Field geology and provenance analyses of the Ganqimaodu accretionary complex (Inner Mongolia, China): implications for early Paleozoic tectonic evolution of the southern Central Asian Orogenic Belt

The early Paleozoic accretionary architecture of the Alxa and Bainaimiao regions in southern Central Asian Orogenic Belt (CAOB) is still ambiguous. In particular, the subduction polarity and tectonic origin of the Bainaimiao Arc are intensely controversial. The Ganqimaodu area in Inner Mongolia provides an excellent window to address this issue. This study presents field and detrital zircon U-Pb-Hf isotopic data of Paleozoic (meta)sedimentary rocks from Ganqimaodu to investigate their provenance and tectonic settings. New field observations show that both the Ondor sum and Hadahushu groups are composed of slightly metamorphosed flysch deposits with pervasive occurrence of "block-in-matrix" structure and isoclinal folds. Two chlorite-quartz schists from the Ondor sum Group yield age peaks at similar to 425, similar to 966, similar to 1590, similar to 1930 and similar to 2564 Ma, and three sandstones from the Hadahushu Group display similar age spectrums with peaks at similar to 443, similar to 586, similar to 960, similar to 1473, similar to 1765 and similar to 2480 Ma. New detrital zircon ages constrain the maximum depositional ages of the Ondor sum and Hadahushu groups to Early Devonian. The Ondor sum and Hadahushu groups are here interpreted as an accretionary complex due to northward subduction of the South Bainaimiao Ocean based on field data and provenance analyses. Our new results combined with previous data suggest bidirectional subduction for the western segment of the South Bainaimiao Ocean during Late Cambrian to Early Devonian. We advocate an archipelago with accretionary complexes, continental fragments, and magmatic arcs for the architecture of the southern CAOB during early Paleozoic, an analogous to modern southwest Pacific accretionary system.