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

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.