Coesite inclusions and prograde compositional zonation of garnet in whiteschist of the HP-UHPM Kokchetav massif, Kazakhstan: a record of progressive UHP metamorphism
Cd. Parkinson, Coesite inclusions and prograde compositional zonation of garnet in whiteschist of the HP-UHPM Kokchetav massif, Kazakhstan: a record of progressive UHP metamorphism, LITHOS, 52(1-4), 2000, pp. 215-233
Coarse-grained whiteschist, containing the assemblage: garnet + kyanite + p
hengite + talc + quartz/coesite, is an abundant constituent of the ultrahig
h-pressure metamorphic (UHPM) belt in the Kulet region of the Kokchetav mas
sif of Kazakhstan.
Garnet displays prograde compositional zonation, with decreasing spessartin
e and increasing pyrope components, from core to rim. Cores were recrystall
ized at T= 380 degrees C (inner) to 580 degrees C (outer) at P < 10 kbar (g
arnet-ilmenite geothermometry, margarite + quartz stability), and mantles a
t T= 720-760 degrees C and P-H2O = 34-36 kbar (coesite + graphite stability
, phengite geobarometer, KFMASH system reaction equilibria). Textural evide
nce indicates that rims grew during decompression and cooling, within the Q
tz-stability field.
Silica inclusions (quartz and/or coesite) of various textural types within
garnets display a systematic zonal distribution. Cores contain abundant inc
lusions of euhedral quartz (type 1 inclusions). Inner mantle regions contai
n inclusions of polycrystalline quartz pseudomorphs after coesite (type 2),
with minute dusty micro-inclusions of chlorite, and more rarely, talc and
kyanite in their cores; intense radial and concentric fractures are well de
veloped in the garnet. Intermediate mantle regions contain bimineralic incl
usions with coesite cores and palisade quartz rims (type 3), which are also
surrounded by radial fractures. Subhedral inclusions of pure coesite witho
ut quartz overgrowths or radial fractures (type 4) occur in the outer part
of the mantle. Garnet rims are silica-inclusion-free.
Type 1 inclusions in garnet cores represent the low-P, low-T precursor stag
e to UHPM recrystallization, and attest to the persistence of low-P assembl
ages in the coesite-stability field. Coesites in inclusion types 2, 3, and
4 are interpreted to have sequentially crystallized by net transfer reactio
n (kyanite + talc = garnet + coesite + H2O), and were sequestered within th
e garnet with progressively decreasing amounts of intragranular aqueous flu
id.
During the retrograde evolution of the rock, all three inclusion types dive
rged from the host garnet P-T path at the coesite-quartz equilibrium, and f
ollowed a trajectory parallel to the equilibrium boundary resulting in incl
usion overpressure. Coesite in type 2 inclusions suffered rapid intragranul
ar H2O-catalysed transformation to quartz, and ruptured the host garnet at
about 600 degrees C (when inclusion P similar to 27 kbar, garnet host P sim
ilar to 9 kbar). Instantaneous decompression ro the host garnet P-T path, p
assed through the kyanite + talc = chlorite + quartz reaction equilibrium,
resulting in the dusty micro-assemblage in inclusion cores. Type 3 inclusio
ns suffered a lower volumetric proportion transformation to quartz at the c
oesite-quartz equilibrium. and finally underwent rupture and decompression
when T < 400 degrees C, facilitating cocsite preservation. Type 4 coesite i
nclusions are interpreted to have suffered minimal transformation to quartz
and proceeded to surface temperature conditions along or near the coesite-
quartz equilibrium boundary. (C) 2000 Elsevier Science B.V. All rights rese
rved.