High-Strain Metamorphic Textures (shear zones)

Figure 22.2. Schematic cross section through a shear zone, showing the vertical distribution of fault-related rock types, ranging from non- cohesive gouge and breccia near the surface through progressively more cohesive and foliated rocks. Note that the width of the shear zone increases with depth as the shear is distributed over a larger area and becomes more ductile. Circles on the right represent microscopic1 views or textures. From Passchier and Trouw (1996) Microtectonics. Springer-Verlag. Berlin. Chapter 23: Metamorphic Textures High-Strain Metamorphic Textures Concentrate on cataclastic processes (shallower levels) relative to ductile behavior (at depth) • Break, crack, bend, crush, rotate

Cataclastic processes continue at intermediate depths: • Slip and shredding of phyllosilicates (along 001 plane) • Undulose extinction is ubiquitous • Clasts- broken remnants of pre-deformational grains

• Porphyroclast- larger remnant in finer crush matrix . Mortar texture (see next slide) . Ribbons (see next slide) 2 a

b

Figure 23.15. Progressive mylonitization of a granite. From Shelton 3 (1966). Geology Illustrated. Photos courtesy © John Shelton. c

d

Figure 23.15. Progressive mylonitization of a granite. From Shelton 4 (1966). Geology Illustrated. Photos courtesy © John Shelton. Chapter 23: Metamorphic Textures Textures of Contact Metamorphism • Typically shallow pluton aureoles (low-P) • Crystallization/recrystallization is “near-static” = lack of deviatoric stress (i.e., after deformation event) . Monomineralic with low ∆ surface energy → granoblastic polygonal (triple junctions with ~120° between them) . Larger ∆ S.E. → decussate • Isotropic textures (hornfels, granofels) 5 Figure 23.9. Typical textures of contact metamorphism. From Spry (1969) Metamorphic Textures. Pergamon. Oxford.

6 Fig. 23.10 Grain boundary energy controls triple point angles

c.

Figure 23.10. a. Dihedral angle between two mineral types. When the A-A grain boundary energy is greater than for A-B, the angle θ will decrease (b) so as to increase the relative area of A-B boundaries. Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. c. Sketch of a plagioclase (light)-clinopyroxene (dark) hornfels showing lower dihedral angles in clinopyroxene at most cpx-plag-plag boundaries. (c. from Vernon, 1976) Metamorphic Processes: Reactions and Microstructure Development. Allen & Unwin, London. 7 Figure 23.11. Drawings of quartz-mica schists. a. Closer a spacing of micas in the lower half causes quartz grains to passively elongate in order for quartz-quartz boundaries to meet mica (001) faces at 90o. From Shelley (1993). b. Layered rock in which the growth of quartz has been retarded by grain boundary “pinning” by finer micas in the upper layer. From Vernon, 1976) Metamorphic Processes: Reactions and Microstructure Development. Allen & Unwin, London.

b

8 The Crystalloblastic Series Most Euhedral Titanite, rutile, pyrite, spinel Garnet, sillimanite, staurolite, tourmaline Epidote, magnetite, ilmenite Andalusite, pyroxene, amphibole Mica, chlorite, dolomite, kyanite Calcite, vesuvianite, scapolite

Differences in development of Feldspar, quartz, cordierite crystal form among some metamorphic minerals. From Best (1982). Igneous and Least Euhedral Metamorphic Petrology. W. H. Freeman. San Francisco. 9 Progressive thermal metamorphism of a diabase (coarse basalt). From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.

10 Progressive thermal metamorphism of a diabase (coarse basalt). From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.

11 Progressive thermal metamorphism of a diabase (coarse basalt). From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.

12 Progressive thermal metamorphism of a diabase (coarse basalt). From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.

13 Progressive thermal metamorphism of slate. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.

14 Progressive thermal metamorphism of slate. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.

15 Progressive thermal metamorphism of slate. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.

16 Metamorphic Textures

• Contact overprint on earlier regional events are common – Thermal maximum later than deformational – Separate post-orogenic (collapse) event • Nodular overprints • Spotted slates and phyllites

17 a

b

Figure 23.14. Overprint of contact metamorphism on regional. a. Nodular texture of cordierite porphyroblasts developed during a thermal overprinting of previous regional metamorphism (note the foliation in the opaques). Approx. 1.5 x 2 mm. From Bard (1986) Microtextures of Igneous and Metamorphic Rocks. Reidel. Dordrecht. b. Spotted phyllite in which small porphyroblasts of cordierite develop in a preexisting phyllite. Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. 18 Depletion haloes

Progressive development of a depletion halo about a growing porphyroblast. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.

