Metamorphic rocks

Types of Metamorphism, Texture, Structure of Metamorphic Rocks

1. Metamorphic Rocks Dr. V. R Ghodake Sinhgad College of Engineering , Vadgaon(Bk ), Pune. Email- [email protected] Mobile- 9764484757

2. Definition of Metamorphism The alteration of a pre-existing igneous, sedimentary, or metamorphic rock (called the “parent rock” or “ protolith” ) by the application of heat and/or pressure. Complete melting does not occur.

3. They are altered or changed beyond their recognition, i.e. change in Chemical composition, texture and structure Metamorphic rocks When rocks are baked by heat of molten magma or squeezed by the movements of huge tectonic plates or by the pressure of overlying thick succession of rocks

4. Fig. Ta 7.1

5. Metamorphism High temperature High pressure Chemically active fluids Agents of Metamorphism

6. The source of temperature is either from magma or due to the depth factor Metamorphism usually result into change in min. comp. and texture of rocks (Ig. and Sed.) which are subjected to temp. Low-grade metamorphism: Occurs within 100 C to 500 C . High-grade metamorphism: Occurs at > 500 C Temperature

7. Uniform Pressure It is present at great depth. The pressure is due to overlying rocks If depth increases the uniform pressure also increases with respect to temperature . Directed Pressure It is present at shallow depth. The pressure operates during folding movements If the depth increases the directed pressure decreases

8. UNIFORM PRESSURE Types of Pressure increases with depth due to increase in overburden . acts vertically downwards and affects the volume of both liquid & solids. increases with depth up to some extent, effective in the upper part of the crust. DIRECT or Differential PRESSURE acts in all direction and affects only on solids resulting into deformation of shape and change in mineral composition High temperature is also associated due to depth factor high temperature is not always associated. to (depth factor) Lithostatic pressure- due to overburden Stress- due to tectonic forces

9. Compressive Stress Shearing

10. Uniform Stress Directed Stress min inter max

11. Granite Granite-Gneiss

12. Metamorphosed Conglomerate

13. Effects of Metamorphism Mineralogical: - change minerals to reflect new P-T (equilibrium) conditions . Recrystallization: change of existing crystal size and shape to form layers (e.g., shale to schist), interlocking crystals (e.g., sandstone to quartzite). Recombination: Recombination of elements in pre-existing minerals to form stable new ones. Addition/subtraction of ions common when fluid is present.

14. Development of Foliation

15. Types of Metamorphism Contact Metamorphism- This type of metamorphism occurs locally adjacent to the igneous intrusion; with high temp. and low stress There is little change in bulk composition of the rock Area surrounding the intrusion (Batholith) is heated by the magma; metamorphism is restricted to a zone surrounding the intrusion, this zone is know as METAMORPHIC AUREOLE . The rocks formed are non-foliated fine-grained rocks called as HORNFELS .

16. Types of Metamorphism: Contact Thermal, local, around intrusions. Size of aureole depends on: Size of intrusion Heat (composition) Fluid content of magma Fluid content of country rock Country rock type

17. Types of Metamorphism Cataclastic Metamorphism This type of metamorphism occurs mainly due to direct pressure e.g. when two bodies of rock slide past one another along a fault zone. Heat is generated by the friction of sliding along the zone, and the rocks tend to crushed and pulverized due to the sliding. Cataclastic metamorphism is mechanical breakdown of rocks without any new mineral formation, however, sometime due to intense shearing few new minerals are formed.

18. Regional Metamorphism metamorphism occurs covering larger area, which is subjected to intense deformation under direct or differential stress. Rocks formed under such environment are usually strongly foliated, such as slates, schist's, and gneisses. The differential stresses result from tectonic forces, e.g. when two continental masses collide with one another resulting into mountain building activity. Compressive stresses result in folding of the rock Regional Metamorphism

19. Types of Metamorphism : Produces non-foliated, granoblastic rocks: Hornfels (if clastic - shale) Quartzite (if sandstone) Marble (if carbonate)

20. Types of Metamorphism: Regional Associated with Mountain buildings Also known as Dynamothermal - produces both foliated and non-foliated metamorphic rocks. Heat & directed pressure on rocks buried deep within the Earth - Prograde or Retrograde . Associated with mountain belts - affects very large areas. Prograde metamorphic reactions liberate a fluid. Retrograde is difficult – pore spaces decrease during prograde so getting fluid back in is not easy.

21. Regional Metamorphism

22. Types of Metamorphism : Meteorite impact – sudden and intense deformation.

23. Types of Metamorphism : High-pressure polymorph of quartz – coesite – can form. Impact melt can form. Lots of rock fragmentation & mineral deformation. Moon – no atmosphere so lots of meteorite impacts (micro and macro!). Produces regolith, rock flour, impact melt, breccias .

24. Water in Metamorphism Provides transport mechanism and can promote reactions. Hydrothermal metamorphism : hot water streams add/remove ions. May promote ore formation.

25. Metasomatism : addition of ions from external source. Water in Metamorphism

26. Water in Metamorphism Metamorphic aureole is greater around granitic plutons than around gabbroic plutons, even though the magma temperature is lower.

27. Ore-bearing veins in a mine

28. Metamorphic Grade : Degree of parent rock alteration, mostly dependent on increasing temperature for increasing grade Prograde : SLATE - phyllite -schist-gneiss- migmatite (melting). SLATE : oriented clay minerals allows the rock to be easily cleaved.

29. Foliation can be deformed

30. Prograde : slate- PHYLLITE -schist-gneiss- migmatite (melting). PHYLLITE : growth of microscopic micas to give strong foliation – rock is “shiny”.

