Abstract:

Zircon is an excellent material to preserve the complex history of ultrahigh-pressure (UHP) metamorphic rocks, whereas mineralogical evidence of UHP conditions is mostly obliterated in matrix assemblages as a result of extensive retrograde overprinting during exhumation. Zircons from the Kokchetav UHP–HP massif contain numerous inclusions of graphite, quartz, garnet, omphacite, jadeite, phengite, phlogopite, rutile, albite, K-feldspar, amphibole, zoisite, kyanite, calcite, dolomite, apatite and monazite, as well as the diagnostic UHP minerals, such as microdiamond and coesite, which were identified by laser Raman spectroscopy. The internal structure of zircon displays a distinct zonation, which comprises an inherited core, a wide mantle and an outer rim, each with distinctive inclusion micro-assemblages. The low-pressure mineral inclusions, such as graphite, quartz and albite, are common in the inherited core and thin outer rim, whereas diamond, coesite and jadeite occupy the mantle domain. The zircon core and outer rim are of detrital and relatively low-grade metamorphic origin, whereas the mantle domain is of HP to UHP metamorphic origin. The mineral assemblages and chemistry of inclusions preserved in zircon have been used to constrain the metamorphic PT path of the Kokchetav UHP–HP rocks, and indicate peak metamorphism at 60–80 kbar and 970–1100 °C followed by nearly isothermal decompression at 10 kbar and c. 800 °C. Sensitive high-resolution ion microprobe U–Pb spot analyses of the zoned zircon indicate four discrete ages of the Kokchetav metamorphic evolution: (1) a Middle Proterozoic protolith age; (2) 537 ± 9 Ma for UHP metamorphism; (3) 507 ± 8 Ma for the late-stage amphibolite-facies overprint; (4) 456–461 Ma for post-orogenic thermal events. This indicates that Middle Proterozoic supracrustal protoliths of the Kokchetav UHP–HP rocks were subducted to mantle depths in the Middle Cambrian, and exhumed to mid-crustal levels in the Late Cambrian. The zonal arrangement of inclusions and the presence of coesite and diamond without back reaction imply that aqueous fluids were low to absent within zircon, and that zircon is capable of retaining minerals of each metamorphic stage. We suggest that the study of inclusions in zircon is a powerful method to clarify the multiple stages and timing of metamorphic evolution of UHP–HP rocks, the evidence for which has been more or less obliterated in the host rock.

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