IX. Metamorphic Rocks Evidence of metamorphism The recipe for metamorphism Prograde metamorphism of shale Classification of Metamorphic Rocks Metamorphism and Plate Tectonics

The Rock Cycle Geological Materials Transformation Processes Rock Metamorphism (Increased T & P) Meta- morphic rock Partial Melting Magma

Metamorphism Metamorphic Rocks Definition: Re-crystallization of minerals While still solid Into minerals that are stable at different temperatures and Different pressures (Usually progressively higher temperatures and pressures)

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

Evidence of Metamorphism 2. Flattened Pebbles Differential pressure “squashes” rock and included features

Evidence of Metamorphism 3. Crystalline Texture Minerals tightly interlocking due to recrystallization under pressure

Evidence of Metamorphism 4. New mineral assemblages E.g., Shale: Clay minerals (some quartz)  Metamorphism Forms Schist: Mica, Feldspar and other silicate minerals 

B. Recipe of Metamorphism 1. Parent Rock Even though minerals will change Most elements are provided by parent rock Except water and some dissolved ions Shale Schist

B. Ingredients of Metamorphism Temp. (ºC) 500 1000 A B C 2. Increased Temperature (high geothermal gradients) Minerals stable at lower temperatures converted to minerals stable at higher temperatures Solid state chemical reactions are accelerated

Ingredients of Metamorphism 3. Increased Pressure (and stresses) Increased Confining Pressure as rocks are buried Compression at convergent plate boundary or Shearing as plates slide past each other

Results of Stress (Pressure) Cause Foliation Compressive Stress Shear Stress

Ingredients of Metamorphism 4. 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 5. Time  Millions of years!

C. Prograde Metamorphism of Shale (and the classification of metamorphic rocks) Shale: Fine grained Clay (and quartz) Minerals stable under low T&P (atmospheric) Compaction due to accumulation of sediment Fissility along laminations Water bound in crystalline structure of clay

Prograde Metamorphism of Shale Metamorphic grade 1: Low Grade  Rock: Slate Low grade metamorphic T&P (Water is expelled from crystalline structure of clay) Minerals stable under low grade metamorphic conditions Mineral composition: Fine grained Mica (and Quartz) Foliation: Slaty Cleavage due to realignment of platy minerals

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

Prograde Metamorphism of Shale 3: Medium to High-Grade Metamorphism  Schist Course grained Mica and Quartz Medium-grade metamorphic T&P Causes minerals to grow Minerals stable under medium grade metamorphic conditions appear: Garnet, amphibole and biotite Schistosity due to alignment of platy and needle likeminerals

Prograde Metamorphism of Shale 4: High-Grade Metamorphism  Gneiss Course grained Feldspar, Quartz, Amphibole, Biotite High-grade metamorphic T&P Causes minerals to separate into bands Fig 7.12 Minerals stable under high grade metamorphic conditions appear: feldspar Gneissic banding bands of dark and light minerals

Prograde Metamorphism of Shale 5: Very High-Grade Metamorphism and partial melting Migmatite Silica rich minerals melt first (quartz and feldspar) Forming silicic magma Injected into fractures resulting in silicic veins if intrusive igneous rock Fig 7.12

Metamorphic Rocks of other Parent Rocks Limestone  Marble Bioclastic calcite  Crystalline calcite

Metamorphic Rocks of other Parent Rocks Quartz Sandstone  Quartzite Granular quartz  Crystalline quartz

Metamorphic Rocks of other Parent Rocks Basalt  Amphibole Schist Granite  Gneiss Uniform texture  Foliation (schistosity and gneissic banding)

Where does Prograde Metamorphism occur? Temp. (ºC) 600 1200 A B C Where temperatures and pressures are progressively increased: Subduction zones and Mountain belts Regional Metomorphism at Convergent plate boundaries

Prograde Metamorphism in Mountain Belts

High Low Northeastern North America Northwestern Africa Metamorphic Grade High Low Northeastern North America Northwestern Africa

Review for Exam For each rock type discussed know: Grain size (rounding and sorting if sedimentary) Texture (clastic or crystalline) Other characteristics Various types of metamorphic foliation Various sedimentary structures Environment of formation History of formation Classification Rock

Reading the Grand Canyon

The Grand Canyon 5. Coconino Sandstone 4. Redwall Limestone Fig. 8.16 The Grand Staircase 5. Coconino Sandstone 4. Redwall Limestone Sedimentary Rocks Intrusive Igneous Metamorphic 3. Bright Angel Shale 2. Grand Canyon Pink Granite 1. Vishnu Schist (and gneiss)

The Grand Staircase Fig. 8.16

Eastern Zion National Park

Checkerboard Butte Zion National Park

Vishnu Schist and Pink Granite