SHRIMP zircon chronology of HP-UHP rocks of the Makbal metamorphic complex in the Northern Tien Shan, Kyrgyzstan

Abstract

In this paper we present the first results of sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon dating obtained for the ultra-high pressure (UHP) metamorphic rocks of the Makbal metamorphic complex in the Northern Tien Shan, Kyrgyzstan. Zircon grains from two HP eclogite samples and one sample of UHP garnet-talk-chloritoid schist have similar bright rims identified on cathodoluminescence (CL) images. The rims are interpreted as zircon overgrowth during UHP metamorphism. Dating of zircon rims from the three samples yielded similar ages of 509±7Ma, 502±10Ma and 498±7Ma. The time span of 510–500Ma is considered as the “minimum” age of UHP metamorphism of the Makbal complex. This age is in accordance with Middle Cambrian faunal age of the Kara-Archa ophiolite in the Makbal area. It is suggested that the Makbal rocks were metamorphosed in the Middle Cambrian subduction zone dipping north under the Northern Tien Shan microcontinent. A deformed granodiorite associated with the Makbal metamorphic rocks yielded an age of 514±5Ma, indicating development of a magmatic arc simultaneously with the UHP metamorphism. Cores of zircon grains from the garnet-talk-chloritoid schist yielded a wide spectrum of ages (1850–800Ma) characteristic for detrital zircons of continental origin. A depositional age <800Ma points to a possible derivation from a Riphean passive margin of the Terskey Ocean. Analyses of zircon grains from two eclogite samples fit well-defined discordia lines with upper intercepts at 1446±25Ma and 1447±29Ma, indicating a homogeneous Mezoproterozoic protolith of these rocks. The possible presence of such protoliths in Makbal is illustrated by a similar age of inherited zircon recovered from the granodiorite (1443±25Ma) and by Mesoproterozoic detrital zircons from the garnet-talk-chloritoid schist. Ages of Kokchetav, Makbal, Aktyuz and Anrakhai HP-UHP complexes in the Kokchetav–Northern Tien Shan belt vary from 537 to 475Ma and register subduction–accretion events during the Caledonian orogeny which finally formed the Paleo-Kazakhstan continent prior to Middle Ordovician.