31. Prograde : slate- phyllite - SCHIST -gneiss- migmatite . SCHIST : strongly foliated with visible micas & feldspars .

32. Garnet Schist

33. Prograde: slate- phyllite -schist-GNEISS- migmatite . GNEISS: strongly banded rock with dark bands of micas & pyroxenes and light bands of feldspars and quartz.

34. Gneissose Banding

35. Prograde : slate- phyllite -schist-gneiss- MIGMATITE . MIGMATITE : strongly veined rock as it has partially melted.

36. 0065

37. STRUCTURES IN METAMORPHIC ROCKS Foliation: when platy, lamellar or flaky minerals (e.g.. sheet silicate minerals the micas: biotite and muscovite, chlorite, talc, and serpentine), occurring in rock orient themselves parallel to one another (i.e. perpendicular to the direction of maximum pressure or stress). Random orientation Of minerals Preferred orientation Of minerals

38. SLATY CLEAVAGE U sually formed during the early stage of Low-grade Metamorphism due to Lithostatic stress. New sheet-structure minerals tends to be parallel to the bedding planes during metamorphism. However , further deep burial along the continental margin; compressional forces will cause deformation (folding). hence , the sheet minerals as well as foliation will no longer be parallel to the bedding planes, such type of foliation in fine grained rocks is called slaty cleavage.

39. Shale Slate

40. SCHISTOSE STRUCTURE Usually formed during intermediate and high grade metamorphism Grain size increases and can be seen by naked eye; grains tends to enlarge with increasing grade of metamorphism; the coarse grained sheet-structure minerals show preferred orientation grain size is the main difference between the slaty structure and schistose structure.

41. GNEISSIC STRUCTURE Usually associated with high-grade regional metamorphism (where differential stress prevails I.e. tectonic forces) where the sheet silicates and other minerals like quartz/feldspars/hornblende/pyroxene are segregated in distinct bands in the rocks- known as gneissic banding.

42. Classification of Metamorphic rocks based on texture/structures PHYLLITE -similar to slate, but slightly coarser phyllosilicate grains -grains can be seen in hand specimen, giving silk appearance to cleavage surfaces -often cleavage planes less perfectly planar than slates SLATE -strongly cleaved rock -cleavage planes are developed due to orientation of fine phyllosilicate grains e.g. Muscovite, biotite, chlorite etc. -individual grains too fine to be visible with naked eye -overall dull appearance

43. SCHIST Parallel alignment of moderately coarse grains ( fabric= schistocity ) -grains are visible by eye -mainly phyllosilicate and other minerals such as hornblende, kyanite etc. GNEISS Coarse grained rock (grain size several millimeters) and -foliated (planar fabric: either schistosity or compositional layering ) Tendency for different minerals to segregate into layers parallel to foliation (gneissic layering): typically quartz and feldspar rich layers tend to separate from micaceous layers. Varieties: --Orthogenesis: rocks formed from Igneous rocks -- paragneiss: rocks formed from Sedimentary rocks -metasedimentary gneisses

44. QUARTIZITE -it comprise equidimensional minerals viz. quartz and feldspars Non foliated; show GRANULOSE STRUCTURE

45. Types of Metamorphic Rocks FOLIATED The common foliated rocks in the order of increasing grain size are SLATE – PHYLLITE – SCHIST – GNEISS NON-FOLIATED Quartzite's and hornfels

46. Importance of Metamorphic rocks SLATES Fine grained impermeable, cleavable and soft Incompetent; cannot withstand great loads But since they are impermeable and split easily; thin large sized slabs of uniform thickness can be extracted for roofing purpose. Economic importance: Since they are bad conductor of electricity– used in electrical industries for switch board base Slate

47. GNEISS Gneissic rocks are rich in SILICA i.e. predominantly Quartz and Feldspars along with garnet, pyroxene, Hornblende etc. Non-porous and impermeable nature increases the strength of the rock Foliated character to some extend improves workability Load perpendicular to foliated planes gives more stronger foundation If mineral assemblage is more or less similar to Granite (with less % mafic minerals) then: It is used as building stone As aggregate for making concrete As road metals etc.

48. Gneiss

49. SCHIST Mainly composed of prismatic or platy minerals, which contributes in development of Schistose Structure. Eg. Hornblende, tourmaline, sillimanite etc (prismatic); chlorite, muscovite, biotite, talc, kyanite etc. (platy) Cleavable nature of Schists is the main reason for their weakness; they are incompetent GARNET-MICA SCHIST Schist Biotite Schist

50. The minerals that compose gneiss are the same as granite . Feldspar is the most important mineral that makes up gneiss along with mica and quartz. Gneiss can be formed from a sedimentary rock such as sandstone or shale , or it can be formed from the metamorphism of the igneous rock granite . Gneiss can be used by man as paving and building stone. PARENT ROCK

51. QUARTZITE SANDSTONE (composed of quartz/feldspars/feldspathoid minerals) when under go metamorphism result into Quartzite. Granulose texture/structure (Granoblastic) makes them most competent rock amongst all other metamorphic rocks. Because metamorphism of sst . Result disappearance of cementing material, bedding planes, fossil content etc. Quartzite's are compact, hard and strong; very less porous and less permeable than the parent sandstone. Predominance of Quartz makes the rock very hard and suitable for road metal; can be used as concrete aggregate etc. Acts as strong foundation for any C.E. structure .

52. MARBLE Latin word “Marmor”– Shining stone. Calcareous metamorphic rock Though it shows granulose structure it is not as hard as Quartzite because of its Calcareous composition; but can withstand reasonable load. Due to its pleasant colour and brilliant appearance when polished it is extensively used as building stone. Marble

53. Marble Mine

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