p-t-d-t paths from granulites - university of oxforddavewa/learning/ispet/2004-3... · 2014. 8....
TRANSCRIPT
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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
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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
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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
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Compositional zoning from retrograde cation diffusion
Pattison & Begin 1994, J Metamorphic Geol 12, 387-410
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Grain boundary or volume diffusion?
• O’Brien 1999
Min Mag 63,
227-238
Mg
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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
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Discordant geothermometry
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Grt-Crd migmatite
• Grt (neosome) and Crd
(palaeosome) physically
separated
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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?
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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
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Isothermal decompression and isobaric cooling paths
Compilation from Harley 1989 Geol Mag 126 215-247
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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
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Spinel + quartz
• Hercynite-rich spinel(Fe,Mg)Al2O4
produced by reactionCrd Hcss + Qtz
Waters (1991) Eur J Min 3, 367-386.
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Use assemblages (facies type)
Establish which minerals
coexisted stably at the
metamorphic peak
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Locate facies type on petrogenetic grid
Waters (1986) J Petrology 27,
541-565
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Reaction sequences
Identify progress of model
divariant equilibria
• Crd + Spl + Crn -> Spr
• Opx + Sil -> Spr + Crd
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P-T path from assemblage changes
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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
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Namaqualand P-T path summary
From Waters (1989) with new monazite age data
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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.
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Neosomes define linear fabric, but...
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Neosomes define later planar fabric
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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
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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).
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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
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Nanga Parbat: constraining rapid exhumation
• Very young ages, e.g. xenotime 0.7-1.1 Ma (Bowring, Hodges,
Searle, Waters, in prep.)
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Nanga Parbat migmatites
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Garnet rimmed by cordierite in neosome
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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