Lecture SIXLecture SIXMetamorphic Grade and

Facies

Metamorphic grade and Facies

Systematic spatial distribution in mineral assemblages that formed

during metamorphism in metamorphic terrains allow to delineate mineral

zonation, using index minerals (e.g. chlorite, biotite, garnet, staurolite,

cordierite, sillimanite, andalusite, wollastonite, diopside… etc.)

- Mineral zone: Zones in the field,

which mark the first appearance of

an index mineral, such as chlorite

zone, garnet zone.

-Mineral isograde: boundary

marked between two mineral zones,

which include:

- Mineral-in: first app

-Mineral-out: the last appearance of

(Younger) Granites

Andesite-rhyolite (Dokhan) Volcanics

Type-VIType-IIIType-I

Undiffrentiatedorthogneisses

Metagabbros-metadiorite/metabasitesPelitic schists

Unmetamorphosed igneous rocks

Metamorphic complex

Mineral assemblagesgrt

grt+st

grt+st+sil

grt+st+and+silgrt+st+crd

grt+st+crd+and+sil

st+crd

Migmatites

(Older) Quartz-diorite

grt+silgrt+crdst+crd+sil

+ +

V V

29° 30'29° 32' 34° 51'+ + + + + + + + + ++ + + + + + + + ++ + + + ++ + + ++ + +

29°

28'30"

T12

T11

T17

T19T18

T27

T8' T9

T10T8T7

T6

T2,2'T0

T1T4

T5 T3

T25

T26

+V

VV V V

VV V V

VV V V VV V V

V V V VV V V

b

V V VV VV VVVVV

W. Kholeifiya

++ ++ + ++ + ++ ++

W . T w e ib a

Example: Wadi Tweiba (S. Sinai) mineral zones

Gabal Yelleq (Sinai) contact aureole mineral zones

Dolorite Sill

20 m

A11

A10A9A8A7

A0Core

Rim

H1H0

H13

Wadi fillingA4A6

Loweraureole

Upperaureole c

Metapelites metamorphic zones (Scotland)

Distribution of minerals within metamorphic zones

Metamorphic grades

Metamorphic grades is a general term for describing the

relative P-T conditions under which the metamorphic rocks

form. The grades could subdivided into:

- Very low grade

- Low grade

- Medium grade

- High grade

- Very high grade

The boundaries between the grades are chosen to

correspond to important discontinuous reactions (could

recognized as major isograde), and they correlate with the

scheme of metamorphic facies.

Metamorphic facies

Metamorphic facies is defined as a set of metamorphic

mineral assemblage, repeatedly associated in space and

time.

Metamorphic grades and facies

Metamorphic gradeMetamorphic faciesIndex mineral assemblages

Pelitic rocksMetabasalt

Very low gradeZeoliteMixed layer clays, sericite

Laumonite, analcite, heulandite, wairakite

Prehnite-pumpellyiteIllite-muscovite, Chl, Ab, Pyr,

- prehnite-pumpellyite, Chl. Ab, Ep, Act, Law.

Blue schistChl, Tlc, Ms, Grt, ChldGlu, Law

Low gradeGreenschistChl, Ms, Ab, Bt, GrtAct, Ep, Ab, Chl, Act, Grt, Hbl

Albite-epidote hornfels

Ms, Bt, ChlPl, Ep, Act, Chl

Medium gradeAmphiboliteGrt, St, And, Ky, Sill, Ms, Crd

Hbl, Pl, Ep, Grt

Hornblende hornfelsCrd, Chl, bt, Ms, And Hbl, Pl, Cumm

High grade GranuliteCrd, Grt, Kfs, Sill, Ky, Hy, Sap.

Cpx, Grt, Opx, Pl, Ol, Hbl

Pyroxene honfelsCrd, And, KfsCpx, Opx, Pl, Ol, Hbl

Sanidinite Cor, Mgt, An, glassNot well defined

Very high gradeEclogiteTlc, ky, Grt, MsOmph (Cpx), grt

Metamorphic grades and facies

Metamorphic series

Graphical representation of metamorphic mineral assemblage

1- ACF and AKF diagrams: Suitable for mafic rocks and calc-

silicates.

2- AFM diagram: Useful for pelitic rocks.

3- CAS (CaO - Al2O3 - SiO2) diagram: Useful for marly rocks

(calcareous mudstones).

4- MCS (MgO - CaO - SiO2) diagram: Useful for ultramafic rocks.

ACF diagram

AKF diagram

Plotting of mineral assemblage and rock categories in

ACF and AKF

AFM plotting

AFM diagram

SCM diagram