High-Pressure Metamorphism of Pelitic Rocks

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

A. Occurrence of high-P metamorphism

-Most is associated with subduction:

-Rocks affected are "immature" pelites, graywackes, basalts
-In these, Ca combines with Al to form plagioclase, epidote, lawsonite.
-Fe-Mg-Al sheet silicates are less important than in t pelites

-High-P metamorphism also occurs during continental collision: true metapelites known from Alps, Greece, Indonesia

B. Characteristics

-High-P pelites develop less well-defined zones than medium-P.
-Recent work has identified characteristic mineral assemblages
-Dominated by phengitic muscovite and talc => whiteschists
-Difficult to distinguish optically

II. High-P reactions

A. Zeolite facies

-Carpholite : distinctive index mineral: (Mn,Mg,Fe+2)(Al,Fe+3)2Si2O6(OH)4 (Most pelites contain Crp + Chl +/- Prl)
-Crp breaks down at higher grade: Crp = Chl + Q + water
-Chloritoid-carpholite: Fe-Cld + Mg-Crp co-exist before Crp completely disappears -
Cld-Crp tie-line => Prl and Chl don''t co-exist in these rocks, as they may in medium-P rocks.

B. Blueschist facies

-Pelites don''t develop typical blue color because no glaucophane
-Kyanite:

-Prl = Ky
-Most pelites aren''t Al-rich enough to develop Ky at this point
-Talc is stable in this zone, but most pelites don''t have the appropriate composition for it to form.
-Garnet is also stable: Fe-Chl + Q = Grt + Tlc

C. Eclogite facies
-Talc:

-Bt + Chl = Tlc + Cld + water
- a tie-line flipping reaction.
-With increasing grade, Grt-Cld-Bt subtriangle migrates right

-Kyanite/garnet:

-Reaction is Tlc + Cld + Ms = Bt + Ky + Q + water.
-This is a tie-line flipping reaction.
-In many pelites, Ky and Grt appear after this reaction.
-Note that most pelites lose Tlc at this point.
-Stable assemblage is now Grt + Bt + Ky

-Inclusions:

-Some ultra-high-P terranes have developed coesite or diamond as inclusions in Grt.
-History
-Crustal rocks subducted to 90 - 150 km depth, without melting.
-Then returned to surface, preserving phases formed at depth => rapid uplift and erosion.

III. Partial melting
A. Medium-P, high-T
-three assumptions:

-Any water in system comes from dehydration reactions.
-Initial melting occurs under H2O-saturated conditions, but only to a few %
-After melting begins, any further H2O released by dehydration reactions dissolves into the melt.

-With these restrictions, melting is triggered by the reaction Ms + Ab + Q + vapor = Al-sil + L
-Next melting reaction is Ms + Ab = Al-sil + Kfs + L:

-This generates up to 20% melt.
-Kfs would be present in pods of melt.
-For P < invariant point 1, Kfs reaction occurs before melting, so Kfs forms porphyroblasts.

-More melting by: Bt + Sil = Grt + Crd + Kfs + L

C. Low-P, high-T

-First melt (point d), where all minerals present melt in the presence of water
-Major melting: Bt + Grt + Q = Crd + Opx + Kfs + L

D. Migmatites

-Mixed rock":

-Granulite (darl)
-Felsic segregations (light)

-Classified by geometry of light and dark parts
-Vein
-Stromatic
-Nets, agamite

-"The Granite Controversy": metamorphic nature of migmatites + felsic character of segregations > suggestion that even large batholiths were metamorphic, not igneous. (Disproved by experiments showing pelites could produce eutectic melts).