Metamorphic Rocks, Part 1 LOWER-GRADE REGIONAL METAMORPHICS

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

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

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

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

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

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

33. Carbonate • Limestone or dolomite • Argillaceous and arenaceous types

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. 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. 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. 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. 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. Elongation Perpendicular to Applied Stress

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

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

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

44. Phyllite Crenulations • Crenulations are often seen in phyllite

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. 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. “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. 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. 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. 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