Geology 12 Presents Metamorphic Rocks Chp 7 Metamorphic

Geology 12 Presents

Metamorphic Rocks

Chp 7 Metamorphic Rocks Change Shape • Process by which heat, pressure and fluids change pre-existing rock in the solid state

• Where? : mainly associated with the continents. Sedimentary rocks (& sediments) cover much of the continents, but they are underlain by mainly igneous and metamorphic rocks making up the basement/craton. Not crouton!! • Where the basement is exposed is the shield. • Basement/shield: very stable over the last 600 ma and as 4 ba. million years billion years

Canadian Shield

Agents of Metamorphism: (3) • 1. Heat: increases rate of chemical reactions – 2 sources of heat: • a) Magma body: heats surrounding rock

– b) geothermal gradient: Earth warms up as you descend • Average of 25’C/km • Question: what is temperature at 30 km? T = 30 km x 25’C/km + 10’C T = 750’C + 10’C T = 760’C

• 2. Pressure: – 2 types • A) Lithostatic Pressure = confining pressure = weight of overlying rock – Applied equally in all directions – Increases 0. 25 Kbars/km of depth » (1 Kbar = 14, 700 psi)

• b) Differential Pressure = directed pressure = pressure due to mountain building – Pressure is unequal so rock is distorted conglomerate Meta-conglomerate

• 3. Fluid Activity: water & CO 2 enhance metamorphism by increasing chemical reactions by moving ions through solution Ex: 2 Mg 2 Si. O 4 + 2 H 2 O Mg 3 Si 2 O 5(OH)4 + Mg. O Olivine water serpentine away in sol’n • 3 sources of water: a) Sedimentary rock pore space (sh, sst) b) Released from magma c) Dehydration of water bearing minerals such as gypsum (Ca. SO 4 2 H 2 O)

Types of Metamorphism (3) • 1. Contact Metamorphism: when a hot body of magma alters surrounding rock with increased temperatures and the release of hot fluids (Hi T/Low P + fluids) Aureole = concentric zones of alteration Country rock Igneous intrusion slight metamorphism (rare) intermediate metamorphism (medium) extensive metamorphism (well-done)

rare medium Well done

• The larger the intrusion (more heat + more fluids) = more alteration – Contact Met’ is generally localized (small areas affected) • i) dikes & sills: only rock in immediate contact affected • ii) laccoliths & batholiths: larger area affected • 2 Types of contact Met’ – i) baked (porcelain) clay: HEAT – ii) as magma cools, water is released making new minerals -> hydrothermal alteration – valuable ore deposits

Contact Metamorphism

Country Rock Contact Metamorphism Intrusion

• 2. Dynamic Metamorphism: very localized alteration around fault zones due to differential pressure (Hi P/Low T) Slikensides or fault breccia

Dynamic Metamorphism

Fault breccia

• 3. Regional Metamorphism: results in most metamorphic rocks – Occurs over large areas – Caused by tremendous pressures, temperatures, and deformation, deep in the crust – Usually along convergent plate margins, but… Oceanic crust Upper mantle Low T/Hi P (differential) zone Continental crust Upper mantle Hi T/Hi P (lithostatic) zone

Regional Metamorphism

• …but, sometimes at divergent boundaries (hi T + fluids) ocean Descending sea water Met’ rocks Hot magma High heat flow

n Hand out note helper WS 7. 1 a

Classification of Metamorphic Rocks • 2. 5 types! • 1. Foliated: minerals have arranged in a parallel fashion…result of differential pressure

Foliated Metamorphic Rocks

Rock Parent Rock Appearance Metamorphic Grade Slate sh, volcanic fine grained, low ash splits into flat pieces Phyllite sh fine grained, low-med silvery sheen Schist sh, carbonates flakey micas low-high mafic ign’ rx Gneiss sh, sst, dark & light high felsic ign’ rx bands “Zebra Rock”

Slate

Phyllite

Notice “slight sheen”

Schist

Gneiss = “Zebra Rock”

“Zebras are gneiss”

• 2. Non-foliated: minerals have no orientation, “equidimensional” (no layers, no banding) Rock Marble Parent Appearance Rock Lst sugary, HCl Quartzite Qtz sst sugary, H=7 Metamorphic Grade low-high med-high

Non-foliated Metamorphic Rocks Marble

Quartzite Ford tough

• 2. 5 If nature of original rock can be recognized… Rock Parent Appearance rock congloflattened Metaconglomerate pebbles Metamorphic Grade low – med

Partially Foliated Rock Meta-conglomerate

Metaconglomerate

• Generally, metamorphic grade is indicated by: – 1. increase in crystal size – 2. increase in foliation, mineral segregation, or granitization of minerals – 3. index minerals: reversal of Bowen’s Reaction Series (move up to dark/Ca plag’ Fs minerals)

Pressure increasing DYNAMIC MET’ diamonds graphite REGIONAL Igneous rx granite gneiss Meltin g poin METAMORPHISM t schist phyllite coal slate Sh marble Sedimentary rocks Lst CONTACT METAMORPHISM Temperature increasing

X Y Z Low P/Hi T Contact Meta’ Y Hi P/Low T Regional Meta’ Z Hi P/Hi T Regional Meta’ X

High T Low P

n n n Do Lab 7. 1 Metamorphic Rocks ID Do WS 7. 1 Do Chp 2 -7 Review WS