Figure 23.13. Light colored depletion haloes around cm-sized garnets in amphibolite. Fe and Mg were less plentiful, so that hornblende was consumed to a greater extent than was plagioclase as the garnets grew, leaving hornblende-depleted zones. Sample courtesy of Peter Misch. Winter (2010) An Introduction to19 Igneous and Metamorphic Petrology. Prentice Hall. Depletion halo around garnet porphyroblast. Boehls Butte area, Idaho

20 Metamorphic Textures Textures of Regional Metamorphism – Dynamothermal (crystallization under dynamic conditions) – Orogeny- long-term mountain-building • May comprise several Tectonic Events –May have several Deformational Phases – May have an accompanying Metamorphic Cycles with one or more Reaction Events

21 Metamorphic Textures Textures of Regional Metamorphism – Tectonite- a deformed rock with a texture that records the deformation, e.g., preferred mineral orientation – Fabric- the complete spatial and geometric configuration of textural elements • Foliation- planar textural element • Lineation- linear textural element

22 Progressive syntectonic metamorphism of a volcanic graywacke, New Zealand. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.

23 Progressive syntectonic metamorphism of a volcanic graywacke, New Zealand. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.

24 Progressive syntectonic metamorphism of a volcanic graywacke, New Zealand. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.

25 Progressive syntectonic metamorphism of a volcanic graywacke, New Zealand. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.

26 Fig 23.21 Types of foliations a. Compositional layering b. Preferred orientation of platy minerals c. Shape of deformed grains d. Grain size variation e. Preferred orientation of platy minerals in a matrix without preferred orientation f. Preferred orientation of lenticular mineral aggregates g. Preferred orientation of fractures h. Combinations of the above

Figure 23.21. Types of fabric elements that may define a foliation. From 27 Turner and Weiss (1963) and Passchier and Trouw (1996). Figure 23.28. Development of foliation by simple shear and pure shear (flattening). 28 After Passchier and Trouw (1996) Microtectonics. Springer-Verlag. Types of foliations

• Crenulation Cleavage- – Actually consists of 2 cleavages – The first may be a slaty cleavage or schistosity that becomes microfolded – Fold axial planes typically form at high angle to the σ1 of the second compressional phase

29 Progressive development (a → c) of a crenulation cleavage for both asymmetric (top) and symmetric (bottom) situations. From Spry (1969) Metamorphic Textures. Pergamon. Oxford.

30 Figure 23.24a. Symmetrical crenulation cleavages in amphibole-quartz-rich schist. Note concentration of quartz in hinge areas. From Borradaile et al. (1982) Atlas of Deformational and Metamorphic Rock Fabrics. Springer-Verlag. 31 Figure 23.24b. Asymmetric crenulation cleavages in mica-quartz-rich schist. Note horizontal compositional layering (relict bedding) and preferential dissolution of quartz from one limb of the folds. From Borradaile et al. (1982) Atlas of Deformational and Metamorphic Rock Fabrics. Springer-Verlag. 32 Figure 23.25. Stages in the development of crenulation cleavage as a function of temperature and intensity of the second deformation. From Passchier and Trouw (1996) Microtectonics. Springer-Verlag.

Development of S2 micas depends upon T and the intensity of the second deformation

33 Types of lineations a. Preferred orientation of elongated mineral aggregates b. Preferred orientation of elongate minerals c. Lineation defined by platy minerals d. Fold axes (especially of crenulations) e. Intersecting planar elements. Figure 23.26. Types of fabric elements that define a lineation. From Turner and Weiss (1963) Structural 34 Analysis of Metamorphic Tectonites. McGraw Hill. Analysis of Deformed Rocks

• If two or more geometric elements are present, we can add a numeric subscript to denote the chronological sequence in which they were developed and superimposed-

• Deformational events: D1 D2 D3 …

• Metamorphic events: M1 M2 M3 …

• Foliations: So S1 S2 S3 …

• Lineations: Lo L1 L2 L3 … • Plot on a metamorphism-deformation-time plot showing the crystallization of each mineral 35 Deformation vs. Metamorphic Mineral Growth

36 Deformation vs. Metamorphic Mineral Growth

• Pre-kinematic – crystals that show the usual characteristics of minerals affected by later deformation – these include:

