Metamorphic Rocks Part 1 LOWERGRADE REGIONAL METAMORPHICS Slate

Metamorphic Rocks, Part 1 LOWER-GRADE REGIONAL METAMORPHICS Slate, Phyllite, “Greenstone” and Schist 1

Metamorphic Rock Definition • A sedimentary, igneous, or previously existing metamorphic rock which has undergone textural, structural, and/or mineralogical changes due to the action of one or more agents of metamorphism 2

Agents of Metamorphism • Changes in pressure or stress • Changes in temperature • Chemically active fluids 3

Recrystallization • Metamorphism involves recrystallization of original minerals in the rock that is undergoing metamorphism • In some minerals, notably quartz, feldspar, and calcite, this may lead to a simple increase in grain size - the orientation of the grain may be modified in the process. • In other minerals, especially clays, chlorites, dolomite, and other carbonates, recrystallization is accompanied by neomineralization, the formation of new minerals not present in the rock prior to metamorphism 4

Stress • Stress (differential pressure) is one of the most powerful factors influencing metamorphic rock textures. • Effects § § Development of mechanical fractures Complex minor folding Development of foliation Rapidly applied stress may result in “crush” rocks 5

Stress Continued • Stress plays a chemical as well as physical role - reduction in grain size increases the surface area available for reaction • Developments of shear planes and fractures provides routes for the movement of chemically active fluids • Stress may also be a source of localized heat - friction of rocks moving past each other may heat up the rocks along the contact 6

Foliation Example Quartz-mica schist • A foliation is any planar fabric in a metamorphic rock • Here, foliation is defined by aligned sheets of muscovite sandwiched between quartz grains 7

Strain • Strain is deformation due to applied stress • Strain in crystals partially breaks bonds, thus facilitating the conversion to new mineral species 8

Temperature • Change in temperature is a potent metamorphic agent • In otherwise unmetamorphosed rocks exposed to the heat from an igneous intrusion, changes in the type of mineral present are often observed 9

Effects of Temperature • The solubility of minerals in water generally increases as temperature rises, although there are many exceptions to this rule (gypsum, calcite, etc. ) • Phase boundaries are crossed § Some minerals become unstable, while others become stable • Chemical reaction rates increase rapidly with increasing temperature § Endothermic reactions are favored 10

Effects of Temperature • Generally minerals with more open crystal structures are favored at higher temperatures • Increasing temperature tends to drive off water and carbon dioxide, which serve to increase the activity of the fluid phase which they enter 11

Effects of Pressure • Pressure tends to counter the effects of temperature § Water and carbon dioxide are retained to higher temperatures as pressure increases • Pressure tends to favor minerals with closed, compact (denser) mineral structures § Metamorphic rocks produced at greater depths are denser than those produced near the surface 12

Effects of Pressure • Pressure is determined largely by burial depth § At great depths, fluids are usually unable to escape and any fluid pressure present will be added to the load pressure § Toward the surface, fluids usually escape § Load pressure is hydrostatic (equal in all directions) § An increase in load pressure tends to prevent stress fractures from forming, and to close that exist 13

Chemically Active Fluids • Many progressive reactions (from lower to higher grade across a metamorphic terrain) liberate water, carbon dioxide, and other mineralizing agents • Mineral assemblage present will vary tremendously in environments high or low in these mineralizing agents, particularly water 14

Fluid Release • Presence of these fluids often depends on the original compositions of the rock. § Wet sediments will release large quantities of water during metamorphism. § Basalts will not § Limestones will release carbon dioxide during metamorphism 15

Combination of Agents • Although it is possible for any agent acting alone to produce metamorphism, in most cases two or more agents will work synergistically. • Many combinations are possible • Several types of metamorphism, involving one or more agents, are commonly recognized 16

Types of Metamorphism • • • Regional metamorphism Contact (or thermal) metamorphism Dynamic metamorphism Impact metamorphism Metasomatism 17

Regional Metamorphism • By far, the most volumetrically important as the name suggests, that these rocks occur over extensive areas • Results from the combined effects of heat, pressure, and stress, with chemically active fluids often playing a role, at least with parts of a regional metamorphic complex • Often regional metamorphism is associated with orogenesis 18

Regional Metamorphism, Cont. • Granitic intrusions are often associated with regional metamorphic complexes § This association raises many questions, especially as to whether the granite is the cause of or the effect of metamorphism • It is often possible to define several zones of progressive metamorphism in regional complexes § Either the “grade” or the “facies” system may be used to classify these zones 19

“Grade” Classification • The grade system is the product of the British petrologists, principally Barrow, Harker, and Tilley • Classification based on the first appearance of certain characteristic minerals • Minerals are chlorite, biotite, garnet, staurolite, kyanite, and sillimanite 20

Facies Classification • The facies system of Eskola is more commonly used in the United States • Classification based on the characteristic mineral associations • The facies system allows a parallel classification with mafic igneous rocks § This means that a rock formed from a magma, by recrystallization from a solid, or from hydrothermal solution, will have similar mineralogical associations • The facies system puts somewhat more emphasis on pressure than the grade system 21

