Early Paleozoic subduction-accretion in the southeastern Central Asian orogenic belt: Evidence from the Baoerhantu accretionary complex, Inner Mongolia, China

The tectonic evolution of the Central Asian orogenic belt is associated with the subduction and closure of the Paleo-Asian Ocean. However, whether the Paleo-Asian Ocean abutting the northern North China craton underwent southward subduction and when the initial subduction occurred remain debated. The Baoerhantu accretionary complex, situated in the northern part of the Bainaimiao arc in the southeastern Central Asian orogenic belt, plays a pivotal role in elucidating the early Paleozoic tectonic evolution of the Paleo-Asian Ocean. According to field mapping, the Baoerhantu accretionary complex exhibits a typical “block-in-matrix” structure of mélanges and a classic ocean plate stratigraphic sequence, characterized by exotic blocks of diverse origins within a metamorphosed siliceous matrix. Laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) zircon U-Pb dating of these blocks and matrix reveals ages of ca. 511−468 Ma for the metagabbro-metabasalt blocks and a maximum depositional age of 458 Ma for the protolith of the metasandstone (matrix), recording subduction-accretion events of the Paleo-Asian Ocean during the early Paleozoic. Geochemical characteristics suggest that serpentinized dunite blocks originated from a depleted, refractory oceanic plate basement that had been influenced by subducted slab−derived fluids/melts. Metagabbros and metabasalts can be divided into mid-ocean-ridge basalt−type and arc-type units. Mid-ocean-ridge basalt−type rocks exhibit affinities to suprasubduction zone−type ophiolites and formed through partial melting of a depleted asthenospheric mantle, whereas arc-type rocks are products of partial melting of a hydrous, depleted mantle wedge. This research identified the first suprasubduction zone−type ophiolite within the Baoerhantu accretionary complex in Inner Mongolia, which formed in a forearc region, recording initial subduction occurring during the late Cambrian. Combined with the coeval Tulinkai and Central Jilin suprasubduction zone−type ophiolites and their associated island arcs, these units constitute an initial “trench-arc” subduction system, recording southward subduction of the Paleo-Asian Ocean abutting the northern North China craton during the earliest early Paleozoic.