P-T-D-t paths from granulites:

a guide to what’s possible

The limits of conventional thermobarometry

Understanding reaction microstructures

Deformation sequences in relation to melting

Linking geochronology to reactions

2

P- and T-sensitive equilibria

Assemblages and petrogenetic grids

• Observed changes of mineral assemblage compared

with predicted stability on petrogenetic grids and

pseudosections

Multivariant equilibria (continuous net transfer

reactions)

• Changes of mineral composition in sliding reactions,

e.g. grossular + 2 kyanite + quartz = 3 anorthite

Ca transferred from plagioclase to garnet with increasing P

Cation distributions (cation exchange reactions)

• e.g. Fe and Mg distribution between garnet and biotite.

Fe prefers garnet, but gets less fussy at higher T.

Solvi and miscibility gaps

• e.g. Cpx-Opx, Cal-Dol, Ab-Or.

• Gaps usually close towards higher T A B

2phases

T

KD

ln K

P

P

T

T

3

Thermobarometry in granulites

Useful thermometers:

Fe-Mg exchange: Grt-Cpx, Grt-Opx, Grt-Crd

Pyroxene thermometry: Opx-Cpx miscibility gap

Most thermometers are exchange equilibria

Useful barometers

P-sensitive, anhydrous equilibria (large DV):

Plag = Grt e.g. En + An = Grs + Prp + Qtz

Crd = Grt e.g. Crd = Grt + Sil + Qtz

Most barometers are net-transfer equilibria

Retrograde changes during slow cooling

affect exchange equilibria:

• without visibly affecting rock texture

affect the net-transfer reactions:

• visibly: reaction rims & intergrowths

4

Compositional zoning from retrograde cation diffusion

Pattison & Begin 1994, J Metamorphic Geol 12, 387-410

5

Grain boundary or volume diffusion?

• O’Brien 1999

Min Mag 63,

227-238

Mg

6

Problems in high grade rocks

• Metapelite, peak assemblage

Grt-Crd-Sil-Qtz

• Thermometer: Grt-Crd Fe-Mg

exchange

• Barometer: Grt + Sil + Qtz =

Crd

1. Cores preserve Grt-Crd-Sil-Qtz

matrix equil;

2. Rims record final closure of

Fe-Mg exchange

“Granulite uncertainty principle” -

peak conditions may not be

preserved

Rims in contact

Grt

Crd

%M

gO

%M

gO

Comp at peakComp at peak

Core Core

Fictive P,Grt + Crd notin equil. withSil + Qtz

Peak P-T (can't be determined)

Fe-Mg exchange ceasesPhases cease to homogenise

Actual P-T path

Apparent P-T path

P

T

12

7

Discordant geothermometry

8

Grt-Crd migmatite

• Grt (neosome) and Crd

(palaeosome) physically

separated

9

Opx-Crd-Bt-Grt gneiss

• Mafic minerals all in contact, easy

exchange?

• But detailed microstructure suggests

garnet growth is late – was it there at

the peak? Is this diffusion zoning or

(down-T) growth zoning?

10

P-T paths from phase relations: Reaction coronas

• Corona shows progress of the continuous reaction

Grt + Qtz = Crd + Opx

• Rock has crossed flat-lying isopleths in down-P direction

Grt + Crd + Opx + Qtz

Temperature

Press

ure

Grt + Opx + Qtz

Grt + Crd + Qtz

11

Isothermal decompression and isobaric cooling paths

Compilation from Harley 1989 Geol Mag 126 215-247

12

Prograde reaction microstructures?

Prograde metamorphic P-T path is an important discriminator.

Can it be determined?

Loss of aqueous fluid reduces mobility of material, despite

higher T.

• reaction rims

• pseudomorphs after amphibolite-facies porphyroblasts.

Ductility contrasts between refractory rocks and enclosing

migmatites.

• strain partitioned into migmatites

• early microstructures preserved in refractory rock-types

13

Spinel + quartz

• Hercynite-rich spinel(Fe,Mg)Al2O4

produced by reactionCrd Hcss + Qtz

Waters (1991) Eur J Min 3, 367-386.

14

Use assemblages (facies type)

Establish which minerals

coexisted stably at the

metamorphic peak

15

Locate facies type on petrogenetic grid

Waters (1986) J Petrology 27,

541-565

16

Reaction sequences

Identify progress of model

divariant equilibria

• Crd + Spl + Crn -> Spr

• Opx + Sil -> Spr + Crd

17

P-T path from assemblage changes

18

P-T segments from different rock types

Metapelites: lost Ms before start of melting, peak at 5 kbar, >800°C

Mg-Al gneisses:Sapphirine-forming reactions constrain slope

Hercynite-quartz metapelites, retrograde reactions

19

Namaqualand P-T path summary

From Waters (1989) with new monazite age data

20

Sequences of deformation and reaction

• Melt generation as a time marker: overprinting older fabrics;

structural control of leucosomes; deformation and recrystallisation of

leucosomes.

• Melting as a control on rock rheology, enhancing lithological contrasts,

allowing preservation of history in low-strain refractory domains.

21

Neosomes define linear fabric, but...

22

Neosomes define later planar fabric

23

Linking geochronology to mineral development

Petrological framework for migmatitic

biotite gneisses

Prograde evolution, up to Sil zone

Biotite gneiss, just Al-saturated (trace

Sil)

Metamorphic peak

Dehydration melting, garnet appears

in neosome only, textural

modification of neosome (melt

present)

Retrograde evolution

Cooling and partial back-reaction with

melt, Grt -> Bt

24

Synthesis of rock evolution

• Early cores show outward decreasing Th,

increasing Y, U. Prograde growth, over ca.

400 - 700°C if xenotime present. No Grt.

• Main zones, lobes Y+HREE depleted,

consistent with Grt growth. Big grains in

neosome = dehydration melting.

• Rims: higher, then lower Th. Outer rims

have v. high Y+HREE, released by Grt

breakdown (this grain in Bt after Grt).

25

Preliminary results and regional pattern

DWN57 migmatitic metapelite

Core zones 1063 ± 24 n = 6

Main growth zones 1038 ± 11 n = 13

Rims 1013 ± 11 n = 14

800850900950100010501100115012001250

DWN57 coresDWN57 mainDWN57 rims

DWN673DWN673 mainDWN673 rims

BP3 coresBP3 main

BP3 high ThBP3 rims

Age (Ma)

Early magmatism Spektakel suite

Koperberg suite

Zircon rim growth

26

Nanga Parbat: constraining rapid exhumation

• Very young ages, e.g. xenotime 0.7-1.1 Ma (Bowring, Hodges,

Searle, Waters, in prep.)

27

Nanga Parbat migmatites

28

Garnet rimmed by cordierite in neosome

29

REE chemistry of xenotimes

Xenotime in garnet has depleted HREE.

Xenotime in cordierite corona has enriched HREE

Chemistry, and so U-Th-Pb system, records the visible reaction