– Undulose extinction – Cracked and broken crystals – Deformation bands – Twins – Kink bands – Pressure shadows – Porphyroclasts with mortar texture or sheared rims (mantles) 37 Pre-kinematic crystals a. Bent crystal with undulose extinction b. Foliation wrapped around a porphyroblast c. Pressure shadow or fringe d. Kink bands or folds e. Microboudinage f. Deformation twins

Figure 23.34. Typical textures of pre- kinematic crystals. From Spry (1969) Metamorphic Textures. Pergamon. Oxford. 38 Deformation vs. Metamorphic Mineral Growth

• Post-kinematic – crystallization either outlasted deformation or occurred in a distinct later thermal or contact event. This results in:

– Unstrained, randomly oriented crystals and cut across an earlier foliation (next slide 23.35b) – Pseudomorphs (next slide 23.35f) – a precursor crystal is replaced by an aggregate of random smaller crystals – Polygonal arcs (next slide 23.35c) – folded, elongate minerals polygonize to an arcuate pattern consisting of smaller straight crystals

39 Post-kinematic crystals a. Helicitic folds b. Randomly oriented crystals c. Polygonal arcs d. Chiastolite e. Late, inclusion-free rim on a poikiloblast (?) f. Random aggregate pseudomorph

Figure 23.35. Typical textures of post- kinematic crystals. From Spry (1969) Metamorphic Textures. Pergamon. Oxford.

40 Deformation vs. Metamorphic Mineral Growth

• Syn-kinematic – mineral growth is probably the most common type in orogenic metamorphism since metamorphism and deformation are believed to go “hand in hand”

41 Syn-kinematic crystals

Paracrystalline microboudinage Spiral Porphyroblast

Figure 23.38. Traditional interpretation of spiral Si train in which a porphyroblast is rotated by shear as it grows. From Spry (1969) Metamorphic Textures. Pergamon. Oxford.

Figure 23.36. Syn-crystallization micro-boudinage. Syn-kinematic crystal growth can be demonstrated by the color zoning that grows and progressively fills the gap between the separating fragments. After Misch (1969) Amer. J. Sci., 267, 43.63.42 Syn-kinematic crystals

Figure 23.38. Spiral Si train in garnet, Connemara, Ireland. Magnification ~20X. From Yardley et al. (1990) Atlas of Metamorphic Rocks and their Textures. Longmans.

43 Syn-kinematic crystals

Figure 23.38. “Snowball garnet” with highly rotated

spiral Si. Porphyroblast is ~ 5 mm in diameter. From Yardley et al. (1990) Atlas of Metamorphic Rocks and their Textures. Longmans.

44 Figure 23.37. Si characteristics of clearly pre-, syn-, and post-kinematic crystals as proposed by Zwart (1962). a. Progressively flattened Si from core to rim. b. Progressively more intense folding of Si from core to rim. c. Spiraled Si due to rotation of the matrix or the porphyroblast during growth. After Zwart (1962) Geol. Rundschau, 52, 38-65. 45 Syn-kinematic crystals

Figure 23.40. Non-uniform distribution of shear strain as proposed by Bell et al. (1986) J. Metam. Geol., 4, 37-67. Blank areas represent high shear strain and colored areas are low-strain. Lines represent initially horizontal inert markers (S1). Note example of porphyroblast growing preferentially in low-strain regions.

46 Analysis of Deformed Rocks

Figure 23.42. (left) Asymmetric crenulation

cleavage (S2) developed over S1 cleavage. S2 is folded, as can be seen in the dark

sub-vertical S2 bands. Field width ~ 2 mm. Right: sequential analysis of the development of the textures. From Passchier and Trouw (1996) Microtectonics. Springer-Verlag.

47 Analysis of Deformed Rocks

Figure 23.43. Graphical analysis of the relationships between deformation (D), metamorphism (M), mineral growth, and textures in the rock illustrated in Figure 23.42. Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. 48 Figure 23.46. Textures in a hypothetical andalusite porphyryoblast-mica schist. After Bard (1986) Microtextures of Igneous and Metamorphic Rocks. Reidel. Dordrecht.

Figure 23.47. Graphical analysis of the relationships between deformation (D), metamorphism (M), mineral growth, and textures in the rock illustrated in Figure 23.46. Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. 49 a

b

Figure 23.23. Continuous schistosity developed by dynamic recrystallization of biotite, muscovite, and quartz. a. Plane-polarized light, width of field 1 mm. b. Crossed-polars, width of field 2 mm. Although there is a definite foliation in both samples, the minerals50 are entirely strain-free. 51