Chapter 7 – Metamorphism & Metamorphic rocks

Metamorphism - alteration of rocks by heat and pressure.

Low-grade metamorphism – Shale to slate – a slight compaction and hardening, slight deformation of features.

Medium-grade metamorphism – significant changes, but parent rock is apparent.

High-grade metamorphism – when changes render the parent rock unclear.

Agents of metamorphism

• Heat – Nearby magma or lava, deep burial

• Pressure – Deep burial or pressure during continental collisions

• Chemical activity (fluids) – Helps mobilize ions.

• Time

Heat provides energy to break chemical bonds and recrystallize existing minerals or cause minerals to combine with other minerals.

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In regional metamorphic setting – continental collisions force some of the rock material to great depths (geothermal gradient), magmas generated by subduction of oceanic plate provide heat. Burial within a deep ocean basin may also trigger metamorphism. Temp. between 1500 and 2000 C (8 km depth) can alter clays to micas.

In a contact metamorphic setting, nearby lava or magma provides heat, if intrusion is “dry”, rock may be “baked”. If fluids are present, more changes take place. Host rock type is important too. Silicate minerals generally are stable and less subject to alteration. Carbonates (limestones, marbles) are more susceptible.

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Biotite Gneiss (regional metamorphism)

Diabase dike intrusion

3 – 5” of contact metamorphism

The host rock – biotite gneiss was

probably originally a granite & under-

went regional metamorphism

during the Paleozoic Era. The

diabase dike was intruded during early Mesozoic

Era. Fluids assoc. with the diabase

caused hydro-thermal alteration

of micas and feldspars in

gneiss.

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Vertical beds at left side of outcrop are con-tact metamorphosed marble & hornfels.

Pink layers – marble. Green layers - hornfels

Confining pressure (usually with increasing depth) causes equal pressure in all directions and compaction, recrystallization of existing minerals.

Differential pressure causes deformation along a single plain. Near the surface, differential pressure may pulverize the minerals into smaller grains. At depth, increased temperature causes the grains to deform in a “ductile” fashion.

When under differential pressure, some minerals rotate while others “flow” internally, i.e., ions flow away from direction of high pressure to area of lower pressure.

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High temperature & intense differential pressure cause flowage (deformation) of the rock. Differential pressure directions can change over time.

Chemically active fluids (primarily water)

Fluids act as catalysts to enhance ion migration.

If different minerals are adjacent – ionic exchange may occur, perhaps changing both minerals.

Higher temperatures may completely expel water from rock, changing the composition of clays, micas, and amphibolites.

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Parent rock – Many metamorphic rocks are chemically similar to their parent rock (shale – slate, limestone – marble, arkose – gneiss, basalt – amphibolite).

Quartz sandstones – stable, little change, except recrystallization.

Limestones – Calcite, more soluble. Dirty limestones (with clays) reactive. Produce a wider range of minerals.

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Metamorphic texture – size, shape, & arrangement of grains.

Deformed metamorphic rocks with platy (mica) and/or prismatic/elongate (amphi-bole) minerals show a preferred orien-tation (alignment) due to the pressure. These are foliated metamorphic rocks.

Foliation forms by 1) Rotation of mineral grains; 2) Recrystallization of minerals to form new grains with preferred orien-tation; 3) Changing the shape of equi-dimensional grains into elongated shapes.

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Text on pp. 251 - 253 describes various mechanisms by which minerals develop a preferred orientation.

Various grades of metamorphism yield various foliated textures – rock (slaty) cleavage, schistosity, and gneissic texture. There are gradational textures in some cases.

Variations in metamorphic grade are due to differences in depth of burial and/or proximity to igneous plutons.

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Rock or slaty cleavage – causes rocks to split into thin tabular slabs. Develops when heat and pressure result in the preferred alignment of platy minerals into thin zones of parallel flakes separated by layers of equigranular (non-platy, non-prismatic) minerals. Over time, the platy minerals will realign perpendicular to the force of pressure, resulting development of rock cleavage at an angle to the relict bedding planes.

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Alignment of clays in shale

Initial alteration of clays in slate

Differential pressure causes development of

foliation at angle to original beds

Other textures –

Non-foliated – Minimal deformation, parent rock composed of equidimensional minerals (calcite, quartz)

Porphyroblastic – Development of individual, large accessory mineral crystals (Staurolite, Garnet, Kyanite) with a matrix showing schistose or gneissic texture (Fig. 8.11, pg. 254). Parent rock mineralogy and heat/pressure conditions may also play a role in development of porphyroblasts.

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Classification of common metamorphic rocks and their probable parent rocks – Figure 8.12, pg. 255.

Slate – clay/mica flakes to small to be visible. Generally has a dull luster, excellent rock cleavage. Metamorphosed shale, mudstone, or siltstone, occas. volcanic ash. Black – organics, Red – iron, Green – chlorite.

