Low-Pressure Metamorphism of Pelitic Rocks

See http://plate-tectonic.narod.ru/metpetrographylinks.html
C http://ijolite.geology.uiuc.edu/08SprgClass/geo436/lectures.html

I. Pelitic rocks


A. Def: fine-grained sediments produced by weathering of continental crust

-Tectonic setting = passive margin, or accretionary wedge off active margin
-Composition:

-Sheet silicates high in Al & K; may be >50% of rock
-Quartz, 10-30%
-Feldspars, oxides, carbonates

-May be represented in the system KFMASH: water is mobile, so AKF & AFM diagrams work.

B. See KFMASH petrogenetic grid

-Low P series is characteristic of contact metamorphism
-At low P, many possible reactions & invariant points: applicable one depend on chemical composition of system
-Facies (with increasing T) :

zeolite;
prehnite-pumpellyite;
albite-epidote hornfels;
hornblend hornfels
pyroxene hornfels
sanidinite

-Note: hornfels = fine-grained non-foliated met. rock.

C. Metamorphic micas

-"White" micas:

-Muscovite: KAl(Si3,Al)O10(OH)2
-Sericite: KAl2(Si,Al)4O10(OH)2 - fine-grained Ms derived from alteration of feldspar
-Pyrophyllite: Al2Si4O10(OH)2
-Phengite: KAl1.5(Mg,Fe)0.5Si3.5Al0.5O10(OH)2
-Celadonite: K(Mg,Fe+2)(Fe+3,Al)Si4O10(OH)2
-Al-celadonite: K(FeAl)Si4O10(OH)2
-Paragonite: NaAl2(Si3Al)O10(OH)2
-Margarite: CaAl2(Si2Al2)O10(OH)2

-Limited solid solution among these.
-Stable into the amphibolite facies

II. Low-P isograds

A.
-Note lowest T = 350 oC, which is above the zeolite facies.
-Rocks at this stage are generally slates and phyllites.

B. Albite-epidote hornfels facies

-Biotite isograd : Chl + Kfs = Bt + Ms + Q + H2O
-Cordierite isograd: Mg-Chl + Al-silicate + Q = Mg-Crd + H2O; first Crd is Mg-rich; with increasing T, Crd incorporates more Fe and Cld less Mg
-Andalusite isograd : may form in Al-rich rocks: Ms + Chl = And + Bt
-New metamorphic minerals (Bt, Crd, And) are coarser and less oriented than pre-existing minerals in slate or phyllite.

D. Hornblende-hornfels facies

-Alternate Cordierite isograd :

-(Mg,Fe)-Chl + And + Q = Crd + Cld + water
-This is a tie-line flipping reaction
-Chl breaks down: Chl + Ms = Crd + Cld + Bt + water

-Staurolite isograd : (P > 0.2 GPa), St may form: Cld + And = St + Chl + Q + water
-Garnet isograd: Grt may form if P > 0.15 GPa
-By this stage, rocks are usually totally recrystallized

E. Pyroxene-hornfels facies

-K-feldspar isograd: Ms breaks down: Ms + Q = Kfs + And + water; see where ubiquitous phases are now Kfs + Q
-Garnet isograd:

-Bt + And + Q = Grt + Crd + Ms + Q
-This is a tie-line flipping reaction
-Note: Grt can form at lower T & P from Ann + Crd

-Onset of melting:

-At low P, wet granite melts ~680 oC. Migmatite may form.
-As P decreases, T of melting increases. But, as P decreases, T of dehydration of-Ms and Bt also drops. Thus very low-P metapelites may dehydrate before melting.

-If T rises high enough, without melting, then . . .

-Orthopyroxene isograd: Bt + Q = Opx + Kfs
-Sillimanite isograd: And = Sil


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