Download Foliated rocks

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)