Lithostratigraphic and geochronological constraints on the evolution of the Central Asian Orogenic Belt in SW Mongolia: Early Paleozoic rifting followed by late Paleozoic accretion

New SHRIMP U-Pb and evaporation Pb-Pb zircon ages, together with a revision of the lithostratigraphy of "suspect" terranes in SW Mongolia, suggest that the collage of continental and oceanic units in this region resulted from recurrent magmatic reworking and deformation of Silurian--early Devonian proximal and distal passive margin sequences of the Paleo-Asian Ocean. The zircon ages from early Ordovician volcaniclastic rocks and syntectonic felsic dikes reveal an heterogeneous stretching of the Precambrian Dzabkhan microcontinent (Lake Zone basement) during the Ordovician, followed by the development of a carbonate platform on a proximal margin (Gobi-Altai Zone), serpentinite breccias and Silurian chert sequences on a distal margin and possibly also the formation of oceanic crust. The assumed early Neoproterozoic South Gobi continental zone may either represent an allochthonous block detached from Dzabkhan or, less likely, the conjugate margin of a Paleo-Asian continental rift. Early Devonian volcanism subsequently affected both types of margins with back-arc spreading centers and arcs located in the core of the future Trans-Altai Zone. During the late Devonian to early Carboniferous a Japan-type magmatic arc developed on the previously stretched continental crust of the Gobi-Altai Zone. This event was associated with shortening of the entire domain, exhumation of the deep arc core and formation of intramontane basins with Devonian and Carboniferous detrital zircons of the adjacent Lake Zone continent. Clastic, flysch-type sedimentation occurred on the former distal margin and in oceanic areas. During this early Carboniferous contraction event the continental and oceanic units were imbricated and accreted to the continent in the north. Subsequently, late Carboniferous volcanic arc sequences and a Japan-type magmatic arc developed on the Trans-Altai oceanic crust and the southern South Gobi Zone, respectively. Finally, a Permian thermal event was localized in the Gobi-Altai--Lake Zone contact domain and was responsible for formation of Permian grabens, bimodal volcanism and substantial melting of the accreted crust.