Texture categories

Grain size reduction, recrystallization

Crenulations, metamorphic differentiation

Foliation development and overgrowths

Pressure shadows

Bent and broken grains

Contact and burial metamorphism

Relict igneous textures

Fault rocks


This is a somewhat haphazard collection of metamorphic textures and rocks seen in thin section, with some discussion of the processes thought to have made them. Over time, this collection should become more orderly and comprehensive, but don't hold your breath.


All images have two views, in plane- and cross-polarized light. Move the cursor over the visible image to see the other view.


Grain size reduction and recrystallization

Grain sizes in metamorphic rocks are, in part, the result of the competition between deformation, which reduces grain size, and annealing, during which less stable small and deformed grains to grow to forme larger, more crystallographically perfect grains. This section shows transformation of sandstone into quartzite, grain size reduction resulting from deformation, and grain growth during annealing.


Sandstone, with rounded quartz grains cemented together with early hematite, later quartz overgrowths, and lastly illite. This rock is undeformed. Source unknown.


Plane/cross-polarized light, field width is 1.2 mm.


W95


Sandstone that has undergone compaction deformation. At grain contacts, where the quartz was under the greatest stress, it dissolved away and either washed away or was reprecipitated nearby as overgrowths. That process is known as pressure solution. Source unknown.


Plane/cross-polarized light, field width is 1.2 mm.


W49


This sandstone has undergone minor ductile deformation, with individual sand grains having been strained and partly recrystallized to form a polygonal texture. The grains have largely lost their identity, and the rock is now a quartzite. Source unknown.


Plane/cross-polarized light, field width is 1.2 mm.


W48

Strongly deformed quartzite. The grain size of the original sandstone is indicated by the several large feldspar grains, which have dark cracks or other diagonal lines through them. Feldspar is considerably stronger than quartz under mid-crustal conditions. The quartz grains are largely elongate, defining a foliation, with undulatory extinction. Bennington, Vermont.


Plane/cross-polarized light, field width is 1.2 mm.


Bennington_Cheshire


After deformation ceases, small, strained grains recrystallize to grow larger, more crystallographically perfect grains. You can also start to see numerous grain boundaries intersecting at about 120° angles, an indication of approach to textural equilibrium. Williamstown, Massachusetts.


Plane/cross-polarized light, field width is 1.2 mm.


NYSGA95-A7-4


After considerable time, the small, strained grains are replaced by large, strain-free grains, and grain boundaries intersecting at approximately 120°. Recrystallization is driven by the reduction in system energy, caused by lower grain surface area and reduced crystallographic strain. New Salem, Massachusetts.


Plane/cross-polarized light, field width is 1.2 mm.


NS-71D


Crenulations and metamorphic differentiation

Foliations are among the most prominent features of deformed metamorphic rocks. They form by the progressive re-orientation of platy grains into parallelism. Crenulations are small folds, typically axial planar to larger-scale folds, that deform earlier foliations. Under some conditions, as crenulations develop, some minerals like quartz tend to segregate into the fold hinges, leaving less soluble minerals in the fold limbs. This process can result in a fine-scale layering and foliation in a completely different orientation than the first.


This pelitic schist has a strong foliation defined by muscovite and graphite, oriented approximately SW-NE. That foliation has been deformed by crenulations, small folds in the foliation. Crenulations can be regularly spaced and parallel, or irregularly spaced and generally anastomosing. These are the latter variety. The impression of topographic relief is an illusion. New Salem, Massachusetts.


Plane/cross-polarized light, field width is 6 mm.


Tape Rock 1


During crenulation development, quartz (especially) can become concentrated in the crenulation fold hinges, leaving behind other, less soluble minerals in the fold limbs. New Salem, Massachusetts.


Plane/cross-polarized light, field width is 3 mm.


A04


The end result of crenulation-related differentiation can be alternating quartz-rich and mica-rich layers. In hand sample and in the field this can resemble pinstriping. New Salem, Massachusetts.


Plane/cross-polarized light, field width is 3 mm.


