Zircon Ages from the Baydrag Block and the Bayankhongor Ophiolite Zone: Time Constraints on Late Neoproterozoic to Cambrian Subduction‐ and Accretion‐Related Magmatism in Central Mongolia

Abstract

Central Mongolia represents a heterogeneous crustal domain of the Central Asian Orogenic Belt and is composed of contrasting lithotectonic units with distinct preorogenic histories. We report single‐zircon evaporation and SHRIMP ages for high‐grade rocks of the Neoarchean‐Paleoproterozoic Baydrag block and for metaigneous rocks of the junction between the late Neoproterozoic Bayankhongor ophiolite zone (BOZ) and the Baydrag block. Zircon ages for metamorphic rocks of the Baydrag block indicate a major tectonothermal event between 1840 and 1826 Ma, coeval with the emplacement of granitic rocks at middle‐crustal level dated at 1839 Ma. A granite‐gneiss yielded a much younger crystallization age of 1051 Ma, the first Grenvillian age reported for this region. Together with predominantly Mesoproterozoic detrital zircon ages for a quartzite lens from the Burd Gol accretionary complex, these data attest to the heterogeneity and long Precambrian history of the Baydrag block. Crystallization ages for granite‐gneisses from the northeastern margin of the Baydrag block indicate prolonged plutonic activity between 579 and 537 Ma, probably related to southward subduction of the Bayankhongor oceanic crust. A syntectonic granite vein yielded a crystallization age 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 472 Ma and suggests that punctuated volcanic centers developed during the early Ordovician in response to protracted convergence.