METAMORPHISM & METAMORPHIC ROCKS Chapter 8

A. Introduction page 168-170

1. Metamorphism is the processes by which metamorphic rocks are formed

2. Definition: Metamorphism is a process of mineralogical and/or textural changes
in a rock in the solid state in response to changes in the physical (and chemical)
environment excluding weathering and diagenesis.

a. Mineralogical change---any formation of new minerals or disappearance of original minerals

b. Textural change---change in size, shape of grains or relationship among grains

c. Solid state---no appreciable melting (not exceeding 5-10 %)

d. Physical environment---only consider temperature, T, and pressure, P

e. Chemical environment---water usually plays a role in metamorphism and can add or subtract element

from the rock being metamorphosed; however, for simplicity, assume NO change in chemical composition

f. Weathering---physical and chemical changes rocks undergo in the near surface environment

g. Diagenesis---minor chemical and mineralogical changes sediment may undergo in become sedimentary rock

B. Types of Metamorphism Note: Book considers 3 types of metamorphism; I consider only 2 types

1. Burial metamorphism (not important)

2. Dynamic metamorphism (not important) (ignore, although books considers this)

3. Impact metamorphism (not important)

4. Contact Metamorphism---see pp. 170-173 ---associated with igneous intrusions

5. Regional Metamorphism—see pp. 173-174---associated with fold-belt mountain systems

C. Metamorphic Environments: T vs. P (actually, depth of burial in km) graph (not in text)

1. Increase of both T & P at increasing depths within the earth

a. Geothermal Gradient----Temperature increase about 15-30 degrees/km
with depth. The Gradient is higher at plate boundaries and also varies
with depth below the surface. See p. 212 (chap 9)

b. Pressure Gradient--Lithostatic Pressure increases with depth—think of stacking blocks of rock

2. NOTE the following points made from the diagram shown in class (not in text)

a. Because of geothermal gradient, there are certain environments that do not occur within the earth

b. The boundaries between the Sedimentary and Metamorphic Environments and, particularly, between

Metamorphic and Igneous Environments are continuous, gradational and indistinct.

c. Contact Metamorphism is “pressure” independent ---T increases without an increase in P

(note: CM is associated with igneous intrusions---therefore, no compressional stress)

d. Regional Metamorphism is progressive from Low Grade (lo T, P) to Medium Grade to
High Grade (hi T, P) as T increases, so does pressure (note: associated with fold-belt
mountain formation at plate boundaries) (see Fig 8.9, page 174; Consider both T&P to increase from left to right across the diagram---not just T as the figure indicates)

D. Metamorphic Textures and Processes

1. Chemical Composition

a. Contact Metamorphic Rks---compositional highly variable (controlled by whatever the intrusion passes through)

b. Regional Metamorphic Rks—for simplicity assume all have about the same composition

2. Textural Change: increase in grain size

a. Grains physically grow (overgrowths, for example) ---both contact and regional

b. Process = Recrystallization—grains are “welded together” through solution/precipitation or melting/crystallizing see Figs. 8.17, 8.18 p. 180

Two examples of Recrystallization (note: no mineral change)

Sedimentary Rock Mineral Metamorphic Rock

Sandstone Quartz Quartzite

Limestone Calcite Marble

3. Textural Change: parallel alignment of flat platy minerals (clays, micas, etc.) = Foliation (regional metamorphics) pp. 175-176

Flat platy minerals orient with the flat faces perpendicular to the direction of stress The process by which foliation develops is (Mechanical) Deformation.

4. Mineralogical Change---Process = Chemical Recombination

5. Time: Metamorphism often occurs over geological (large) spans of time

E. Progressive (or “Prograde”) Metamorphism (regional metamorphism)

Consider processes and changes going from sedimentary rock (shale) to igneous (granite) see the bottom of Figure 8.9, page 174 again

shale (sed. rock) -----slate (low grade meta.)----phyllite----schist----gneiss (high grade)----granite (igneous rock)

Know this progressive (Prograde) sequence with increasing T&P

F. Types of Metamorphic Rocks (see Table 8.1, p. 175 ---you do need to know all of this information, just that given below.

1. Non-foliated Metamorphic Rocks: tend to be CONTACT metamorphic rocks

Know: marble, quartzite, hornfels

2. Foliated Metamorphic Rocks: tend to be REGIONAL metamorphic rocks
Know: slate, phyllite, schist, gneiss