Metamorphic Textures IN THIS LECTURE Processes important in the development of metamorphic textures Removing strain and recrystallisation High strain metamorphic textures Contact metamorphic textures Triple point junctions

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

Metamorphic Textures The processes of deformation, recovery, and recrystallization are central to the development of metamorphic textures 1. Cataclastic Flow Mechanical fragmentation and sliding, rotation of fragments Crush, break, bend, grind, kink, deformation twins, undulose extinction, shredding of micas, augen, mortar, etc. Technically not metamorphic 2. Pressure Solution Products: fault gouge, breccia, “cataclasite”

Metamorphic Textures Pressure Solution Pressure solution in grains affected by a vertical maximum stress and surrounded by a pore fluid. 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. 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.

Metamorphic Textures 3. Plastic Intracrystalline Deformation No loss of cohesion Several processes may operate simultaneously Defect migration Slip planes Dislocation glide Deformation twinning

Metamorphic Textures 4. Recovery Loss of stored strain energy by vacancy migration, dislocation migration and annihilation Polygonization- general term for formation of low-strain subgrains Cataclasis + recrystallization (delicate interplay) Cataclasis promotes recrystallization- lattice strain E & smaller crystals

Metamorphic Textures 5. Recrystallization Grain boundary migration Subgrain rotation Solid-state diffusion creep at higher T Crystalplastic deformation (general term) Grain boundary sliding and area reduction Coalescence- recovery and recrystallization by which large grains form by the addition of smaller strained grains by grain boundary migration Coalescence- general term for the collective process of recovery and recrystallization by which large grains form by the addition of smaller strained grains by grain boundary migration Building back up following polygonization which reduces

Formation of two strain-free subgrains via dislocation migration Metamorphic Textures Formation of two strain-free subgrains via dislocation migration

Metamorphic Textures a b (a) Undulose extinction and (b) elongate subgrains in quartz due to dislocation formation and migration

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

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 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.

High-Strain Metamorphic Textures Concentrate on cataclastic > ductile (shallower) Break, crack, bend, crush, rotate Slip and shredding of phyllosilicates Clasts- broken remnants Porphyroclast- larger remnant in finer crush matrix Mortar texture Ribbons Pseudotachylite

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

High-Strain Metamorphic Textures Nice simple classification based on textures

An alternative classification that attempts to consider the processes Good for understanding the processes, but not for naming rocks Figure 22-3. Terminology for high-strain shear-zone related rocks proposed by Wise et al. (1984) Fault-related rocks: Suggestions for terminology. Geology, 12, 391-394.

Contact Metamorphic Textures Typically shallow pluton aureoles (low-P) Crystallization/recrystallization is near-static Monomineralic with low D surface energy ® granoblastic polygonal Larger D S.E. ® decussate Isotropic textures (hornfels, granofels) Relict textures are common May be some stress due to moderate P or emplacement of the pluton Minerals are often equidimensional- Quartz and carbonates may be isotropic even when regionally met and other minerals foliated Elongated minerals typically randomly oriented

Progressive thermal metamorphism of a diabase (coarse basalt) Progressive thermal metamorphism of a diabase (coarse basalt). From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco. Progressive thermal metamorphism of a diabase (coarse basalt) Incipient met- lots of relict textures from diabase Plag -> albitic so loses Ca and Al (Ca -> calcite) Augite hydrates -> chlorite, actinolite

Progressive thermal metamorphism of a diabase (coarse basalt) Progressive thermal metamorphism of a diabase (coarse basalt). From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco. Stage 2- closer to igneous body- textures now metamorphic, thoroughly recrsytallized Epidote forms as more Ca released

Progressive thermal metamorphism of a diabase (coarse basalt) Progressive thermal metamorphism of a diabase (coarse basalt). From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco. Warmer yet, Actinolite, Chlorite, Epidote, Calcite become unstable as Hornblende and more calcic plagioclase become stable Note coarser grain size as well

Progressive thermal metamorphism of a diabase (coarse basalt) Progressive thermal metamorphism of a diabase (coarse basalt). From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco. Right up against gabbro body (hot), dehydrate at high grade -> Cpx + Plagioclase rock Note that mineralogy is now ~ same as original diabase, but the texture is completely different

Progressive thermal metamorphism of slate. From Best (1982) Progressive thermal metamorphism of slate. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco. Progressive thermal met of slate- pelite 1) Original slate texture- already low-grade regional met.

Progressive thermal metamorphism of slate. From Best (1982) Progressive thermal metamorphism of slate. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco. Coarser Random (isotropic) fabrics Low-P minerals- And + Crd (pelite mineralogy very sensitive to P and T) Poikiloblasts are common

Progressive thermal metamorphism of slate. From Best (1982) Progressive thermal metamorphism of slate. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco. Higher T -> coarser grains and less porphyroblastic Random texture New hi-T mineral: sillimanite coexists here with andalusite implying what?

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.

Contact Metamorphic Textures Typical textures of contact metamorphism. From Spry (1969) Metamorphic Textures. Pergamon. Oxford.

Grain boundary energy controls on triple point angles a) A-A GBE > A-B -> larger dihedral angle b) A-A GBE < A-B -> smaller dihedral c) Sketch of plag-cpx in which cpx has lower angle a. Dihedral angle between two mineral types. When the A-A grain boundary energy is greater than for A-B, the angle  will decrease (b) so as to increase the relative area of A-B boundaries. Winter (2001) 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.

Grain boundary energy controls on triple point angles Drawings of quartz-mica schists. a. Closer 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. a b Micas have high surface energy -> Q-Q boundaries meet mica at 90o Also closer spacing of micas + 90o angle -> elongation and foliation to Q layers b) Finer micas “pin” quartz to finer grain sizes

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

a b 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 (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

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. Depletion halo around garnet porphyroblast. Boehls Butte area, Idaho Progressive development of a depletion halo about a growing porphyroblast. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.