DECIPHERING TWO STAGES OF OPHIOLITIC MÉLANGE BELTS IN THE EASTERN NORTH TIANSHAN OCEAN DURING THE PALEOZOIC: IMPLICATIONS FOR THE EVOLUTION OF THE PALEO-ASIAN OCEAN

The North Tianshan Orogenic Belt, between the Junggar-Tuha and Yili-Central Tianshan Blocks, occupies a pivotal geological position in the southwestern Central Asian Orogenic Belt. Discontinuous Paleozoic ophiolitic mélanges along this belt record the subduction, accretion, and collision history of the North Tianshan Ocean (southern Paleo-Asian Ocean). However, its evolutionary history remains highly debated. Some researchers suggest that all these mélanges collectively represent the remnants of the North Tianshan Ocean with a long-term history spanning from the Cambrian to the Permian or even into the Triassic. Others propose that the main ocean closed before the Early Carboniferous, with a limited, re-opened oceanic basin developed thereafter. This study reveals two distinct mélange systems, the Dacaotan mélange and the Kangguer mélange, which represent two completely different oceanic basin systems.

The Dacaotan mélange (Ordovician-Middle Devonian) contains SSZ-type ophiolites, accreted ocean plate stratigraphy, and forearc flysch deposits. Provenance and paleobiogeography analyses indicate that the North Tianshan Ocean, represented by the Dacaotan mélange, served as a tectonic barrier until the Middle Devonian, then evolved into a remnant basin and closed during the Late Devonian, evidenced by unconformities, provenance interpenetration, and post-kinematic intrusions.

The Kangguer mélange, in contrast, displays less diverse and dismembered ocean plate stratigraphy, including basalts, gabbros, cherts, limestones, and turbidites. It lacks of mantle-derived components and has a relatively short time frame from Carboniferous to Early Permian. Geochronological and geochemical evidence suggests it formed in a short-lived re-opened back-arc basin, likely triggered by northward roll-back subduction of the South Tianshan Ocean, emerging within the amalgamated Junggar-Tuha-Central Tianshan Block during the Middle to Late Devonian.

These findings demonstrate the North Tianshan Ocean and the Kangguer Ocean are two distinct oceanic systems with separate evolutionary histories, challenging the notion of a single, long-lived North Tianshan Ocean. This refined understanding provides new insights into the complex tectonic architecture of the eastern North Tianshan Orogenic Belt.