Metamorphic Textures Textures of Regional Metamorphism 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 It’s your task to unravel the complex history of these rocks...

Chapter 23: Metamorphic Textures Textures are small-scale penetrative features Relict Textures Inherited from original rock “Blasto-” = relict Any degree of preservation Pseudomorphs of minerals or pre-metamorphic textures/structures Relict bedding by alt qtz & Al-rich layers or opaques Blasto-porphyritic Confusion with suffix “-blast” meaning of true metamorphic origin Porphyroblast

Chapter 23: Metamorphic Textures The Processes of Deformation, Recovery, and Recrystallization Pressure Solution Pressure solution in grains affected by a vertical maximum stress and surrounded by a pore fluid. Figure 23-2 a. Highest strain in areas near grain contacts (hatch pattern). b. High-strain areas dissolve and material precipitates in adjacent low-strain areas (shaded). The process is accompanied by vertical shortening. c. Pressure solution of a quartz crystal in a deformed quartzite (s1 is vertical). Pressure solution results in a serrated solution surface in high-strain areas (small arrows) and precipitation in low-strain areas (large arrow). ~ 0.5 mm across. The faint line within the grain is a hematite stain along the original clast surface. After Hibbard (1995) Petrography to Petrogenesis. Prentice Hall.

Chapter 23: Metamorphic Textures b Figure 23-4 a. Undulose extinction and (b) elongate subgrains in quartz due to dislocation formation and migration Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Dislocations in the crystal lattice 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).

Recrystallization by grain boundary migration and sub-grain rotation Bulging -> serrated boundaries Figure 23-6. Recrystallization by (a) grain-boundary migration (including nucleation) and (b) subgrain rotation. From Passchier and Trouw (1996) Microtectonics. Springer-Verlag. Berlin. Figure 23-7a. Recrystallized quartz with irregular (sutured) boundaries, formed by grain boundary migration. Width 0.2 mm. From Borradaile et al. (1982).

High-Strain Metamorphic Textures (shear zones) Truly cataclastic where shallow- narrow fault zone with gouge, breccia As -> deeper get wider, more distributed, more ductile component 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 microscopic views or textures. From Passchier and Trouw (1996) Microtectonics. Springer-Verlag. Berlin.

a Progressive mylonitization of a granite, San Gabriel Mts, California Excellent mortar in (b) b Figure 23-15. Progressive mylonitization of a granite. From Shelton (1966). Geology Illustrated. Photos courtesy © John Shelton.

c Grain-size reduction Increased foliation Ribbon texture is forming in both (d) is a classic ultramylonite d Figure 23-15. Progressive mylonitization of a granite. From Shelton (1966). Geology Illustrated. Photos courtesy © John Shelton.

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 Feldspar, quartz, cordierite Least Euhedral Crystalloblastic Series Differences in development of crystal form among some metamorphic minerals. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.

Depletion haloes 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 (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Progressive development of a depletion halo about a growing porphyroblast. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.

Progressive syntectonic metamorphism of a volcanic graywacke, New Zealand. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco. Progressive syntectonic metamorphism of a volcanic graywacke, NZ Both plag and Kfs in isotropic matrix Alteration to fine sericite and secondary Ca-Al silicates

Progressive syntectonic metamorphism of a volcanic graywacke, New Zealand. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco. 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. Fine-grained schist with larger crystals- no relict gy textures Good muscovite and biotite define schistosity Some metamorphic differentiation to layering Qtz and Ab are polygonal mosaic in mica-free layers (even though dynamothermal)

Progressive syntectonic metamorphism of a volcanic graywacke, New Zealand. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco. 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 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 Note in (e) that linear minerals may also define a foliation if randomly oriented in a plane Use S-surface terminology So – S1 – S2 – S3 … Figure 23-21. Types of fabric elements that may define a foliation. From Turner and Weiss (1963) and Passchier and Trouw (1996).

a b Continuous schistosity by dynamic recrystallization of biotite, muscovite, and quartz Minerals are entirely strain-free so crystallization outlasted deformation 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 minerals are entirely strain-free.

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

Symmetric crenulation cleavage 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.

Asymmetric crenulation cleavage 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.

Post-kinematic: Si is identical to and continuous with Se Pre-kinematic: Porphyroblasts are post-S2. Si is inherited from an earlier deformation. Se is compressed about the porphyroblast in (c) and a pressure shadow develops. Syn-kinematic: Rotational porphyroblasts in which Si is continuous with Se suggesting that deformation did not outlast porphyroblast growth. From Yardley (1989) An Introduction to Metamorphic Petrology. Longman.

Pre-kinematic crystals Bent crystal with undulose extinction Foliation wrapped around a porphyroblast Pressure shadow or fringe Kink bands or folds Microboudinage Deformation twins Figure 23-34. Typical textures of pre-kinematic crystals. From Spry (1969) Metamorphic Textures. Pergamon. Oxford.

Post-kinematic crystals Helicitic folds b. Randomly oriented crystals c. Polygonal arcs d. Chiastolite e. Late, inclusion-free rim on a poikiloblast (?) f. Random aggregate pseudomorph Helicitic folds in Si. b. Randomly oriented elongate crystals. c. Polygonal arcs of straight crystal segments in a fold. d. Chiastolite with central Si concordant with Se. e. Late, inclusion-free rim on a poikiloblast. The rim may be post-kinematic. f. Random aggregate of grains of one mineral pseudomorphing a single crystal of another (chlorite after garnet). The chlorite is post-kinematic. Figure 23-35. Typical textures of post-kinematic crystals. From Spry (1969) Metamorphic Textures. Pergamon. Oxford.

Syn-kinematic crystals Paracrystalline microboudinage Spiral Porphyroblast Si and Se for internal and external S in porphyroblasts Question as to whether porphyroblasts rotate or matrix rotates around them 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.

Syn-kinematic crystals Spiral Garnet in schist 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.

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. Snowball Garnet in schist

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. Assume S3 -> bent micas

Coronas and Reaction Rims Figure 23-53. Reaction rims and coronas. From Passchier and Trouw (1996) Microtectonics. Springer-Verlag.

The orthopyroxene-clinopyroxene coronas and subsequent garnet developed as the anorthosite cooled during burial and compression due to thrust stacking and nappe development The amphibole corona developed during uplift as water became available, and later rapid uplift and decompression at relatively high temperatures resulted in the decomposition of garnet to produce a symplectitic intergrowth (“S”) of fine-grained orthopyroxene + plagioclase  spinel  clinopyroxene. Figure 23-54. Portion of a multiple coronite developed as concentric rims due to reaction at what was initially the contact between an olivine megacryst and surrounding plagioclase in anorthosites of the upper Jotun Nappe, W. Norway. From Griffen (1971) J. Petrol., 12, 219-243.

Photomicrograph of multiple reaction rims between olivine (green, left) and plagioclase (right).

Coronites in outcrop. Cores of orthopyroxene (brown) with successive rims of clinopyroxene (dark green) and garnet (red) in an anorthositic matrix. Austrheim, Norway.

Sense of shear indicators 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.