile - fluids from magma

Metamorphism Types

• Based on rock area / volume affected

  • Local Metamorphism

    • driven by small-scale processes (e.g. intrusions, faults) and are this limited to a small volume (<300 km³)

  • Regional Metamorphism

    • generated by large-scale processes typically associated with orogenesis (mountain building)

      • Example:

        • Pillow lavas - exhibit serpentinization, when mafic/ultramafic is metamorphosed to serpentinites

• Based on nature of chemical affected

  • Isochemical metamorphism

    • No new elements are introduced into the system

    • The relative concentrations of previously present elements remain constant throughout the metamorphic process

    • Involves textual change (recrystallization and compression)

    • Example:

      • Contact - marble, quartzite

      • Dynamic Metamorphism - fault breccia, mylonite

  • Allochemical metamorphism

    • Integrates elements introduced into the system during the reaction process into the newly formed mineral species

    • Involves chemical change (metasomatism) and formation of new minerals

    • Example: Hydrothermal metamorphism - basalt —> serpentinite

• Based on dominant agent of metamorphism

  • Contact / thermal metamorphism

    • Dominant T; low P

    • Magma invading pre-existing rock creates an aureole around the intrusion

      • Aureole

        • zones of alteration wherein the metamorphic grade is controlled by the distance from the intrusion

      • Creates non-foliated metamorphic rocks

      • Bake-and-chill margins

        • textural contrast between an intrusion and the country rock caused by contact metamorphism

        • clear boundary where crystallization happens

  • Dynamic Metamorphism

    • Dominant P (deviatoric stress)

    • Rocks are subjected to shear forces along the shear zone

      • Brittle Deformation - occurs at shallow depth and low geothermal temperature

        • Fault breccia - loosely coherent rock from pulverized rock

      • Ductile Deformation - occurs at great depth with higher temperatures

        • Mylonite - apparently foliated; formed from intense ductile deformation

  • Burial / static metamorphism

    • Dominant P

    • Massive amount of materials accumulating in a subsiding basin induces low-grade metamorphism in the deepest layers

    • Associated with subduction zones

  • Dynamothermal / regional metamorphism

    • Dominant T and P (high grade)

    • Occurs during orogenesis where large segments of the crust are subjected to high T and P conditions

    • Associated with collision zones

    • They would create folded or compositional banded rocks (like gneiss)

  • Hydrothermal metamorphism

    • Dominated by chemically active fluids

    • Hot, ion-rich fluids circulating through fissures and cracks in rocks associated with the emplacement of magma

    • Can also occur at shallow crustal depths in regions where geysers and hot springs are active

  • Impact / shock metamorphism

    • Very high P and T in a short span of time

    • Produces impactites (rich in shock qtz)

    • Energy from impacting meteorite converted into heat and shockwaves melting, pulverizing, and shattering the surrounding rocks

Metamorphic Rock Textures

• Foliation

  • Planar arrangement of mineral grains or crystals formed from the directed compression that shorten rocks. This causes flattening or reorientation of pre-existing mineral grains as well as compositional banding

    • Flattening of spherically-shaped grains along the maximum stress direction

    • Rotation of platy minerals perpendicular to the maximum stress direction

    • Recrystallization of new minerals perpendicular to the maximum stress direction

  • Foliated Metamorphic Rocks

    • Rock / slaty cleavage

      • Rocks split into thin planar slabs across the bedding surfaces

      • Occurs in slates and phyllites (but also occur in higher grade foliated schists and gneisses)

      • Prominent in low-grade metamorphism of shales

    • Schistosity

      • Planar and layered structures from the parallel alignment of platy minerals (msc, bt) that have been recrystallized from minute mica and chlorite flakes to discernable size due to higher T-P conditions

      • Intermediate-grade metamorphism

      • Prominent in schists

      • look like crumpled paper of rocks

      • Examples:

