Coesite inclusions and prograde compositional zonation of garnet in whiteschist of the HP-UHPM Kokchetav massif, Kazakhstan: a record of progressive UHP metamorphism

Abstract

Coarse-grained whiteschist, containing the assemblage: garnet+kyanite+phengite+talc+quartz/coesite, is an abundant constituent of the ultrahigh-pressure metamorphic (UHPM) belt in the Kulet region of the Kokchetav massif of Kazakhstan. Garnet displays prograde compositional zonation, with decreasing spessartine and increasing pyrope components, from core to rim. Cores were recrystallized at T=380°C (inner) to 580°C (outer) at P<10 kbar (garnet–ilmenite geothermometry, margarite+quartz stability), and mantles at T=720–760°C and P H20=34–36 kbar (coesite+graphite stability, phengite geobarometer, KFMASH system reaction equilibria). Textural evidence indicates that rims grew during decompression and cooling, within the Qtz-stability field. Silica inclusions (quartz and/or coesite) of various textural types within garnets display a systematic zonal distribution. Cores contain abundant inclusions of euhedral quartz (type 1 inclusions). Inner mantle regions contain 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 developed in the garnet. Intermediate mantle regions contain bimineralic inclusions with coesite cores and palisade quartz rims (type 3), which are also surrounded by radial fractures. Subhedral inclusions of pure coesite without 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 stage to UHPM recrystallization, and attest to the persistence of low- P assemblages in the coesite-stability field. Coesites in inclusion types 2, 3, and 4 are interpreted to have sequentially crystallized by net transfer reaction (kyanite+talc=garnet+coesite+H 2O), and were sequestered within the garnet with progressively decreasing amounts of intragranular aqueous fluid. During the retrograde evolution of the rock, all three inclusion types diverged from the host garnet P– T path at the coesite–quartz equilibrium, and followed a trajectory parallel to the equilibrium boundary resulting in inclusion overpressure. Coesite in type 2 inclusions suffered rapid intragranular H 2O-catalysed transformation to quartz, and ruptured the host garnet at about 600°C (when inclusion P∼27 kbar, garnet host P∼9 kbar). Instantaneous decompression to the host garnet P– T path, passed through the kyanite+talc=chlorite+quartz reaction equilibrium, resulting in the dusty micro-assemblage in inclusion cores. Type 3 inclusions suffered a lower volumetric proportion transformation to quartz at the coesite–quartz equilibrium, and finally underwent rupture and decompression when T<400°C, facilitating coesite preservation. Type 4 coesite inclusions are interpreted to have suffered minimal transformation to quartz and proceeded to surface temperature conditions along or near the coesite–quartz equilibrium boundary.