Metamorphic Textures 2 - Metamorphic Textures 2 - RegionalRegional

TThis part contains several color reproductions of Figures and Tables in your text. I will pass over them with a quick comment on their importance without reading their captions. You can do that at home.

Textures of Regional Textures of Regional MetamorphismMetamorphism

– DynamothermalDynamothermal (crystallization under (crystallization under dynamic conditions)dynamic conditions)

– OrogenyOrogeny- long-term mountain-building- long-term mountain-buildingMay comprise several May comprise several Tectonic EventsTectonic Events

–May have several May have several Deformational Deformational PhasesPhases

– May have an accompanying May have an accompanying Metamorphic CyclesMetamorphic Cycles with one or more with one or more Reaction EventsReaction Events

Textures of Regional Textures of Regional MetamorphismMetamorphism

– Tectonite- Tectonite- a deformed rock with a a deformed rock with a texture that records the deformationtexture that records the deformation

– Fabric-Fabric- the complete spatial and the complete spatial and geometric configuration of textural geometric configuration of textural elementselementsFoliation-Foliation- planar textural element planar textural elementLineation-Lineation- linear textural element linear textural element

Example 4Example 4

Progressive Syntectonic Progressive Syntectonic Metamorphism of a volcanic Metamorphism of a volcanic graywackegraywacke..

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

Phase 1: Both plag and Kfs in isotropic matrix•Feldspars altered to fine sericite and secondary Ca-Al silicates

•Sericite is a fine grained mica, either muscovite or Illite. Sericite is a common alteration mineral of orthoclase or plagioclase feldspars

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

Phase 2: Pervasive foliation develops due mostly to shear•Grain size reduction•Porphyroclasts common and rounded•Matrix recrystallized and new minerals form (Qtz, Ep, Sericite, Ab, Chl) •Chl & Ser enhance foliation

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

Phase 3: Fine-grained schist with larger crystals- no relict textures•Good muscovite and biotite define schistosity•Some metamorphic differentiation to layering•Qtz and Ab are polygonal mosaic in mica-free layers

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

Phase 4: Good schist with coarser grains•More enhanced segregation into layers•Plag no longer Ab- accepts more Ca at higher T•Garnet is a new isograd mineral

Fig 23-21 Types of foliationsFig 23-21 Types of foliationsa.a. Compositional layering Compositional layeringb.b. Preferred orientation of platy Preferred orientation of platy

mineralsmineralsc.c. Shape of deformed grains Shape of deformed grainsd.d. Grain size variation Grain size variatione.e. Preferred orientation of platy Preferred orientation of platy

minerals in a matrix without minerals in a matrix without preferred orientation otherspreferred orientation others

f.f. Preferred orientation of Preferred orientation of lenticular mineral aggregateslenticular mineral aggregates

g.g. Preferred orientation of Preferred orientation of fracturesfractures

h.h. Combinations of the above Combinations of the above Figure 23-21. Types of fabric elements that may define a foliation. From Turner and Weiss (1963) and Passchier and Trouw (1996).

Figure 23-22. A morphological (non-genetic) classification of foliations. After Powell (1979) Tectonophys., 58, 21-34; Borradaile et al. (1982) Atlas of Deformational and Metamorphic Rock Fabrics. Springer-Verlag; and Passchier and Trouw (1996) Microtectonics. Springer-Verlag.

Figure 23-22. (continued)

Analysis of Deformed RocksAnalysis 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 (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

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.

Crenulation: A slaty cleavage or schistocity that becomes microfolded

Quartz grains commonly dissolve by pressure solution from the steep limbs and precipitate in the hinge

Figure 23-27. Proposed mechanisms for the development of foliationsa.Mechanical rotation. b. Preferred growth normal to compression. c. Grains

with advantageous orientation grow whereas those with poor orientation do not (or dissolve). d. Minerals change shape by ductile deformation. e. Pressure solution. f. A combination of a and e. g. Constrained growth between platy minerals. h. Mimetic growth following an existing foliation.

Mimetic – growth during non-shear conditions follows old orientation

Figure 23-28. Development of foliation by simple shear and pure shear (flattening). After Passchier and Trouw (1996) Microtectonics. Springer-Verlag.

Figure 23-28. Development of foliation by simple shear and pure shear (flattening) a. Beginning with randomly oriented planar or linear b. Beginning with equi-dimensional crystalsc. Beginning with pre-existing foliationShaded figures represent an initial sphere and the resulting strain ellipsoid

Development of an axial-planar cleavage in folded metasediments. Circular images are microscopic views showing that the axial-planar cleavage is a crenulation cleavage, and is developed preferentially in the micaceous layers. From Gilluly, Waters and Woodford (1959) Principles of Geology, W.H. Freeman; and Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.

Figure 23-3. Plastic deformation of a crystal lattice (experiencing dextral shear) by the migration of an edge dislocation (as viewed down the axis of the dislocation).

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 S i 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.

Syn-kinematic crystalsSyn-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.

Figure 23-19. Mantled porphyroclasts and “mica fish” as sense-of-shear indicators. After Passchier and Simpson (1986) Porphyroclast systems as kinematic indicators. J. Struct. Geol., 8, 831-843.

Figure 23-18. Augen Gneiss. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.