Chapter 23: Metamorphic Chapter 23: Metamorphic TexturesTextures

TexturesTextures are small-scale features of rocks are small-scale features of rocks

http://en.wikipedia.org/wiki/List_of_rock_textures

We have already examined the progressive metamorphism of Pelites, mudrocks.

Relict TexturesRelict Textures– Inherited from original rockInherited from original rock

Porphyroclast is a mineral fragment of the Porphyroclast is a mineral fragment of the original rock before dynamic recrystallization original rock before dynamic recrystallization produced the YOUNGER groundmass. produced the YOUNGER groundmass.

A porphyroblast is a large mineral crystal in a A porphyroblast is a large mineral crystal in a metamorphic rock which has grown within the metamorphic rock which has grown within the finer grained groundmass.finer grained groundmass.

Example 1Example 1 Mylonitization of GraniteMylonitization of Granite Mylonite is a fine-grained, compact Mylonite is a fine-grained, compact

rock produced by recrystallization of rock produced by recrystallization of the constituent minerals, and the constituent minerals, and resulting in a reduction of the grain resulting in a reduction of the grain size of the rock.size of the rock.Mylonites are crushed rocks, typically

in shear zones. http://myweb.facstaff.wwu.edu/~talbot/

aa

bb

a. Undeformed Granite

b. Mortar texture in which large porphyroclasts of quartz are surrounded by cataclastic matrix

A porphyroclast is a clast or mineral fragment in a metamorphic rock, surrounded by a younger groundmass of fine grained crystals.

Cataclastic textures are produced by mechanical crushing without significant recrystallization

Progressive mylonitization of a granite

Quartz strong, not crushed

dd

cc

Progressive mylonitization of a granite.

c,d .Foliation evident and grains elongate and become smaller

Example 2Example 2

Progressive Thermal Metamorphism Progressive Thermal Metamorphism of a Diabase (usually a shallow dike, of a Diabase (usually a shallow dike, basaltic in composition)basaltic in composition)

Progressive thermal metamorphism of a Diabase (coarse basaltic)

Stage 1: lots of relict textures from Diabase Augite and Plagioclase

• Plag becomes Albite NaAlSi3O8, so it loses Ca and (Ca -> calcite)

• Augite (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6

hydrates -> Chlorite(Mg,Fe)3(Si,Al)4O10(OH)2

.

(Mg,Fe)3(OH)6

Actinolite• Ca2(Mg,Fe)5Si8O22(OH)2.• Color is greener

Sphene is CaTiSiO5

Progressive thermal metamorphism of a diabase (coarse basalt)

Stage 2- closer to igneous body, thoroughly recrystallized

Epidote forms as more Ca released from Calcite

Even greener

Epidote is Ca2Al2(Fe+3Al)(SiO4)(Si2O7)O(OH)

Progressive thermal metamorphism of a diabase

Stage 3: Warmer yet, Actinolite, Chlorite, Epidote, Calcite become unstable as Hornblende and more calcic plagioclase become stable “Amphibolite”

Note coarser grain size as well

Progressive thermal metamorphism of a diabase (coarse basalt)

Stage 4: Right up against a magma (hot), dehydration at high grade -> Cpx + Plagioclase rock

Note that mineralogy is now ~ same as original diabase, but the granoblastic texture is new

PorphyroblastsPorphyroblasts

A porphyroblast is a large mineral crystal in a A porphyroblast is a large mineral crystal in a metamorphic rock which has grown within the metamorphic rock which has grown within the finer grained groundmass.finer grained groundmass.

Porphyroblasts have difficulty nucleating, are Porphyroblasts have difficulty nucleating, are separated by large distancesseparated by large distances

Surrounding matrix gets depleted in componentsSurrounding matrix gets depleted in components Components must therefore travel long distances Components must therefore travel long distances

by diffusion.by diffusion. Crystallization occurs via single diffusing ions, Crystallization occurs via single diffusing ions,

not by assembly of clumps as in crystallization not by assembly of clumps as in crystallization from liquids.from liquids.

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.

Depletion HaloesDepletion Haloes 3NaAlSi3O8 + CaAl2Si2O8 +3Ca2Mg5Si8O22(OH)2 =3NaCa2Mg4Al3Si6O22(OH)2 +2Ca3Al2Si3O12 +Mg3Al2Si3O12 +18SiO2