Zircon Ages from the Baydrag Block and the BayankhongorOphiolite Zone: Time Constraints on Late Neoproterozoicto Cambrian Subduction- and Accretion-RelatedMagmatism in Central Mongolia

摘要

Central Mongolia represents a heterogeneous crustal domain of the Central Asian Orogenic Belt and is composed ofcontrasting lithotectonic units with distinct preorogenic histories. We report single-zircon evaporation and SHRIMPages for high-grade rocks of the Neoarchean-Paleoproterozoic Baydrag block and for metaigneous rocks of the junctionbetween the late Neoproterozoic Bayankhongor ophiolite zone (BOZ) and the Baydrag block. Zircon ages for metamorphicrocks of the Baydrag block indicate a major tectonothermal event between 1840 and 1826 Ma, coeval withthe emplacement of granitic rocks at middle-crustal level dated at 1839 Ma. A granite-gneiss yielded a much youngercrystallization age of 1051 Ma, the first Grenvillian age reported for this region. Together with predominantly Mesoproterozoicdetrital zircon ages for a quartzite lens from the Burd Gol accretionary complex, these data attest tothe heterogeneity and long Precambrian history of the Baydrag block. Crystallization ages for granite-gneisses fromthe northeastern margin of the Baydrag block indicate prolonged plutonic activity between 579 and 537 Ma, probablyrelated to southward subduction of the Bayankhongor oceanic crust.Asyntectonic granite vein yielded a crystallizationage of 519 Ma, probably linked to accretion of the BOZ onto the northeastern active margin of the Baydrag block.Lastly, a felsic metavolcanic rock from the southeastern termination of the BOZ yielded a crystallization age of 472Ma and suggests that punctuated volcanic centers developed during the early Ordovician in response to protractedconvergence.