Neoproterozoic ophiolite and related high-grade rocks of the Baikal–Muya belt, Siberia: Geochronology and geodynamic implications

Abstract

We report zircon for from ophiolitic and high-grade rocks of the Neoproterozoic Baikal–Muya belt of Siberia that occupies an arc-shaped area on the southeastern margin of the Siberian craton. It consists of arc-related plutonic, metavolcanic and metasedimentary rocks as well as fragmented ophiolites and high-grade metamorphic assemblages. Magmatic zircons from two plagiogranite dyke samples of the Mamakan ophiolite complex in the Sredne–Mamakan massif of the eastern Baikal–Muya belt yielded similar and concordant SHRIMP mean 206Pb/238U ages of 640.0±4.1 and 650±6Ma, respectively, that reflect the time of dyke emplacement and from which we suggest an age of ca. 645Ma as the most likely time of ophiolite formation.Enderbitic gneisses of the North Baikal area, in the western part of the Baikal–Muya belt, contain complex zircon populations that reflect variable recrystallization, Pb-loss and metamorphic overgrowth during granulite-facies metamorphism. LA-ICP-MS dating of these zircons yielded inconclusive results that led us to undertake a detailed study of cathodoluminescence images combined with U–Pb SHRIMP dating. Well-preserved magmatic domains in zircons from enderbite sample 2821 yielded concordant results with a mean 206Pb/238U age of 640±5Ma, slightly higher but broadly comparable with the data obtained by LA-ICP-MS. The zircon populations of two more enderbitic gneiss samples are more complex, and their LA-ICP-MS data constitute broad swaths along concordia between ca. 840 and 600Ma, reflecting two end-member isotopic components, namely an igneous crystallization event at ca. 800Ma and a Pb-loss and recrystallization event at ca. 600Ma. SHRIMP analyses of magmatic zircon domains of these samples yielded concordant data with identical mean 206Pb/238U ages of 826±7.5Ma and 826±8Ma, respectively, whereas low-U metamorphic rims crystallized at 640±7Ma. Newly crystallized ball-round metamorphic zircons in one sample produced a mean 206Pb/238U age of 640±6Ma. We suggest that the protoliths of the enderbitic gneisses crystallized at 826±7.5Ma and experienced granulite-metamorphism at 640±6Ma. The LA-ICP-MS analyses are fully compatible with this interpretation.Our geochronological data and previously published ages for Neoproterozoic igneous rocks of the Baikal–Muya belt define two age groups at 830–780 and 650–640Ma. We interpret that the older group reflects the evolution of a large arc system in the Baikal–Muya belt and the eastern Sayan–northwestern Mongolia region, whereas the younger group documents collision between the above arc system and the southern margin of the Siberian craton.