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.

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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