Textural and Mineralogical Changes
The new orientation will be essentially perpendicular to the direction of stress
Textural and Mineralogical Changes
The resulting mineral alignment usually gives the rock a layered of banded
appearance termed foliation (‫)التورق‬
Origin of pressure in metamorphism
Textural and Mineralogical Changes


Lower-grade Metamorphism: Slaty Cleavage )‫(قصم األردواز‬.
 Alignment of very fine-grained micas
)‫(اصطفاف البلورات الدقيقة لمعدن المايكا‬
Higher-grade Metamorphism: Schistosity )‫ (التورق المنقسم‬.
 At intermediate and high grades of metamorphism,
grain size increases.
 Alignment of large mica (diameter is about one cm)
flakes is called schistosity.
)‫ لمعدن المايكا‬-‫ سم‬1 ‫التي يبلغ قطرها حوالي‬- ‫(اصطفاف البلورات الكبيرة‬
Textural and Mineralogical Changes
Not all metamorphic rock have a foliated texture. Metamorphic rocks composed
of only one mineral that forms equidimensional crystals )‫ (البلورات متساوية األبعاد‬are
generally nonfoliated )‫(عديمة التورق‬
Common metamorphic rocks
(‫`)الصخور المتحولة الشائعة‬

Nonfoliated rocks (‫)الصخور عديمة المتورقة‬
 Quartzite
(‫)الكوارتزيت‬
Formed from a parent rock of quartz-rich
sandstone
 Quartz grains are fused (combined) together
 Forms in intermediate temperature and pressure
conditions

Sample of
quartzite
Thin section
of quartzite
Common metamorphic rocks

Nonfoliated rocks

Marble (‫)الرخام‬





Coarse, crystalline
Parent rock usually limestone or dolomite
Composed of calcite crystals
It fizzes in acid
Fabric can be random or oriented
Marble (Random fabric = annealing; nonfoliated)
Common metamorphic rocks

Nonfoliated rocks
 Hornfels
(‫)هورنفلس‬
A fine-grained, tough, dense, hard, and massive
rock
 It forms through contact metamorphism
 The parent rock is commonly siltstone or basalt

Common metamorphic rocks

Foliated rocks (‫)الصخور المتورقة‬
 Slate
(‫)االردواز‬
Rocks become more dense and compact
 Very fine-grained
 Excellent rock cleavage
 Made by low-grade metamorphism of shale
 Dull luster
 Clay and mica minerals

Example of slate
Common metamorphic rocks

Foliated rocks
 Phyllite
(‫)الفياليت‬
Grade of metamorphism between slate and
schist
 Intermediate temperature and pressure
 Made of small platy minerals
 Shiny luster
 Composed mainly of muscovite and/or chlorite
minerals

Phyllite (left) and Slate (right)
lack visible mineral grains
Common metamorphic rocks

Foliated rocks
 Schist
(‫)الشيست‬
Medium- to coarse-grained crystals
 High temperature and pressure
 Comprised of platy minerals (micas)
 The term schist describes the texture
 To indicate composition, mineral names are
used (such as mica schist)

Mica Schist - note well developed foliation
A mica garnet schist
Common metamorphic rocks

Foliated rocks
 Gneiss
(‫)نايس‬
Medium- to coarse-grained crystals
 Banded appearance
 High-grade metamorphism (high temperature
and pressure)
 Composed of light-colored feldspar layers with
bands of dark mafic minerals

Gneiss displays bands of light and dark
minerals
Common metamorphic rocks

Foliated rocks

Migmatite (‫)المغماتيتات‬
A very high grade metamorphic rock that has been
subjected to such high temperatures that it has
partially melted
 It is intermediate between the metamorphic and the
igneous rocks

Change in metamorphic grade with depth
Metamorphism and plate tectonics
(‫)التحول و حركة االلواح‬

Most regional metamorphism occurs along
convergent plate boundaries
 Compressional
stresses deform plate edge
 Occurs in major mountain belts: Alps,
Himalayas, and Appalachians
Location of metamorphic zones
in a subduction zone
Metamorphism and plate tectonics

Metamorphism at subduction zones
 Cores
of subduction zones contain linear
belts of metamorphic rocks
High-P, low-T zones near trench (‫)غور محيطي‬
 High-T, low-P zones in region of igneous activity
(‫( )النشاط الناري‬volcanic arc) (‫)االقواس البركانية‬

Location of metamorphic zones
in a subduction zone
Metamorphic Environments (‫)بيئات التحول‬

Index minerals (‫ )المعادن الدالة‬and
metamorphic grade
 Certain
minerals, called index minerals,
are good indicators of the metamorphic
conditions in which they form
Index Minerals in metamorphic rocks
Metamorphic Environments

Metamorphic grade
 A group
of minerals that form in a
particular P-T environment
Zeolite (really low T,P; <200 C)
Greenschist (low T, P; 200-450 C, 10-15 km)
Blueschist (low T, high P - subduction zones)
Amphibolite (high T, P; 450-650 C, 15-20 km)
Granulite (super high T, P; > 700 C, >25km)