A12A


Foliation development and mineral overgrowths

Foliations develop and change as a result of rock deformation. Metamorphic minerals grow as a result of the progress of chemical reactions at different P-T conditions. In some cases, porphyroblasts overgrow the foliation present at the time of growth, preserving it. Such foliation relics can be helpful in the reconstruction of deformation history.


In most cases, porphyroblasts and porphyroclasts contain little or nothing in the way of overgrown foliation. Here, a large, lens-shaped cordierite porphyroclast (an augen) has the foliation wrapped around it. The cordierite was there first, then deformation caused foliation development around the grain. Sillimanite-cordierite-biotite schist, Wales, Massachusetts.


Plane/cross-polarized light, field width is 6 mm.


WE-1


In this case, the albite porphyroclast in the center overgrew an early foliation, oriented SW-NE, defined best by numerous tiny graphite flakes in the albite. Later deformation produced the prominent external foliation, oriented NW-SE, which is absent from the porphyroclast interior. The texture demonstrates that the porphyroclast overgrew an older foliation prior to development of the present, prominent foliation. Florida, Massachusetts.


Plane/cross-polarized light, field width is 3 mm.


NYSGA95-A7-7


This shows a chlorite porphyroblast that grew with its cleavage approximately at right angles to the prominent foliation, defined by muscovite and graphite. As it grew, the chlorite grain overgrew and included graphite flakes. The fact that the overgrown foliation is continuous with that outside the chlorite grain demonstrates that this grain grew after foliation development. Muscovite schist, Charlemont, Massachusetts.


Plane/cross-polarized light, field width is 1.2 mm.


Charlemont Goshen


In this last example, the garnet has included foliation that is NW-SE in the garnet center, changing gradually to N-S at the edges. That can be interpreted to indicate continuous garnet growth along with continuous change in foliation orientation, with the garnet rim foliation matching the present external foliation. Muscovite schist, North Windham, Maine.


Plane/cross-polarized light, field width is 3 mm.


NEIGC86-A4-4


Pressure shadows

Pressure shadows form around relatively rigid grains as the local rock is extended around them. As the matrix rock pulls away from the rigid crystal in the extension direction, fluids precipitate minerals in the potential void spaces. Pressure shadows can be symmetrical, characteristic of flattening, or asymmetrical, characteristic of shear or varying extension direction.


In this case, the rock extended in the E-W direction, opening pressure shadows to the left and right of the biotite porphyroblast. Note that the biotite also includes a relict foliation oriented NW-SE, contrasting with the surrounding E-W foliation. Muscovite schist, Charlemont, Massachusetts.


Plane/cross-polarized light, field width is 3 mm.


Charlemont Goshen


This pressure shadow, on the end of a pyrite grain, contains fibrous quartz that presumably grew in the opening direction. The oldest quartz is in the upper-right part of the shadow, farthest from the pyrite, and the youngest is adjacent to the pyrite, just below its right-most corner. The shadow therefore extended almost N-S first, then extension rotated to the E-W direction, then to NW-SE. Phyllite, Littleton, New Hampshire


Cross-polarized light, field width is 3 mm.


NEGSA13-5


Bent and broken grains

Deforming rocks commonly affect porphyroblasts and other crystals much like logs moving along in a stream, without any significant deformation of the minerals themselves. Weaker minerals, or even strong minerals under the right conditions, can bend and break under the right circumstances. Sheet silicates, for example, are commonly seen bent around porphyroblasts and crenulation hinges. Here are some other examples.


Bent kyanite grain in a mylonitic garnet-kyanite schist. The bending was not smooth, but rather is somewhat kinked, with with thin regions where the extinction angle changes quickly, and thicker regions where it changes more slowly. Visible cracks are recent and not directly related to original bending. Fjørtoft, Norway.


Plane/cross-polarized light, field width is 3 mm.


NOR-35


Broken and boudinaged kyanite, indicating rock extended in the NW-SE direction to pull the two broken ends apart. Notice how the surrounding foliation, and a quartz vein, folded into the potential void space. Chlorite, quartz, and muscovite also grew in the potential void, filling part of the space. Lancaster area, Pennsylvania.


