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Big Island Field Trip

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1 Metamorphic Rocks

Metamorphism: to change form Metamorphic : any rock (sedimentary, volcanic, plutonic, metamorphic) that has undergone changes in texture or mineralogical composition in the state

No Single Mode of Origin

• Metamorphic rocks result from the partial or complete recrystallization of in rocks over long periods of time • Rocks remain essentially solid during

2 Metamorphic Processes HEAT: stability region of sensitive to T. With increased T, pore fluid decreases PRESSURE: greater pressure tends to decrease space available; metamorphic mineral tend to be dense Increased P can come from any directed (burial, differential stress) Differential stress will bring about a preferred orientation of minerals

FLUIDS (H2O, CO2): acts as catalysts during metamorphism; aids the exchange of ions between growing

Metamorphic Grade

Refers to the intensity of metamorphism

High grade: high T, P Low grade: low T, P

3 Pressure and Temperature vs. Depth

Temperature and Pressure Conditions of Metamorphism

4 Metamorphic Grade

Evidence of Metamorphism e.g., Sedimentary Rocks – Made of minerals derived from of a – Stable at atmospheric temperatures and pressures (low T & P) – Originally horizontal, continuous and uniform layers • 1. Bent (deformed) layers

5 Evidence of Metamorphism

• 2. Flattened Pebbles

Conglomerate Differential pressure “squashes” rock and included features

6 Evidence of Metamorphism

Quartz

Quartzite • 3. Crystalline Texture Minerals tightly interlocking due to recrystallization under pressure

Evidence of Metamorphism

• 4. New mineral assemblages e.g., Shale: minerals (some ) Æ

Metamorphism (Mid-grade) • Forms : , and other silicate minerals Æ

7 Metamorphism

•Parent Rock –Even though minerals will change Shale – Most elements are provided by parent rock – Except water and some dissolved ions

Schist

Metamorphism Temp. (ºC) C 0 600 1200 A B 600 1200

A B C

• Increased Temperature (geothermal gradients) – Minerals stable at lower temperatures converted to minerals stable at higher temperatures – Solid state chemical reactions are accelerated

8 Metamorphism

• Increased Pressure (and stresses) – Increased Confining Pressure as rocks are buried – Compression at convergent plate boundary or – Sheared as plates slide past each other

Types of Metamorphism

Regional: Widespread changes in temperature and pressure bring about changes in rocks due to tectonic forces : Intrusion of against colder rocks (affected area is proportional to the size and temperature of the intrusion but always only a local phenomenon) Deformational: Changes in rocks associated with faulting and folding (regional or local)

9 Regional Metamorphism

Contact Metamorphism

10 Contact Metamorphism adjacent to a Intrusion

Contact Metamorphism of

11 Contact Metamorphism of &

Deformational Metamorphism

12 Types of Metamorphism (cont.)

Burial: Changes in a rock due to the gradual changes in T and P due to successive burial (regional) Impact: Changes due to rapid increase in pressure (only localized)

Plate Tectonics and Metamorphism

13 and Metamorphism

Metamorphic Reactions

Mineralogical changes (e.g., clay to mica): Many complicated reactions — depend on pressure, temperature, composition

Common metamorphic minerals include , , mica, , and

Textural changes: recrystallization (grain boundaries more compact) and (preferred orientation of minerals)

14 How Much Can a Rock Change?

The amount of change during metamorphism depends on:

• grade of metamorphism

• duration of metamorphism

• composition of the rock

15 Changes in Texture Grain size • Recrystallization • Mineral size can either decrease or increase Orientation of minerals • Recrystallization • Directed stress will orient minerals: – Lineation – Foliation

GraniteGranite

16 GneissGneiss

Granite Gneiss

17 Metamorphic Foliation

18 Metamorphic Foliation

Direction of Compressive Forces

19 Slaty metamorphic

Regional stresses

Slate with Foliation and Relict Bedding

20 Slaty Cleavage

Formation of Slaty Cleavage

21 Classification of Foliated Rocks

Metamorphism of Shale (and the classification of metamorphic rocks)

Shale: Fine grained Clay (and quartz) • Minerals stable under low T&P (atmospheric) • due to accumulation of sediment • Fissility along laminations • Water bound in crystalline structure of clay

22 Metamorphism of Shale

1: Low Grade MetamorphismÆSlate • Fine grained Mica, (Chlorite and Quartz) • Low grade metamorphic T&P (Water is expelled from crystalline structure of clay)

• Minerals stable under low grade metamorphic conditions • Slaty Cleavage due to realignment of platy minerals

Slate

23 Metamorphism of Shale

2: Medium-Grade Metamorphism Æ Phillite • Courser grained Mica and quartz • Medium-grade metamorphic T&P Causes minerals to grow • Slaty cleavage becomes rippled and rock has a sheen

Phyllite

24 Metamorphism of Shale

3: Medium to High-Grade Metamorphism Æ Schist •Course grained Mica and Quartz • Medium-grade metamorphic T&P Causes minerals to grow

• Minerals stable under medium grade metamorphic conditions appear: Garnet, and • Schistosity due to alignment of platy and needle- like minerals

Schist

25 Schist in

26 Metamorphism of Shale 4: High-Grade Metamorphism Æ Gneiss • Course grained Feldspar, Quartz, Amphibole, Biotite •High-grade metamorphic T&P Causes minerals to separate into bands • Minerals stable under high grade metamorphic conditions appear: feldspar • Gneissic banding bands of dark and light minerals

Gneiss

27 Metamorphism of Shale

5: Very High-Grade Metamorphism and partial meltingÆ • Silica rich minerals melt first (quartz and feldspar) • Forming silicic magma •Injected into fractures resulting in silicic veins if intrusive

28 Migmatite

Quartzite

29 Quartzite in Thin Section

Marble

30 in Thin Section

Garnet

Schist Matrix

31 Stability of Minerals

• Most minerals are stable over a relatively narrow range of P and T (e.g., ice unstable above 0°C) • The stability range of different minerals overlap and provide constraints on the metamorphic history of rocks

Mineralogic Changes in Metamorphosed Shales

32 Mineralogic Changes in Metamorphosed Shales

33 Mineralogic Changes in Metamorphosed Rocks

Changes in Composition

Chemical composition little changed during metamorphism

Addition and loss of volatile constituents (H2O, CO2) Non-volatile constituents (anions, cations) redistributed – bulk chemical composition constant Principal changes are in mineral assemblages Mineral assemblage for a given rock composition determined by temperature and pressure

34 • Addition or removal of fluids (and elements) – Water (and other fluids) within rocks and minerals – Moving during metamorphism – Accelerates solid-state chemical reactions and – May change rock composition

Metasomatism • Metamorphism with a high water:rock ratio – water:rock ratio 10:1 – 100:1 – Metamorphism water:rock ratio ~ 1:10 • Water acts as a conduit for changes in chemical composition – Fluids can add and remove materials from rock • Metasomatism associated with – Contact metamorphism – Alteration of rocks at mid ocean ridge systems

35 Submarine hot springs

Metamorphic

• A given set of metamorphic conditions • Each facies is characteristic of particular tectonic environments and will have certain minerals that are diagnostic of those conditions • Therefore, the minerals in a rock can be clues to the (P,T) history of the rock

36

Metamorphic Reactions

Prograde: Mineral changes that take place during an increase in temperature Retrograde: Mineral changes that take place during an decrease in temperature

37 38 Plate Setting and Metamorphism

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