Metamorphism and Metamorphic Rocks (Chapter 8)

I. The same elements that originally went into igneous minerals were incorporated into sedimentary rocks by fragmentation and dissolution, and have now been heated, pressurized (and possibly chemically altered) to form metamorphic rocks.

A. Conditions have gone far beyond those found near the surface of the Earth (i.e., temperature >200-300^oC & pressure greater than a few hundred bars) due to burial, plate collisions, and intrusion of magma.

II. Agents of metamorphism

A. Heat may be most essential because thermal energy breaks bonds & increases reaction rates. Heat source is intruding magma or the normal geothermal gradient [i.e., rate of increase of temperature with depth within the Earth - (20-30^oC/km; Fig.4.18)]

B. Pressure produces denser minerals & recrystallization by compressing the preexisting minerals. It’s called directed pressure when not applied uniformly and results in foliation (e.g., rock cleavage, slaty cleavage, and schistosity) {Figures 8.6-8.9}

C. Chemically active fluids introduce new elements.

III. Common metamorphic rocks (Figure 8.12)

A. The foliated rocks below are listed in order of increasing grade or intensity of metamorphism (Figures 8.9-8.11, 8.13-8.15):

SLATE - PHYLLITE - SCHIST – GNEISS:

1. These rocks show increasing grain size, increasingly higher grade index minerals, & more distinct foliation from rough parallelism of platy minerals to distinct bands of differing minerals.

B. Marble and quartzite are non-foliated rocks (metamorphic equivalents of limestone and sandstone, respectively; Figures 8.16-8.18).

IV. Types of metamorphism

A. Contact metamorphism-near contact between magma and rock during intrusion; causes new minerals, larger grains and possibly introduction of new elements by hydrothermal solutions. Temperature most important, but chemically active fluids also required.

1. Large intrusions cause aureoles of decreasing intensity of metamorphism to develop around them (Figs. 8.19-8.20).

B. Metamorphism along fault zones-caused by purely mechanical processes as blocks of crust grind past one another pulverizing the existing minerals (Figure 8.23).

C. Regional metamorphism-great bulk of metamorphic rocks subjected to both increasing temperature and pressure-not a localized process but instead form from regional heating and deformation (often directed pressure causing foliation) See Figures 8.24 and 8.26.

1. DEPENDING ON THE COMPOSITION OF THE ORIGINAL ROCK, DIFFERENT MINERAL GROUPS WILL RESULT FROM VARIOUS COMBINATIONS OF TEMPERATURE AND PRESSURE

2. Index minerals = good geothermometers & geobarometers (Fig.8.25 and Box 8.2)

a. Aluminosilicate polymorphs (Al₂SiO₅) (Understand phase diagram shown in Box 8.2 and explained in class)

V. Minerals found almost exclusively in metamorphic rocks

1. Al₂SiO₅ polymorphs, garnet, epidote, staurolite

VI. Other important minerals (not exclusively metamorphic)

1. Quartz, mica, plagioclase, amphibole, pyroxene

VII. Understand the relationships shown in Fig. 8.25.