Early Carboniferous backarc basin in the Bidz ophiolitic terrane, Mongolia: Geochemical and geochronological constraints and tectonic implications

The tectonic relationship between the Chinese-Mongolian Altai domain and the East Junggar−Trans-Altai domain in the Mongolian collage remains debated, largely due to the poorly constrained nature of intervening oceanic basins. To address this, the Bidz ophiolitic terrane located at their junction was investigated by using integrated geological, geochemical, and geochronological approaches. The terrane comprises gabbro, basalt, and andesite units embedded in a chert-sandstone-tuff matrix. Gabbro can be roughly classified into high- and low-Mg# subgroups, where the former shows transitional mid-ocean-ridge basalt and island-arc basalt geochemical affinities, while the latter exhibits overall arc-like geochemical signatures. Basalt displays comparable geochemistry with respect to the high-Mg# gabbro, and both of them resemble backarc basin basalt. In contrast, andesite shows calc-alkaline arc geochemical characteristics. Zircon U-Pb dating of gabbro samples yields formation ages of ca. 350 Ma. These findings suggest that the Bidz terrane represents a fragment of an early Carboniferous backarc basin. Regional synthesis suggests that this basin, together with its western counterpart (the Erqis backarc) and the southerly East Junggar−Trans-Altai arc domains, formed a Devonian−Carboniferous marginal sea−island-arc system to the south of the Altai domain, driven by southward (present-day coordinates) subduction retreat. Our data also indicate that opening of the Erqis-Bidz backarc coincided with eastward-propagating crustal extension in the Altai domain, reflecting gradual eastward rollback of the subduction system, analogous to the geodynamic evolution of the modern Southwest Pacific oceanic domain.