Differentiating advancing and retreating subduction zones through regional zircon Hf isotope mapping: A case study from the Eastern Tianshan, NW China

Abstract

Advancing and retreating subduction zones at convergent margins, primary sites of continental crust growth and loss, are the cornerstones of tectonic evolution of accretionary orogens. Yet, distinguishing the tectonic mode (advancing, retreating, or switching) of subduction zones in fossil orogens is not always straightforward due to the scarcity of strong evidence. As an alternative approach, we utilized zircon Hf isotope mapping of arc magmatism (previously-published data) from different tectonic units in the Eastern Tianshan and the northern Tarim, revealing that (a) an early Paleozoic (≥ca. 450 Ma) southward subduction of the Junggar oceanic plate (subduction zone advance) resulted in compression and crustal thickening in the northern Tarim (upper plate), as evidenced by increasingly more enriched zircon Hf isotopic compositions, and (b) a slab rollback/retreat of the Junggar oceanic plate probably since ca. 450–440 Ma (tectonic switching from subduction zone advance to retreat), as manifested by progressive increases in zircon εHf(t) values, induced crustal extension and opening of the South Tianshan back-arc basin in the northern Tarim, separating the Central Tianshan continental arc as a micro-continent sliver. This case study highlights the great potential of regional zircon Hf isotope mapping, besides traditional investigations, in helping reveal the enigmatic tectonic mode of subduction zones in ancient orogens.