Phyllite – small grains, glossy sheen, sometimes wavy.

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Schist – Dominated by platy minerals (muscovite, biotite, chlorite), sometimes with lenses of quartz, feldspar. Other less common schists include talc and graphite.

Gneiss – Medium- to coarse-grained with bands of aligned minerals. Light colored gneisses may be derived from arkose or granitic/rhyolitic igneous rocks.Sometimes called Granitic Gneiss.Examples: Lithonia Gneiss.

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Dark gneisses – amphibolites – from basalts/gabbros - Wolf Creek Fm. (Lawrenceville area), Pumpkinvine Creek Fm. (I-75 North of Marietta). If significant quartz/feldspar is present – quartz or feldspar amphibolite. If quartz/feldspar >50%, amphibole gneiss. Pyroxenites – gneisses composed of pyroxene are fairly rare.

Layers of platy/elongate minerals within gneiss may referred to as “schistose” layers.

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Non-foliated RocksMarble – Coarse, crystalline rock derived

from limestone or dolomite, composed of calcite. Impurities produce color varieties and accessory minerals (chlorite, muscovite, garnet, wollastonite) may produce a banded appearance. Popular for carvings, building purposes, but susceptible to acid rainfall. Georgia Marble is preserved within Murphy Marble Belt (or Murphy Syncline). Isolated marble occurrences are present in the Brevard Fault Zone, Buford area, Gainesville area,…

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Quartzite – In moderate- to high-grade metamorphism, quartz sandstone is subject to fusion of individual grains, i.e., in a sandstone, fracturing occurs between grains, whereas in quartzite, fracturing occurs across grains.

Marble or Quartzite may be similar in appearance. To discern between them, calcite in marble is softer than steel & effervesces (fizzes) in HCl. Quartz in quartzite is harder than steel & does not fizz.

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Metamorphic Environments – 1) Contact; 2) Hydrothermal; 3) Regional; 4) Burial; 5) Impact; 6) Fault-associated

Contact Metamorphism – adjacent to molten igneous rock. “Aureole” – zone of alteration. Small aureole – diabase dike in Vulcan quarry (slide 3). Large aureole (near batholith) may have distinctive zones of alteration, e.g., garnet – adjacent; chlorite – margin.

Directed pressure is usually not present, thus foliation usually doesn’t form.

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Hydrothermal Metamorphism – caused when hot, ion-rich fluids circulate through rock fractures, causing mineral alterations. Example – in Vulcan quarry, diabase dike caused alteration of feldspars and other silicates to sericite (a fine-grained muscovite). In submarine vulcanism, hydrothermal metamorphism can play a role in mineral enrichment of fractured rocks near seafloor vent (pg. 258 - 259). Fractured rocks provide more surface area.

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Regional Metamorphism – Most widespread type, regionally, Blue Ridge and Piedmont Provinces formed during Paleozoic continental collisions when rocks caught between continents was forcibly buried (Fig. 8.21, pg. 261). Metamorphic Grade increases toward the “core” of the mountain belts, e.g., Shale-Slate-Phyllite-Gneiss.

Burial Metamorphism – Sedimentary rocks at the bottom of a thick accumulation undergo alteration by heat, while pressure is uniform, not directional.

Fault-zone Associated Metamorphism – Rocks caught in fault zones are sheared and shattered, taking on a foliated or lineated appearance, example: Mylonites.

Impact Metamorphism – Meteorite impact causes melting and passage of shock waves. Lightning fused sand – stishovite might be included. Article – pg. 266 – Tektites have been found in south Georgia.

Index Minerals and Grade – certain minerals are good indicators of heat and pressure conditions within metamorphic rocks (see Fig. 8.25, pg. 262). For instance, the presence of garnets in the mica schists on the GUC campus offers evidence of Intermediate-grade metamorphism. One-half mile from the old GPC campus, chlorite schist was present. Both rock units are probably in the Wolf Creek Formation.

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Migmatites – intense metamorphism causes light-colored silicates to melt, while dark-colored silicates remain solid.

Plate Tectonic Settings – Subduction Zone - Fig. 8.28 – Low Temp./High Pressure – initial portion of subduction zone. High Temp./High Pressure – zone of Partial Melting & Rising plutons (though pressure is uniform). Near surface – High Temp./Low Pressure.

High Temp./Differential Pressure – Cont. plate collisions.

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High Temp/ Lo Pressure

Low Temp/ High Pressure

Pluton

High Temp/ High Pressure

Different Temperature/Pressure settings in Subduction Zones

Continental Shields – discussed more fully later.

Core areas of continents. Composed of ancient igneous and metamorphic rocks, surrounded by mountain belts “built” by continental collisions.