TY - JOUR
T1 - Ion micro-probe U-Pb zircon geochronology of peak and retrograde stages of ultrahigh-pressure metamorphic rocks from the Kokchetav massif, northern Kazakhstan
AU - Katayama, Ikuo
AU - Maruyama, Shigenori
AU - Parkinson, Christopher D.
AU - Terada, Kentaro
AU - Sano, Yuji
N1 - Funding Information:
We are grateful to J.G. Liou, K. Ye, T. Ohta, K. Okamoto for discussion and constructive comments, and Y. Tsutsumi for help in SHRIMP analysis. Field survey and sample collection were assisted by Y. Kaneko, M. Terabayashi, H. Yamamoto, R. Anma, M. Ishikawa, T. Ohta, Y. Nakajima, H. Masago, K. Yamauchi, J. Yamamoto, who are thanked. This paper was improved considerably by U. Schaltegger, J. Claoue-Long and anonymous reviewers. This study was financially supported in part by a project on Whole Earth Dynamics, from the Science and Technology Agency of Japan, and by a Research Fellowship of the Japan Society for the Promotion of Science for Young Scientists to I.K. [AH]
PY - 2001
Y1 - 2001
N2 - Sensitive high resolution ion micro-probe (SHRIMP) U-Pb analyses of zoned zircons were performed to constrain the age and geodynamic significance of the Kokchetav ultrahigh-pressure (UHP) metamorphic rocks, which recrystallized at P > 60 kbar and T> 1000°C. Cathodoluminescence images were correlated with micro-assemblages of mineral inclusions in zircons from diamond-bearing gneisses. These revealed UHP mineral-bearing cores and low-pressure mineral-bearing rims, some zircons also retain inherited cores. Our SHRIMP data identified four discrete stages: Middle Proterozoic protolith age, 537 ± 9 Ma for UHP metamorphic conditions, 507 ± 8 Ma for the late-stage amphibolite facies overprint, and 456-461 Ma for post-orogenic thermal events. Thus, we conclude 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 exhumation rate calculated from the SHRIMP data and relative P-T conditions is approximately 5 km/Ma. The rapid exhumation rate for the massif is in the same range as other UHP metamorphic terranes, and suggests that slab decoupling may be a driving force for exhumation of UHP metamorphic rocks from mantle depths.
AB - Sensitive high resolution ion micro-probe (SHRIMP) U-Pb analyses of zoned zircons were performed to constrain the age and geodynamic significance of the Kokchetav ultrahigh-pressure (UHP) metamorphic rocks, which recrystallized at P > 60 kbar and T> 1000°C. Cathodoluminescence images were correlated with micro-assemblages of mineral inclusions in zircons from diamond-bearing gneisses. These revealed UHP mineral-bearing cores and low-pressure mineral-bearing rims, some zircons also retain inherited cores. Our SHRIMP data identified four discrete stages: Middle Proterozoic protolith age, 537 ± 9 Ma for UHP metamorphic conditions, 507 ± 8 Ma for the late-stage amphibolite facies overprint, and 456-461 Ma for post-orogenic thermal events. Thus, we conclude 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 exhumation rate calculated from the SHRIMP data and relative P-T conditions is approximately 5 km/Ma. The rapid exhumation rate for the massif is in the same range as other UHP metamorphic terranes, and suggests that slab decoupling may be a driving force for exhumation of UHP metamorphic rocks from mantle depths.
KW - Kokchetav Kazakhstan
KW - Mineral inclusion
KW - Polym metamorphism
KW - Probe
KW - SHRIMP data
KW - U/Pb
KW - Zircon
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U2 - 10.1016/S0012-821X(01)00319-3
DO - 10.1016/S0012-821X(01)00319-3
M3 - Article
AN - SCOPUS:0034987696
SN - 0012-821X
VL - 188
SP - 185
EP - 198
JO - Earth and Planetary Sciences Letters
JF - Earth and Planetary Sciences Letters
IS - 1-2
ER -