INITIATION OF SUBDUCTION BELOW THE ZAVKHAN TERRANE OF MONGOLIA AND THE ONSET OF ACCRETIONARY OROGENESIS IN THE CENTRAL ASIAN OROGENIC BELT

Previous studies of the Paleozoic Central Asia Orogenic Belt (CAOB) have focused on documenting accretionary processes, but little has been done to address how and why accretion began. The Zavkhan Terrane of Mongolia is a Proterozoic cratonic fragment that forms the core of the CAOB—accretion nucleated around the Zavkhan Terrane in the Ediacaran to early Paleozoic and the composite terrane later collided with the Siberian margin. Here we present new geochronological, stratigraphic, and mapping data that constrain Ediacaran to early Cambrian arc-continent collision, slab reversal, and the initiation of subduction on the southern margin of the Zavkhan Terrane. Although there is no exposed crystalline basement older than ~850 Ma on the Zavkhan Terrane, detrital age spectra from Neoproterozoic siliciclastic rocks provide evidence for basement sources with age peaks at 2050, 2200, and 2500 Ma. The Zavkhan Terrane rifted from its parent craton by 750 Ma and was covered by glacial deposits and carbonate on a passive margin from 720-580 Ma. New geochronological data from the allochthonous zone on the southern margin of the Zavkhan Terrane suggest late Ediacaran obduction of the Khantaishir Arc and the initiation of subduction related magmatism by the early Cambrian. In addition, we have identified extensive ~500 Ma magmatism in the southern part of the Zavkhan Terrane, but the tectonic significance of this event remains enigmatic. The model we present here for the transformation of a passive margin to an active margin with arc-continent collision and slab reversal is analogous to slab reversal in Taiwan and to new models of the early stages of arc-continent collision in the Taconic orogeny of the New England Appalachians.