Eskola Facies System • The diagram shows the principal regional metamorphic facies • Note that pressure increases downward 22

Contact Metamorphism • Contact metamorphism is the result of heat from an intrusion of magma altering the country rock around it • This type of metamorphism was formerly called “thermal” metamorphism because it was believed that heat was the only agent of metamorphism involved - this may sometimes be true 23

Contact Metamorphism, Cont. • However, the heat often releases water and carbon dioxide, and these fluids play an important role in many cases • Thus, the name contact metamorphism is more appropriate 24

Dynamic Metamorphism • Dynamic metamorphism occurs when rocks move along fault zones • Stresses involved are large • Frictional heat and fluids also may plate a role • Fluid metamorphism is often temporally later than the stress metamorphism § Volumes involved are small § No samples are available, and we will not examine this type of rock 25

Impact Metamorphism • Result of large scale impacts, usually of meteoritic origin • Temporary pressures of megapascals are possible • Temperature spikes of short-duration may also play a role 26

Impact Metamorphism, Cont. • This type of metamorphism was important during the early history of the earth, but has become less frequent with time • The volume of rock metamorphosed is not large 27

Impact Metamorphism, Cont. • The major importance of impact metamorphic studies are in recognizing and/or confirming the occurrence of events such as the K-T impact that caused mass extinctions • The establishment of the frequency of these events is far more significant than the petrographic study of the rocks 28

Metasomatism • Metamorphosis through the action of chemically active fluids • Frequently occurs during other types of metamorphosis, but has begun to be recognized as a type of metamorphism in its own right • We will examine rocks, often carbonate containing, that may be either regional or contact § These rocks likely involve metasomatic changes, sometimes with the aid of other agents 29

Parent Rock Compositions • Original composition of the country rock plays a tremendous role in the type of metamorphic rock formed • While a complete discussion of this topic is nearly a course in itself, but some of the principles can be outlined here • We can recognize five basic categories of country rock 30

Pelite • A sediment or sedimentary rock composed of the finest detritus, clays or mud-size particles, or a calcareous sediment composed of clays and minute quartz particles • Often these sediments are aluminous • Pelite is sometimes used to mean the metamorphic equivalent of an argillaceous 31 rock

Psammite • A clastic sediment or sedimentary rock composed of sand-sized particles • Synonymous term is arenite • Sometimes called the metamorphic equivalent of arenite 32

Carbonate • Limestone or dolomite • Argillaceous and arenaceous types 33

Felsic to Intermediate Igneous Rocks • “Granite”, Diorite or equivalent among the intrusive rocks • Extrusive igneous rocks are less commonly metamorphosed, but may be if they are buried - Rhyolite to Dacite 34

Mafic to Ultramafic Igneous Rocks • Gabbros, Peridotites, Pyroxenites may be metamorphosed • Extrusive rocks such as basalt are frequently metamorphosed, because they are dragged down a subduction zone, or cut in an accretionary prism melange 35

Slates and Phyllites • Slates and phyllites characteristically form from pelitic rocks. • Little difference in mineralogy from the original rock • Typical principal minerals are quartz, feldspar, sericite, and chlorite • At slightly more advanced metamorphic levels, biotite will be present, usually with either chlorite or sericite 36

Slates and Phyllites • A close relative of biotite, stilpnomelane, is another new mineral that may form in these rocks • If manganese is present, garnets may form in the biotite zone • Some loss of water occurs during the formation of these rocks 37

Foliation in Slates and Phyllites • Both slates and phyllites show welldeveloped rock cleavage • The cleavage may be parallel to the original bedding in some phyllites • In other phyllites and in most slates, the cleavage cuts across the bedding 38

Elongation Perpendicular to Applied Stress 39

Slaty Cleavage • Gray slate showing well-developed slaty cleavage 40

Slate Photomicrograph • Note the fine grain size and the unimpressive foliation in this weakly -metamorphosed rock • Locality: Vermont 41

Andalusite • Upper (CN): The distinct chiastolite cross that is characteristic of andalusite is easily seen in this section • Lower (PP): The distinct chiastolite cross that is characteristic of andalusite is easily seen in this section • First order white/gray interference colors • Moderately high positive relief 42

Phyllite • Phyllite showing the sheen typically associated with it • Larger grain size of phyllite produces the sheen 43

Phyllite Crenulations • Crenulations are often seen in phyllite 44

Phyllite Photomicrographs • Sample: Ira Phyllite • Note the wavy foliation and the overall fine-grain size of this rock • Location: Vermont • Upper photo CN • Lower photo PP 45

Slates and Phyllites of Psammitic Origin • Psammitic rocks show little changes in hand specimen under low-grade metamorphism • The grains of quartzitic sandstones may become elongated and interlocking, but this can only be seen by microscopic examination of thin sections 46

“Greenstones” • Greenstones are usually mafic to ultramafic rocks that formed under conditions of hightemperature and often high-pressure • If these rocks undergo low-grade metamorphism, the formation condition is greatly different than the conditions of metamorphism 47

