Inclusion study in zircon from ultrahigh-pressure metamorphic rocks in the Kokchetav massif: An excellent tracer of metamorphic history

Ikuo Katayama, Shigenori Maruyama

    Research output: Contribution to journalArticlepeer-review

    33 Citations (Scopus)

    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 lowpressure 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 P-T path of the Kokchetav UHP-HP rocks, and indicate peak metamorphism at 60-80 kbar and 970-1100 8C followed by nearly isothermal decompression at 10 kbar and c. 800 8C. 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 postorogenic 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.

    Original languageEnglish
    Pages (from-to)783-796
    Number of pages14
    JournalJournal of the Geological Society
    Volume166
    Issue number4
    DOIs
    Publication statusPublished - Jul 2009

    ASJC Scopus subject areas

    • Geology

    Fingerprint Dive into the research topics of 'Inclusion study in zircon from ultrahigh-pressure metamorphic rocks in the Kokchetav massif: An excellent tracer of metamorphic history'. Together they form a unique fingerprint.

    Cite this