Plane/cross-polarized light, field width is 6 mm.


NEGSA14-1


These biotite crystals are elongate parallel to the dominant foliation, which is defined by muscovite and graphite. The biotite crystals overgrew the foliation, also parallel to the external foliation. Unusually, the biotite cleavage is at right angles to the foliation and the direction of biotite long axes. Extension late in the episode of biotite growth broke the grains along their cleavages and, as they extended, more biotite, but graphite-free, grew in the potential void spaces. Crenulations visible in the foliation outside the biotite crystals are not apparent in the included foliation, indicating that initial growth took place prior to crenulation development. Muscovite-garnet schist, New Salem, Massachusetts.


Plane/cross-polarized light, field width is 3 mm.


Tape Rock 1


This is a biotite-cordierite-sillimanite-garnet gneiss, metamorphosed in the lower granulite facies. During late deformation, a series of mylonitic shear surfaces developed, one of which intersected and broke this garnet, offsetting the two sides by about 0.5 mm. Sturbridge, Massachusetts.


Plane/cross-polarized light, field width is 3 mm.


NEIGC03-C1-14


Contact and burial metamorphism

Burial metamorphism implies relatively high temperatures, without deformation or intense heating from obvious nearby heat sources. The concept is that simple burial gradually allows rock temperature to rise, and metamorphic reactions to progress.


This is a tholeiitic basalt that was relatively warm in the presence of aqueous fluids. The sub-ophitic textures are still evident, although the plagioclase has experienced extensive alteration, most of the pyroxene and interstitial material has transformed into actinolite, and other interstitial material has turned into fine-grained, brown sheet silicates. In the upper-right is calcite, filling an amygdule. Just to the left of the calcite is a crack, along which are colorless epidote crystals that have bright birefringence. South Hadley, Massachusetts.


Plane/cross-polarized light, field width is 3 mm.


HB-1


This is a contact metamorphosed slate, with compositional layering that includes fine-grained graded beds. During contact metamorphism, reactions allowed andalusite to grow. In the center is a relatively large porphyroblast of andalusite, poikilitic with quartz, muscovite, and biotite. Hornfels, Greenville, Maine.


Cross-polarized light, field width is 6 mm.


NEIGC83-C1-1


Slate in which relatively coarse andalusite (center), radially-twinned cordierite (smaller, light-colored spots in plane light), biotite, and muscovite grew during contact metamorphism. The cordierite grains are all surrounded by thin rims of retrograde chlorite, which is light-brown in plane light, black in cross-polarized light. Hornfels, Millinocket, Maine.


Plane/cross-polarized light, field width is 3 mm.


NEIGC94-A3-RR


Originally, this rock was a biotite schist. During contact metamorphism next to a gabbro pluton, this rock experienced very high temperatures, melting, and loss of the melted fraction. The rock is now composed almost entirely of orthopyroxene, cordierite, plagioclase, and magnetite, with small quantities of spinel. It contains no sheet silicates, quartz, or alkali feldspar. About 80% of the light-colored mineral visible here is faintly purple cordierite, which can be compared the colorless plagioclase. Granofels, York, Maine.


Plane/cross-polarized light, field width is 1.2 mm.


4-8-84Ea


Relict magmatic textures in metamorphic rocks

Relict igneous textures in metamorphic rocks are relatively common, mostly requiring that deformation has been limited enough to allow them to survive. These are a few examples of igneous feature that survived regional-scale deformation.


This is a porphyritic basalt that was regionally metamorphosed to epidote amphibolite facies. Despite considerable regional deformation, this particular outcrop remained almost undeformed, and retains intact igneous textures. The original plagioclase phenocrysts are the light-colored blocks in the lower-left and upper-left of this image. They have been transformed into a mass of epidote and albite. The surrounding matrix is made of quartz, albite, chlorite, hornblende, rutile, and magnetite. Epidote amphibolite, Charlemont, Massachusetts.


Plane/cross-polarized light, field width is 3 mm.