        • Talc schist

        • Garnet mica schist

        • Blueschist

        • Mica schist

    • Gneissic / Compositional Banding

      • Segregation of dark (bt, amp) and light (qtz, fsp) minerals caused by ion migration due to P-T conditions

      • High-grade metamorphism

      • Prominent in gneiss

• Non-Foliated

  • Does not exhibit layered or banded appearance that may be either due to minimal deformation, dominance of equidimensional crystals and absence of platy minerals

  • Occurs in contact and hydrothermal metamorphism

    • Granoblastic

      • exhibit coarse-grained, equant and equigranular crystals with boundaries that intersect at 120­° triple junctions

    • Hornfelsic

      • exhibit fine-grained (<1 mm), irregular and interlocking grains in a non-foliated fabric that forms from contact metamorphism of fine-grained protolith

  • Metamorphic rocks exhibiting bimodal size distribution

    • Porphyroclastic

      • contain porphyroclasts that can reveal the direction of shear

      • Porphyroclast

        • large relic grains from protolith, from before cataclasis, that have experienced deformation but retained its original composition

          • Augen - oval-shaped fsp porphyroclasts

          • Flaser - oval-shaped qtz porphyroclasts

    • Porphyroblastic

      • contain porphyroblasts that reveal local tectonic and metamorphic evolution

      • Porphyroblast

        • large crystals that experienced neocrystallization and growth in response to the new P-T conditions that typically develop crystal faces

      • Examples:

        • Garnet mica schist

        • Pinite hornfels

        • Staurolite schist (iron cross)

        • Porphyroblast is called idiomorphic when it is well-formed, sub-idiomorphic when partly well-formed, and allotriomorphic when it is not well-formed

Metamorphic Rock Classification

• Metamorphic rocks are typically classified by their:

  • Texture - size, shape and relationships of constituent minerals

  • Protolith - metamorphic rock retains characteristics of protolith

  • Mineralogy - abundance of mineral phases present

  

Metamorphic Grade

• Describes the relative T-P conditions under which metamorphic rocks form

• Low Grade

  • Temperature: 200 - 400°C

  • Pressure: 1 - 6 kbars

• High Grade

  • Temperature: 500 - 1000°C

  • Pressure: 12 - 40 kbars

• Prograde

  • Mineralogy reflects lower P-T to higher P-T transformation

• Retrograde

  • Mineralogy reflects higher P-T to lower P-T transformation

• Index minerals

  • used to distinguish among different zones of regional metamorphism

Metamorphic Facies

• Set of rocks representing the full range of possible chemistries within a limited range of metamorphic conditions

  

• Example: Metamorphism at 3.5 kbar and 300°C

  • Marble (limestone): calcite, dolomite, and minor quartz

  • Metabasalt (basalt): chlorite, albite, actinolite, epidote, and minor quartz

  • Pelitic schist (mudstone): quartz, chlorite, muscovite, and albite

• Different mineralogically (because of the protolith) but metamorphosed under the same conditions and thus represents the same region (facies)

Resources from Metamorphic Rocks

• Metals from hydrothermal deposits

  • Includes economically important minerals such as copper, gold, molybdenum, silver and zinc

  • Metasomatism causes mobility of metal ions which subsequently concentrates within faults and fractures

• Marble

  • Formed from contact metamorphism of limestones, used primarily in construction (e.g. cement, tiles)

• Asbestos

  • Formed from hydrothermal metamorphism of ultramafic rocks, used for insulation and fireproofing

Metamorphic Rocks in the Philippines

• Schist - foliated and metamorphosed volcanics (Suyo Schist in Burgos, Ilocos Norte and Alicia Schist in Bohol)

• Amphibolite - composed of foliated amphiboles and plagioclases; protoliths; gabbros and diorites (Dalrymple Amphibolite in Palawan)

• Serpentinite - composed of serpentine as a product of hydrothermally altered peridotites (Sibuyan Ophiolitic Complex)

• Marble - metamorphosed limestones (Romblon Metamorphic Complex - Romblon Marble)