Protoliths
Image credit: Scott Brande
Pathways from Protoliths to Metamorphic Rocks
ALL Metamorphic Rocks Have Parents!
Any pre-existing rock - sedimentary, igneous or metamorphic - may be heated and/or squeezed within the earth. The product of this process is a metamorphic product that has been transformed from the pre-existing rock, termed a protolith. The possible pathways of transformation are as simple or complex as the geological events producing heat and pressure, from a simple igneous intrusion, to complex plate tectonic motions. Metamorphism can even proceed backwards, termed 'retrograde'!
The diagram below is an extreme simplification of the many possible pathways for the origins of a limited set of common metamorphic rocks.
Click to enlarge.
Quartz Sandstone to Quartzite
- quartz sandstone features
- medium-grained (sand-sized grains)
- variable mineral composition
- quartzite features
- medium-grained (sand-sized grains)
- tightly interlocked grains (enlarge image below to see)
- typically not foliated (but may be foliated by later deformation)
Click image to enlarge.
quartz sandstone
Image credit: R. Weller, Cochise College
Click image to enlarge.
quartzite
Image credit: James St. John
Fossiliferous Limestone to Marble
- fossiliferous limestone features
- sedimentary rock
- minerals (calcite, aragonite, dolomite)
- fossil shells, fragments
- +/-matrix (mud, sand, gravel)
- marble features
- fine- to coarse-grained crystals
- lack fossils
- may be various colors
- typically not foliated (but original sedimentary layering of different beds may resemble foliation )
Click image to enlarge.
fossiliferous limestone
Image credit: James St. John
Click image to enlarge.
marble
Image credit: James St. John
Conglomerate to Metaconglomerate
- conglomerate features
- sedimentary rock
- gravel-sized, rounded fragments
- +/-matrix (mud, sand, gravel)
- variable composition for fragments and matrix
- metaconglomerate features
- gravel-sized, rounded, elliptical fragments
- elliptical fragments generally parallel (long axis)
- foliation seen as parallel alignment of long axes
Click image to enlarge.
conglomerate
Image credit: James St. John
Click image to enlarge.
metaconglomerate
Image credit: R. Weller, Cochise College
Shale to Slate
- shale features
- fine-grained
- typical sedimentary layering
- tends to split into thin, flat sheets
- slate features
- fine-grained
- foliation present as cleavage that differs from original bedding or lamination
Click image to enlarge.
shale
Image credit: R. Weller, Cochise College
Click image to enlarge.
slate
Image credit: James St. John
Slate to Phyllite
- slate features
- fine-grained
- relatively dull (non-reflective) surface
- phyllite features
- fine-grained
- distinctly reflective surface from flat mica crystals in parallel layers
- foliation seen in parallel alignment of rippled surface (crenulations) and microscopic crystals
- may exhibit few, medium- to coarse-grained, mineral crystals (e.g., garnet)
Click image to enlarge.
slate
Image credit: James St. John
Click image to enlarge.
phyllite
Image credit: James St. John
Phyllite to Schist
- phyllite features
- fine-grained
- distinctly reflective surface from mica crystals
- foliation seen in parallel alignment of rippled surface (crenulations) and microscopic crystals
- may exhibit few, medium- to coarse-grained, mineral crystals (e.g., garnet)
- schist features
- medium- to coarser-grained (individual crystals distinct without magnification - "grainy")
- variable minerals
Click image to enlarge.
phyllite
Image credit: James St. John
Click image to enlarge.
schist
Image credit: James St. John
Granite to Gneiss
- granite features
- coarse-grained (phaneritic)
- felsic composition (feldspar, quartz), some mafic minerals (e.g., biotite mica)
- gneiss features
- coarse-grained (phaneritic)
- felsic composition (feldspar, quartz), some mafic minerals (e.g., biotite mica)
- segregation of felsic and mafic minerals into distinct layers (bands)
- foliation seen as parallel alignment of layers in bands
Click image to enlarge.
granite
Image credit: R. Weller, Cochise College
Click image to enlarge.
gneiss
Image credit: James St. John
Mudstone to Hornfels
- mudstone features
- fine-grained (mud-sized grains)
- variable mineral composition
- may show (or not) developed sedimentary feature(s) (e.g., mudcracks, burrows, fine layering)
- hornfels features
- fine-grained (mud-sized grains)
- variable color
- irregular fracture
- lacks sedimentary features
- distinctly non-foliated
mudstone
with mudcracks
Image credit: R. Weller, Cochise College
hornfels
Image credit: R. Weller, Cochise College