10-13-91D


This shows a large patch of plagioclase in a basalt that was metamorphosed to upper amphibolite facies. This amphibolite has a matrix made of hornblende and plagioclase, with minor magnetite. The patch of plagioclase consists of a rim of relatively low-Ca plagioclase, in sharp contact with an inclusion-rich core is made of somewhat higher-Ca plagioclase. The core has mottled birefringence, caused by compositional variations, and grayish patches of opaque oxide dust (not visible at this scale) that may have unmixed from minor Fe dissolved in the original phenocryst. Quabbin Reservation, Massachusetts.


Plane/cross-polarized light, field width is 6 mm.


938Na


This is a metamorphosed gabbro that has been metamorphosed to pyroxene granulite facies under dry conditions. Although the region was subject to severe regional deformation, the strong, dry gabbro resisted deformation. The igneous textures of cumulate plagioclase, with interstitial Fe-Ti oxides and augite, are still visible and largely intact. Metamorphic reactions developed thin "corona" reaction assemblages between Fe-Ti oxides and plagioclase (biotite, hornblende, augite, and garnet), and between igneous augite and plagioclase (hornblende). the reactions also resulted in clouding plagioclase and interstitial augite with millions of minute spinel and Fe-Ti oxide inclusions. Saranac Lake, New York.


Plane/cross-polarized light, field width is 6 mm.


LL


Fault rocks

Fault rocks are a special variety of metamorphic rock that is often overlooked. Fault rocks have a range of characteristics from brittle to ductile. Brittle faulting, foliation development, and grain size reduction tend to make faults weaker than surrounding rocks, a process known as strain-softening. That tends to restrict faults to narrow zones. Other processes, such as deformation speeding up reaction rates, can make deforming rock stronger, especially if strong anhydrous minerals grow at the expense of sheet silicates. That results in strain-hardening, which can allow deformation zones to widen as deformation proceeds, possibly to encompass major parts of orogenic belts. Faults can also reset the mineralogy to that stable at the time of deformation, allowing conditions, and possibly even timing, of faulting to be determined.


Brittle fault zone, where a granitic rock has been broken into angular, offset fragments at different scales. Here, small fracture zones cut larger grains of alkali feldspar and quartz. Alteration of the finest-grained material has produced barely-visible white micas and the rusty stain. Mendham, New Jersey, sample courtesy of Bruce Taterka.


Cross-polarized light, field width is 3 mm.


Mendham_NJ


Protomylonite (grain size reduction 10-50%)that was derived from a granodiorite. More than half of the rock is still porphyroclasts of feldspar left over from the igneous protolith, the rest has undergone severe grain size reduction to form ribbons of biotite and quartz. Annealing, with grain growth, has been minimal, and the fine-grained matrix is made mostly of small, strained grains. Hudson, Massachusetts.


Plane/cross-polarized light, field width is 3 mm.


NEIGC84-A5-5B


Mylonite (50-90% grain size reduced) apparently derived from a granodioritic protolith. Large feldspar porphyroclasts still remain, though less abundant than in the protomylonite immediately above. Ihe matrix contains quartz and feldspars, plus epidote, muscovite, biotite, and titanite. Thrust fault at the base of the Jotun Nappe, Norway.


Plane/cross-polarized light, field width is 3 mm.


NOR-54


Ultramylonite (>90% grain size reduced). This view is part of a sample that varied in composition from basaltic on one end, to granodioritic here, to pelitic at the other end. This granodioritic part shows small garnet porphyroblasts in a fine-grained, foliated matrix of muscovite, biotite, quartz, and feldspar. Bratvåg, Norway.


Plane/cross-polarized light, field width is 3 mm.


NOR-37


Ultramylonite (>90% grain size reduced) from the same sample as that immediately above, but from the pelitic end. This area has considerably more muscovite, and less feldspar. This view shows some muscovite "tectonic fish," porphyroclasts that resemble fish as seen from above. Bratvåg, Norway.


Plane/cross-polarized light, field width is 3 mm.


NOR-37