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

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Presentation on theme: "1 Metamorphic Rocks, Part 1 LOWER-GRADE REGIONAL METAMORPHICS Slate, Phyllite, “Greenstone” and Schist."— Presentation transcript:

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

2 2 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

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

4 4 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

5 5 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

6 6 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

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

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

9 9 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

10 10 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

11 11 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

12 12 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

13 13 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 those that exist

14 14 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

15 15 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

16 16 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

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

18 18 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

19 19 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

20 20 “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

21 21 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

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

23 23 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

24 24 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

25 25 Dynamic Metamorphism Dynamic metamorphism occurs when rocks move along fault zones Stresses involved are large Frictional heat and fluids also may play 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

26 26 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

27 27 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

28 28 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

29 29 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

30 30 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

31 31 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 rock

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

33 33 Carbonate Limestone or dolomite Argillaceous and arenaceous types

34 34 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

35 35 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

36 36 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

37 37 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

38 38 Foliation in Slates and Phyllites Both slates and phyllites show well- developed 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

39 39 Elongation Perpendicular to Applied Stress

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

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

42 42 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

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

44 44 Phyllite Crenulations Crenulations are often seen in phyllite

45 45 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

46 46 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

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

48 48 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

49 49 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

50 50 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

51 51 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

52 52 Mica Schist Continued As metamorphism intensifies, typical metamorphic minerals include chloritoid, garnet, staurolite, and kyanite Cordierite and 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

53 53 Mica Schist Mica makes this schist quite shiny

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

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

56 56 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 occupied by coesite, the high- pressure polymorph of quartz Quartz-kyanite-garnet- muscovite schist Metamorphic rocks from the Dora Maira Massif show other evidence of being exhumed from EXTREMELY deep levels in thickened crust (>28 kbar) Photo: K. Stewart

57 57 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

58 58 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

59 59 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.

60 60 Stilpnomelane Stilpnomelane in a characteristic sheaf- like arrangement Plane polarized

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

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

63 63 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

64 64 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

65 65 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

66 66 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

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

68 68 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 diamond shaped


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