GEOLOGY 12 NOTES KEY CHAPTER 7 METAMORPHIC ROCKS. Major Concepts

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1 GEOLOGY 12 NOTES KEY CHAPTER 7 METAMORPHIC ROCKS Major Concepts 1. Metamorphic rocks form when increases in heat and/or pressure change the physical and chemical conditions of an existing rock. a. Metamorphic temperatures are greater than those required for DIAGENESIS (i.e. changes during lithification - compaction, cementation, recrystallization, dolomitization) and less than those required to melt a given rock. b. Sources for heat include the normal increase of temperature with depth in the earth's crust (or GEOTHERMAL GRADIENT and, more locally, plutonic activity. c. Sources for pressure include the normal confining pressure at depth and, more locally, regional directed stress associated with TECTONIC activity. d. Chemical conditions may also facilitate metamorphic changes if FLUIDS pass through the rocks during metamorphism. 2. The effects of metamorphism include changes in mineralogy and textures of the initial rocks. a. As temperatures INCREASE, minerals stable at low temperatures break down to form minerals stable at higher temperatures. b. With increasing confining pressure, DENSER minerals may form in metamorphic rocks. c. With increases in directed stress, compositional or textural BANDING or layering may develop in metamorphic rocks. (aka foliation) d. CHEMICAL changes are more difficult to generalize, and also play an important role during metamorphism. 3. Metamorphic rocks are classified on the basis of texture as either FOLIATED or NON-FOLIATED. a. FOLIATED rocks may form when directed stress is involved in metamorphism. b. NON-FOLIATED rocks form under a variety of conditions. 4. Many geologic conditions exist in which metamorphic rocks may form. a. Regional metamorphism, contact metamorphism, and fault-zone metamorphism describe the major conditions in which metamorphism occurs. b. The conditions of temperature and pressure in which metamorphism has occurred can be described in terms of metamorphic FACIES and metamorphic grades. c. The presence of specific MINERALS defines particular metamorphic facies and metamorphic grades.

2 Metamorphism A. Types of Metamorphism Rocks are changed when they are subject to physical or chemical conditions which differ from those under which they were formed. 1. CONTACT Metamorphism a) localized, shallow depth b) zone of metamorphic rock around a pluton in country rock known as a contact aureole 2. FAULT-ZONE Metamorphism a) localized b) generally associated with high pressure/directed stress c) fault breccia formed from angular fragments of rock broken up in a fault zone 3. REGIONAL Metamorphism a) major igneous intrusions cooling slowly b) extreme pressure and heat associated with deep burial c) very widespread migration of hot gases throughout a region (condensation of gases and precipitation of crystals called hydrothermal metamorphism) B. Mineral Changes 1. RECRYSTALLIZATION - small crystals slowly convert to fewer, larger crystals (i.e. slate phyllite schist) 2. NEO-METAMORPHISM - process by which minerals not only recrystallize but also form different minerals. (i.e. muscovite garnet) 3. METASOMATISM - addition or loss of elements through hydrothermal metamorphism (i.e. ion exchange processes from fluids) C. Metamorphic Textures 1. FOLIATED - parallel planes of platy minerals (mostly micas) that have been realigned due to effects of pressure and recrystallization a) SLATY cleavage - low grade b) PHYLLITIC - parallel or wavy foliation of fine grained platy minerals (mostly micas and chlorite) having a silky or metallic lustre, low-intermediate grade c) SCHISTOSITY - parallel foliation of medium to cease grained platy minerals. Intermediate to high grade d) GNEISSIC texture - parallel foliation of medium to coarse grained platy minerals in alternating layers of different compositions, distinctly banded, Intermediate to high grade page 2

3 e) MIGMATITE texture - swirled banding due to melting of lighter colored minerals, darker minerals contorted by their flow, very high grade metamorphism due to partial melting, 2. NON-FOLIATED Textures - no parallel planes, predominantly equidimensional grains, may have stretched fossils or pebbles (i.e. marble, quartzite, ALSO INCLUDES metaconglomerate, amphibolite) D. Metamorphic Grade A METAMORPHIC FACIES (fae-sheez) consists of all metamorphic rock types that formed under similar T and P conditions. Rocks in the same facies have the same mineral assemblage. (see p.145 and diagram next page) E. Index Minerals 1. The most useful index minerals are stable over a narrow range of pressuretemperature conditions. (see p.147) 2. Many FERROMAGNESIAN minerals are useful index minerals. 3. Index minerals provide an ESTIMATE of the metamorphic grade achieved by the rocks that contain them. 4. Quartz and potassium feldspar are common and stable over a large range of T/P conditions. Therefore, they are NOT good INDEX MINERALS. Metamorphic Grades and Their Representative Index Minerals Low Metamorphic Grade (low temperatures and pressures) - about 200ºC rock types = slate and phyllite minerals = chlorite, muscovite, biotite Intermediate Metamorphic Grade rock types = schist minerals = garnet, staurolite High metamorphic grade - 800ºC (verging on melting) rock types = gneiss and migmatite, sillimanite minerals = pyroxene KNOW THE GRADES OF METAMORPHISM COMPLETE THE CHARTS ON THE FOLLOWING PAGE. (See last page for help.) page 3

4 METAMORPHIC GRADES METAMORPHIC FACIES page 4

5 Types of Stress Placed on Rocks CONFINING COMPRESSIVE TENSILE SHEAR PLASTIC rocks will deform under stress (foliations, folds and crystal growth) BRITTLE rocks will rupture under stress (fractures and faults) page 5

6 Coal Formation. (due to increasing depth / temperature & pressure) ANTHRACITE COAL Anthracite is a Metamorphic / Organic / Non-Foliated Rock which is ~95% carbon. (Note: further metamorphism produces the mineral graphite = 0% Carbon) page 6

7 Sample Charts: METAMORPHIC GRADES METAMORPHIC FACIES page 7