Greenstones Continued • As a result, they undergo almost a complete mineralogical change. • Chief changes involve addition of water and carbon dioxide • Many of these rocks are undeformed, and may preserve igneous textures • The most important new mineral is usually chlorite • The feldspar is often albitized 48

Schists • Schists exhibit much larger grain sizes than slates or phyllites • They correspond to phaneritic igneous rocks. • Hand specimen examination of schists reveals much more about composition than from the examination of slates and phyllites 49

Mica Schist • • Most common type of schist Not all schists are micaceous Mica schists are typically of pelitic origin The micaceous minerals include sericite, muscovite, chlorite, biotite or stilpnomelane, and sometimes talc 50

Mica Schist Continued • Quartz is almost invariably present • Unless formed from a graywacke or similar rock, the quartz is recrystallized and larger than in the original rock • Feldspars include microcline, perthite, and/or sodic plagioclase (albite to oligoclase) • Generally speaking, the higher the grade of metamorphism, the higher the calcium content in plagioclase, although the availability of calcium affects this generalization 51

Mica Schist Continued • As metamorphism intensifies, typical metamorphic minerals include chloritoid, garnet, staurolite, and kyanite • Cordierite andalusite are typical of contact metamorphosed pelites, but may be present in regionally metamorphosed terrains in which metasomatism is involved • Sillimanite is indicative of the highest grades of metamorphism in schist, although it is more commonly found in gneisses 52

Mica Schist • Mica makes this schist quite shiny 53

Garniferous Mica Schist • Garnets often develop in higher-grade schists 54

Crenulation Cleavage Photomicrograph, CN • The vertical foliation in this rock is a crenulation cleavage, and developed after the horizonal foliation • Muscovite-biotite -garnet schist • Location: New Mexico 55

Whiteschist • This is a photomicrograph of a high-pressure schist from the famous Dora Maira massif in Parigi, Italy • The region of coarser-grained quartz in the upper center portion of this photomicrograph was probably originally Metamorphic rocks from the occupied by coesite, the high. Dora Maira Massif show other pressure polymorph of quartz evidence of being exhumed • Quartz-kyanite-garnetfrom EXTREMELY deep levels in thickened crust (>28 muscovite schist kbar) Photo: K. Stewart 56

Glaucophane Schist • Upper (CN): Glaucophane schist with a large, isotropic garnet in the center, surrounded by highly birefringent glaucophane and white mica • Lower (PP): A mixture of lightly colored glaucophane, colorless white mica, and dark, high relief epidote in a low grade schist 57

Glaucophane • Occurrence: In low grade metamorphic rocks, associated with white mica, albite, quartz, chlorite, epidote, and occasionally lawsonite and jadeite • Strong pleochroism, blue to violet to colorless to pale brown; amphibole cleavage • Photos at left are in plane polarized light, illustrating the unique pleochroism of glaucophane 58

Stilpnomelane • Golden stilpnomelane in plane polarized light (below) and under crossed nicols (above) • Stilpnomelane occurs in some low grade, burial metamorphic rocks, with quartz, white mica, garnet, etc. 59

Stilpnomelane • Stilpnomelane in a characteristic sheaflike arrangement • Plane polarized 60

Quartz Elongation in Schist • Quartz grains have been stretched perpendicular to the direction of stress 61

Tourmaline Schist • Hexagonal crystals of tourmaline clearly visible • Tourmaline commonly occurs in metamorphosed sedimentary rock 62

Psammitic Schist • Psammitic schists may be similar to pelitic schists if the rock is exposed to the activity of alkaline solutions, which convert quartz to mica, or if the rock is arkosic or a feldspathic sandstone 63

Carbonate and Schist • Limestones and dolomites associated with schists are often deformed by plastic flow • Silication reactions (reaction of carbonate with silica) may occur if the rock is an impure carbonate, with a clay or silica component, or if siliceous fluids from hydrothermal or pegmatitic fluids occurs • Typical minerals include tremolite, actinolite, diopside, epidote, phlogopite, scapolite, and serpentine 64

Greenstone Schists • Commonly form from mafic igneous rocks • Hornblende is a major mineral • May be accompanied by epidote and albite in the epidote part of the amphibolite facies, or by plagioclase more calcic than albite in the amphibolite facies • Hornblende becomes darker as grade increases, the grains are coarser, and the habit more pronounced 65

Greenstone Schist Continued • Large structures, such as pillows, may be preserved • Fine-grained mafic lavas generally lose their structure as the result of recrystallization • Primary diabasic texture from dikes or sills may be preserved 66

Hornblende Schist • Hornblende schist is a typical greenstone schist, probably derived from a mafic igneous rock 67

Actinolite • Upper (CN): Actinolite in a groundmass of Mg-rich chlorite photo shows the upper first-order to mid second-order interference colors of actinolite • Lower (PP) Actinolite in a groundmass of Mg-rich chlorite • Crystal form: columnar, bladed, or acicular crystals, elongate parallel to the c-axis, basal sections (cleavage visible) are 68 diamond shaped